Jump to content

Changing Wheel Size To Overcome Power Problem On Hills


Recommended Posts

Hi Guys

 

Vitesse engine survived Shelsley (I think) So now I'm looking at the times and data logs to see where I might improve.

 

Looking at all the class times I can tell I'm as quick of the start and through the twisty bits as the bigger cars, but I know I'm down on capacity and around 30-60 bhp and it really shows on the hill pulls where I just can't get up to the same speed.

 

So as getting more power isn't easy, I'm wondering if a change in gearing by changing wheel size would help me move the usable power band to my advantage given that I have lots of revs available, and that maximum top speed isn't an issue.

 

The engine is developing max torque between 3.5-4.5K and max hp at 6.5K  (which is the point we stopped) i.e. the hp is still climbing at max sensible (just) revs.

 

If you look at the graphs below you can see the problem, on the flat in all gears or on a hill in first and second I have enough power to accelerate quickly but on a hill when I get into 3rd or 4th at around 4k rpm (max torque) I end just gathering momentum slowly.

 

A few graphs below to show the problem.

 

Colour code:  Yellow is TPS, Red is RPM, Blue is manifold Vacuum, and Purple & Black AFR's

 

post-2759-0-94272000-1467714261_thumb.jpg

 

This is a data log from my standard on the flat WOT pull in 1st thru 4th as you can see in first and second I'm up to 6K in a couple of secs, slightly slower in 3rd but still accelerating briskly, and slower again in 4th but still accelerating until I shut.

 

 

post-2759-0-19103400-1467714304_thumb.jpg

 

This is a data log from my local test hill which is fairly steep constant and long showing the pull through 2nd-4th where 2nd is still fairly brisk, 3rd is starting to look like 4th on the flat, and 4th whilst still accelerating its now gradual.

 

 

post-2759-0-10998900-1467717660_thumb.jpg

 

 

This is the log from the top esse to the finish at Shelsley in 2nd & 3rd, the blip at 18:57 is through the top esse and then its WOT in second where I am accelerating nicely up the hill to 6K and then change to 3rd at approx 4K where as you can see I just hit a brick wall and the revs just don't rise, i.e. the power available with the gearing is only just capable of matching the increasing slope of the hill and I'm not really gaining speed. In fact if I had stayed in 2nd and carried on up to 6.5K+ and held it I would have been going quicker. The problem being that would be an extra 5 secs reving the bollocks of it which doesn't seem quite fair as I'm trying to keep this years engine in one piece.

 

So I'm wondering if I reduce wheel size and rolling radius, I will in effect reduce the overall gearing in 3rd and maybe just get over the hurdle.

 

I'm running a 4.11 diff and 195/55/15 tyres which have a rolling radius of 73.65" if I swap to 175/60/13 wheels and tyres with a rolling radius of 66.81"  it would have the same effect on gearing as changing to a 4.5 diff.

 

So am I kidding myself that changing the overall gearing like this is going to help?

 

Alan

Link to comment
Share on other sites

  • Replies 51
  • Created
  • Last Reply

Top Posters In This Topic

 

So am I kidding myself that changing the overall gearing like this is going to help?

 

Alan

 

 

No.  Surely the easiest way to change the gearing.  You don't even need to change the front wheels (on the track at least, not sure about legality for road use).

 

What it might end up doing (in some places anyway) is forcing you to make more gear changes, which will also cost you time.  I've also know people (hillclimbers) raise the final gearing so that they can use fewer gears on the run.  Maybe not the best strategy on a Vitesse where 1st & 2nd are already quite high and you've not got a very high rev limit to play with.  There are those who might say that with your carefully built bottom-end would survive to 7k at least, but I can see why you'd be a bit wary of that!

 

Other route is weight reduction.......

 

Nick

Link to comment
Share on other sites

I change wheel (or rather wheel and tyre) size to change the gearing. For track etc I run 185/60 13. And as I am off for a cruise to le mans they will be swapped for 185 55/15 to give me a 6% change in gearing. That is significant....and very noticeable.

Link to comment
Share on other sites

No.  Surely the easiest way to change the gearing.  You don't even need to change the front wheels (on the track at least, not sure about legality for road use).

 

What it might end up doing (in some places anyway) is forcing you to make more gear changes, which will also cost you time.  I've also know people (hillclimbers) raise the final gearing so that they can use fewer gears on the run.  Maybe not the best strategy on a Vitesse where 1st & 2nd are already quite high and you've not got a very high rev limit to play with.  There are those who might say that with your carefully built bottom-end would survive to 7k at least, but I can see why you'd be a bit wary of that!

 

Other route is weight reduction.......

 

 

I only have a few pounds to shed to get back to fighting weight  :yes:

 

I have a set of 13" with List 1A tyres so will probably give it a try in a fortnight back at Shelsley so will have a back to back.

 

On raising final gearing, I had gone to the 4.11 from 3.89 as with the 3.89 whilst it was pulling well and further up the hill I was running out of revs and still had a distance to go so was having to go to third when it was almost too late, so the change to 4.11 was in the hope that I would get up to 3rd earlier and make the change worthwhile, the first bit of the plan worked out nicely I'm now getting to change point with sufficient distance to make it worthwhile, however it has the discovered downside that its a waste of time.

 

Just wondering if going to a 3.63 and seeing if I can keep to 2nd might be an answer, seemed counter intuitive before as it would slow outright initial acceleration, but saving a couple of gearchanges and covering a few hundred yards at 70mph instead of 60mph might be a better result.

 

Alan

Link to comment
Share on other sites

No.  Surely the easiest way to change the gearing.  You don't even need to change the front wheels (on the track at least, not sure about legality for road use).

 

What it might end up doing (in some places anyway) is forcing you to make more gear changes, which will also cost you time.  I've also know people (hillclimbers) raise the final gearing so that they can use fewer gears on the run.  Maybe not the best strategy on a Vitesse where 1st & 2nd are already quite high and you've not got a very high rev limit to play with.  There are those who might say that with your carefully built bottom-end would survive to 7k at least, but I can see why you'd be a bit wary of that!

 

Other route is weight reduction.......

 

 

I only have a few pounds to shed to get back to fighting weight  :yes:

 

I have a set of 13" with List 1A tyres so will probably give it a try in a fortnight back at Shelsley so will have a back to back.

 

On raising final gearing, I had gone to the 4.11 from 3.89 as with the 3.89 whilst it was pulling well and further up the hill I was running out of revs and still had a distance to go so was having to go to third when it was almost too late, so the change to 4.11 was in the hope that I would get up to 3rd earlier and make the change worthwhile, the first bit of the plan worked out nicely I'm now getting to change point with sufficient distance to make it worthwhile, however it has the discovered downside that its a waste of time.

 

Just wondering if going to a 3.63 and seeing if I can keep to 2nd might be an answer, seemed counter intuitive before as it would slow outright initial acceleration, but saving a couple of gearchanges and covering a few hundred yards at 70mph instead of 60mph might be a better result.

 

Alan

Link to comment
Share on other sites

I change wheel (or rather wheel and tyre) size to change the gearing. For track etc I run 185/60 13. And as I am off for a cruise to le mans they will be swapped for 185 55/15 to give me a 6% change in gearing. That is significant....and very noticeable. 

 

http://www.mintylamb.co.uk/gearspeed/ use that and you can see where your gearchanges will leave you in terms of rpm.

 

I was finding with my old diff when I changed from 2>3rd, I was dropping out of the "best" rev range. Much better now, but harder on the ears on a long journey.

 

Interesting that's about the wheel/tyre combination that I was thinking of swapping to, and also a similar effect.

 

Yes the mintylamb charts are a great resource, it where I just worked out what the change to 13" wheels would do ratio wise.

 

Thanks for the input

 

Alan

Link to comment
Share on other sites

If you are struggling for traction off the line and out of slow bends then raising the gearing is probably going to cost very little "off the line", while saving you some gear changing.  3.63 gives great cruising manners as well.  Not that that is the main concern.....

 

Nick

Link to comment
Share on other sites

I guess the plan of attack has to be try the 13" in a fortnight, if it works and doesn't compromise the handling, then I'll try at a few other venues. If it doesn't work then I'll keep my eye's open for 3.63 diff and swap it in over the winter and try it when I have some time.

 

Wish I had tried 13" with the 3.89 diff before changing to the 4.11, but at that point it was a choice between buying new wheels and tyres, or using a brand new 4.11 Gripper LSD that I had obtained, and playing with the LSD won  :yes:

 

Alan 

Link to comment
Share on other sites

  • 2 weeks later...
Ok interesting results on the road tests so pondering about what to fit for Shelsley this weekend.

 

Spent most of the last week actually playing with fuelling, and have found that this engine loves running a bit too rich at idle and cruise and just a tad rich at full WOT. Before at protracted WOT I was going too rich which seems to be a problem with CD type carbs although interestingly more talked about on MC forums than on car forums (maybe car drivers don't actually hold WOT as much as bike riders).

 

Anyway by having a needle that is a bit rich at idle/cruise and nominally a bit lean at WOT it appears the transition going to WOT is smoother as its already a bit rich (thus avoiding the normal lean spike) and then whilst the logs show the AFR's still heading downwards at protracted WOT a slightly lean needle in this area seems to hold it above 11-11.5 below which power really drops off.

 

But anyway yesterday I was ready to do some back to back tests between the 195/55/15 and the 175/60/13 profiles, and the results surprised me!

 

I'm very lucky that within 2 miles of home I have two very quiet test roads, one about a mile dead flat and straight with no joining roads and great visibility, and the other a nice straight long and fairly steep hill, so I can go and keep repeating tests and compare results, without annoying anybody!

 

My two standard tests are a flat WOT run from 2K in 1st up through 5.6K in third, and the hill run is 2K in 1st thru 5.6K in 3rd, with the 15" wheels this just about equates to 20mph - 75mph

 

Now to compare the 13" to 15" I had to adjust the rev range for the 13" as although they should get to 5.6K quicker that is actually slower than 5.6K on the 15" and what I'm trying to measure is how the gearing affects how fast I actually get to a given speed, not how fast the engine spins up. The Mintylamb gear speed calculator was useful in getting the results in sync. The change between wheel/tyre rolling diameter is almost identical to going from the 4.11 to a 4.55 diff ratio.

 

So the results:

 

FLAT

 

15" 20mph - 75mph WOT in 1st second third           11.47 secs

13"                                "                                           11.96 secs

 

HILL

 

15" 20mph - 75mph WOT in 1st second third            17.04 secs

13"                               "                                             18.20 secs

 

So with lower overall gearing whilst from the logs I can see the revs rise a bit quicker, any gain is offset by not gaining speed as quickly and having to use higher revs.

 

I know from the rolling road that this engine is generating most torque in the 3-4.5K band, so it appears that using that torque with higher gearing is actually an advantage. So maybe plan b to look for a 3.63 diff over the winter might be a good idea.

 

Leaves me in a conundrum for Shelsley on Sunday, as my plan had been to do a back to back comparison between 15" times from last weekend with the 13", but as I know that the fueling changes have improved my test times by about 1 sec, I have slightly messed up that plan. Don't change two things at once!

 

Oh well have a couple of days to make my mind up

 

Alan

Link to comment
Share on other sites

Given that you've altered the engine and your tests show that 13" is slower, surely it's a no-brainer to stick with the 15" on for the next runs?  Wonder if 3.63 might be to big a jump?  3.89?  Though not very different from 4.11...... decisions decisions.......

 

Nick

Link to comment
Share on other sites

Given that you've altered the engine and your tests show that 13" is slower, surely it's a no-brainer to stick with the 15" on for the next runs?  Wonder if 3.63 might be to big a jump?  3.89?  Though not very different from 4.11...... decisions decisions.......

 

Nick

 

Well I wanted to try the skinny 175 13" anyway, as one of my options for next year is instead of trying to make the car go faster, to remove a couple of things that put me in the modified class and return to roadgoing and thus have better bogey times, as they say "if your having trouble raising the bridge, try lowering the water" :-)  Its one of the reasons I have also stuck with and tried to get the best I can out of 1.5" carbs, and in fact doing the runs on the 13" has I think highlighted that they are a restriction.

 

So anyway did all the runs on 13" at Shelsley, couple of minor? problems, its a bit twitchier through the Esses on the narrower rubber, got used to it, but am wondering about damp handling.  Can't get off the line without spinning wheels, with the 15" I could hover at 3K drop the clutch get a tiny bit of spin and a squeel and was off. With the 13" I can do a professional Tyre Burn, slowly tried dropping the revs to 2K but was still getting wheel spin off the line but managing to take 0.1 secs of the 64ft time. I suspect that part of the problem apart from the gearing is that the narrower 13" List1A tyres aren't as sticky as the 15" List1B tyres.

 

How did the runs go?

 

Quite well actually, all runs including practice were quicker (just) than my previous PB and managed to shave 1.04 secs on the final run.

 

Not sure if anyone is interested but here are a couple of data logs, first is 15" the other weekend and second is 13" from this weekend followed by some comments as it's all a bit weird (or obvious) depending on your view point.

Red = RPM

Yellow =TPS

Blue=MAP

Purple & Black = Front & Rear Carb AFR's

 

post-2759-0-62427400-1468946064_thumb.jpg

 

This is the 15" run note terminal speed at the Esses entry (3rd-2nd change) 59 mph, and the throttle lift at 9:45 in 2nd for the first bend.

 

post-2759-0-62085400-1468946088_thumb.jpg

 

This is the 13" run note terminal speed at Esses entry is only 1 mph faster at 60, and I know from the the Shelsley timing system that I actually made 99.5% of the 1 sec gain by the Esses split. Also note no throttle lift for the first bend instead because of the lower gearing I managed to have the revs up and pull the change to 3rd. So spent a lot longer in 3rd at WOT on the 13" on the pull up the hill. Note however that although the terminal revs are higher the actual speed isn't, well only a smidgen.

 

Likewise the final bit of the logs through the Esses to the finish, although on the 13" I held it in 2nd longer (actually took it to 6750) and less time in 3rd it was an almost identical time for that section. I have a couple of earlier run logs where I changed from 2nd at 6K but those were actually about 0.5 secs slower through that section.

 

So whilst 1 sec is a reasonable gain, I didn't really hit my target of higher speed at either Esses entry or the finish. So it looks like lower gearing isn't a solution.

 

One subtle thing you may spot in the logs which I have highlighted  with a red ! is the manifold vacuum increasing at high rpm at protracted WOT (5v is atmospheric) I think what this is telling me is that twin 1.5" even with a very nicely ported inlet manifold just won't flow enough air for this engine at high revs. And yes I have spent a lot of time testing different carb bell mouths and these ones appear the best. And also for this final logged run I actually unclipped the ITG filter box just to see if it might be a restriction, but it made no difference.

 

So I think I may be back to where I was on the last post wondering if lower gearing and higher revs doesn't really make a significant difference,   if  I try pulling everything back down the rev range (where I have torque) by using a taller ratio diff.

 

I have a good 3.89, but think I'd really like to try a 3.63.

 

Back at Shelsley mid September, so may try and find one and swap it in just to see.

 

Alan

Link to comment
Share on other sites

Sounds like an interesting weekend and not without success - more than 1s taken off PB not be sniffed at!

 

Interesting, and somewhat counter-intuitive, that your 64 foot time is better in spite of the wheelspin.......  You spend a respectable amount of time at full throttle I note - would moving up to 1.75" carbs carry a class penalty?  The fact that you don't ever seem to see atmospheric pressure in the manifold (not above 3k anyway) and that it drops as the rpm rises does show the carbs are too small IMO.  Any scope for modding the carbs themselves in a similar way to Vizard's SU mods?  Or have you done this already?

 

I can smoke my 175/70 13s pretty convincingly (2nd gear too) with a 3.63 diff, but the Toyota box has lower first and 2nd gears and no LSD either so probably only smoking one and therefore not a good comparison!

 

Nick

Link to comment
Share on other sites

Sounds like an interesting weekend and not without success - more than 1s taken off PB not be sniffed at!

 

You spend a respectable amount of time at full throttle I note - would moving up to 1.75" carbs carry a class penalty?  

 

Any scope for modding the carbs themselves in a similar way to Vizard's SU mods?  Or have you done this already?

 

 

 

If I could get terminal speeds up to around 70, which looking at similar age engine sized vehicles times doesn't seem too ambitious I could easily knock another 2-3 secs off which would have me snapping at the heals of the TR6's in my class :-)

 

Yes if I stay in modified I could use 1.75 carbs or even fuel injection  :yes: without penalty, but if I want to compete in road going I'm supposed to stick to standard induction and choke sizes.  My ambition was too see how much one could ring out of a 2Ltr on original carbs, but I think I'm fairly close now unless I'm missing something.  May try some further needle fiddling to see if I can get the AFR's up to low 13's at protracted WOT as this may release a bit more power.

 

Re Vizard SU carb mods, I have read about a few people who have tried and given up as it made the car undrivable but I'm not sure what they were aiming at.  I have an old pair of carbs (somewhere) it might be fun to try some experimentation.

 

Think I'll try and find a 3.63 diff to try as that's an easy swap and test, although finding one for reasonable money may not be easy.

 

Alan

Link to comment
Share on other sites

I modded some 1.25s years ago (they'll probably end up on the Spit as I think I still have them) for my fierce Herald 1300.  Nothing big - just blended the edges of the jet bridge and the front end of the piston where it doesn't go fully up.  I think I also modded the butterfly valve screws and reduced the shaft diameter a bit.  Seemed to work as moving up to 1.5" made damn all difference until 6k plus.

 

I would say you have found the airflow limit of your Stroms as they stand - this explains both the decreasing manifold pressure as the revs rise at WOT and the going rich - which is exactly the same thing - more vacuum at the jet bridge drawing more fuel out.

 

There's a bit on it in Vizards A-series tuning book and I'd think the same basic ideas apply to the Stroms.

 

Nick

 

PS. For diffs see

http://www.club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1464459772/

 

Marcus is a very interesting and talented man.  You won't agree on Europe though!

Link to comment
Share on other sites

Ok question folks as I can't find a definitive answer, and its to do with manifold vacuum and re the above that I'm seeing it slowly start to increase at protracted WOT high RPM.

 

What if any manifold vacuum should I be getting in ideal conditions at WOT (piston fully lifted) i.e. some or none?

 

From reading its confusing as even authoritative SU texts keep mixing their metaphors sometimes referring to to them as Constant Depression and sometimes as Constant Velocity.  And their operation also gets mangled between saying that the vacuum in the inlet manifold causes a depression in the carb which causes the air piston to lift, but  then says that the air velocity passing below the air piston  causes the piston to rise.

 

So at WOT on every inlet stroke we create a partial vacuum in the cylinder, and air/fuel from the inlet manifold and thus air/fuel from the carb rushes in to fill it, during which process there must be a partial vacuum in the inlet manifold or air wouldn't try to fill it via the carb. So even at full WOT there has to be some vacuum in the inlet manifold, correct? or not?

 

How much? there must be an ideal but I can't find it referenced anywhere.

 

Alan 

Link to comment
Share on other sites

As quite always happens rephrasing the question and following a few blind alleys can lead to an answer:

 

I did several WOT runs and recorded the maximum vacuum recorded from all the runs. On this particular car, the faster the car goes, the more is forced into the intake snorkel which sits over top of the radiator and sucks air from the grille area. Therefore, my highest vacuum was seen in 1st gear - a great design by the way, thanks Toyota. Typically, if the car does not suck up air from the grille area or some other forward facing area, the highest vacuum will be seen at peak power in 2nd gear usually.

The highest vacuum recorded was 9.1 inches of water, which, is actually very little. For those of you who are used to PSI, that's 0.327psi of "negative boost". Generally speaking, getting below 5 inches of water for "total intake restriction" (what we've just measured) is extremely difficult and would be considered superb.

To give you an idea, a bad intake system would maybe register something like 27-30 inches of water. A phenomenally good one would be well, around 4-9 inches of water.

I don't know how good an answer it is, but I found it  here

 

http://www.tuneruniversity.com/blog/2011/05/intelligent-modification-measuring-the-intake-restriction-on-project-lexus/

 

What was interesting was the comment:

 

"Typically, if the car does not suck up air from the grille area or some other forward facing area, the highest vacuum will be seen at peak power in 2nd gear usually." 

 
As this is exactly what I was getting if you look at the trace in #12 above.
 
So it looks like one of my tasks is either to try and cross reference voltage output from my MAP sensor back to vacuum, or find a good differential pressure gauge, so that I can find out if the results I'm getting are good, bad or indifferent.
 
Started work on modifying a spare inlet manifold to take 1.75 carbs, just going to be a pain getting loads of metal added just to grind it away porting one again.
 
Alan
Link to comment
Share on other sites

My impression looking at your logs is that when you say prolonged WOT what is not being mentioned is that over this WOT period the revs are also climbing and therefore the airflow requirement is increasing as the engine is pumping more air.  In your case the carbs are providing enough of a restriction to be visible.

 

In an ideal world you want the manifold pressure to be the same as atmospheric at WOT.  On my EFI system it is pretty much.  With carbs I suppose there always has to be some drop as the carbs need a differential pressure across them to work.  I don't know what numbers would be regarded as normal*. However, I think yours drop too much at the top end  and this seems to be proven by their inability to maintain calibration suggesting they are maxed out.  I guess it might be possible to increase the spring load which would presumably increase manifold vacuum across the board but might enable them to maintain mixture control a bit longer.

 

*http://www.speedtalk.com/forum/viewtopic.php?f=1&t=29700

 

This seems to be saying somewhere between 0.5" and 1.5" HG  (16.9 - 50.8 mBar less than whatever atmospheric is at the time).  Would be very useful to have a calibration on your sensor......

 

Nick

Link to comment
Share on other sites

 In your case the carbs are providing enough of a restriction to be visible.

 

 With carbs I suppose there always has to be some drop as the carbs need a differential pressure across them to work.  I don't know what numbers would be regarded as normal*. However, I think yours drop too much at the top end  and this seems to be proven by their inability to maintain calibration suggesting they are maxed out.  I guess it might be possible to increase the spring load which would presumably increase manifold vacuum across the board but might enable them to maintain mixture control a bit longer.

 

*http://www.speedtalk.com/forum/viewtopic.php?f=1&t=29700

 

This seems to be saying somewhere between 0.5" and 1.5" HG  (16.9 - 50.8 mBar less than whatever atmospheric is at the time).  Would be very useful to have a calibration on your sensor......

 

 

Well "THANK YOU NICK" for that link (interesting forum) just when I think I'm starting to get a handle on things some Bstard comes along and shows you that its time to go back to kindergarden class!   :yes:

 

Don't think stronger springs will do anything as I think I'm at full lift by the time the problem occurs and if stronger springs held the piston down I think I'd be getting even more depression/velocity at the jet and possibly going even richer. A couple of weeks ago I did play with modifying a pair of nominally week needles by thinning them in the cruise-full lift range to give correct AFR's in this range but then step back out at the full lift point to nominally give a lean AFR at that point but then allow the extra protracted WOT depression to pull it back richer. Only did a few test runs which showed I didn't have both needles the same, so switched back to some better? profiles. But I may return to this experiment.

 

Yes your right I need to find a way to calibrate the MAP sensor back to hg to see what's really happening, I think currently knowing that my inlet vacuum drops from 4.4v to 4.35v at WOT doesn't tell me much! other than its changed from x to y where I don't know what x is and I don't know what y is  :unsure:

 

So much to learn, so little of life left to do it!

 

Alan

Link to comment
Share on other sites

Speed Talk is an interesting forum with plenty of smart people.  They know American V8s best obviously.  You might also try posting here

http://kaskastnersforum.yuku.com/

and see if you can get a response from Kas himself.  He pushed the boundaries with 1.5" Stroms as that was all the rules allowed him.

 

Stronger springs:
Yes, this would send you back to point 1 on calibration. But would possibly allow you to achieve calibration over a greater range of airflow by preventing the piston from reaching full lift before the engine's maximum air demand - maybe - I don't know, I'm speculating.  

I think this would also cause higher manifold vacuum (lower absolute manifold pressure) across the range as a result of slightly increased restriction by the carb - so it might well not help your power anyway.

 

Sensor calibration:

 

nominally voltage should be directly proportional to pressure in a straight line (unless your data sheet for the sensor shows different - what is it?) so it should be possible to ratio it.

 

Examples figures (plucked from thin air)

 

Range 0 - 1000 mBar (abs)

Output 0 - 5v

 

(4.35/5) x 1000 = 870 mBar abs or 130ish mBar below atmospheric.

 

I'd prefer to do it with a calibrated gauge and vacuum pump but not often a practical option.  What voltage does it show with the engine off and does it vary a bit with the weather?

 

Nick

Link to comment
Share on other sites


 

 He pushed the boundaries with 1.5" Stroms as that was all the rules allowed him.

 

That would be an interesting line to investigate, as as far as I recall I seem to remember some claims that they were getting close to 180bhp from the 2ltr (although what size horses they were seems up for question), and as far as any other research goes 2 x 150CD might be good for a theoretical 150bhp down hill with a following wind using 110 fuel on an engine with close to 100% VE.

 

 

Sensor calibration:

 

nominally voltage should be directly proportional to pressure in a straight line (unless your data sheet for the sensor shows different - what is it?) so it should be possible to ratio it.

 

(4.35/5) x 1000 = 870 mBar abs or 130ish mBar below atmospheric.

 

I'd prefer to do it with a calibrated gauge and vacuum pump but not often a practical option.  What voltage does it show with the engine off and does it vary a bit with the weather?

 

Its a clone GM 1bar sensor. Took a reading last night 4.88V and this morning 4.95V so looks like it does vary a bit, I'm only about 150ft above sea level here so shouldn't be too much altitude adjustment to make. Looking at the chart Nick referenced a while back at http://www.aa1car.com/library/map_sensors.htm 4.9V for atmospheric looks a good number.

 

I also think I'd like to actually calibrate it as the vacuum figures at WOT between good and bad are actually close if you are trying to read using voltage.

 

In Tuner Studio I can redo the calibration into inHG, Bar, PSI or inHGa. It looks like actually using inHGa (water) may actually give the best granular readings.  Now I'm not that experienced with Tuner Studio yet as up until now I have just been using it to view and compare logs bit below is how I think I correctly alter the calibration for the MAP sensor. If anyone know better please let me know.

 

post-2759-0-43010300-1469279230_thumb.jpg

 

 

So if thats correct, then taking a bit of a Shelsley log we get 

 

post-2759-0-15171700-1469279893_thumb.jpg

 

Which seems to show at best going to WOT at 7:15 at 2.8K I have a miserable 25.85 inHGa and that at 7:30 at 4.9K its increasing to 28.65 inHGa and by the time I get to 7:50 max WOT 6.3K its climbed to 39.86 inHGa. I think the shut reading at 8.0 of 383.48 seems to be good given that -1bar is 401 and according to the Map chart I should be getting 0.3V at -1bar.

 

So if all this is to be be believed and the following statement is true

 

"To give you an idea, a bad intake system would maybe register something like 27-30 inches of water. A phenomenally good one would be well, around 4-9 inches of water."

 

Then I think we can say that despite best efforts so far I have a "bad intake system" !

 

So this has got me thinking!

 

As even going WOT at low rpm I have 25 inHGa this says this engine want's more air that it can get, also looking at the AFR logs the carbs can supply more than enough fuel, but maybe not enough air?

 

So if the carbs can supply more than enough fuel, can I give it more air?

 

So whilst idly pondering modifying carbs to flow more air, and looking at inlet manifold wondering if I could introduce more air to mix with the over rich air/fuel from the carbs, it suddenly dawns on me that decades ago I threw away the Crank Case Emission Valve (as who want oily air going back into the inlet manifold) and just externally vented the rocker, blocking off the port in the inlet manifold (actually now I'm using it for the MAP sensor).

 

What if the CCEV didn't just relieve crank case pressure and clean up emissions a bit but also supplied more air into the inlet manifold?  Reading the manual all is as clear as mud, but I certainly get the impression that most of the time it allowing the manifold to suck air from the rocker/crank case.

 

So here is my theory: When at WOT and the carbs can't flow enough air the manifold vacuum is increasing so more and more air is pulled through the CCEV thus relieving the vacuum in the inlet manifold, and thus the excess depression at the carbs which is causing the mixture to go over rich. The air pulled through the CCEV is introduced into the balance pipe chamber in the manifold so easily mixes evenly with the air/fuel mixture from the carbs thus leaning this mixture (Ok it also adds a bit of oil vapour) but overall my theory is the CCEV generally always leans out the mixture.  

 

Could this also explain why when looking at old needle profile recommendations for Vitesse and GT6 especially in the full lift area I look at them and struggle to see why they nominally are so rich, where my data logging shows that a leaner needle in this area is required.

 

Was the CCEV doing something undocumented, by accident, or have I gone off on another blind alley?

 

Alan

Link to comment
Share on other sites

Alan?

 

I like the idea, but the first thought that springs to mind is this:

 

"Where is the air coming from?".

 

To me, your idea seems logical except that it is sucking the air from the crankcase. So, how does that air get in there in the first place? And in enough quantity?

 

Phil

Link to comment
Share on other sites

Alan?

 

I like the idea, but the first thought that springs to mind is this:

 

"Where is the air coming from?".

 

 

 

 

Yes, but what would happen is you let it suck clean air?  The description in the GT6 manual says

 

In the system employed in the GT6 the oil filter cap is sealed and air is drawn in through a retrictor hole on the clean air side of the air cleaner, this air together with blow by gasses is drawn via the ECV into the combustion chamber via the inlet manifold?

 

Just wondering if instead of letting the valve suck dirty blowby gases from the rocker, it was just allowed to suck clean air?  May take a bit of playing working out how much air to allow and when, but thought I might try a passenger, a long length of tube, and a thumb over the end of it and data log a few runs to see what happens to AFR's.

 

Also came across an interesting piece about blowby gasses at high rpms, where someone was claiming that on the whole they aren't just burnt exhaust gases, but actually a mixture of burnt, fresh air fuel mixture and water vapour, the last two of which would actually be beneficial to recycle.

 

Its all conjecture, but at least worth an experiment.

 

My naive theory is sort of along the lines if  2xCD150 can't flow enough in  controlled manner at full wot, then if I had a third carb (not that I think conventional  3 CD carb set ups are good on a 6) in the balance tube then they might flow enough, and that in effect  introducing air in the balance pipe to over rich mixture from two carbs is a similar set up.

 

Its probably all Bol..ks

 

Alan

Link to comment
Share on other sites

 

The highest vacuum recorded was 9.1 inches of water, which, is actually very little. For those of you who are used to PSI, that's 0.327psi of "negative boost". Generally speaking, getting below 5 inches of water for "total intake restriction" (what we've just measured) is extremely difficult and would be considered superb.

To give you an idea, a bad intake system would maybe register something like 27-30 inches of water. A phenomenally good one would be well, around 4-9 inches of water.

 

 

 

 

 

In an ideal world you want the manifold pressure to be the same as atmospheric at WOT.  On my EFI system it is pretty much.  

 

This seems to be saying somewhere between 0.5" and 1.5" HG  (16.9 - 50.8 mBar less than whatever atmospheric is at the time).  Would be very useful to have a calibration on your sensor......

 

Nick

 

 

 

Yes your right I need to find a way to calibrate the MAP sensor back to hg to see what's really happening, I think currently knowing that my inlet vacuum drops from 4.4v to 4.35v at WOT doesn't tell me much! other than its changed from x to y where I don't know what x is and I don't know what y is  :unsure:

 

Alan

 

 

 

Sensor calibration:

 

nominally voltage should be directly proportional to pressure in a straight line (unless your data sheet for the sensor shows different - what is it?) so it should be possible to ratio it.

 

I'd prefer to do it with a calibrated gauge and vacuum pump but not often a practical option.  What voltage does it show with the engine off and does it vary a bit with the weather?

 

Nick

 

Have tweaked the Tuner Studio software to show me inHGa calculated from the voltage, which gives me a more detailed picture but doesn't tell me how good the calibration is. So today thought I'd hook up a vacuum gauge and compare output from the map sensor to see if I can believe the log readings.

 

NIck had pointed me to this chart for the GM sensor I'm using

 

map_sensor.gif

 

 

So could do a few static tests and with a passenger and the vacuum gauge and data logger display in view take some reading whilst driving.

 

With the vacuum gauge needle bouncing between 8-14 inHG at fast idle I was getting 2.5 - 3.02V from the MAP sensor.

 

At a steady(ish) 15 inHG I was getting 1.83-2.0v

 

Cruising with a light throttle holding the inHG at a steady(ish) 20 inHG I was getting 1.08-1.16

 

All of these seem to just about tie in with the expected ranges from the above chart so I think on the whole I can believe the output from the MAP sensor and the conversion range I'm using to inHGa.

 

Thought we would try a few WOT runs and see what the vacuum gauge showed.  Unfortunately all it really highlighted is that a typical inHG vacuum gauge is just about useless as a precision instrument for measuring inlet vacuum. It might be great for an idle,cruise indicator but hit it with a WOT condition and it goes to zero against the stop and hold there, whereas from the MAP sensor output we could see a variation of around 0.3V which is a hell of a lot of variation if we convert to inHGA (water).

 

Did try a single Hill WOT run to see what we could see, and its just possible that the needle wasn't held on the zero stop and might have been hovering towards the 1, but the scale is just to course in inHG to give any meaningful information, as about 1 inHG just about covers the entire range between a completely crap choking inlet and a really good one.

 

So for the moment I'm going to have to believe the MAP sensor and the LogWorks conversion to inHGa to give some insight into this area, but will see if I can find anyone with an inHGa vacuum gauge and pump to see if I can really calibrate that critical area between 4.5-4.9v

 

Alan

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...