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Data Logging Su And Stromberg Carbs


oldtuckunder

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Well this forum section doesn't seem to get much traffic these days as you have all switched to PI  :yes:

 

Well as I now have the engine running again, I get a chance to pick through the data logs from a couple of recent events and can see that the age old problem of CD carbs going over rich at prolonged WOT is still lurking there.

 

I'm data logging each carb individually for AFR's with a WB sensor in each exhaust branch, and I'm also logging TPS, but all that really tells you is what I was asking for "TPS" and what the end result is "AFR's"  what you never know is where that dam air piston is and thus what position the needle is in.  Now I've seen people set up video cameras to record the piston lift and then try an tie it back to a data log (but you need to remove the filters and inlet ducting to do that, and I've seen people drill holes in the carbs and drive about with the bonnet off to try and watch what happens (although not on a Vitesse, GT6 or Spitfire!).  And I've also seen how a Manometer can tell you lift at one specific time point.

 

So I have been playing with a project for a couple of years to tie exact piston lift into the data logs, in a way that it can be installed and left without affecting the operation of the carbs.

 

Lets say I've worked on dozens of idea's and prototypes to try and find a method that would work with all types and sizes of SU and Stromberg's, and the current one I have on the bench seems to do it, but I need some scrap SU carbs to check out a few final things.

 

So does anyone have any scrap SU's?  I'm happy to pay postage. I have every size of Stromberg and HIF SU's but could do with a 1.25, 1.5 and 1.75  H and HS SU.  It can be broken, chipped, damaged, it doesn't need a float bowl, or butterflies just a body, dome, piston, damper rod and cap, ideally with a spring (but not necessary).

 

If anyone has one please PM me!

 

Alan

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I've seen something about this subject before, either on here or on CT.......  Was talk or using linear encoders or somesuch but don't think it was ever completed, or at least the write-up wasn't.

 

I certainly have a 1.25" HS carb and quite likely a 1.5" HS too.  Won't have any of the others.

 

The full rich problem might be you maxing out the carbs...... once the pistons are fully up they can't meter any more.  Stronger damper springs (with needles to match)?  Next size up carbs?

 

Nick

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The full rich problem might be you maxing out the carbs...... once the pistons are fully up they can't meter any more.  Stronger damper springs (with needles to match)?  Next size up carbs?

 

 

I know at somepoint that I need to go to 1.75" but I'm a bit perverse and want to find out what you can get from points and standard carbs, before changing.

 

The problem with an over rich mixture is that you can get it for two reasons, either fully lifted and still too much depression sucking too much fuel out because the carbs won't flow enough air, or not enough lift causing too much depression and too rich a mixture, and as soon as your AFR's go below 11 everything goes to pot as the engine efficiency get worse and power drops, even worse on a hill as on the flat you can quite often accellerate through the problem and everything comes back into balance, especially as on the flat holding WOT for more that a second or two without needing to change up again is almost impossible.

 

I have more springs and magic weight oils that I have tested and data logged than you can shake a stick at, and I can put together combinations that will do perfect standing starts, but go rich in mid range, or will give over rich or over lean starts, are perfect up to 4K WOT and then go off track, but with all of these I never know where that piston was and what part of the needle is in play.

 

I'm not deperately unhappy with my current settings, but know there is a fair bit more to be gained, I also know if I upgrade the carbs I'll be back in the experimentation stage, whereas if I know what's happening it becomes more of a science than an art form.

 

Alan

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My dissertation in computer science was pretty much on this before i started my PhD the other year, well on SU carbs and tuning using piston position with an SU carb against AFR. I can share most parts of the writeup with you and the Software if you want via private message. I cannot share it all as my uni was a bit funny as I signed a bit of paper that said that they owned my intellectual property so it means they wanted to market it and sell it and i said no so it sits there in a stale mate for 7 years. 

Did quite a bit of testing on the rolling road and it all worked very well. 

Using a base sequence you can them map upper and lower limits of vacuum against needle as every car is different, use then the base map to map against needle position and AFR

 

 

Screen%20Shot%202016-05-11%20at%2019.46.

Screen%20Shot%202016-05-11%20at%2018.58.

Screen%20Shot%202016-05-11%20at%2018.59.

 

One question though what is the PI forum?

Edited by GT6_sleeper
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Sorry I meant that a lot on this forum had switched to Petrol Injection, and thus the sub forum on Carbs didn't see much traffic  :yes:

My dissertation in computer science was pretty much on this before i started my PhD the other year, well on SU carbs and tuning using piston position with an SU carb against AFR. I can share most parts of the writeup with you and the Software if you want via private message. I cannot share it all as my uni was a bit funny as I signed a bit of paper that said that they owned my intellectual property so it means they wanted to market it and sell it and i said no so it sits there in a stale mate for 7 years. 

Did quite a bit of testing on the rolling road and it all worked very well. 

Using a base sequence you can them map upper and lower limits of vacuum against needle as every car is different, use then the base map to map against needle position and AFR

 

One question though what is the PI forum?

 

Really impressive/interesting your work on SU digital tune, and that you were using vacuum readings (from the manifold?) And I can see that with a rolling road you could map and then match to AFR's, one of the problems I have been struggling with which is why I wanted real time logging of needle position (apart from not having a rolling road) is that if you change spring rates or damper oil, where the piston is and how long it takes to get there for any given throttle opening changes (I deduce from AFR's). I have also found that how hot the carb gets can also dramatically affect its transient responses as the oil thins, I now duct air to ensure that my carbs stay as cool as possible when driving, another reason I want the oil/water heat exchanger fitted as I discovered that idling for 25 mins to get the engine oil up to temp also raised the carb body temp by 25C which completely buggered the lift response I had engineered. After the first run at MIRA I had to suck all the hot oil out the dash pots and refill them with fresh cool oil just before the next run to get things working correctly.

 

When I've got the logging working, I'd love to contact you and see where the data between measuring lift and AFR's correlates to your data set. 

 

Alan

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Sorry I meant that a lot on this forum had switched to Petrol Injection, and thus the sub forum on Carbs didn't see much traffic  :yes:

My dissertation in computer science was pretty much on this before i started my PhD the other year, well on SU carbs and tuning using piston position with an SU carb against AFR. I can share most parts of the writeup with you and the Software if you want via private message. I cannot share it all as my uni was a bit funny as I signed a bit of paper that said that they owned my intellectual property so it means they wanted to market it and sell it and i said no so it sits there in a stale mate for 7 years. 

Did quite a bit of testing on the rolling road and it all worked very well. 

Using a base sequence you can them map upper and lower limits of vacuum against needle as every car is different, use then the base map to map against needle position and AFR

 

One question though what is the PI forum?

 

Really impressive/interesting your work on SU digital tune, and that you were using vacuum readings (from the manifold?) And I can see that with a rolling road you could map and then match to AFR's, one of the problems I have been struggling with which is why I wanted real time logging of needle position (apart from not having a rolling road) is that if you change spring rates or damper oil, where the piston is and how long it takes to get there for any given throttle opening changes (I deduce from AFR's). I have also found that how hot the carb gets can also dramatically affect its transient responses as the oil thins, I now duct air to ensure that my carbs stay as cool as possible when driving, another reason I want the oil/water heat exchanger fitted as I discovered that idling for 25 mins to get the engine oil up to temp also raised the carb body temp by 25C which completely buggered the lift response I had engineered. After the first run at MIRA I had to suck all the hot oil out the dash pots and refill them with fresh cool oil just before the next run to get things working correctly.

 

When I've got the logging working, I'd love to contact you and see where the data between measuring lift and AFR's correlates to your data set. 

 

Alan

Hi Alan,

Yeah no problem. 

the main problem with carbs are they are only two dimensional tuneable with regards to the needle. As you point out the oil temp, spring rate even the the air pressure can have an effect, however the carb is only tuneable for whatever setup and use on a particular car. This is why there was no way when doing the stuff i did i could ever tune the end of the needle(the bit used when the choke is on) this was because every choke operation is so different and often not repeatable as owners push them back in when they feel it is time too. Therefore I only looked at tuning a needle when the following was checked first: oil grade, spring operation, linkage setup and adjustment corrections. Any change to the setup then all previous data must me scrapped and fresh data recorded so no bias is given. Also the engine must be up to full operating temp. 

 

What are you using to log your data? I can provide you with a way of logging data cheaply with a combination of off the shelf parts if you have not yet bought the hardware?

 

During the 18 month project I logged over 400 engine running hours and 20 rolling road hours on a total of 33 different setups and there was always common things the program picked up. Towards the end of the project we got the self calibration settings so good that a new car with a new setup could be added to the hardware and within 40 minutes of driving around in normal traffic the software could show a good use suggestion to an alternative needle if one was needed. 

The software currently works out the perfect needle profile then depending on mode will:

in basic mode suggest the closest standard needle to your ideal profile.

advanced mode select a needle that is just thick enough in all areas then advise you at what postion on the needle it must be sanded to and by how much (as seen in the bottom image n my above post)

The next stage of the project was for the software to automatically produce a CAD file for an optimum setup that could be used to make a brand new needle or cut an oversized needle to size automatically. Just need a Masters student to pick this up as a project as it falls way outside my PhD work.

  

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Amazing.....  :bow:

 

Was this before or after your venture into MS land?  I suppose after......

 

Nick

way after. i finished the digital SU tune stuff summer 2014

 

I only did it because they said i could and helped by chucking a load of money at the project. The original idea was to give it away all free along with the research but the uni had other ideas. Hence I cannot post online (but i can send to people for further development/testing ;) )

You can build the hardware for less than £100 if you buy some bits second hand and the user software all just runs on a java platform on both mac and windows. The program auto detects which operating system it running on and works from there. the windows one in an .EXE and the Mac is a .JAR 

the hardware runs independent of a computer on its own software. you just load the user software on the computer plug it into your usb and press sync. 

I created a fake dataset for a rover P6 so all functions can be loaded on the software without plugging in the hardware. 

 

 

Edited by GT6_sleeper
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If you ask me Nick if this would mean i would choose SU over injection the answer is NO!!!!
if anything i really and truly understand how simple the carb is but at the same time how rigid the tuning capabilities are in comparison to a modern fuel injection setup. 

 

For me it always has to be fuel injection with a mappable ignition system

 

BTW Nick if you also want a copy of the user software let me know and i can send it over. 

Edited by GT6_sleeper
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What are you using to log your data? I can provide you with a way of logging data cheaply with a combination of off the shelf parts if you have not yet bought the hardware?

 

 a new car with a new setup could be added to the hardware and within 40 minutes of driving around in normal traffic 

 

What's this traffic stuff?   :P

 

Fairly amazing stuff you were working on.

 

I'm currently logging to an Innovate LM2 so apart from AFR's anything that I can generate a 0-5 volt analog signal from I can log like TPS and the piston lift sensor I'm working on. I'm assuming that you were logging manifold vacuum?  what kit were you using to do it, as I may as well add it as part of the current work as it looks as though it may well be usefull.

 

I had only got as far as wanting to know where the needle is, whereas as it seems you are on the next rung off the ladder working out the changes required to the needle!

 

I joked to Nick at Gurston that playing with oil weights and springs and logging AFR's you can play tunes with CD carbs, and that if I knew where the needle was I thought that given a standard needle with a straight taper proved the low and high points were correct anything else in between was almost irrelevant :-)

 

In reading your text, it keeps reminding the difference between the SU and the Stromberg in that on the SU you have that funny choke jet movement thing to factor in,  I could never work out why people keep talking about needle profiles way beyond the range that the piston moves. i.e. on a 1.5" carb there are 13 1/8" dimensions, whereas in operation a 1.5" carb only moves the piston about 1&1/8th of an inch i.e. only 9 of the 13 needle positions are ever operative.

 

Do we really think the designated choke points on an SU needle ever see use? its just that you need an awfull amount of piston lift for them to be effective, and with a cold engine that's one hell of a lot of manifold vacuum to get that amount of lift which basically means flooring the throttle on a stone cold engine?

 

Sorry gone off track :-)

 

Alan

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What's this traffic stuff?   :P

 

Fairly amazing stuff you were working on.

 

I'm currently logging to an Innovate LM2 so apart from AFR's anything that I can generate a 0-5 volt analog signal from I can log like TPS and the piston lift sensor I'm working on. I'm assuming that you were logging manifold vacuum?  what kit were you using to do it, as I may as well add it as part of the current work as it looks as though it may well be usefull.

 

I had only got as far as wanting to know where the needle is, whereas as it seems you are on the next rung off the ladder working out the changes required to the needle!

 

I joked to Nick at Gurston that playing with oil weights and springs and logging AFR's you can play tunes with CD carbs, and that if I knew where the needle was I thought that given a standard needle with a straight taper proved the low and high points were correct anything else in between was almost irrelevant :-)

 

In reading your text, it keeps reminding the difference between the SU and the Stromberg in that on the SU you have that funny choke jet movement thing to factor in,  I could never work out why people keep talking about needle profiles way beyond the range that the piston moves. i.e. on a 1.5" carb there are 13 1/8" dimensions, whereas in operation a 1.5" carb only moves the piston about 1&1/8th of an inch i.e. only 9 of the 13 needle positions are ever operative.

 

Do we really think the designated choke points on an SU needle ever see use? its just that you need an awfull amount of piston lift for them to be effective, and with a cold engine that's one hell of a lot of manifold vacuum to get that amount of lift which basically means flooring the throttle on a stone cold engine?

 

Sorry gone off track :-)

 

Alan

I think we need a good chat Alan, You don't happen to be at classic Spa this weekend do you?

With regards to vacuum I found that the OEM MAP sensors work well. I like the Peugeot ones. 0-5v readings like your after. 

arduino uno r3 with a nice shield board will easily take inputs you need and can log to an SD card and rather cheap. 

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I think we need a good chat Alan, You don't happen to be at classic Spa this weekend do you?

With regards to vacuum I found that the OEM MAP sensors work well. I like the Peugeot ones. 0-5v readings like your after. 

arduino uno r3 with a nice shield board will easily take inputs you need and can log to an SD card and rather cheap. 

 

No not going to Spa :-(

 

Just looking at MAP sensors, do you happen to recall what manifold vacuum ranges you were typically seeing? just seeing if I can find one that I can fit remotely and just run a short hose to the manifold, rather than fit the sensor direct into the manifold, some of the GM Vauxhall ones look like they may fit the bill.

 

Alan

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m

 

No not going to Spa :-(

 

Just looking at MAP sensors, do you happen to recall what manifold vacuum ranges you were typically seeing? just seeing if I can find one that I can fit remotely and just run a short hose to the manifold, rather than fit the sensor direct into the manifold, some of the GM Vauxhall ones look like they may fit the bill.

 

Alan

mine were remote ones. 

I used these Peugeot ones

Screen%20Shot%202016-05-12%20at%2015.48.

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Hi many thanks, and I did look at solutions like that which are way better than nothing, but the problem is that they completely mess up the damper action, and the most misunderstood thing about (semi)Constant Depression Carbs is the effect of the damper at snap WOT, Standing Starts and very rapid gear changes, which are the real problem areas. Setting up for cruise isn't too much of a problem.  The other oft quoted falicy (even in the expert books) about the dampers is that they only have an effect on piston lift, whereas it only takes a few data logs of AFR's using different weight oils to show that is completely untrue, and how fast/far the piston falls can have a dramatic effect on transient mixtures during WOT gear changes. I can show the effects the only thing I don't know is exactly where the piston is and how fast its rising and falling, hense the desire to have a mechanism that can data log exactly what is happening all the time without affecting the operation of the carb.

 

Alan

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It is possible to get proximity sensors that can sense magnets through aluminium, so with a small but powerful magnet riding on the piston and sensors on the outside you could get digital signal(s) at least.

 

All other methods I can think of would be invasive to a degree and there inevitably have some effect on the working of the carb.  Done well it would only be a small effect though.

 

Nick

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It is possible to get proximity sensors that can sense magnets through aluminium, so with a small but powerful magnet riding on the piston and sensors on the outside you could get digital signal(s) at least.

 

All other methods I can think of would be invasive to a degree and there inevitably have some effect on the working of the carb.  Done well it would only be a small effect though.

 

 

Yes I played with those, the problem is that the only place you can fit a magnet in a common place is on top of the piston damper tube, which means you can only have a circular magnet about 2mm thick and 2 mm tall otherwise they interfere with the operation of the damper, and all the magnetic contactless sensors I found require a very strong magnet pull even when the magnet is very close. I even tried flex sensors inside the chamber, the problem with those is that they have a memory and very rapidly loose their range response, well way to quickly to leave in there!

 

I think I have all the bits now for my solution so hoping in a week or two to be able to post some logs.

 

Alan

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Hi many thanks, and I did look at solutions like that which are way better than nothing, but the problem is that they completely mess up the damper action, and the most misunderstood thing about (semi)Constant Depression Carbs is the effect of the damper at snap WOT, Standing Starts and very rapid gear changes, which are the real problem areas. Setting up for cruise isn't too much of a problem.  The other oft quoted falicy (even in the expert books) about the dampers is that they only have an effect on piston lift, whereas it only takes a few data logs of AFR's using different weight oils to show that is completely untrue, and how fast/far the piston falls can have a dramatic effect on transient mixtures during WOT gear changes. I can show the effects the only thing I don't know is exactly where the piston is and how fast its rising and falling, hense the desire to have a mechanism that can data log exactly what is happening all the time without affecting the operation of the carb.

 

Alan

 

You're really missing a circuit if that's what you're after. 

 

Better to just use a Weber. With a tuner who really knows what they're doing, they are very hard to beat, even against EFI. 

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You're really missing a circuit if that's what you're after. 

 

Better to just use a Weber. With a tuner who really knows what they're doing, they are very hard to beat, even against EFI. 

 

I agree if I was after absolute maximum gain then Webers or EFI would be the way to go, however the down side is that would bump me up a class and my target bogey times would drop to the point where any extra power delivered would be eaten and then some, hense the desire to achieve the maximum I can from the standard set up.

 

Is also a personal challenge to see what's possible from a pair of twin CD's with the tools available these days, as as your comment indicated most people gave up on them long ago!  I think there's still a few more unexploited horse power left in them.

 

Alan

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I get it. It's a cool project! 

 

Some things to consider:

 

Carburetors work off pressure differential. Everything that happens downstream of the carburetor will effect how it meters fuel. 

 

Going back further, how well the camshaft is timed, the velocity through the ports (both intake and exhaust), and how well the exhaust is designed will have a huge impact on engine performance, but WOT and part throttle. Really what we're talking about is 'transient response'.

 

Road racers and circle track racers tend to use the term 'transient response' while drag racers tend to use 'shift recovery'. Either way, they're after the same thing. 

 

So what am I talking about? It's really the engine's ability to recover and start.

 

From a post by Dr. Neels Vannik, agreeing with Calvin Elston (a pioneer and one of the top exhaust designers/fabricators in the US (NASCAR, NHRA, WoO, IMSA, etc.)):

 

 

During a shift, whether with short throttle blip, or momentarily engine cut or whatever screws up the wave action and flow through the engine. So it does not fall back to the maximum line (torque or power) but below it. Transient response is how quickly the engine recovers back to this max line

 

So where am I going with this? You need to get the rest of the package sorted out before you can really try to measure and make any meaningful changes at part throttle and the recovery portion of the shift. 

 

Also, look at the thread I linked to. Jet signal is huge as this is what the jet sees. Improve the jet signal, and the carburetor will respond a lot faster as well to changes in depression. 

 

I'll post more later. 

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Thanks for the post, I'd like to think that I have all the fundamentals in the engine build done well (but don't we all?) and I'm not unpleased with its performance, and with the AFR data logging over the last couple of years where I haven't been pleased with it I have almost always been able to see that the AFR's are not where they need to be, and adjust them by trial an error. The main problem always being that I have had to guess what's happening to the piston, although by changing things like oil weights and Springs and repeating tests I get a feeling for what's happening.

 

A couple of logs below show what just changing oil does.

 

Red = RPM, Yellow is TPS, Black is cyl's 123 (carb1) Purple is cyl's 456 (carb2)

 

post-2759-0-12911800-1463310373_thumb.jpg

 

1st, 2nd, 3rd at WOT using 20/50

 

post-2759-0-81304000-1463310374_thumb.jpg

 

1st, 2nd, 3rd at WOT using 80/90

 

post-2759-0-43161800-1463310376_thumb.jpg

 

30 - 70 WOT Pull using 20/50 (NB had to ease back on the throttle on this run as AFR's dropped to 10 at which point power starts disappearing in a rich misfire) AFR's below 11 and you may as well be pressing the brake peddle.

 

post-2759-0-22223500-1463310371_thumb.jpg

 

30 - 70 WOT Pull using 80/90 (NB Just manages to hold AFR's above 11)

 

 

Alan

 

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I had a go at similar stuff using linear pots - worked partially using a techedge logging unit but allot of lag and not very repeatable. Lost interest but could have got there with an oil bath maybe...

It was a bit of fun though... Carbs have central TPS and vacuum log on each.

 

http://sideways-technologies.co.uk/forums/index.php/topic/6165-twin-su-lambda-readings/page-2

 

Maybe of interest...

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I had a go at similar stuff using linear pots - worked partially using a techedge logging unit but allot of lag and not very repeatable. Lost interest but could have got there with an oil bath maybe...

It was a bit of fun though... Carbs have central TPS and vacuum log on each.

 

http://sideways-technologies.co.uk/forums/index.php/topic/6165-twin-su-lambda-readings/page-2

 

Maybe of interest...

 

Interesting thread, I also had a play with magnetic sensors (pity I hadn't read your thread first) and found that the readings were a bit erractic just moving the piston slowly by hand. Also impossible to adapt to the Strombergs as the piston is at least an inch away from the ouside of the carb and there is no vertical surface to mount against. 

 

Interesting to note the differences between your logging results and mine, you were getting a rich spike on lift and going lean at high RPM's whereas I have engineered lean on lift, going rich on sudden WOT and staying rich (and getting richer at sustained WOT high rpm's).  NB I do this all so far by playing with oil weights and spring weights and not touching the needle.  That's next when I know where the dam needle actually is  :yes:

 

Alan

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OK MAP Sensor Guru's Input required!

 

About to install the MAP sensor just to add to the data logging picture and I have a couple of questions. 

 

First one should be easy. I have the MAP sensor (its supposed to be a 1bar model) sat on the bench and hooked up to a 5V supply and have a Voltmeter hooked up across Vout and Grnd.  At turn on i.e. no vacuum I am getting 5v constant and when I apply a vacuum (using a syringe) I can pull the signal down to about 2v. Is this the correct way around? For some reason I thought I'd get 0v with no vacuum and pull up to 5V as vacuum was applied.   Have played with swopping wires around but this appears to be the only combination that makes the sensor work.

 

Second question. I'm trying to decide where to take the vacuum from. Bearing in mind what I'm trying to do is just to log manifold vacuum so that I can see how it correlates to carb piston lift and AFR's not to drive an EFI system.

 

I seem to have two choices. Go straight into inlet manifold which should give me a consistant reading, or use one of the spare carb vacuum ports that is normally use to pull distributor vacuum advance. The easy one would be to use a spare carb port, but these ports are carefully positioned to that when the throttle is shut the butterfly plate just blocks off the port to kill any vacuum advance, so the effect I think on the MAP sensor would be that the vacuum suddenly drops to zero, whereas in the manifold I think (but almost certainly could be wrong) the vacuum (especially on a shut at high rpm) has suddenly shot sky high.  I guess as I'm trying to correlate manifold vacuum to the vacuum pulling the carb piston up I'm probably better using the carb port rather than tapping into the inlet manifold, its just rather interesting that when you read of people installing vacuum gauges on normally aspirated cars to help with tuning (and those old fuel efficiency gauges) they always seem to tap into the inlet manifold.

 

It also makes me think that in the past I have often seen set ups where the vacuum advance is taken from the inlet manifold, not the carb, so what happens in those sets ups on throttle shut does the timing suddenly go to max vacuum advance?.

 

Of course thinking about this as I'm writing it does occur to me that I could be wrong about that carb vaccum port, when the butterfly closes just about over it instead of shutting of the vacuum signal may it also sees a huge vacuum as air is pulled through the tiny gap around the butterfly.

 

OK answered my own question, just consulted the DesHammill Ignition Systems books, and of the course the answer is it depends. Some distributors are set to use manifold vacuum, and some are set to use a reduced vacuum taken from next to the butterfly. Mixing these up can cause problems and is apparently often seen on Rover V8's when people replace the SU's or Strombergs with a Holley or Webers and then take the vacuum advance from the manifold when the distributor is set up to use the reduced vacuum from the carbs! excessive idle & cruise advance is a result, however acceleration and WOT isn't affected.

 

Doesn't answer where the best place to take my MAP readings from is, but I guess I can try carb port first (as its easy) and if the results look a bit compressed try moving to the manifold.

 

Alan

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My first answer to your sensor output signal was that I don't know 'cause MS deals with it and provides me with the numbers......

 

However, this suggests that what you are seeing is normal

http://www.aa1car.com/library/map_sensors.htm

 

As to the take off point - as you say, it depends.  Some carbs have a suitable port that shows manifold vacuum.  Others have a tapping that is swept by the butterfly and probably would not give the right result.  I'd connect to manifold proper myself to be sure.

 

Nick

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