Andrew's Mk IV Spitfire

[egret]

I've finally done some calibrated runs and am now pretty sure my car is running a speedo drive for a 4.11 diff, but actually has a longer 3.63 diff in it.

I'd like to get a 4.11 into it and, like all triumph bits, there appear to be several subtle differences in the differentials. Spitfire graveyard have what they are calling a "1500 us spec" diff. It then goes on to say it has a 6 stud pattern, but I don't know what that means. The drive pinion gear connection flange looks like the round one on my car, so I think it'll fit.  Can anyone help clarify? Photo of propshaft and diff connection for info

 

 

[Nick Jones]

Round input flanges usually mean 3.89, 3.63 or 3.27. These also have the larger output shaft flanges with 3/8 UNF bolts.  Most 4.11 diffs have square input flanges with smaller 5/16” bolts and smaller bolt PCD. I believe there are some oddball ones with larger, round flanges but not sure they were ever production items. Output flanges are the same but on later 4.11 diffs the stub shafts interchange with the others.

6 bolt refers to the number of drillings for the spring mount. 6 is for fixed spring. Swing spring have 4 but you can always plug the extras.

FWIW I would use a 3.89 on a road-going 1300, not a 4.11. They aren’t enormously different in effect and are direct swap!

[egret]

I've been using it more as it's running really well.  Having a bit more time to mull over the various foibles as I'm driving with it has been useful. 

I noticed that the minor gearbox issues I've been having, mainly first to second changes, seem to stem from the engine taking it's time to lose revs on a closed throttle. So when you're at high revs in first, you have to wait for a while for the engine revs to get to a point where the syncromesh isn't being asked to work very hard.  The following is my musings on the subject, and I'm very much open to advice please!

Attached is a video showing the issue, I'm snapping the throttle closed every time and it seems to hesitate at around 2,000 rpm.  Am I expecting too much of old carburettors, or is there an issue here?

 

Edited November 11 by egret

[Escadrille Ecosse]

2nd synchro is usually the first to get tired simply because it has the biggest speed differential to cope with. And more so on the wide ratio Spitfire box compared to the closer ratio GT6/Vitesse.

And the standard flywheel on the 1500 is a fairly heavy beast which adds to the inertia.

Lightening the flywheel helps the engine speed up/slow down more quickly but probably the 'best solution' is a gearbox overhaul, or at least new synchros.

The impecunious youngster solution back in the day was to strip the box, swap 4th synchro (usually least wear) for 2nd and put the whole lot straight back together.

[Nick Jones]

Looks like you have pretty late HS4s so possibly you have the spring loaded “anti-vacuum discs” in your throttle butterflies.  I remember them causing strange effects on my Herald in its 1500 era. I soldered them shut. 
 

That said, it doesn’t seem that slow to return.

[Escadrille Ecosse]

Soldering up those discs is a well trodden path and worth doing regardless of anything else.

As Nick says they are a source of "strange effects".

[egret]

Thanks both, I removed those anti vacuum things long ago.  Knowing the return isn't excessively lazy is really useful info.  It sounds like I need to continue to be extra sympathetic on 1st to second gear changes.  I'll file it away under "reasons for machining the flywheel" and add that to the savings figure I need to achieve before I pull the engine for the camshaft/piston rings, and now flywheel works.

[PeteStupps]

On 11/12/2024 at 1:38 PM, egret said:

Knowing the return isn't excessively lazy is really useful info

Doesn't sound unusual to me, that apparent hesitation around 2000rpm could just be the gauge rather than the engine. By ear it sounds normal. 

If it makes you feel any better, my gearbox was rebuilt in 2021 with new synchros and NOS gearset, and it's already a pig to get into 2nd. Probably says more about my rebuild efforts than the design and hardware quality though...

[egret]

So, carburettors.  Frustrating.

As mentioned in the today I bollocks thread I was fiddling with the carburettors to visually check throttle plate idle openings and thought I'd do an in situe drop test and noticed that one piston wouldn't fully rise, and that the damper seemed to be the thing preventing full rising.  As can be seen from that post, the damper tubes on the dashpots are not the same height, one is seemingly obviously taller.  Frustratingly I missed this during the selection and minor refurbishment process for these carburettors.

These non-matching carbs are ones which came in a box when I got the car as a spare set.  They must be a bitsa and appear to be late HS4s with the spindle mounted return springs and ball bearings on the piston dampers.  They have the wider tops and large diameter springs.  The taller unit stops about 1/4" of travel at the top, meaning that carburettor will be subjected to larger depression, but smaller jet opening.

I'm not actually sure the engine is performing enough to really pull the pistons to the top of their travels as it's a mk3 engine, not the 1500 that had HS4s, but it feels right to at least try and make the carburettors I've got match up.

After a cup of coffee I think the best solution is to use the tops (bell, piston, damper assembly) from the other HS4s I have.  I'm can't be sure these are any less of a bitsa, but they at least appeer to be matched pair under closer inspection.  I'll loose the ballbearings, but that's not a big deal.

I could mill the top off the mismatched one.  Feels a bit brutal, but given that I've never heard of different heights being an issue, I'm not sure anyone will come after me for butchering historically significant parts...

 

[Gt64fun]

55 minutes ago, egret said:

I could mill the top off the mismatched one.

I actually had the height of the top, damper chamber and damper rod of my Stromberg 175 CD2s reduced by 6mm to create bonnet clearance.

By careful measuring I worked out that this could be achieved whilst still allowing the slide to fully open...not that it'll ever get there!

Ian

[Escadrille Ecosse]

6 hours ago, egret said:

After a cup of coffee I think the best solution is to use the tops (bell, piston, damper assembly) from the other HS4s I have.  I'm can't be sure these are any less of a bitsa, but they at least appeer to be matched pair under closer inspection.  I'll loose the ballbearings, but that's not a big deal.

Probably simplest. Never know what you might find when you take them apart.

5 hours ago, Gt64fun said:

I actually had the height of the top, damper chamber and damper rod of my Stromberg 175 CD2s reduced by 6mm to create bonnet clearance.

Hardcore

[Nick Jones]

So, if I’m understanding correctly….. the dashpots match and the damper rods match, but the ground guide tube on one of the pistons is longer than the other and too long for the dashpot?

I have a vague memory from long ago that the ball-bearing setup is slightly shorter…. But that might be wrong or a generalisation.  Assuming the guide tube depth into the piston is the same on the longer one I’d be tempted to just lop a bit off it (a few seconds work with a 1mm slitting disk). However, before doing this you might want to consider that your pistons and dashpots are clearly not matched sets and you might want to experiment within your collection to see which combinations give the best (and best matched) drop-test results. 
 

https://youtu.be/IfU47Oqq9wA?feature=shared

[rogerguzzi]

Hello All

              They did dashpots with a shorter top tube bit so I assume that is what the one is and the other for the taller tube bit?

Roger

[egret]

All good advice above, thank you.

I did some drop tests and the pairs I have are reasonable.

The 'older' dashpots were installed for a run this afternoon. They had the needles swapped into them and the softer of the two narrow springs I have.  I'm pretty sure these are slightly stronger than the 'newer' dashpot ones.

Another dynamic at play is that the dampers are different (later dashpots have larger diameter threaded sections for the damper) and I think better, at least they allow the pistons to drop faster. 

So I have the same oil, same needles and bodies, slightly stiffer springs with the new dashpots, different dampers, and finally bushings on the pistons.  I had to reduce the idle, but didn't change the jet heights although I probably should have.

The result is a somehow sharper(not quite the right word) engine, more responsive/eager and clunky through the drivetrain. Runs hotter at motorway speeds, but doesn't seem any faster under load through the revs.

People talk about fast drop dampers, and I think this is what I'm noticing in the more responsive engine at town speeds. Probably running leaner at motorway speeds too, probably a bit too lean really.

While I'm aware fuel injection has it's difficulties, the big difference in engine feel just from a spring and damper change really makes me wonder about the huge range of variables that are difficult to keep track of, let alone control with carbs, and SUs are supposed to be the simple ones!

I think Christmas present might have to be a wide band lambda sensor so I can remove some of the guesswork here.

Photos below show pistons at full travel, one is clearly not the right set.

[Nick Jones]

Running noticeably hotter is lean. Likely too lean. You might want to try the stronger springs as that should make it a bit richer with the same needle.

Though, iirc, basic setup would be be aim for a the spring that gives a fully (or near fully) raised piston at max airflow and select the needle from there. In this case the carbs are a bit big so you might not actually want them to open all the way. Witchcraft…..

[rogerguzzi]

Hello 

            Those are the short top type and a tall top piston!

This what tall tops look like

Shop Pair of HS4 Carburettors for a MGB 4 cyl 1969-71 | SU Carbur | SU Carburetters

Roger

[Sigma hurricane]

I have a 1300 engine at the moment with HS4’s (obviously the wrong carbs for the engine) however took it to a good guy with a rolling road who managed to to completely overhaul the way the engine ran by custom machining the needles to get complete combustion through the rev range. Was only just over £100 so would be potentially well worth it and less faff than having a permanent lamda sensor?

 

(They will also be going spare once the engine swop starts in the near year)

[egret]

30 minutes ago, rogerguzzi said:

Hello 

            Those are the short top type and a tall top piston!

This what tall tops look like

Shop Pair of HS4 Carburettors for a MGB 4 cyl 1969-71 | SU Carbur | SU Carburetters

Roger

Pretty sure I could go full efi for that £££

[Escadrille Ecosse]

11 minutes ago, egret said:

Pretty sure I could go full efi for that £££

Yes.

But Roger was trying to show the different daspot shape/height rather than nsell you more carbs

[egret]

Hard to convey tone sometimes in text.  I was not suggesting that Roger, who seems to be fully sold on the EFI route, is prodding me to buy new carbs and was being a little flippant on the prices.  When you consider what goes into making a set of carbs then just over a grand with the VAT is really not bad at all.  I've been spoiled with finding bargains and being able to utilise what often amounts to other peoples clutter to slowly take my car from an unreliable wreck to a sort of reliable roadworthy thing!

It's interesting to note the various iterations of carburettors, when a cursory gathering of information suggests that HS4s are HS4s.  I'll have a good look at all 4 sets of dashpots and pistons to see how interchangable they all are and maybe find a better combination of bits.

[egret]

One of my daydreams that I've been paying more attention to recently is switching to fuel injection.  I've ruled out single point injection as the gains not being worth the effort, so I'm left with single throttle body port injection or ITB port injection.  I'll preface the below with the fact that I have no capability (or ability) for welding, so my goals will be to try and simplify any welding and keep it to a minimum.

I like the idea of motorbike ITBs and happen to have a set for a 2007-12 CBR600rr which are good up to about 120hp in the bike configuration.  I like that the fuel rail, injectors, TPS, idle control, and vacuum balancing are all bundled in an OEM package, which means lots of the tricky manufacturing tolerances all resolved in a single bolt on item.

I believe this makes much of the fabrication simpler as you're not having to mount fuel injectors with tight tolerances.  The ITB module can connect to an inlet manifold with stubs of large diameter silicone tubes and jubilee clips, which should allow for some manufacturing tolerances in the inlet manifold.  These ITBs do restrict you to a fixed inlet runner spacing and port size, which are both some way different from the triumph head, so this will add some complexity to the inlet manifold as the runners need to taper and offset.

One of my concerns is that this setup will end up with throttle butterflies and injectors further from the valves and slightly different inlet runner lengths, compared to most of the EFI setups on here e.g. Nick, Mike and Roger.  I calculate 3% difference in airpath lengths from injector to the head, with the injectors about 130mm from the head.  My understanding is that this could cause issues at idle an low throttle openings if the fuel drops out of the airflow.  I'm assuming it will be ok because with the exhaust so close these things are unlikely to be cold once it's started.

I am interested in the thoughts of others who better understand the difficulties of fuel injection setups.  I'm concerned that my recent carburetor frustrations are leading me to consider that EFI will magically solve all my problems.

The overall plan will be to compartmentalize any troubleshooting by doing it in phases.  So changing from the current megajolt to a speeduino (with smart coil) and at the same time install a low pressure electric fuel pump in the boot while retaining the carbs, as a phase 1.  Then the ITBs, manifold, high pressure fuel system, can come as phase 2 once the phase 1 is up and running effectively.  Wideband O2 can come anytime after Speeduino, but really needs to be in to allow me to tune it.

So how many assumptions have I made that are wrong, what have I missed, and is this really going to give me fewer headaches than the SUs?

[JohnD]

Those you mentioned will explain why they used the length  they did.   But longer may not be a disadvantage.

Like exhaust primary length, inlet length may be tuned to advantage, although for inlets the standing pressure wave needs to be positive at the valve.  I'm on my phone, else I would post a chart of optimal length vs resonance revs.

John

[Nick Jones]

1 hour ago, egret said:

is this really going to give me fewer headaches than the SUs?

No. Not until you’ve got it all developed and dialled in anyway, which could take a while. After that, maybe. I’ve a slightly jaundiced view of ITBs just now as holding the balance is critical, and mine don’t. Though as you plan to use a complete OEM setup this is probably a me problem, not a you problem!

Throttle butterfly position really not an issue, injectors not really either. Yes, you will get some fuel drop-out at cold idle but, as you say, it won’t last long.

As regards the carbs, if you can find a localish RR operator who knows SUs (getting trickier as time passes), he’d likely get it resolved in a couple of hours and optimise your MJL timing map too.

[egret]

On 11/29/2024 at 2:02 PM, Nick Jones said:

As regards the carbs, if you can find a localish RR operator who knows SUs (getting trickier as time passes), he’d likely get it resolved in a couple of hours and optimise your MJL timing map too.

I did the RR back in may and it was expensive, but excellent value.  The carbs needed so much doing to them (springs, needles, new jets) that we didn't get time to do the ignition beyond a pull to confirm that it was a bit advanced at the top end.  It was the most expensive single thing I've done to the car, but also made the biggest improvement to driving enjoyment.  My issue is that it was a lot of money which will get the current setup right, and needs to be spent again every time something changes.

So with a bit of contemplation and the input from my Dad's less impetuous viewpoint we came to the conclusion that an ECU with better logging and O2 capabilities should give me the tools to fix my own problems.  So to that end I've ordered a speeduino kit and smart coil, so I can replace the megajolt setup with this new ECU.  This will allow me to datalog more variables, but most importantly it will allow me to fit an O2 sensor and review RPM, MAP, and lambda which should allow me to optimize spring and needle options and get the car running well.  It will also run 4 channels of ignition and fuel injection but that's getting ahead of myself, and my current budget.

To keep the troubleshooting as simple as possible, I'll start with just matching what the megajolt is doing, and add the O2 in the future.  If this doesn't help and I lose all patience with the SUs this is good step on the road to fuel injection!

[mpbarrett]

do you have good software tools with Speeduino? Does the TunerStudio and MegaLogViewer  work with Speeduino or is there an equivalent?
Analysing the data from the system is very interesting and complex. You quickly realise when you drive the car around the streets there are very few driving periods when everything is stable! This makes analysing the data more challenging or fun!
 

Going down the EFI route can be a lot of fun but still takes a lot of time to get the mapping optimised. Mine is good enough and works well but could be better, certainly uses more fuel than Rogers! But once you have the mechanics of the hardware sorted (I spent lots of  money on the hardware, Jenvey throttle bodies and weber inlet manifold rather than trying to develop my own hardware) then its down to tweaking the software to tune it.
I did the whole ignition and fuel injection in one go, I had just retired so had time and funds  to do it but if I did it again I would do ignition first and then fuel.
As I wanted to play with it I brought all the connections to terminals so I could easily access all the signals and I took a lot of time planning the wiring to minimise the any earthing noise issues. 

We could have talked about this at the club meeting on Monday rather than benefits or otherwise of electric cars and heat pumps!

mike