Unibody Mk4 Spitfire 6

[BiTurbo228]

So, in a completely unrelated soujourn to youtube I stumbled upon this video:

The whole channel is excellent, with the premise of incrementally changing and testing different modifications to NA and turbo engines with dyno runs to demonstrate the differences. This one is particularly pertinent as it tests all sorts of different intake setups.

Some interesting observations:

• As expected, a longer runner produces more power in the midrange, with a slight drop at higher rpm (the power tails off earlier).
• He's revving out to 9000 which is mightily optimistic for most of the engines we're working on.
• A bigger diameter runner (57mm vs 51mm) had minimal effect in the midrange, but the top end goes further before dropping off. Possibly a smoother curve, although that's not something he seems particularly fussed about so long as the new line is fatter than the old one. Might make more of a difference for us in our shorter rev ranges.
• A very simple bellmouth has a noticeable effect over just a cut tube end, mostly at the top end.
• An even simpler conical end seems roughly similar to the simple bellmouth.
• A proper bellmouth is better than both of these other options throughout the rev range.
• A 45 degree bend in the tube had negligible effect.
• A 90 degree tight radius cast bend had minimal effect lower down, but cut the top end noticeably.
• Interestingly a 45 degree silicone bend seemed to smooth out the curve a little (although it was slightly lower), and also held the power a little longer at the top end. It would be interesting to know if he had any blending of the tubes into the greater diameter of the silicone or whether there was a step in the runner (which I always thought was a Very Bad Thing).
• The really big bellmouths needed 45mm extensions as the effective length was shorter, but once that change was made they performed identically to the regular (but good) bellmouths.
• I kinda wanted to see what the 1m long intake runners did! 

Edited October 28, 2021 by BiTurbo228

[Escadrille Ecosse]

2 hours ago, BiTurbo228 said:

• I kinda wanted to see what the 1m long intake runners did! 

Me too.

And the thought of turning up at an event with peach tins stuck on the inlets kind of appeals

Good video. Although I got a bit distracted at times by the nut on the vent pipe precessing with the rising and falling revs. But that probably says more about me

[BiTurbo228]

Yeah I was thinking that too  tinned pineapple slices for me I think. Would need to be a yellow car as well

You and half of the comment section!  found the little mobile nut quite amusing!

Oh, and I've just discovered he has done a test with the metre long tubes. Conclusion: complete junk.

Made less power everywhere, and didn't start making more power until it got back down to 200-ish mm (close to the 185mm extensions he was running in the previous video). Seems the negative effects of having the waves interfere with each other at higher rpm causes big issues.

Edited October 28, 2021 by BiTurbo228

[BiTurbo228]

Oh, and peaches beat traffic cones:

 

[BiTurbo228]

Here's an interesting thing from the extra long trumpets test:

Being a 9000rpm engine, the issues at high rpm are stark. However, if you have, say a 6000rpm 2.5l OHC/4.0l AJ6/Rover 2600 then it's less cut and dry.

I'm not sure how much of a bad thing having a lumpy power curve is, but if that's fine then the 200mm extensions are noticeably higher than his short trumpet setup all the way to 5900rpm.

If it works a similar way with exhaust pulse tuning it might explain why BMWs always seem to have 6-2-1 exhaust manifolds. The aim of the game would be to minimise pulse tuning, not try to take advantage of it.

[Nick Jones]

My main surprise in the first video was how little difference the changes made.

As observed already, it’s all rather dependent on the character of the engine it’s attached too. Suspect the longer runners would work better on a lower revving engine. Also feel that the diameters are all too big and that’s the main reason that he gets minimal differences except at monster rpms.

 

[BiTurbo228]

Agreed it's all to be taken with a pinch of salt if it's on a different engine, but at a glance his cam timings aren't a million miles different from the 2.5PI cam (244deg duration @ 1mm lift, 284deg @ 0.3mm). 

Interestingly he's done that one too. Going smaller hurt the top end (again, 7000rpm up), although the lower end was fatter (but again, lumpier, which he attributes to aggressive cam overlap).

The surprise entry one was a tapered 62mm-51mm one which was pretty much the same as the regular intake lower down but held the power for longer higher up.

I suspect you're right in that it's the rpm that's throwing people off as lots of people suggested smaller runners. I get the impression tuning cars for ~6000rpm and tuning them for ~9000rpm requires significantly different approaches. But still, it's interesting to see what happens low down. He might be willing to trade 15hp at 4000rpm for 10hp at 7500, but we wouldn't!

[BiTurbo228]

Next question I'm wondering. If we want to keep the big fat gains of having a long intake manifold under 6000rpm, is there anything we can do about that big dip he was experiencing at 4600ish rpm?

If it's aggressive cam overlap that's causing that then I'm sure a more mild cam would suit our engines anyway as we're unlikely to need to tune for 9000rpm. However, I suspect that if it's caused by resonance it'll still be there.

If it's all to do with frequencies I wonder if you could tune it out with one of those resonator boxes you find in OEM intakes:

On regular intakes they're tuned to the frequency of drones in the intake system to cancel them out and cut NVH. I wonder if you could tune one to the frequency produced somewhere around 4600rpm and cancel out the negative effect. You'd still have a dip, but it might not be quite so harsh.

Fitting the thing in the engine bay might be a challenge though!

Unless the frequency is set and you'd just cancel out the whole thing...

[Escadrille Ecosse]

I would suspect that the optimum inlet diameter depends on the size of the engine, the valving area, porting efficiency, cam and revs. Basically how much air the engine is pulling in through each inlet which will obviously depend on the engine and the state of tune.

And whether it is injection or carburettor (and no doubt which type of carb).

As for tapered trumpets there is nothing new

1963 Lightweight E-Type with Lucas mechanical injection

Like you I suspect the dip is caused by a resonance or reversion effect due to an interaction between the valve timing and the diameter/length of the inlet (and possibly exhaust too ) and if so then probably the only way you would be able to tune it out would be by (expensive) trial and (similarly expensive) error. Unless you have access to your own dyno!

Good luck on that one!

[Nick Jones]

3 hours ago, BiTurbo228 said:

but at a glance his cam timings aren't a million miles different from the 2.5PI cam (244deg duration @ 1mm lift, 284deg @ 0.3mm). 

You’ll not be revving a Triumph 2.5 to 9k….. not more than once or twice anyway. 
 

In pure mass airflow terms there’s not such a huge difference between a 1.6 at 9k (7,200 lpm) and a 2.5 at 6k (7,500 lpm), though the different number of pots changes things again. 1,250 lpm per pot on the 6 vs. 1,800 lpm per pot on the 4 pot screamer.

lumps and bumps in the curve could be down to the exhaust….. or the interaction between inlet and exhaust where there’s lots of overlap 

[BiTurbo228]

Man that XK is pretty! Props for mechanical injection as well.

So, some minor breakthroughs on how inlet resonance works. Turns out, the whole ;intake valve closes, setting off a high pressure wave which reflects off the open runner and back into the cylinder' isn't really how it works. Or, rather, it does happen but it's nowhere near being a significant effect.

What actually happens is a negative pressure wave is set off by the downward motion of the piston (when it reaches its fastest rate of travel down the cylinder bore). This negative pressure wave travels up the bore, up the inlet and bounces off the (nearer) atmospheric pressure at the end of the runner as a positive pressure wave (apparently it changes sine here). The tricky thing is that the atmospheric pressure doesn't stay at the end of the runner, but creeps down the length of the runner a little as it tries to fill the low pressure area created by the inlet cycle. The distance it travels down the runner is dependent largely on the diameter of the runner.

The trick with inlet tuning is to time this cycle so that the pressure wave returns at a point of low suction in the cylinder, either as the piston reaches its lowest point in the cycle or even as it starts to rise back up the other side. There's also a secondary benefit if you can align a later set of waves to the next cycle's valve overlap period.

Unfortunately, to calculate all of this stuff you really need a proper engine simulator which is rather expensive, but I've put together a rough calculator of sorts to hopefully get people in the ballpark. It's based on actual measured cam/valve specs (as the late, great Alan 'Oldtuckunder' found that they differ quite a lot from those published!), and figures to hit 3/4 and 7/8 of an inlet cycle. The idea with that is as the pressure wave is actually quite broad, if you get it roughly in the right place it should hit roughly where you want it to. Enough to cut out some of the error in trial and error anyway!

Oh, and from what I gather you're on the money @Nick Jones. The real dips and wiggly power deliveries are down to interactions between exhaust and inlet. Getting the wrong setup means you end up with comparatively low pressure in the inlet port and high pressure in the exhaust causing reversion and stuff like that.

Oh, and in something a little less spreadsheety, I've picked up another dead complete 2600 engine and a known good 2600 long block, so thought I'd weigh them and see what we have.

It's not a featherweight!

The long block (block, head assembly, water pump, damper, thermostat, dipstick, 6 spark plugs, 2 lifting eyes and a sump full of oil) came in at 161kg dead.

The complete engine (all of the above, minus 6 spark plugs but plus dizzy, alternator, PS pump, viscous fan, HT leads, inlet, carbs, airbox, exhaust manifold, coolant pipes, engine mount brackets, engine mounts, flex plate and torque converter) came in at a chunky 219.9kg. 

When I pull the 2.5 TC engine out of the T2000 I'll see if I can weigh that in a similar state of dress. I expect it's been done before, but another time couldn't hurt!

 

Edited May 11, 2022 by BiTurbo228

[BiTurbo228]

So, it's been yonks since I've done anything on the Spitfire, but the offer of a chappy on FB who does roto upright machining made me dust some bits off and have a go at some of the remaining tasks.

First off, just for posterity, a fully dressed Triumph 2.5 twin-carb from a 2500TC (so the long manifold and power steering, full oil, starter motor, pipercross filter, cast manifold) plus A-Type overdrive saloon gearbox weighed 269.1kg.

A J-Type OD gearbox is 41.1kg and the starter is 4.8kg, so if the A-Type is broadly similar then the engine alone would be 223.2kg which is a fair chunk heavier than I was expecting! Not sure if there was still some coolant sloshing around in the block, but even still it's not going to be nearly 40kg-worth (I've seen a pic of a Triumph OHC weight at 183kg, but I'm now doubting that!). Power steering pump will add something, as will the heavier pulley, but again. 40kg is a big difference to make up...

Either that or my scales are duff 

Now, onto new stuff!

Shiny new uprights machined for MGF bearings

Next job has been to have a look at rear brake caliper options. I know people have used MGF calipers and Peugeot 106/Citroen Saxo calipers before so I started with those.

I found that if you redrill a 278mm rear disc from an 06-11 Yaris then you can just bolt a flat plate onto the rotoflex upright and have it set at the right height, but ideally I'd prefer a disc that doesn't need machining.

I like the Peugeot calipers from a weight perspective, so I bought a set to do some mocking up. Took some measurements using an MGF 240mm disc and came up with this:

...but when making a mockup in MDF I discovered a couple of potential issues. The top bolt fits in ok, but the bottom of the caliper clashes on the locating boss for the radius arm:

It's not a dealbreaker as I can grid that bit off (I'm not using radius arms), but it's all looking mightily close.

So I thought of trying the 247mm disc the Peugeot comes with (or, rather, a front axle one from a 205 as they've got the same 66mm centrebore as the MGF disc so should make redrilling easier). They looks something like this:

While the extra 7mm diameter on the disc makes upright clearance a bit better, the shorter height of the disc makes the second problem I've encountered worse:

The handbrake cable wants to exit directly into the upright. The above pic is with the MGF disc, and the Peugeot disc makes it worse. I suppose you could machine a flat or something onto the Peugeot caliper to bolt a tab to and pop the cable mount back 20mm and skirt past the back of the upright, but that's more machining.

I know both of these calipers have been used on the back of Spitfires before, but it's occurred to me I don't know what the rest of the setup was. I don't suppose anyone here knows!

[BiTurbo228]

Well, I committed the cardinal forum sin of not using the search function before posting a question. Looks like when Marcus fit his he swapped the pivots around and drilled through them for the standard cable. Dead simple.

[Nick Jones]

Good to see some movement on this again. So what is the one that you have advertised for sale elsewhere?

And who’s the fella doing the machining?

[BiTurbo228]

Yeah definitely! I've been wanting to get back on with it for ages, but the garage it's in is totally rammed. Need to get some other cars together to unearth it from all the crap  either that or build a new garage, whichever comes soonest 

The other one is a shell I bought way back because it was at a price I couldn't refuse! Then it occurred to me that with all of the bits I've taken off various cars I could make 90% of a Spitfire with it, which was the plan. But too little time, too many projects so it's up for sale ahead of time.

It was a chap down as Michael Triumph on the Modified Triumph Cars Facebook page, though his payment details were for a Haydn Owen so who knows what his name is  not tried putting anything together yet, but it all looks very nicely done

[BiTurbo228]

So, on the positive side if you grid off the little lip on the vertical link then the calipers do clear on an MGF-sized disc:

However, I popped the boot off the back of one of the calipers to see how easily the arms flip over to the opposite side and it turns out they're drilled off-centre:

Looking at some pics of OEM calipers I think that's a feature rather than a bug, which means that flipping them over will put quite an angle on the little plate that pushes the pin in (you can see it in this picture).

Maybe that's not so much of an issue. Marcus did run his for quite some time. Just got me wondering if some of the pictures I've seen are also repros, and the OEM ones are drilled centrally...

It's a blurry picture, but it looks like the ones on Marcus' car are different entirely. I wonder if they're from a different set of calipers, or have had a different arm welding onto them...

Edited September 11, 2023 by BiTurbo228

[BiTurbo228]

Found another pic on Triumph Experience

Looks like Marcus (or Markus that should be) did weld a different arm onto the Pug caliper, but it's still pointing the wrong direction to my eyes. If you swap the arms around to have them poking out the top and keep the right orientation of the offset drilling, you should have the arm facing in towards the vertical link rather than out towards the tunnel.

He also mentioned that he didn't think the Peugeot calipers were very good. Had to keep the handbrake cable tight to have any handbrake at all which caused the pads to rub. Will have to have a play around with pivot arms to see if I can tweak that out. Either that or it was caused by the offset pin situation...

Edited September 11, 2023 by BiTurbo228

[Escadrille Ecosse]

I can't help with the caliper question as I have not done this conversion.

But I do have a couple of related questions 

1. Will the standard GT6 drum brakes fit with the MGF hub conversion. 

2. Anyone tried fitting the Spitfire drums to the GT6 uprights. With the MGF or GT6 hubs. Reason for me would be to keep the weight down and so I could fit the alfin style drums.

Apologies for thread creep...

[BiTurbo228]

No worries!

I think the chappy who did my machining had GT6 drums on MGF hubs on his car, so there must be a way of getting them all to play nicely together.

Alfin drums are lovely things to have! I can tell you the mounting pattern for the GT6 backplates are 4x101.6 if that helps. If the  MGF bearings are machined to put the hub face in the same place as the stock rotoflex hubs, then there's 45.8mm from the hub mounting face to the backplate mounting face.

I suppose if those two match closely enough then there's no reason the Spitfire drums wouldn't work...

[Escadrille Ecosse]

9 minutes ago, BiTurbo228 said:

No worries!

I think the chappy who did my machining had GT6 drums on MGF hubs on his car, so there must be a way of getting them all to play nicely together.

Alfin drums are lovely things to have! I can tell you the mounting pattern for the GT6 backplates are 4x101.6 if that helps. If the  MGF bearings are machined to put the hub face in the same place as the stock rotoflex hubs, then there's 45.8mm from the hub mounting face to the backplate mounting face.

I suppose if those two match closely enough then there's no reason the Spitfire drums wouldn't work...

Thank you. Some mejuring then...

[Nick Jones]

12 hours ago, Escadrille Ecosse said:

Anyone tried fitting the Spitfire drums to the GT6 uprights. With the MGF or GT6 hubs

I think you may struggle. The obvious issue is the backplate. Not sure that the Spitfire plates have enough flat area in the middle to take the bigger hole and PCD.  Even if it does, the bearing housing is quite chunky and you may struggle to fit the brakes around it, especially getting the springs in top and bottom. Marginally worse with MGF hubs as the are thicker.

Happy to be proven wrong….

 

PS we just stuck GT6 brakes on Chris’s Spit as the fronts were anyway - smallest diameter rear cylinders as a nod towards brake balance. Actually seems fine.

[BiTurbo228]

Does Chris' car have a rotoflex rear? If not, did the GT6 backplates just bolt straight on or did they need new holes drilling?

[Nick Jones]

Has roto links machined for MGF bearings and CV shafts. Twin adjustable lower links similar to Markus’s.

The CV/MGF bearing conversion is designed to have the GT6 brakes. Only additional mod is a ring to match the MGF hub spigot with the drum centre hole so it’s properly centred.

[BiTurbo228]

On 9/12/2023 at 8:45 PM, Escadrille Ecosse said:

Thank you. Some mejuring then...

If you're playing around with Spitfire brakes would you mind popping them on a set of scales? Trying to work out how much heavier my disc conversion's going to be...

[Escadrille Ecosse]

On 9/16/2023 at 9:58 AM, BiTurbo228 said:

If you're playing around with Spitfire brakes would you mind popping them on a set of scales? Trying to work out how much heavier my disc conversion's going to be...

Will see what I can do