High Compression

[tubbyG]

Does anyone have any experience with building a high compression engine?

Im hoping to raise my compression to a level that i can produce 200 brake with 20psi or less. What I would like to know is if I should go as high as 9:1, or is this an acceptable level or overkill?

I have had a few searches of past projects and I have not found huge amounts of info.

Currently my cylinder head is 73.4mm, im ideally just looking at having the head skimmed to give an increase in the c/r, or should I be looking at additional work to the combustion chamber while its getting skimmed? Im not interested in going all out on fancy head work such as big valves etc

Any opinions/thoughts on this are welcomed

[Ian S]

To do it, you need to increase the swirl so as to increase the mix and reduce the det.

I think Brad from CTM, with a big Jim setup was on 9:1 or maybe 10:1 after a few more failures and skims.

The chamber was welded up and reshaped. And the piston top dished. Also piston protruding a bit so as to reduce the squish area, or something like that.

It seemed to work. Apparently his engine made, lets say, more than 220 BHP at 18psi. But then a piston cracked around a ring land, so apart again. After that it apparently made 244 but I don't know at what boost. He wasn't much into rolling roads, but on the road it went very well, at it's zenith, for the first time, it sped up slightly faster than mine. I had an older development and was scheduled for that setup, I still have the welded up head, but never did it. Instead wanted to go 2 litre instead. Never did that either.

There was a photo of the head on here somewhere years ago.

[Matty]

The first I would do is weld up the two tapered parts in the head and introduce a beak between the valves opposite the plug, and shape it better. When you know what CC the head ends up, dish the Pistons to suit. I would avoid decking the Pistons completely (running down the bore) as this doesn't help with squish.

EFI and mappable igntion if you haven't already got them, makes it far easier to avoid det.

[Ian S]

weld up the two tapered parts in the head and introduce a beak between the valves opposite the plug, and shape it better.

Like this? Brad's 5GTT head from maybe Jan 2005.

Jim told me he based it on racing mini heads.

[tubbyG]

Thanks for the input.. Some good information to chew on.
Welding up the combustion chamber then shaping sounds like it could be expensive with the amount of labour involved so that more or less rules that out for now.
As it happens I have a vnt and mappable ignition install and running already. Well I did untill the engine was removed before Christmas

[Ian S]

Jim told me that was £100 of welding. Probably about 2007. Don't know about fettling cost.

[Matty]

Yeah just like that!

A lot of the mini boys used to remove the beak to lower the CR on engines with large rebores, but suffered from det. Now the route to take isdeeper dished Pistons and keep the combustion shape. I believe the beak is very important with non cross flow engines, improves the swirl, and scavenging, and the beak being opposite the plug speeds up the burn, which increases your max cylinder pressure before you run into det.

Like this? Brad's 5GTT head from maybe Jan 2005.

Jim told me he based it on racing mini heads.

[tubbyG]

I hope I am getting the jist of this...

So by just skimming the head the compression would be increased (a little), but the the cc of the chamber would be reduced and the swirl/mixing effect could potentialy be reduced, potentially meaning any gain from the increase in compression would be lost to the drop in cc and increased danger of det and inability to mix the air/fuel correctly........ is that why the other techniques are implemented also?

[Ian S]

I not an expert but I'm surmising and guessing a bit.

I don't think that skimming a few thou off will change the intrinsic swirl of the OE chamber.

My head was skimmed 1.5 mm or something, I forget now. But the chamber was much smaller, I buretted it at 44 cc maybe.

Jim could then take maybe 10 cc out on the piston tops in a dish shape, the idea to maintain compression ratio, but increase swirl and reduce det. and so increase boost.

Next step to increase the swirl is the chamber shape, maintaining, or reducing somewhat, the chamber size, that increases the mix and allows more compression, either from higher ratio or from more boost.