High Compression head?

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kiwi
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High Compression head?

Post by kiwi »

Been watching this car on Trademe thats the Kiwi version of online auction site.

Reading the sellers comments I am wondering what the heck a High Compression head is?

http://www.trademe.co.nz/Trade-Me-Motor ... 324971.htm
1991 BX19 TZS 04/01/91 (Deceased)
1990 BX19 TRS 27/10/89 (Reborn)
1992 BX19 TXD (Ex UK - K 744 SDF) 15/06/92
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Matt H
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Post by Matt H »

Why bother with that when you could have...

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Way2go
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Re: High Compression head?

Post by Way2go »

kiwi wrote: Reading the sellers comments I am wondering what the heck a High Compression head is?
Just a cylinder head that creates a higher compression ratio than standard to give you increased power and acceleration. Used to be a very popular mod in the days of long-stroke engines.
Skimming of the head will also cause an increase in compression ratio. How much of a benefit that you can achieve on a BX, others will be able to inform you better than me.
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DavidRutherford
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Re: High Compression head?

Post by DavidRutherford »

Way2go wrote:How much of a benefit that you can achieve on a BX, others will be able to inform you better than me.
Given that the XU engine has a flat head face with no combustion chamber to speak of, there's very little that can be done to the head to increase the CR. About the most you might expect is an increase of 0.5%. Maybe. And that's with the head skimmed so much that the valves are flush with the head face, which wouldn't be recommended. For an XU, it would have to be high compression pistons rather than anything else to actually make a difference.

Also, the XU9 engine is already about 9.5:1 isn't it? Any increase over that will mean a reliance on higher octane fuel to avoid knock, otherwise you'll actually be down on power.

Strikes me that the seller either doesn't know what he's on about, or is waffling.
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Post by jeremy »

Its a TRi - so petrol and I thought the heads had combustion chambers with flat or dished pistons rather than the Renault 21/19, Rover 2000, etc with Heron heads giving a flat head face with all the combustion chamber in the piston.

Citroens didn't use a particularily high compression - and are not a short stroke engines. The compression can be increased I suppose - but it can make the engine require higher octane fuel.

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Post by Philip Chidlow »

Matt H wrote:Why bother with that when you could have...

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That GT is gorgeous! It's got a computer and those seat covers are genuine Citroen accessories. I would love to have that car.
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Post by Tim Leech »

Me too, but im just greedy!
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cavmad
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Post by cavmad »

Philip Chidlow wrote:
Matt H wrote:Why bother with that when you could have...

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That GT is gorgeous! It's got a computer and those seat covers are genuine Citroen accessories. I would love to have that car.
Great 4 BXAGON!
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Philip Chidlow
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Post by Philip Chidlow »

A long way to go and fetch it from!
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kiwi
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Post by kiwi »

Oh thanks guys for confusing the thread with the GT thats on sale there as well :roll:

Anyways
jeremy wrote:Its a TRi - so petrol and I thought the heads had combustion chambers with flat or dished pistons rather than the Renault 21/19, Rover 2000, etc with Heron heads giving a flat head face with all the combustion chamber in the piston.

Citroens didn't use a particularily high compression - and are not a short stroke engines. The compression can be increased I suppose - but it can make the engine require higher octane fuel.
So basically this BX has had a head job and you dont really do head jobs unless thers something wrong?

Talking of the Fuel though I think I will go and ask about what grade of Fuel it uses :wink: Have heard from couple people who own TRIs they have been told they can use 91 Octane (nats piss).
1991 BX19 TZS 04/01/91 (Deceased)
1990 BX19 TRS 27/10/89 (Reborn)
1992 BX19 TXD (Ex UK - K 744 SDF) 15/06/92
1990 BX19 TZS Auto 06/11/1989
1992 BX TZD Turbo Estate (Ex UK) 1/07/91

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Kitch
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Post by Kitch »

It's probably blown a head gasket at some point and overheated, so it's been skimmed. Not really an issue and if a thicker gasket is used theres no difference.
It wouldn't put me off....you could whack just about any XU petrol head on there and it'll run! What would put me off is that gearbox :lol:

My valver's got a skimmed head. It's had quite a bit lopped off by the sounds of it, but they do have domed chambers, so it works fine. Pinks it's tit's off at low revs...anything under 4k really :roll:

Goes well above :D
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Post by jeremy »

Interestingly it seems that in UK TRi is a GTi estate - so I don't know what its spec is. Kitch may well be right. Alternatively the NZ spec whenever it was made may well have had a reduced compression (which could be effected by a thicker head) to cope with the available fuel. UK spec ones had a compression of 8.4:1 (in 1989)which is remarkably low for an engine of that spec - ie GTi - many of its competitors being around 10:1.

Its output was very healthy even on 8 valves so someone designed it carefully - but what the effect of raising the compression will be I cannot say - except to say that if there was an easy 10 BHP there then the manufacturer would probably have taken it.

Some tuning ifo on 8 valve (and 16's) on Pums site

http://www.pumaracing.co.uk/

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Kitch
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Post by Kitch »

I'm not sure they would have taken any more easy power. They're knocking on all the doors of the competitors at those sort of figures, and with the planned valver running 160bhp (as much as it could economically and in anything close to a refined manner) the gap between the two would have been too close. Plus the GTi was always supposed to be a hauler, rather than a screamer.

However, I think the catalised cars had lower compression ratios and I know the rules on cats differed in other countrys....either way I doubt there would be much difference anyway. I put a 1.9 head on that old 16RS mk1 I had. Not sure what year of car it came off, but it bolted in place and ran the same as the one before had....fine :)
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Post by DavidRutherford »

jeremy wrote:UK spec ones had a compression of 8.4:1 (in 1989)which is remarkably low for an engine of that spec - ie GTi - many of its competitors being around 10:1.
Bear in mind that the "headline" compression ratio is meaningless without being related to the cam timing. The engine could have a 20:1 compression ratio, but if the inlet valves don't close until 110 degrees ABDC it would still be a low compression engine.*

*this is an extreme example, but you can see what I mean.

I still recon the increase in compression ratio you get from skimming a head is both halves of practically bugger all. Each cylinder is 476.25cc, and even if you machine 10thou off a head (which is an awful lot) you've only removed less than 0.5cc. (as all you've done is bring the valves a bit closer) You most likely gain all of that back and a fair bit more by fitting a "repair" (IE a thick) gasket anyway.
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