Power difference between 8-1/9-1 at same boost.
Turbo Mini Forums

Just wondering what the power difference would be at same 10psi of boost with increasing the cr from
8-1 to 9-1. Surely it must just be maths formula.

Edited by alaskanow0 on 28th Jun, 2012.

Class C 3rd Place Avon 2011 14.18 @101mph

(BHP/8)*9 ? must be good enough for a ball park figure
but then the tune may need changing depending how close to dett the 8:1 is

although with 9:1 the off boost performance will be better and i would expect a generally nicer drive

Edited by Brett on 28th Jun, 2012.

Yes i moved to the darkside

Instagram @jdm_brett

Theoretical Otto cycle gives:

The efficiency is equal to the one minus one above compression ratio to the power of gamma minus one. Gamma is about 1.4.

So Efficiency = 1 - (1/(CR)^0.4))

So efficiency at 9:1 is 58.5%

At 8:1 - 56.5%

I was talking to Vizard about this the other week. It's all bollocks in the real world.

My computer says quite the opposite when you use tuned length runners. More space in the combustion chamber allows more air to be burnt. You just need to make sure it is clean air :)

EDIT: Said Carnot but it's actually the Otto cycle.

Edited by Paul S on 28th Jun, 2012.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

thats alot less than i expected tbh

Yes i moved to the darkside

Instagram @jdm_brett

head..........hurt............**************

You like maths???

You'll like this even more:

http://en.wikipedia.org/wiki/Otto_cycle

Equation 6 is what I've repeated above.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

That may as well be written in Chinese for all I can make out of it. It's all just random letters on a page to me lol.

Reminds me of when we studied the Otto cycle in primary school.... :S

Audi s4 b5 - 470bhp & 486ft lbs

On 30th Apr, 2012 Brett said:
yeah stick the bit in and give it a wobble

ot·ter (tr)
n. pl. otter or ot·ters
1. Any of various aquatic, carnivorous mammals of the genus Lutra and allied genera, related to the minks and weasels and having webbed feet and dense, dark brown fur.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

You see the heading above?

It says "Technical Chat". The guy asked for a mathematical answer.

If you guys don't understand the technical stuff, then why trivialise these threads with inane drivel ?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Tough crowd

Audi s4 b5 - 470bhp & 486ft lbs

On 30th Apr, 2012 Brett said:
yeah stick the bit in and give it a wobble

Thanks Paul,

Class C 3rd Place Avon 2011 14.18 @101mph

Interesting, that is for an NA engine, I wonder how it holds true with boost.

The figures suggest a moderate increase in power, but it must be based on too many assumptions. Cam profile for example, to optimise an engine with 8:1 CR you would have a different cam profile than that of an engine with 9:1.

And for a turbo, you would compromise the cam profile against CR to allow boost.

Just too simple, and as you say Paul, not quite right.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Like most things relating to turbo-charging, there is an optimum compression ratio for maximum power and it may not be as high as you can get away with. However, for driveability, it certainly is.

It would be a very interesting exercise to take two otherwise identical engines and test different CRs. However that's impractical.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Impossible to have this discussion without a mention of the SAAB variable compression engine. High compression for off boost torque/driveability. Lower compression for less ignition timing compromise when on boost.

Went to the Indian for a takeaway and the bloke asked me if i'd like to try a chicken tarka. "Tarka?" i asked. whats that. "It's like chicken tikka only a little otter"...........I'll get my coat.

At that then in short! You could gain a minimum! 4% power at 1 atmosphere

I'm on 8.6:1 and on 10psi my engine makes considerably less power than Wintersurfers on 9:1.

They share the same cam, same basic engine arrangement. The only real difference is CR and side mount turbo.

In the 13's at last!.. Just

Class C 3rd Place Avon 2011 14.18 @101mph

From what I understand of all the information Paul has written, there is more power to be had from correctly designed manifolds (inlet and exhaust) than from a small compression ratio change.

I would also suspect that the side mount turbo has a bigger impact than the CR difference between 8.6:1 and 9:1.

Jean

http://www.jbperf.com/

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

That makes sense.

Class C 3rd Place Avon 2011 14.18 @101mph

yep, I agree with this,

essentially, although pauls may make more power at 10 psi.... it'll never see 20 psi like Garys's (or not for long anyway!) its a case of make 150 hp at 12psi and 9:1 or 150hp at 15 psi and 8.6:1.....

like wise, with mine with its big cam... it should make less still at 10 psi, but I can run 28 psi with no octane booster....

Swings and round abouts....

That said Gary, you shold still get rid of the servo and start cutting up an LCB...

I am very tempted.

In the 13's at last!.. Just

...and you need to get stuff sorted so you can run this alleged 28psi!

In the 13's at last!.. Just