Compression ratio

[hissingsid]

I've bought some Compertition conrods and have found that they are 0.4mm shorter than standard 172 conrods. I put it all together (because I had no time to relocate other's) and checked compression, which was 165 psi across all four cylinders, after inital run down the road have found that it is down on power, so I was wondering if anyone has checked there compresions and what psi (or bar) is thanks

 

[Fred@BTM]

BenR will know

but you will have lost compression

what pistons are you using??

185psi sounds familiar on our demo engine, i maybe wrong

 

[BenR]

Std compression ratio is 11.2:1 which gives a rough cranking psi of roughly 190psi, but thats for a std cam setup.

If you are running rods with a 0.4mm reduced centre to centre length then your static CR drops to 10.77:1 and cranking pressue is reduced to about 180psi.

When you built your engine did you check the deck height?
How did you measure conrod length, was it centre to centre and what did you use to measure it?

Why did you not just skim the block face to bring it back upto the stock cars deck height clearance?

is everything else on the engine stock?

 

[BenR]

My 12:1 static CR 172 engine had a cranking pressure of just under 300psi.

 

[Fred@BTM]

My 12:1 static CR 172 engine had a cranking pressure of just under 300psi.

which engine was that??

 

[MarkM]

My 12:1 static CR 172 engine had a cranking pressure of just under 300psi.

Seems like a big jump over a standard engine!

Is it the pressure generated in a cylinder at the top of the stroke?

What modifications do you have to make to stop the engine blowing itself apart?

What sort of gains do you see from increasing the compression ratio and is it just a case of skimming the head?

 

[Fred@BTM]

loads of ways to increase the comp ratio, rods, pistons, head, block, fairy dust

 

[rmccardal]

Just because you want 300 psi

. o O (where is my PM)

 

[hissingsid]

Thanks for the figure's peep's , gonna have to skim the block 0.4mm or even 0.5 just to save me putting high comp piston's in.

 

[BenR]

pistons fail under sustained high rpm, renault sport themselves say std pistons WILL fail if you live above 7200rpm.

Skim the block to give std deck height, 4 thou isnt going to make a nats difference.

 

[hissingsid]

Got the engine all back together and in car . Took 0.7mm of block and we now have 205 psi across all 4 cylinders, well chuffed , will get on rollers to see if we have inproved power in a week or so.

 

[BenR]

you took 28 thou off the block!!!!!!!! lol

Hope you checked your deck height and piston head clearance, as your basically sitting a 8 thou deck clearance and 32 thou total piston/head clearance MAX on std gasket.

 

[hissingsid]

Ben how old are you can you tell me your figure's in metric. remember that I have only raised the piston .3mm to the standard height

 

[BenR]

to get metric, multiply the imperial (inch) number by 25.4.

 

[BenR]

i'm 24

 

[GordonD]

i'm 24

Is that in metric or imperial?

 

[BenR]

I'm from China......so EMperial

 

[Raving Drifter]

if my new cam come with a pre-lift at TDC (IN 2.00 / EX 1.55mm), the actual pressure will drop theoretically right?

 

[BenR]

If your lift @ tdc is actually those figures in the dephased state then the cranking pressure will be significantly lower yes. You'd be lucky to hit 100-120psi.

 

[Raving Drifter]

Hi BenR,
Does that mean my cam is too wild if i didn't do something correspond to it?
Do i need to raise the cranking pressure? And how?

BTW, my cam is Cat Cam 5502411 (duration of 1.0+ mm lift is 239 / 230°). What's your opinion about it? Is it too wild to a long stroke engine? If yes, which kind of cam will be your recommendation?

Raving Drifter

 

[BenR]

from my experience that cam is too wild for a stock engine and ECU. But if your engine is modified and you have appropriate engine management, then i see no problem.

Cranking pressure has very little concern to us apart from providing info on condition of seals.

 

[Raving Drifter]

i see. thanks!