Compression Ratio

mandarinvx · August 5, 2009

For once Wikipedia has let me down

It simply sates:

Petrol/gasoline engine with pressure-charging
In a turbocharged or supercharged gasoline engine, the CR is customarily built at 9:1 or lower.

So a lot of people will be using 8.9CR pistons, but is there a way of working out the optimum for a Z22 running ~300bhp? And what disadvantages are there to simply fitting a decompression plate (apart from the fact there are then two head gaskets to cause potential problems)

winstar · August 5, 2009

hmm I'd say it's abit out of date with modern engines and mapping you can run alot higher CR if your only after 300 bhp then you could use 10:1. There are two advantage of this is that your not relying on the charger to make the power so have a better torque curve and you'll need less boost to make the power so will be more efficient.

mandarinvx · August 5, 2009

So standard pistons "should" be OK up to 300bhp from a compression ratio perspective

So presumably accurate fuelling on the map becomes vital, is this likely to change over time (wear, tear, deposit build up, filter efficiency etc) I guess what I'm trying to ask is am I likely to have to get the map checked regularly if sticking with stock pistons at that power level

FLD · August 6, 2009

Decompression plates are a bodge at best. It does work but you loose all the squish and quench pads / design aspects of the piston to head area. This can result in a poor burn with related symptoms. I think its best to bowl out pistons keeping the various protrusions where possible. (I've not pulled my spare Z22 apart to look at whats where). Ross pistons in the US are generally good at making bowled pistons that retain all these features.

alanoo · August 6, 2009

mandarinvx said:
So standard pistons "should" be OK up to 300bhp from a compression ratio perspective So presumably accurate fuelling on the map becomes vital, is this likely to change over time (wear, tear, deposit build up, filter efficiency etc) I guess what I'm trying to ask is am I likely to have to get the map checked regularly if sticking with stock pistons at that power level

Not the fueling, the ignition map will be vital
Stock pistons won't work at that level, you're talking about more than twice the power they are rated for. As you said from the CR perspective yes, from the strength one, definitely no

vocky · August 6, 2009

it's not worth the gamble for 300 bhp, fit some forged pistons (and steel rods) then there's no worries

mandarinvx · August 6, 2009

Yeah, I read through the GM build book again last night and found this:

The stock 2.2LECOTEC pistons have been tested to power
levels approaching 300 hp. However these pistons should
be replaced with a stock forged-type piston for applications
over the 300 hp level. JE, Wiseco and Diamond offer pistons
in various compression ratios that work for most
applications. We recommend a compression ratio of 9:1 or
less for blown gas applications, and 10-11:1 for normally
aspirated with premium fuel. GM Performance Parts offers
a stock replacement piston that is a direct drop in when used
with the LSJ connecting rod. (Fig. 49)

Should have guessed it would be right on the limit :roll:

So how busy are you Vocky

I'm not going for 300bhp straight away, I'd rather start out with a tamer set-up <250bhp, and as the Harrop GB may be on and there's little price difference between it and the Eaton one I may as well future-proof as much as I can

I wonder what the Harrop would make with no CC / laminovas notsure.gif

vocky · August 6, 2009

not that busy really, just the harrop rebuild, another stage 2 sc and possibly a quick engine rebuild and then my winter mods will begin

alanoo · August 7, 2009

But 250 SC is not 250 for the pistons

mandarinvx · August 7, 2009

alanoo said:
But 250 SC is not 250 for the pistons

Very true, and the buid book doesn't state what engine the pistons were tested in - NA/SC/Turbo/Nitrous etc

I'm happy at 250bhp, as it's a tested level; tho of course I'm not sure whether the characteristics of the Harrop would affect the pistons / CR much with regard to inlet temps - especially if I were to use a Harrop with no CC system

techieboy · August 7, 2009

I was quite surprised at the difference between the power levels of the high compression and low compression pistons. When I moved from the OE pistons to the low compression (8.9:1) Wiseco pistons, I lost 19bhp on the original high compression map that Courtenay had put on my ECU. Klasen tweaked the map a few times, added back 2 degrees of ignition advance (that was removed on the original high compression conversion) and did some more partial throttle mapping to stop over fuelling in the mid-range, I got back up to 251bhp which is 9bhp down on the original conversion and 3lbft down on torque. Fuelling is absolutely spot on across the entire rev range now but the car does feel slightly less keen now. Hard to describe but it just feels like it lost a bit of urgency.

Hopefully, we'll finally get the new uprated fuel pump sorted shortly and I'll be able to get my rev limit raised back up to a more sensible level and have a play with pulley sizes. The current 6,500rpm limit is really beginning to bug me now. :x

alanoo · August 7, 2009

Yep, low compression sucks

Looking at getting something near 300hp at 7800/8000 rpm with the M62 personally

techieboy · August 7, 2009

What pulley size would you need to get to that point? Or will your increased rpm's deliver that kind of power on the standard pulley? I'm not sure what impact (other than presumably more heat) that stepping down the size of the pulley has to boost level and what power that increase in boost equates to.

Given my time again, I'm not sure I would have got the low compression pistons. Would maybe have been better with standard compression forged replacements or somewhere in between.

alanoo · August 7, 2009

Not sure yet, but think a 3.2 or 3.1 pulley would be fine
+ cams + ported head + dual pass + 10.5 CR...

vocky · August 7, 2009

all you need to do is fit a standard head gasket or have the head skimmed, that will get the compression ratio back up

alanoo · August 7, 2009

only thing is I am yet not sure if I do this with my 2.0 Z20NEx crank, rods and pistons, or with the Eagle 2.2 one, eagle rods and JE pistons...

I'd prefer keeping the Eagle crank for an extreme N/A build...

exmantaa · August 8, 2009

Reading this, I remember reading some US build that the already low(er) LSJ compression did not work that well with hotter cams installed. (more overlap lowers the dynamic compression.) So they changed back to standard cams...

Like Alano; a ported head, cams, ~10:1 CR and a 3"pulley should do for around 300HP. Only not sure if the 2.0 LSJ will make for a finer engine than the 2,2 stroker.

nickb777 · August 9, 2009

O thanks I hope I won't have trouble when I put the s/c cams back in. I know for a fact the standard cams can't go past 315 bhp even with a smaller pulley on. If I do have trouble wonder if I can skim the head to up the CR? also thinking valve clearence.

Just awaiting Jon to return from hols and I will get the car booked in again for the cams and smaller pulley

exmantaa · August 9, 2009

nickb777 said:
O thanks I hope I won't have trouble when I put the s/c cams back in. I know for a fact the standard cams can't go past 315 bhp even with a smaller pulley on. If I do have trouble wonder if I can skim the head to up the CR? also thinking valve clearence.

Just awaiting Jon to return from hols and I will get the car booked in again for the cams and smaller pulley

What pistons and head do you run now? Not sure, but think the LSJ.Saab metal head gasket is a tad thinner than the 2,2's, so maybe that can up your compression a bit.

joe_589 · August 9, 2009

The Saab head gasket is thicker I seem to remember