harvey

iTrader: (48)

Morray McKenzie : In one of the power threads now locked you postulated about the effect of compression ratio on power output.

As far as I know standard compression ratio for recent 2litre cars is 8.0:1 and some of the older cars had 8.5:1. Andy Forrest runs just below standard whatever that might be as he has STi dished pistons, Christian runs 8.0:1, and Bob Rawle and Steve McCulloch are 8.5:1.

A higher or lower compression ratio on one of these engines will have little effect on ultimate power output PROVIDING the turbo is well specified. Where an increased compression ratio benefits most is when the car is running naturally aspirated ie. in vacuum/off boost. In that condition the higher compression ratio makes the car sharper and more responsive but I expect you know that anyhow.

When my engine was rebuilt last March at around 14,000 miles, I was persuaded to go for 7.45:1 CR. That was a mistake which was compounded with my longer gearing. Below 3000rpm the car was lethargic. When the transmission was out in August I took the opportunity to remove the engine and replace the 1.6mm head gaskets with 0.6mm head gaskets which would have brought compression to 8.0:1 but at the same time rather hurriedly, shaved 20 thou off the heads and my compression is now around 8.4:1 or just less. I was planning to further raise this to around 9.3:1 but have shelved these plans because I already have adequate off boost performance. Since the 327 ft/lbs at Well Lane last October this has now increased in stages to the current 418 ft/lbs, and while this is obviously not a measure of off boost performance, it too has been increased progressively. Incidentally, the engine now has done around 31,000 miles. ie. 17,000 miles since rebuild.

Roger Clark Motor Sport who originally built the engine now offer as standard a rebuild with Omega pistons at either 8.5:1 or 9.5:1 but this is in the interests of good off boost performance and from results to date my strong belief is that the higher compression ratio has no effect on ultimate power whatsoever.

I hope you find this helpful.

Adam M

useful info harvey, think you should look up the effects of compression ratio in more detail. there is lots of interesting info out there, especially on the web. More importanly look at dynamic compression ratio as it is far more complex that just x:1.

pat

Unfortunately an engine's performance is dictated by a lot of factors, mechanical compression ratio and turbocharger choice being just a couple of these. For an engine to give the highest BMEP all the components must be working in harmony with eachother. The single most important thing is to achieve a rapid, thorough combustion process, and it is more effective to concentrate on the combustion space and cam / port design; get this right and the mechanical compression ratio is pretty much incidental, get it wrong and the mechanical compression ratio can cause the thing to be sluggish, or det like there's no tomorrow!

Cheers,

Pat.

harvey

iTrader: (48)

I accept that this is a very complex and detailed subject and obviously the C.R. has an effect on the volumetric efficiency and hence fueling. The very noticable effect of increasing the C.R. for me has been off boost performance but I was unable to detect an improvement at high boost even though more fuel was added.

Adam M

would have thought that higher compression up the top would cause you to be forced to back off on the ignition to avoid det.

You should still make the same power, butwould be surprised if the timing map was not affected.

Considering what pat is saying, I wonder how much of the perceived increase from raised compression comes from the decrease in distance between the head and the piston crown which will seriously increase squish and effectively give rise to a charge concentrated close to the spark. This should seriously improve the combustion efficiency which pat is talking about.

Am still undecided on the compression I am going to run, hvaing chnaged my mind again. There is merit to dropping into the low 7s and upping the boost to increase engine longevity.

MorayMackenzie

Thanks for dragging this back out of the depths Harvey.

I believe that many of the phase two cars, including, I think, the STI v5 and v6, have an OE static CR of 8.5:1

I think Pat has summarised this issue best. With the correct combustion chamber design, camming and headflow work, the actual static compression ratio becomes much less important, the dynamic compression ratio being the interesting bit.

Ignition advance is still not the be-all and end-all of everything, if an engine can get full clean efficient combustion with bu&&er all advance then thats better, in my opinion, than running a 7:1 static CR and requiring masses of advance in order to get any useful torque until making silly levels of boost.

carlos_hiraoka

Yesterday I was checking the FIA homologation papers for the GC8 555 grp. N and it stated that a max comp. of 8.8:1 could be used (wonder how with stock parts, that is stock pistons and head gasket). And the max comp. in the homologation papers of a EVO 5 grp. N was 9.3:1

IIRC the static compression of the EVO 4,5,6 and 7 has been 8.8:1.

Like Moray has stated the newer STi's have more CR than the previous versions (STi ver 3/4 had 8~8.2:1), and run more ign timing from factory, wonder if the CR on the newer STi ver 8 is even higher, in the search for a better low end and off boost response .....

carlos_hiraoka

BTW the homologation of GC8 555 belongs to the STi ver 3 & 4.

carlos_hiraoka

A nice web link to calculate the effective compression ratio, of a turbocharged engine:

http://www.turbofast.com.au/TFcompB.html

the VE is affected by CR, so the lower the CR the lower VE .....

Bob Rawle

Moray, the std comp ratio for phase 2 is 8:1, raising the comp ratio normaly would bring about an increase in VE given all other parameters are able to support it, there would be a limit as timing would have to be reduced but not as much as you may think due to the "cleaner" combustion process, piston design in doing this also figures highly. The alternative comp ratio found on some Wagons was 9:1 btw.
Harvey runs a higher comp ratio but also is still using the traditional squish shape which could explain why he has not noted a startling change in feel on boost although his car clearly makes decent power.

carlos_hiraoka

Bob, got any idea of how Prodrive increase the CR of their grp. N engines ?

chg.

harvey

iTrader: (48)

Hi Moray: All the information I have indicates that STi 5+6 are standad 8.0:1 and that was my C.R. when the engine was original. If anybody has factual info to the contrary I am interested to hear about it.
All I was trying to do, in answer to your original postulations was point out that, IMHO, the C.R. was not a particularly signifigant factor in a high power turbocharged EJ20 and it certainly could not account for the higher output cars figures bearing in mind Bob and Steve are 8.5:1 and 465/389 bhp respectively, Andy is under 8.0:1 and 432 bhp and at 8.3/8.4 I am around 425 bhp +.As already stated I think it has a noticable effect on off boost performance but that is only to be expected.
Once the system is capable of flowing high volume cold air in and hot gas out it is down to a carefully matching of the turbocharger characteristics and that is why I believe Bob, Andy and I have our present outputs.
Also out of interest, I know some rally teams run at 10.5:1 with water injection and from memory Stan S runs 9.3:1. I have no idea on the engine life at 10.5:1 but if a whole rally is 100mls,who cares.

ustolemyname??stevieturbo

iTrader: (1)

You mention swapping the 1.6mm head gaskets for 0.6mm head gaskets.
When I did this on my own engine, it had 2 effects.
Below 6000rpm, the engine had no power( as if cam timing or something was out?? but pulleys were aligned ), and anything above 1200-1300rpm, the was a metallic noise from the engine, that did not sound good.
I only tried the gaskets, as they were ordered by incorrectly by the dealer, and I decided what harm could it do to try?
The inlet manifold bolts also were a bad fit due to the heads coming closer together by almost 2mm.

I can only assume that valves, or something was making contact, although no damage was done, and there were no visible indications of contact anywhere. Cam timing was lined up as close as possible to the markings on the pulleys ( I havent actually seen an engine yet where they line up perfectly?? ), and everything else on the engine wise was in good order.
Replacing the head gaskets with the thicker ones fixed all the problems I had with mine. Power was restored, and a nice quiet engine. My Engine at the time was basically an early wrx type.

harvey

iTrader: (48)

Hi Stevie: I am not an expert on phase 1 engines but I think the standard gasket on an STi 4 for instance is 1.6mm whereas on phase 2 STi 5 and 6 the standard gasket is .6mm. By reducing the clearance by 40 thou I suspect your valves were kissing the piston tops (just). When I shaved my heads I took off 20 thou having decided anything over 25 thou could possibly result in problems.

MorayMackenzie

Hi Bob,

I am suprised about the phase two CR you mention. I am sure I have read 8.5:1 in the subaru workshop manual for the MY99. I will check again when i get home.

Moray

Bob Rawle

Moray ... its wrong !!

Adam M

I was under the impression that all standard saloons were 8:1 and some of the wagons were 8.5:1 but certainly the difference didnt manifest itself across the engine reincarnation.

steve McCulloch

Harvey

Please dont compare the power specs between mine and Bobs car from two different companies RR figures. Plus as you know Bobs car has a bigger turbo and different manifold and runs higher boost figures than mine at present although I know why you portrayed the numbers

Re Compression. I thought it had long been known that raising compression increases power and response...(if everything is compatible) well this has been known for a long time in Cossie Tuning

In my Escort Cossie, if I remember the CR was about 7-7.2:1. I had a 2.2 kit put on and at the same time the cylinder heads were modded to raise the compression to 8:1. The driving difference was remarkable.. but I was still running silly boost, though I suspect they did reduce the advance levels....
That said its power was an unstunning 480bhp now...but very driveable

harvey

iTrader: (48)

Hi Steve: Point taken. Bob did 407bhp at W.L. whereas your figure on the day was 389bhp and Bob then went on to produce 465 bhp at Power Engineering.
For some time it has been obvious that extracting power from EJ20's is way behind the 2 litre Cosworths but is that because the Cosworth has more potential, is a better design for high output in the first place or just that it has had more development over a longer period?

MorayMackenzie

Bob,

The subaru workshop manual is wrong? How do we know this? It does _claim_ that the C/R is 8.5:1.

Moray

Bob Rawle

Moray its wrong cos Subaru admit its wrong and also cos if you check an engine you will find its 8:1. I guess they never bothered to send an errata out with them.

cheers

harvey

iTrader: (48)

Was 8.0:1 from calcs at the time and this corresponded with both workshop manual figures and owner hand book info. for STi ver 5+6.
Earlier Wagons cica '96 did have 8.5:1 compression.
Above info also checks out on cross reference on chasis plate numbers.
Hope this clears up any doubts.

MorayMackenzie

Okay, thanks for clearing that up Bob.