fishfire

Hi all,

Sorry if this is a bit of a bone question, but I am changing the pistons and rods in my engine, but not sure what commpresion ration pistons I need.

Is it 8.0:1 or 8.5:1?

I have a MY97 WRX 2.0 Turbo (ej20)

Please help

dansti300

where ever you buy pistons from will be able to tell you if give them engine number
cheers dan.

500

Speak to Mark at Lateral or David at Api, they will be able to advise you which pistons and which head gaskets to use.

I'd say 8.0:1

mcphilimey

hi mate it's 8.5:1, i can supply you with some forged race pistons at a very cheap price. All above board, got a contact who works for an engine components company. let me know what you want i'll get some prices

fishfire

Been told 8.0:1 up to year 2000 ? SIDC FAQ also says this. Can someone confirm this?

fishfire

Engine
1994-6 1997 1998 1999 - 2000
Type 4 cylinder, boxer layout, 1994cc
Construction Aluminium alloy head and block
Valve Gear 4 per cylinder, DOHC
Compression ratio 8.0:1
Power 208 bhp@6000 rpm 215 bhp@5600 rpm
Torque 201 lb ft@4800 rpm 214 lbft@4000 rpm
Bore x Stroke 92.0mm x 75.0mm

fishfire

Anyone confirm the JDM ej20's are still 8.0:1 please ?

mcphilimey

if you look at forged pistons on the internet they all state a 8.5:1 ratio. spoke to my contact who sells loads of these and they fit all ej20's regardless of year

Triple X

iTrader: (1)

If you have a mappable ECU it wont really make much difference, difference being the 8.5 might give slightly better response when off boost whereas the 8.0 will take slightly more boost.

fishfire

Thats great just didnt want to install the wrong pistons and end up with a broken engine!

Thanks for the advice guys

Mongo

Lower the CR, the greater the lag though mate..

I'm looking at this myself, and having spoken to mark sheid, he reccomends using 8.5:1.

mcphilimey

standard compression is 8.5:1 as fitted in a standard built engine. You can probably get other compression ratio pistons. Like i say, let me know i'll get some prices.

scooby abuser

WHY?

Mongo

Which bit?

8.5:1 was a good comprimise between lag and usability.

Lower CR creating more lag. I'll have to ask

scooby abuser

Lowering the compression ratio just makes the motor produce less power with the same spec as a higher compression motor, the lower CR just allows safe running of higher boost levels without detonation

APIDavid

iTrader: (4)

Exactly as fishfire says.

8.5 :1 would be dangerous on an unmapped / unmappable engine. For the very same reason I explain carefully to customers about not skimming Impreza turbo heads ***** nilly.

UP TO 8.5 : 1 is fine on a properly built for it engine using other components compatible with that power / energy level. If the rest of the car / engime is basically standard then stick to original spec.

David APi

Mongo

Ah I see what your on about now. I'm talking about a re-mapped ecu

Bob Rawle

Std comp ratio is 8 to 1 plus/minus 0.2, so thinking 8.5 to 1 is std is very wrong. As David says any increase on the std comp must have a remap, the "best" compromise for a road car is 8.3 to 1 from my experience, makes for a crisp drive and will still take good ignition when remapped properly and run with decent fuel with a good degree of det tolerance. Subaru engines cannot be compared to Evo or Ford lumps in this respect.

cheers

bob

harvey

iTrader: (48)

Yep. Standard C.R. on Classic Subaru is 8.0:1 but an exception to that at 8.5 :1 were some of the early wagons.
I have experience of various CRs from 7.45 (Cosworth Pistons) to around 9.2:1 on road cars. I know of rally cars operating at and above 12:1 but that has no relevance to road going engines built for reliability. Unless you are building a special engine with the right components and are prepared to accurately measure cylinder head combustion bowl, deck height, gasket thickness, , gasket diameter, actual bore, piston bowl and do laborious calcs, just stickto standard 8.0:1
If buying pistons, other than O/E or direct replacements then check out the piston bowl and deck height with the manufacturer.
A head skim of 20 thou and + .25 pistons will raise CR on an otherwise standard 2 litre from 8.0 to nearly 8.3 approx :1 on a Classic STi5/6 etc.

GF8CL8D

mine is a early MY 95 wagon... dutch car probably... EJ20BUG engine running with TD05H.
Someone knows more about this engine?!?! Compression ratio, etc...

silent running

I'm interested in this as I'm about to embark on a rebuild myself and I've got to say, I'm looking to buy from the States, where it seems you can pick up a set of Wiseco/CP/JE forged pistons and Eagle rods for under £400. They are suited to 92.5mm, so obviously my block will need a slight overbore, but the CR is something a bit more vague. 8.5:1 seems to be the most often quoted CR but presumably that's on standard block/deck height, standard gasket, standard head? Is there a reliable way to calculate CR changes from any combo of block/gasket/head? It seems there are a lot of variables such as whether the deck or head have been skimmed, what thickness of gasket is used etc. Can anyone give me an idea of how far you can move the CR by for example taking 1mm out of the combustion chamber height at TDC?

I'm looking at this from the perspective that I'd be better to go slightly higher compression than lower, as this is a daily driver that I want acceptable fuel economy from and I spend a lot of time on cruise. I'm powered by a TD05-16G and although I want to build my engine strong I won't want more than 350bhp and my Power FC is of course mappable. Is there anything wrong with going for a 8.5 to 9:1 ratio if it can be mapped for it? Heat management is all under control, I'll be using V-Power, and I definitely want flexibility in the midrange. I'll be gutted if after the build I need to run the turbo ragged and rev the nuts off it to get the thing to go anywhere - I'd be much happier with a good solid midrange.