Thoughts on Compression Ratios? And CR of some common builds

smh0101 · Post by **smh0101** » Mon Oct 12, 2009 6:41 am

So I found a Compression Ratio Calculator for common subaru engines...

Stock ej22t = 8.17:1
NA ej25d = 9.4:1
Stock ej257 = 8.55:1

Common Hybrid Motors found on LC:

Stock 22t block w/ JDM Legacy RS 20g w/ 22t HG = 6.8:1 FAIL
Stock 22t block w/ LGT DOHC Heads w/ 22t HG = 7.6:1
Stock 22t block w/ JDM WRX/STi 20g w/ 22t HG = 7.3:1
Stock 22t block w/ JDM WRX/STi 20k w/ 22t HG = 7.7:1

ej22t w/ Wiseco 8.5 CR w/ JDM Legacy RS 20g w/ 22t HG = 7.4:1
ej22t w/ Wiseco 8.5 CR w/ LGT DOHC Heads w/ 22t HG = 8.3:1
ej22t w/ Wiseco 8.5 CR w/ JDM WRX/STi 20g Heads w/ 22t HG = 8.1:1
ej22t w/ Wiseco 8.5 CR w/ JDM WRX/STi 20k Heads w/22t HG = 8.5:1

ej235 Stroker w/ Wiseco Pistons w/ JDM Legacy RS 20g w/ 22t HG = 7.6:1
ej235 Stroker w/ Wiseco Pistons w/ LGT DOHC Heads w/ 22t HG = 8.4:1
ej235 Stroker w/ Wiseco Pistons w/ JDM WRX/STi 20g Heads = 8.2:1
ej235 Stroker w/ Wiseco Pistons w/ JDM WRX/STi 20k Heads = 8.5:1

Combustion Chamber Size:
JDM Legacy RS 20g Heads = 57cc
USDM LGT Heads = 46.2 cc
JDM WRX/STi 20g Heads = 49cc
JDM WRX/STi 20k Heads = 45.2

My build ended up being HELLA high compression...

Basically an ej257 (257 rods and drop in CP Pistons) w/ LGT DOHC Heads w/ ej253 THIN Metal HG = 9.1:1

Holy NA High Compression Motor batman! No wonder I have great off boost response and have a td04 spinning at 1600rpm LOL

Whats funny is that the Stock ECU doesnt seem to care that I have a higher compression and higher boost and bigger injectors! These old ecus are better than we give them credit for.

So basically for my build I can run Higher Boost on a big turbo, just gonna have to pull timing up top. Thats fine with me because I'll still have better throttle response than someone with Low Compression but crazy high boost and mega advance timing.

But THANK god I dont have a stock 2.2L Block w/ Roller Rocker 20g heads! Yipes! LOW Compression

Now lets see if I can find the link to the Excel Spreadsheet Calculator haha

Just figured I'd share this with everybody, If you post up what hg you have and the block and the heads and the piston dish size I can just plug the numbers in if you are lazy like me lol

And keep in mind these may be off by +/- .1 or so... Its really hard to keep absolutely everything correct when you calculate these

smh0101 · Post by **smh0101** » Mon Oct 12, 2009 7:38 am

Annnnnndddd that was a lot of frikkin math lol

I just finished editing and recalculating and adding things almost an hour after the original post haha

kleinkid · Post by **kleinkid** » Mon Oct 12, 2009 7:45 am

Way to go Einstein!

Mattheww044 · Post by **Mattheww044** » Mon Oct 12, 2009 7:56 am

damn, that will definately come in handy for when I get that deep into my build (God only knows when that will be!) Thanks man

smh0101 · Post by **smh0101** » Mon Oct 12, 2009 8:02 am

No prob

Hope this helps some people decide on combos...

Dave was telling me how on 25lbs of boost with a t3/t4 turbo on a 2.2L w/ Legacy RS Heads he was only dynoing at about 260 wheel... well I think that was when he had stock pistons... so a super low comp ratio probably killed him... he also had a standalone

PhyrraM · Post by **PhyrraM** » Mon Oct 12, 2009 1:09 pm

Where did you quote the EJ20G and EJ20K heads from? The most commonly quoted that I have seen is 47cc for both.

If my machinist would get off his lazy bum, I would have hands on quotes for both.

Post by **Legacy777** » Mon Oct 12, 2009 5:44 pm

smh0101 wrote:Whats funny is that the Stock ECU doesnt seem to care that I have a higher compression and higher boost and bigger injectors! These old ecus are better than we give them credit for.

It may be "working" but I'm sure it's far from ideal. I'm sure you would see better performance with a properly setup ECU.

tturnpaw · Post by **tturnpaw** » Mon Oct 12, 2009 10:55 pm

I think the best compression ratio for boost, turbo specifically tends to be around 8.5:1

Most high horsepower cars that are reliable are running near or at that for a good combination of off boost response and safety margins of power.

smh0101 · Post by **smh0101** » Mon Oct 12, 2009 11:41 pm

The safety margins rely on the tuning more than anything else.

And Josh, I'm sure if I had a standalone it would make more power, but my AFRs are good, and its pretty fast so thats why I'm saying it seems to be working okay... but a retune would definitely net some gains.

And Phyrra... They were in the calculator as that.

log1call · Post by **log1call** » Tue Oct 13, 2009 2:16 am

I wonder how long the rings and bearings are going to last.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 2:28 am

On my motor? Why wouldnt they last? As long as there is no detonation everything will last just dandy

TUNING

tturnpaw · Post by **tturnpaw** » Tue Oct 13, 2009 4:16 am

tuning is obviously important. were debating compression ratios, and a higher safety margin would be lower. You can tune a 12:1 turbo engine, but its not going to be as safe or reliable as a 8.5:1 running more boost with the same quality tune no matter what fuel you run.

93forestpearl · Post by **93forestpearl** » Tue Oct 13, 2009 5:14 am

I'm going to miss the 8.5:1 of my old setup, but an extra 60 cfm in head flow will be worth it. It won't really matter at 16 lb/min at 3000 rpm anyways.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 5:36 am

tturnpaw wrote:tuning is obviously important. were debating compression ratios, and a higher safety margin would be lower. You can tune a 12:1 turbo engine, but its not going to be as safe or reliable as a 8.5:1 running more boost with the same quality tune no matter what fuel you run.

Well a 12:1 motor is a BIG difference than a 9.1:1 motor

tturnpaw · Post by **tturnpaw** » Tue Oct 13, 2009 7:58 am

true...but you talk to an engine builder that builds boosted engines he will tell you that 8.5 is significantly better than a 9:1 for streetability and for flexability on the tuners part

If you had 8.8:1 we wouldnt be having this conversation.

but i will say that if its remaining under 280whp-300whp you will be fine.

Also, i suggest you install an egt gauge and monitor your temps on long pulls. Do back to back runs on a dyno for instance. AFRs can be perfect but your egt could be in the 1350s-1400+ without you even knowing it. Another reason Im no longer in the dsm world haha. ---poor head + intake design.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 8:02 am

I'd disagree. I was talking to Doug Vincent and Dave Bush about this... They BOTH agreed that 9:1 is better for off boost response AND streetability. Doug said that low compression is great for drag racing, Massive Boost w/ Low Compression.

Doug is here on the boards with his creations and Dave is the ex rallitek tech who went to Tuning School with Tim Bailey from Surgeline.

So I think you've got it backwards there.

log1call · Post by **log1call** » Tue Oct 13, 2009 8:02 am

When you raise the compression the thermal efficiency goes down. When you raise the compression so much you have to back off the advance then the volumetric efficiency goes down. When you lessen the combustion chamber size you get less air/fuel mix in there. It's been proven to be best to have as larger combustion chamber as possible with as higher compression(courtecy of the turbo) as can be run with as much advance as possible gives the best power and life to the engine. Raisnig he compression ratio negates the advantage of having a turbo.

Those are factors that effect power.

Reliabilityis a different matter but it's influence by most of those factors as well.

Subaru(and all the other manufacturers) know quite a bit about these things.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 8:04 am

I'd disagree. I was talking to Doug Vincent and Dave Bush about this... They BOTH agreed that 9:1 is better for off boost response AND streetability. Doug said that low compression is great for drag racing, Massive Boost w/ Low Compression.

Doug is here on the boards with his creations and Dave is the ex rallitek tech who went to Tuning School with Tim Bailey from Surgeline.

So I think you've got it backwards there.

tturnpaw · Post by **tturnpaw** » Tue Oct 13, 2009 8:08 am

log1call's got it. You have a LARGE margin for tuning and fuel adjustments with low compression turbo setups than you ever will with higher compression. Once you get to where youre losing 5-10-15hp just off one to two degrees you know youre at the edge. Any tuner will tell you you dont want that kind of a situation for a street car.

Yes, off boost behavior will be better because its practically stock. But you dont have to worry about your engine melting a piston or ring lands at idle-3000rpm. So its your opinion i guess. I believe if i build a car that isnt extremely reliable to where i can drive it on the street, its useless.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 8:09 am

log1call wrote:When you raise the compression the thermal efficiency goes down. When you raise the compression so much you have to back off the advance then the volumetric efficiency goes down. When you lessen the combustion chamber size you get less air/fuel mix in there. It's been proven to be best to have as larger combustion chamber as possible with as higher compression(courtecy of the turbo) as can be run with as much advance as possible gives the best power and life to the engine. Raisnig he compression ratio negates the advantage of having a turbo.

Those are factors that effect power.

Reliabilityis a different matter but it's influence by most of those factors as well.

Subaru(and all the other manufacturers) know quite a bit about these things.

Sure all true... But I suppose the answer to all this lies in what you want from the car? Whats your driving style? What is YOUR preference...

Personally I like immediate response with good power and the ability to accelerate without either downing a gear or waiting for the turbo to spool.

Obviously my build isnt for someone who wants a mega top end monster... but rather fast turboed car with great response.

And btw... From what I've read... Most turbo charged cars run 9:1 or less. So I'm only a tad over that... Big whoop.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 8:11 am

tturnpaw wrote:log1call's got it. You have a LARGE margin for tuning and fuel adjustments with low compression turbo setups than you ever will with higher compression. Once you get to where youre losing 5-10-15hp just off one to two degrees you know youre at the edge. Any tuner will tell you you dont want that kind of a situation for a street car.

Yes, off boost behavior will be better because its practically stock. But you dont have to worry about your engine melting a piston or ring lands at idle-3000rpm. So its your opinion i guess. I believe if i build a car that isnt extremely reliable to where i can drive it on the street, its useless.

Once again... Sure...

This is why you get a good tuner

And I'm not worried one bit about melting a piston (I dont know how the fuck I'd do that without massively running lean.... once again... tuning) And I'm not worried about it not being reliable.

The biggest turbo that will most likely be on this car is a 20g or a PE 1820

tturnpaw · Post by **tturnpaw** » Tue Oct 13, 2009 8:12 am

Yeah im looking out for you man. Might wanna pick up a stand alone soon.

log1call · Post by **log1call** » Tue Oct 13, 2009 8:13 am

When the thermal efficiency goes down it's because the heat is going into the motor... the pistons in particular... which softens the rings.

smh0101 · Post by **smh0101** » Tue Oct 13, 2009 8:14 am

Oh I will!

I'm planning on getting a Link once I get the money... until then keep the turbo small or the boost low. lol

And I'll be keeping an eye on the afrs with my wideband until then.

tturnpaw · Post by **tturnpaw** » Tue Oct 13, 2009 8:16 am

93forestpearl wrote:I'm going to miss the 8.5:1 of my old setup, but an extra 60 cfm in head flow will be worth it. It won't really matter at 16 lb/min at 3000 rpm anyways.

Thats what im talking about, make up for it in CFM!!

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Thoughts on Compression Ratios? And CR of some common builds

Thoughts on Compression Ratios? And CR of some common builds

Re: Thoughts on Compression Ratios? And CR of some common bu