legacycentral bbs

smh0101 wrote:
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

That was those heads and in particular the cams within them. The CR didn't cause that. Just look at Xephyr's old threads. He was making more than 50 more whp than that with a similar turbo with a CR in the low 7's with different heads. You really have ZERO empirical evidence to back up your claim that CR caused that.

As for the 9:1 thing? Can't be that high since Subaru did it from the factory on a number of their mid to late 90's turbo cars, including the Foresters and Legacies.

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.

Sorry...but there's a lot of false in the above post.

-Timing and volumetric efficiency are totally unrelated. VE is a factor of how much of the mixture fills the cylinder...timing is NOT a factor in this regard.

-The size of the combustion chamber also has no effect on cylinder filling...that has to do with port sizing/shape, combustion chamber shape, valve timing and intake/exhaust manifold design.

I also find it funny that some of you are arguing about what the best comp. ratio is for turbos...as if there's only one number. There's soooo many factors that should be accounted for to determine this...like vehicle weight, gearing, desired fuel, engine displacement, desired power band/usage, etc. There's plenty of very well sorted-out, street driven turbo cars with compression ratios ranging anywhere from 7.5:1 all the way to 11:1. As the compression ratio goes up, tuning becomes more critical for sure, but the upshot of a higher C/R is that the car becomes more drivable, thanks to more torque when off-boost.

Even better is DI. Makes running high CR with turbo much safer. Come on, Subaru, dammit!

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.

Thermal efficiency increases with mechanical higher compression (see chart below). Ignition timing has little effect on volumetric efficiency, its more the other way around, the better the VE the less advance you need because a full cylinder burns faster than a less full cylinder.

I will agree that the factory engineers know quite a bit about these things.



Like every thing else in engine performance compression ratio is a compromise and what’s “best” covers a broad range. In general a lower CR is more forgiving and requires a less precise tune and it may also provide more flexibility in tuning.

For me, off boost and transitional performance is very important in a street car. And a street car that will be used year round in all conditions will by nature have a more conservative tune. I believe that relatively high CR and lower boost are a better option for a street car. My build (which seems to be taking a lifetime to complete), will be 9:1 and stroked to 79mm. I’m shooting for full boost by 3000RPM with about 20psi max (max boost will ultimately be limited by detonation on pump gas).

Just some thoughts,
Gary

After driving the old setup for two years, and some stock S's and WRX's, I prefer power a little bit later. In my opinion, the VF11 and TD04 come on too early. It was more challenging to be easy on gas with something that spooled so damn fast.

My motor with the 22T heads was snappier off-boost than my N/A motor ever was, but there was a 200k mile disparity there, lol. I'll be at about 8.0:1 which will be fine by me. I like a nice smooth progression of power, which will be nicer than what I had because the whallop of torque in the midrange then a lack of balls up top makes the car harder to drive fast. I had to short shift the damn thing before 5900 rpm, which was really annoying.

I feel a boost threshold of about 2500 rpm coupled with a full spool by 4000-4500 and rapping it out to 7000 rpm with nice top end makes for a nice driving, easily modulatable power band.



Its all just preference.

Going from the N/A to turbo block N/A, I felt almost no difference in power until higher rpms. The lower compression engine started to roll over and die over 5k.

Yeah ok guys, I concede that volumetric efficiency was a bad choice of words. I meant overall efficiency more.

Thermal efficiency is to do with pressure which isn't just compresion ratio in this case and my point was that too much compression pressure will cause heat in the pistons. My comments were in reply to the question about whu the rings and bearings may not last long.

We can, as has been suggested retune our motors as we wish. The designers at subaru however have developed motors to a very fine degree to give a high performance whilst retaining both reliability and durability. As we modify any one factor the effect is compounded by other conciderations which can throw the entire design criteria out. The line between reliability, durability and disaster is very. very fine.

That's my observations anyway.

what about the dynamic compression ratio? isn't that a more meaningful number than static CR albeit harder to calculate? even if you get the same static CR from 2 different builds cylinder pressure may not be the same because of cam profiles. i have not seen dynamic CR discussed much.

Bringing up an OLD thread here, Anyone know what the ratio would be for 22T block with OEM HG and 20H heads?

OLD Thread lol

I have an excel spreadsheet somewhere that I can calculate it with. I'll check on my other computer once I get home.

Thanks man

CJ here's a matrix of blocks and heads, done by Marnix on Nasioc:

http://www.b4et.com/docs/cr_matrix.pdf