I know how stupid that sounds, but it got your attention! I am probably nearing the limits of my S475 T6 (1.32AR) at 10.1:1 c/r @ 17psi (trapping just over 120 in the 1/8th). I don't "know" this for certain as the car has never "flat-lined" and always picks up MPH w/each additional psi, but from what I've read that turbo is only good for ~800rw.

If I drop my compression a point will it reduce that 800rwhp level of my current turbo? If so by how much. I'd assume that it would take more air to fill the larger combustion area, but in the grand scheme of things it is only 11cc per cylinder of extra volume.

Any input here?

 

Its not filling anymore or less volume, but reducing compression reduces the efficiency of the combustion process (less power for the same airmass).

 

so how much more airmass would i need to equalize the reduction in compression?

 

You will lose Volumetric Efficiency.

That turbo makes 21-23psi on two forged 370's here locally. Both have a good set of aftermarket/worked over cylinder heads. They trap 129-132mph. One weighs 3420ish and has trapped 129 in the 1/8th and 161 in the quarter while the other at 3250 has been 132 in the 1/8th.

I'd say you have some room left to turn the wick up on that turbo you have now, but now would be a good time to go ahead and upgrade.

You can send the turbo in and have the compressor side upgraded to a billet wheel 76mm wheel that the new S476R's are using, you can upgrade it to a 80mm wheel, 82mm, 83mm or you can ditch the S400 chassis altogether and step up to a S500 series turbo with the 111mm turbine versus the 96mm turbine. You will most likely either need to advance the ICL of the cam you have now and possibly a new grind, but advancing it first would be what I would do.

There is also a S488 out there that uses a 108mm turbine wheel and a 88mm compressor wheel. Bullseye power makes one with a 1.0, 1.10 and 1.32 AR housings. I would use the 1.10 or 1.32.

 

Ya, money is too tight for swapping turbo's right now. I'll just have to see how much I can get out of this 75/96mm wheel. Any idea what they get to bump it to an 80, 82, or 83? I presume the 80 would flow more then enough hp for 900+rwhp which seems to be the limit of the 4 bolt stock heads anyways.

 

I always tell folks 1 point is equivalent to about 3% in power. All other things being equal.

Not to nitpick, but from my understanding VE should stay the same. It cuts peak compression/peak cylinder temperature so the whole process is a smidge less efficient.

I think you'll spool a little slower as well - just something to keep in mind when you get your other turbo.

 

Yes, VE will remain the same, just more wasted heat during combustion process (slightly higher EGTs).

 

I figured it would drop a little bit, I guess not.

Don't know how much they get to upgrade a compressor wheel. 80mm wheel will be plenty for 900rwhp on the boost you run.

 

IF you are reaching the limits of the turbo, then altering the CR will achieve nothing.

You need to establish if the turbo is the problem or not. You know boost pressure, but what is turbine pressure like ?
What is your tuning like ? What fuel are you using ? What are intake temps like ?

If your fuel was a limitation, then a lower CR would allow running more timing for any given boost which could help power.

 

You mean if he is at the limit of the turbo, then lower his compression ratio will hurt power a little. A compression ratio increase will increase it slightly (assuming he has fuel that will tolerate it).

 

I would guess another 3% of air flow, or about 1 psi more of boost.

There are alot of variables here though. Especially, if you are on pump gas at the boost level you listed. You may find that the lower compression will allow you to advance your timing to a more optimal point....in which case, you wouldn't be down any power.

 

I am on pump gas. I went down in compression trying to buy myself a little room before detonation. I am currently spraying one 14gal nozzle of meth as well but thinking about adding another 10 or 12 gal nozzle as well to help out even more.

 

The drop in compression should help a lot. Some folks aren't fans of lower compression. Its always served me well though.

I'm also a fan of specing the pistons to achieve a healthy amount of rise to minimize the squish - seems to help in building a detonation resistant motor. I don't know what the trend is now days, but I know a lot of shops used to build a zero rise or even deck piston motor.

Also on the list of gizmo's that help alot with detonation is your intercooler - if you haven't looked at it already.

Methanol works great, I'm just leery of pushing people towards it because I've seen a couple motors give up the ghost as the result of a methanol system failure.