A couple people might be interested in this. Jeff Evans did the comparison. Allot of variables but when isnt there going to be with this kind of comparison?

I had an opportunity to measure the difference between 10:1 compression set-up, and than dropped to 9:1 compression. The previous engine was a stock OEM gsr bottom end. The bottom end was swapped out with a 9:1 CP pistons/Eagle rod 0.020" gsr bottom end.

The set-up was as follows:

Inline pro manifold
Gt28rs
2.5" downpipe, 2.5" exhaust with an itr muffler (def. a restriction)
Tial 38mm wastegate (9psi wastegate spring)
Spearco intercooler
Drag IC piping
RC 550cc injectors
Walboro 255lph HP pump
3bar map sensor
Neptune Tuned

What was changed:

Bottom end
9:1 CP pistons, eagle rods, ACL bearings

No boost controller, the pulls were done on a 9psi Tial wastegate spring.

Here are the results:



To show that there was no difference in boost pressures, here is the boost pressure plot:



This is probably the most accurate test that you can perform. NO other changes were made to the system. What was interesting between the two tunes is that it took 6-7 degrees more timing than the previous 10:1 compression tune everywhere in boost to get to MBTT (maximum brake torque timing).

One pressure plot is like 0.25~0.5psi higher than the other. They are at approximately 8.5~9psi. With wastegates running off of just a spring, and no boost controller i ofter see 0.5~1psi fluctuations in the boost pressure from the wastegate getting hot.

One thing to remember is that the turbo that is being used is a gt28rs, and its a log manifold. The log manifold will have a higher exhaust pressure in the manifold, as well as needing less exhaust energy to spin the smaller turbine wheel in the gt28rs.

 

That is a sweet test. Thanks for sharing.

What jumps out at me right away is that spool up didnt really change at all with the higher compression. The car just makes a little more power everywhere after peak boost is reached. I have heard that this was true, but its nice to see that theory put to the test.

 

Nice test man. Is this the engine that I just did the machine work on?

 

Nope. That one is going to make a little more power than that I will shoot you some pictures Tuesday or Wenesday of the assembly.

 

So the engine made less horsepower with one point compression less at 1/2lb less boost. No surprises. What would have been a good learning experience is which compression makes more power before detonation appears. Its no secret higher compression motors make more power for a given boost pressure.

5 more horsepower before boost hits. Which does very little to help spool the turbo. Been telling that to people for years.

 

Exactly, the whole reason to lower the compression is to raise the boost pressure beyond previous pressures that would cause detonation. But a neat test nonetheless. I would like a test where the boost was maxed out on each setup to see the power deifferences or if anyone can throw up a formula for calculating the approx. increase of pressure you could raise it would be cool too.

 

it would have lost more power if it was a standard bore block. and thats also why the spool up time didnt change. so no, not an entirely accurate test, considering the constants became variables. and to go on with jerry's comments, a better test would have been @ 10:1 compression, how much more timing could be added on the .020" over bottom end before detonation occured. also, the pressure drop could be attributed to the over bore on the 2nd bottom end.