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GSP vs. JRB Round 1
The tale of big valves...

When you custom build an engine it is only natural to want to test it on a dyno in order to ascertain your results, both in terms of design and planning, as well as execution.

Jim Butterworth and I started rebuilding our engines at the same time. Both rebuilds were prompted by spun rod bearings, probably a result of detonation or oil starvation. We were both committed to going the modified 2.5L route. Since Jim and I were working for the same firm we did a lot of planning together, and we agreed to share the cost of many of the special tools such as dial indicators and micrometers. It should be no surprise that our engines ended up being virtual clones of each other. Not just similar, but actually identical in almost all respects.

Our blocks were machined by the same shop at the same time. We used the same crank, pistons, rings, rods, cams, cam-timing, flywheels, electric fans, computer chips, intake system, injectors, etc... The full Evo III treatment for the most part, except that we planned on using a higher compression ratio. We basically tried to build our engines to match the software which was available, in this case the Conforti 2.5L chip for Evo III injectors.

Jim's engine and mine differ in three ways that we are aware of:

Jim Gustave
Turner Stage III Cyl. Head Don Fields "Stage II" Cyl. Head
Evo III 48 mm Throttle Bodies Don Fields Ported 48 mm Throttle Bodies
0.0030" skirt-to-wall clearance 0.0035" skirt-to-wall clearance

Jim's Turner Stage III head is enhanced in that it uses larger intake valves (38.5 mm vs. 37.0 mm) and larger exhaust valves (33.0 mm vs. 32.0 mm). The TMS head also incorporates aggressive porting, as opposed to the slightly more moderate porting on the Don Fields head. Don retained the stock valve diameters for my head. The Evo III throttle bodies are 2 mm larger than the stock 2.3L units (48 mm vs. 46 mm, a 9% increase in cross-sectional area). It is possible to machine the stock TB's to accept the larger 48 mm butterfly valves, and then port the rest of the body to match the butterfly's. But there is one particular section in the TB that just cannot be made as large as an Evo III unit because there is simply not enough "meat" in the casting.

Since Jim planned on using his engine as a daily driver, a streetable piston skirt to cylinder wall clearance was chosen. I don't drive my car on the street much, except to go to events, so I went with a looser clearance. On a race only engine an even larger clearance of 0.0040" could be spec'd.

OK, so enough of that, how did the two engines compare on the dyno? Well let's find out. Both Jim and I dyno'd at Dick Chiang's Dynospot Racing facility in Mountain View CA. Which takes at least that particular variable out of the comparison.

Click here to see a comparison dyno plot between my engine and Jim's.

Both Jim, Dick and I were surprised by this result. Jim and I fully expected that his engine would be below mine in the lower rpm region and then climb above mine as the rpm's got past say 5000 rpm. But as you can see, Jim's results for both torque and HP were uniformly lower than mine. At the 7800 rpm rev cutoff Jim "just" equaled my engine.

Speculation? Well, it is possible that Jim is running a lower compression ratio than I, even though we have the same pistons, rod length, deck height etc... It is possible that to fit the larger valves into the Stage III cylinder head, that some meat was removed and that his combustion chamber volume is therefore larger than mine. We both cc'd our heads and Jim calculated a 10.5:1 compression ratio where I calculated 11.4:1. But Jim said he was hasty and he does not have much confidence in his cc results. I was very meticulous and am very confident in mine.

Also, although we both degree'd our cams and set them to the Schrick factory specs, it is possible that Jim's engine requires less overlap to function properly since his low lift breathing is probably much better. Some trial and error dyno testing while adjusting the cam gears will shed some light on this and is being planned. Variability in weather and conditions is also a possibility, although it is unlikely to have a very large impact, if any.

Note: This dyno comparison is a bit premature. Jim had installed a new stainless 3" exhaust with Burns Y-Pipe and Evo III header for his runs. I have an identical system sitting in the garage but it is not installed yet. When I did my runs my car still had the SuperSprint center resonator and stock muffler with 2.3L Gp A header (smaller secondaries than the Evo III). We will have to see the result when I re-dyno my car with an identical exhaust to Jim's (see below).

Click here to see dyno charts of my engine with the new exhaust...

Click here to see a more recent comparison of Jim's engine to mine...

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