comparing 1990 to 2001 build for unleaded premium fuel
Oct 24, 2012
I was digging through some old data and dyno sheets last night from the back of my old file cabinet that tells a tale around the improving cylinder head technology of modern engines. These are four engines two of which the data is from 1990 and two others from 2001; both pairs are small block Chevy Gen I’s in today’s understanding of Chevy engine evolution. Both pairs represent the technology of building a 362 and 356 horsepower 355 also a 400 and 401 horsepower 355 basically hot street performance engines in the technology available when they were built.
A big message I took from this old data is the effect that more efficient combustion chambers have on power output and how that drives backwards into camshaft selection and compression ratio. The older heads take a lot more camshaft to develop similar power to modern head designs using milder cam timing. Also, seen is that the longer duration cams drive you to higher Static Compression Ratios in-order to recover the dynamic Compression ratio which sometimes just can’t get there. Further along this thought in the engine with the Magnum 292 cam one can see where a dome is required to push the SCR up which fails to restore the DCR to a high enough level to recover lower end torque while having a dome large enough to impede the burn. The very obvious effect of this is seen where the torque peaks at a lower value 389 pound feet at a higher RPM 5000 with a horsepower peak of 401 but at 6250 RPM compared to the Vortec headed engine with 430 pound feet at only 3700 RPM with a horsepower peak of 401 at 5900 RPM. The Vortec headed engine is more street and automatic transmission friendly. To bring up the torque curve on the Bow-Tie headed engine would require about a 14 to 1 SCR, which will not be unleaded fuel friendly, the 12 to 1 did not work well on 91 octane pump gas without running the engine quite cold and the mixture rich with less than the best amount of ignition advance. With 100 plus octane race gas this engine could be corrected where it developed another 20-30 horses. It needed the Street Ram to get enough breathing to achive 400 horses as it just had to wind further to get there. The Vortec headed engine was totally happy on 91 octane unleaded with normal operating temps 180-190 degrees and normal mixtures and typical 34-36 degree advance for that head. Not to mention it ran against the stock converter of a 700R4. This consistency also continues between the older 360 and newer 356 horse 355 as well; where the larger cam and old 292 large valve head of the earlier engine shows a lower peak 370 pound feet of torque at a higher 4500 RPM than the L98 headed engine which produced 380 pound feet at 3500 RPM which is an even thousand RPM lower, while both engines pulled their horsepower peak at the same 5500 RPM with the L98 showing only 6 fewer hp. The L98 went into my baby brother’s 79 which couldn’t pull your grandmother off a pin cushion. What a difference, it’s very streetable and pins you to your seat. The Vortec version would be 46 more horsepower of the same action.
Last edited by oldbogie; 10-26-2012 at 06:05 PM.