Engine Question

[Ray C.]

After restoring two Corvettes it is time for me to become more knowledgeable about engine performance. I have a 1965 327CI-250HP car with 336 gears and a 1961 283CI-270 HP with 370 gears. Could someone please explain the differences in performance capability of the above engines or engines that differ in cubic inches and horsepower like the 283CI vs. 327CI?

Thanks Ray

[Gene M.]

Re: Engine Question

Ray,
I would put my money on the 283 because the 250 horse 65 327 has the baby 1.72 intakes. The 327 will run out of breath on the intake side. This is assuming all original engine parts.

This could be rectified with some head and valve rework and a little carb massage.

[Duke W.]

Re: Engine Question

The 283/270 has medium port small valve heads (1.72"/1.50") and a Duntov cam. The 327/250 up to '64 has very limited performance potential because it has similar heads to the 283s with medium ports and small valves. The '65 327/250 is an anamoly. The larger 461 heads are installed but with the small port inlet manifold and 400 CFM WCFB carb, but Chevrolet did not increase the power rating. This engine could essentially be "upgraded" to 300 HP spec with the addition of the large port CI manifold and AFB, and the 2.5" exhaust manifolds, which were used on 300 HP engine with manual trans, but the 300 HP with PG were equipped with 2" manifolds.

All other 327s and later 350s have "big port" heads (1.94"/1.5" valves) with SHP versions after '63 having slightly larger valves (2.02"/1.6").

In general, for each displacement the primary difference in engine performance is the camshaft, but changes to CR and manifolding/carb also contribute to engine performance. (The two '61 FI engine options were equipped with the "big port" 461 heads, which is why they have higher rated power than the their '60 antecedents) The more aggressive SHP cams shift the torque curve up the rev scale, which increases top end power, but reduces low end power relative to the base engine camshaft.

Another issue to consider is that after 1971 GM only quoted "SAE net" power and torque, which is "as installed" in the vehicle with all front end accessories, vehicle exhaust system, emission control equipment (including air preheaters) and production fuel and spark calibration.

In 1971 both SAE net and gross were quoted. Prior to 1971 only SAE gross was quoted, which is on a lab dynamometer with no front end accessories, fuel and ignition calibration optimized, which did not necessarily represent production configuration with observed results corrected to standard sea level conditions. SAE net is corrected to 990 millibars and 25C dry air which results in about 4.5 percent less air density than standard sea level conditions and has about the same percentage effect on power.

A final note. Beginning in 1971 all CRs were lowered to allow all GM engines including SHP to operate on regular unleaded gasoline. This alone dropped Corvette engine torque and power nearly ten percent relative to their high compression premium fuel antecedents.

The lower compression ratio and conversion to SAE net rating is what caused the 1970 LT-1 to drop from 370 HP to 255 HP in 1972.

Duke

[Duke W.]

PS

Unfortunately, most enthusiasts fall in the "horsepower trap" when comparing engines, using only peak horsepower ratings to compare one to another.

This is a very narrowly focused one-dimensional view of engine performance since we rarely see peak power revs. What accelerates a car is the average horsepower available over the operating range.

For example, though the 283/270 has higher rated peak power than the 327/250, the later has considerably more peak torque and low RPM torque, which means more power is available in the normal low to mid operation range since power, which is energy per unit time, is the product of torque and revolutions. (T x N/5252).

A better way to characterize engine performance is peak torque and 80 percent torque bandwidth, which is the rev range that the engine produces 80 percent or more of peak torque. For a medium performance engine this range is typically under 1500 to about 5000. For SHP engines is it typically about 2000 to 6000.

Duke

[Ray C.]

Re: PS

Thanks Gene!

Duke when you are ready to teach an on-line course sign me up!

Ray

[Dave F.]

Me too Duke!! *NM*