In May of 1955 Hot Rod Magazine did an article on Frank McGurk's dyno tests on a Chev 261 I-6. It appears that factory ratings are a bit optimistic, something I've long wondered about as they are signed off by the engineer in charge. They probably do battle with the marketing dept. regularly. Also Mc Gurk's dyno could have read low. I don't believe it was certified. But I think the relative increases are meaningful.
Anyway the max HP on the stock engine on McGurk's dyno was 114 at 3500 RPM and torque was 199@2500.
Changing to the Corvette dual exhaust manifold netted a 6% increase in HP at the same RPM and 3% on torque.
Test 3 added 2 Stromberg 1bbl carbs on McGruk's manifold for a 16% increase in HP and 18% over stock @ 4000 RPM and 10% more torque, again at 2500RPM.
Test 4 added porting and polishing the head and bumping the compression ration to 7.85:1 This netted a bump in HP of 9% and torque of 4%. So now we're up to 29% HP over stock @4000 and and 15% torque still at 2500RPM.
Test 5 added a reground camshaft. I should note that various reports are that the stock 261 cam is the same part no. as the Corvette cam, considerably hotter than the 235 powerglide cam. The test cam was a "3/4" grind with 245 deg. intake and 274 deg. exh duration with .421 and .410 respectively. They don't say at what lift these measurements were taken. The result was a 6.1 increase in HP and 2.6 torque. Relative to stock performance we now see 36% and 18% respectively.
Further increases were with the aid of a boring bar going 1/8" over stock and hotter cams not really suitable for the street. So fro this you can see that power improvements come incrementally and without a boring bar the best one can hope for is about what test 5 shows. I was surprised to see that the most effective way to increase torque is by increasing the compression ratio.
On my 235 engine I have made all the mods except porting and polishing and boring. The cam is slightly hotter than McGurks and advanced from split-timing by 5 degrees, as much as I dared because of possible valve-piston interference. The head is an 848 milled 0.010." The engine has been in service for 15 years in its current configuration and about 100,000 miles. From driving it, I would judge it's torque peak to be 2800-2900 RPM. The fire pretty much goes out below about 2400-2500RPM but it will pull smoothly down to about 1200RPM. It is a bit peaky but with a 12-speed gearbox (4x3) I can easily keep it on the boil.
