In a book called "Souping the Stock Engine", published by Floyd Clymer in 1950, Roger Huntington included a section on estimating peak horsepower when souping a stock block. He used a method using a factor for each type of modification. The following are his multiplier factors for each souping step and are for flatheads only... OHV's are different. [please note that CR stands for Compression Ratio]
Special aluminum heads........from 1.08 for 7.5:1 CR..... to 1.16 for 12.0:1 CR
Adding carbs and intake manifold........ 1.10 for 1 barrel per 3 or 4 cyl. 1.15 for 1 barrel per 1 or 2 cyl.
Reground cam........ from 1.08 for a semi grind..... to 1.22 for a super race grind Porting and relieving..... 1.05 to 1.08 (varies depending on severity)
Boring and stroking...... 1.00 plus 0.7 times the percentage increase in displacement (use 1.0 if larger valve are used)
Methanol..... 1.10 (scale down with lower alcohol content)
How about an example? Suppose we started with a 100hp Merc. that was 239 cubes from the factory. Say we added 9.5:1 heads, dual carbs, 3/4 cam, ported with a mild relief, bored and stroked to 286 inches, and will run it on gas.
Starting from the top and going in sequence.....
The 9.5:1 heads should have a factor of about 1.12.
Adding the dual two barrel carbs gives us 4 barrels for 8 cylinders or 1 barrel per 2 cylinders and a factor of about 1.15.
A reground 3/4 cam factor should be about 1.16.
Ported and mild relieving factor is about 1.05.
Boring and stroking from 239 to 286 results in a percent INCREASE of 286 divided by 239 is 19.7% [we only want the % of INCREASE] or 0.197 and using the constant 0.7 from above, the factor becomes 1.00+(0.197)(0.7)=1.138.
There is no multipliying factor for running on gas.
Now we multiply these factors together or (1.12)(1.15)(1.16)(1.05)(1.138)= 1.78 to get our total factor. The estimated peak hp of our modified engine is the original hp rating times the total factor or (100hp)(1.78)= 178 hp.
Whew... some work out huh? Anyway we have something new to play with now on cold winter nights. I had an opportunity to dyno an engine I had estimated using this method.
It was surprisingly close!!!!! rumble seat