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Old 06-20-2013, 09:41 AM
oldbogie oldbogie is offline
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Quote:
Originally Posted by SPEED MASTERS CC View Post
I am considering changing cylinder heads from the current 64cc chambered 200 cc Sportsman 2's to 50cc chambered 220 cc heads from engine quest.

The engine is a 408 sbc, 11.7-1 compression, 250 duration at .05/.575 lift on a 106 lsa. The intake is a edelbrock dual quad. The fuel is E-85.

Would there be a worth while gain in output for the above combo?
The thing that counts is the Dynamic Compression Ratio (DCR) to get at this requires a timing card with the intake closing point in crankshaft degrees. To make the calculation the rod length and stroke are also needed because the math turns the rotational degrees of the crank into a trig function to compute the amount of stroke used up at the point where the intake valve closes.

A caution in that many timing cards give the event timing starting from .050 inch lift which is not yet the point where the valve is closed on its seat. Many models have you add 15 degrees for this situation but this rather models modern fast ramp cams, older design and muscle car era cams and many required by rule limitations of racing associations specify cams that tend to have long ramps which may hold the valve off its seat for 25 to 30 or more degrees. While admittedly this is much not of a gap between the valve and the seat one needs to keep in mind that reverse pumping is occurring with the compressive force of the rising piston behind it. You can calculate the total ramp if you have the total duration and the .050 duration, then subtracting the latter from the former, dividing by 2 will give the ramp degrees between .050 and totally closed. This assumes the ramp is symmetrical on both sides of the lobe.

As far as gains go, this depends upon the cam and the DCR. If the DCR is optimized ((keep in mind that the DCR is always less than the Static Compression Ratio (SCR)) then there is little gain to be had 2-3% maybe. If the DCR is sub-optimized then the gain can be substantially larger. Generally the DCR needs to be at least 8 to 1. I have a range I try to hit that goes to 9 but there are qualifiers to ratios above 8 and maybe a couple decimal points more. A street engine or any engine that's working with a lot of vehicle weight and high gear ratios needs to be lower where lighter weight and stiffer gearing will work with more. Lower octane pump fuel needs to stay around the 8:1 figure, high octane can tolerate more. Iron heads need to stay around 8, aluminum can push .5 to 1 full point more depending on octane, vehicle weight and gearing. Small chamber modern fast burn heads can tolerate more; way more than old smog era open chambers and some more than the old pre smog small chambers, basically this is where the spark plug is located in these older heads. Piston shape is important flat tops and D-dish pistons will favor higher ratios, round dishes and stepped pistons the latter looks a lot like a D dish but doesn't contain the dish in a bowl on the spark plug side, these will not tolerate as much compression ratio as a F/T or D dish but do better than a round dish. Domes are sometimes a necessary evil if large chamber heads are used they're not efficient but they are effective.

Calculation of the DCR is dependent upon calculation of the SCR, to get to the desired DCR you've got to play the two back and forth a few times.
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