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In a message dated 11/19/2003 10:40:33 AM Eastern Standard Time, canarder@frontiernet.net writes:
<... bridge ported 13B in GT-2 was 305 HP at 9,300 RPM with two 44MM chokes in a Webber 48 IDA ... as in E production cars, 235 HP at 9000 RPM with 38MM chokes ....> Does anyone know how much power these engines produce at our operating ranges of 6500 RPM or so? What are 44,48 mm chokes?
Just wonderin' ... Jim S.
The choke is the restriction inside of a carburetor that creates a high flow velocity used in generating a pressure differential between the throat and the fuel supply. The carb size is generally the size of the body at the level of the butterfly shaft. So even though the carb is a Webber 48 IDA, (48MM hole out the bottom) the amount of air it can flow is controlled by the removable choke, or in the case of GT-2 cars, 44MM. The Sports Car Club of America alters the choke size of different cars in the rule book to balance competition potential. (slow down the Mazdas).
The intake systems being built by most are more than adequate. Compare the single 44MM diameter area per rotor to, the area of the two tubes per rotor most are using.
My thinking here is that since the runner length is going to be much longer anyway, that the tube diameter should be a bit larger.
I have only one dyno sheet that goes down to 6,500 RPM because we are looking for peak power at 9,400 RPM (244.9 HP). But that is for a smaller 12A engine with bridgeports. At 6,500 RPM, torque is 139.6. HP is 173.7.
EGTs 1515 and 1491. BSFC is .666 (the devils own efficiency) Oil temp was 162. Oil pressure was 99.9 pounds. Air Fuel ratio was 12.3.
So what good was that? The engine was smaller than a 13B. The engine was bridgeported, and has the runners in the cast iron blended very well.
Every engine will be different, even from the same builder, so, in general, I think the stock porting is adequate for this RPM range, and the average installation should produce between 165 and 175 HP. Matching up the ports would help, and street porting would be a big help. Then later, if you want to go nuts, or put the Sun&Fun record away for good, you could bridgeport the streetported engine, and go up to maybe 185 HP.
This power (work) measured on a dyno at crankshaft speeds. So now multiply the 173.3 times the 2.17 speed reduction ratio to get 376.06 foot pounds of torque at the prop flange. Now look up a list of piston airplane engines, and see what is the advertised HP of the engine that has 376 foot pounds of torque (at the prop flange)?
And now we know how Tracy wins the race.
Lynn E. Hanover
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