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Old 12-07-2008, 11:44 PM
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Jac Mac Jac Mac is offline
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Join Date: Feb 2006
Location: Gore. New Zealand., SI
Cobra Make, Engine: DIY Coupe, F/T ,MkIV.
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It would be fair to assume that most road going naturally aspirated motors produce maximum torque within a 2500 / 4000 rev range so your 35° advance is in for most of that- now another critical factor here is rod ratio & how long in terms of crank degrees the piston is parked around TDC. Apart from all the mechanical reasons like less piston skirt thrust loads etc , this time at TDC allows the flame front to burn thru the fuel mix & create more pressure at the point where the piston/rod assy is getting the maximum leverage on the crankshaft throw. In a short rod combo the piston would be further away down the bore to the point where it is virtually running away from the pressure ( an over exageration, but should help explain it ). Low compression ratios and lots of ignition advance can be quite scary because if there is not proper mixing of the fuel/air you will get lean and rich areas in the fuel mix and this can /does lead to preignition & detonation given the right circumstance's even at relatively low RPM-- once this happens your on a downward spiral to trouble as the temps will spike in that cyl and increase these problems.
Main thing is to be aware of what might/can happen & back off timing & throttle at the first hint of problems-- most of which you only learn by doing it wrong---once!
Need to add a bit more to this--you must address what the engine wants/needs at all times and in regard to acceleration even get a step in front to what its about to need, dynos can only do so much and you have to be aware continuously of the what effect airflow can have at speed,particularly around carbs and ram tubes etc. I have seen motors that produced great dyno numbers only to lose 25/30% of that on installation in the car due to carb air supply alone. while you might have been acheiving close to 100% VE on the dyno, you wont get that in the car if your drawing hot air off the exhaust/radiator or even off the exhaust of a competitors car if your drawing it from up front. Again this is only one facet of the engine package & the guy that wins has all or more of his ducks in a row with everything on the car- handling, brakes, etc mean as much if not more than outright HP. Its not easy to drive for any length of time in a high HP car if the thing wont stop & turn when you want to.
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Last edited by Jac Mac; 12-08-2008 at 03:21 AM.. Reason: xtra
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