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Old 07-15-2003, 09:36 AM
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Redline is determined by power production, if an engine produces 400Hp at 6200 rpm then the redline should be set at the rpm at which a single gear shift places the engine back onto it's torque peak. For example if the above engine made 400Hp at 6200 and peak torque occured at 4000 rpm then the redline should be placed between the two peaks at a point that is above the peak torque but below the peak HP. In the above example we could say anywhere between 4000-6200 rpm would be a good redline mark. Remember torque is what accelerates the vehicle...not horsepower.

The question then becomes "should I turn it to 6000 rpm or 4700 before I shift"? This question is answered by the actual loss in rpm between shifts. Lets say your 1-2 shift causes a 1200 rpm loss in rpm so ideally you would want the shift to occur anywhere between 5200 and 6200 rpm to accomodate the rpm drop so the engine never falls below it's torque peak. Each gear will have a certain rpm loss so it is entirely likely you will have a different shift point for each gear. The actual chosen shift point is determined by experimentation or calculation of the torque area under the power curve which results in the most power available for accelerating the car, weight of the car and rear axle gear ratio directly affects where the shift point is placed. Suffice to say heavier cars with stiffer gear ratios need more rpm between shifts and shifts should occur closer to redline to obtain the best times. On lighter cars it may be found there is little effect shifting closer to redline.

The question then becomes should I build my 400 Hp@6200 rpm engine to handle 6500 rpm or 7000? In general 1000 rpm is a good cushion to build into the motor so your peak power at 6200 rpm should handle a peak rpm of 7000 in case you miss a shift and also to build some durability into the engine. Valvetrain dynamics close to valve float is a dangerous combination of harmonics and spring surge so the engine ideally should never approach those conditions. Building your valvetrain to handle 8000 rpm but never turning it higher than 7000 is a good way to make an engine bullet proof, in this case more is usually better.

This is a complex issue and everyone will have their own opinion of how to do it, since engines will handle all kinds of abuse you may find people shifting their peak power at 6200 rpm engines at 7000 rpm with no ill effects whatsoever, of course how close you want to cut things is up to you. Durability takes a serious hit if the engine is revved too close too valve float territory. Frequency of abuse directly influences how you should build it. Most engines die from valvetrain failure resulting from valve float/surge at high rpm.

Another factor to consider is durability of your rotating assembly, just because you can cam and spring it to take 8000 rpm does not mean your stock Chevy 350 rods can handle it, off hand I would say a stock 350 rod will take 7000 rpm for extended periods of time assuming good balance and no weak links. A ford 302 would have similar limitations but a 351C Ford might be capable of 7500 rpm. By comparison my 400 Ford will never see the high side of 5500 rpm because of it's weaker rods, the build of the engine kept this limitation in mind.

Does this help?
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