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Some time ago there was discussion about advancing cam timing 4 degrees to increase torque. I am building a 283 using the 151 cam. The question is - how will this cam timing affect upper RPM limit? Will it reduce it or will it be barely noticeable? Appreciate any info on this, John.
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Re: 283 CI cam timing
John;
Since you are building a 283 this cam will act more radical than if it were in a 350 or larger small block. advancing the cam timing usually will bring down your power in the upper range, but in this engine it will happen much later than the larger CI motors. If you also use the stock intake and heads, (283), then this combo may work very well as the air flow will drop off early anyway, say 6300 RPM's, and with the added lift of the 151 cam and small ports the velocity into the cylinders will be high enough at low speeds to be strong from there all the way up. Sounds like the right thing to do to me. David -
Re: 283 CI cam timing
Consider that the centerlines of the 151 cam are 110 degrees ATC for the inlet and 118 BTC for the exhaust. The inlet centerline is fairly late and indicative of a high performance cam. The relatively early opening of the exhaust is good for a small block, because the exhaust port is restictive relative to the inlet port. The early opening allows a slightly higher pressure blowdown, which helps move the exhaust gas out.
By way of comparison, the Duntov cam has inlet/exhaust centerlines of 108/112 and the LT-1 cam is 110/122. For the LT-1 cam these are actually the points of maximum lift. The actual centerlines are a couple of degree later because of its asymmetrical lobes.
The L-46/82 cam (3896962)is an interesting study because the centerlines are both at 114 degrees and it has marginally more duration than the 151 cam. You could say that the 151 cam is the 962 cam with 4 degrees of advance, or, conversely the 962 cam is the 151 with 4 degrees of retard.
A later phased inlet cam will shift the torque curve up the rev scale, which means more top end power at the expense of the low end, and for a given cam, shortening the stroke will shift the peak torque point up the rev scale. This is why longer stroke engines can work with more aggressive valve timing, and why the 962 cam is retarded relative to the 151,which was designed for the 327's 3.25" stroke.
The 151 cam will create about the same torque curve in a 283 (or any other displacement) as the Duntov cam - a little weak down low with good mid-range and top end. If you want more on the low end at a slight expense to the top end, advancing the 151 cam 4 degrees on a 283 is a good idea, but I wouldn't recommend it on a 327 unless you have rather tall gearing.
DukeComment
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Re: 283 CI cam timing
Thank you gentlemen. I appreciate your insight and expertise. I will go with the 4 degree advanced cam timing.
John.Comment
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