h/c/i questions

[97desertCobra]

I'm wondering where in the rpm range does the 96-98 Cobra experience valve float? The reason I ask is because I know the stock cams can rev to 7500rpms. Whats stopping them from making power all the way up to that point is the intake right? So, with a short runner, imrc deletes, and re-degreed cams and a good tune it should be able to rev to 7500rpms and make power. Am I right on this? So from there the concern is valve float and the other components(springs, valves etc.) Will those need to be replaced. And lastly a good friend has a pair of lincoln mark IIIV heads lying around. They are identical to the Cobra B heads right? Thanks in advance.

 

[na svt]

No, the intake is not the bottleneck. The stock intake keeps peak power rpm at about 6200 and a short runner (1.5" removed) raises that to about 6500-6600rpm, but even then the power drops off substantially the further you go up in rpm. Stock springs are reliable to 7k and at roughly 7200 they float. Your stock cams, like any stock cams can rev to 7500rpm, but their duration keeps the peak power at a much lower rpm. You can move the chagne the overlap to slow the drop-off after peak power is made but there are penalties when doing so like loss of power down low. You will want peak power to occur at 7000-7200rpm or so if you want to shift at 7500.

In a purpose built motor like those that race NMRA factory stock, the 96-98 cams are used and they shift at roughly 7500rpm. However, the heads are much better, the intake provides much higher velocities and the compression is a lot higher. There is a lot to overcome when working with B heads and the B head intake...especially when using stock cams.

Below is a dyno chart from a stock cammed, B headed, short runner intaked motor. If the rpm is carried out to 7500rpm the power will be about 75rwhp lower than peak.


Why do you want to turn 7500 with stock cams?

 

[sho amo]

re-degreed cams and a good tune it should be able to rev to 7500rpms and make power

hes not saying stock cams.

what exactly are you trying to rev to?

i have heard this motor shouldnt really be revved higher than stock because of how far the stroke is. you may need to first bore out and destroke before you can make peak power beyond 7k

 

[na svt]

hes not saying stock cams.

what exactly are you trying to rev to?

i have heard this motor shouldnt really be revved higher than stock because of how far the stroke is. you may need to first bore out and destroke before you can make peak power beyond 7k

These motors can spin to the moon safely with good rods and pistons and it's very easy to make power way beyond 8k with the stock stroke. Heck, you can make power beyond 7k with a stock 5.4 stroke. BTW, he said re-degreed stock cams.

 

[na svt]

A good example of a non-restrictive intake and cam setup are the FR500 cams and intake. When they are installed in a 281 cubic inch motor the peak power is around 7300rpm and those cams have about 20deg more duration @.050 than the stock cams.