Kerns

Okay, so in a 99-04 mustang GT the redline is 5800RPM's. In a 05-10 its 6250RPM's... Cobra's are 6500RPM's. I am aware that the redline is in place for a couple of reasons. 1) that you stop making power past that point and 2) you can damage your engine by going past that point (repeatedly). For me with a centri supercharger I pretty much just keep making power. I know valve float is one reason that our cars don't really go past 5800, but my question is this. By upgrading to performance valve springs and retainers what is the limiting factor for your RPMs? I belive you don't wanna rev the stock crank past 7k but if say I ran stock cams with upgraded valve springs could I push my car to 6800 instead of 5800? If not, why? If so, do i need .550 lift springs? high load springs? Why can the stock crank not handle 7k RPMs? In general, is there any reason not to get .600 lift springs over .550 springs?

Kerns

/bump

0949er

i hope somebody can answer this.. I am interested as well.

ShadowDrake

I'll be looking for a more definite answer too, but in our case I'm not so sure it's the valve springs, but the stock cams. Even with a blower my power is well on the way down at 6000 RPM, I hit peak around 5700 RPM.

The crank's probably ok but I doubt the rods will be. Stress on those moving parts increases vastly with speed... a 1000 RPM speed increase results on much more stress on the parts than you think. Then again, the 99-01 cobras have the same rods and pistons as the 99-04 GTs and they have a 6800 RPM redline - just a forged crank.

I have to say, I'm interested in trying, my next motor will be built for a 7000 RPM redline, but I could stand to increase it now if there were an easy way.

Cam experts help?

mrtstang

I've been told the connecting rods are the main weak link here. The valve springs are fairly strong, but i personally wouldn't trust them with boost and 6k plus shifting for very long..

DS002

The culprit probably is the valve springs, especially if the 2V's have the same rods as the 4V's. Other issues could be rod bearings or rod bolts, if I were going to spin to 7k Id probably upgrade my rod bolts along with the valve springs. And if youre going to swap your valves you mine as well just pick up some blower cams for that centri while youre at it.

Mustang_NTriangle

OP you are doomed if plan on spinning your stock motor past 6,000 even with new springs and retainers. My stock motor with 400 rwhp and 4.10 gears and I had my rev limiter set at 6250. I did not throw a rod but ended up cracking the top of my piston as I was consistly rev motor high. The weak link is rods and pistons.

JerseyTrueBlue

The fact is that you don't make any power above 5500, after that your power curve drops like a rock. So even if you could spin higher, you would be wasting your time, the extra 1000+ rpms would be creeping along like you are half throttle. Why is everyone so fixed on spinning higher? You don't gain rpm's because you want to, you only shift higher if you are making power there.

SVTeeshirt

First off GTs redline at 5850 don't they? i know only 50 more, cobras redline at 6850, But have 4 valves per cylinder, i belive this is why they can rev to 6850 without harm, there are more springs to do the work thus an easier workload per spring. The problem with spinning in the rotating assembly if im not mistaken is the Crank pulley, 4vs have a forged crank and a different crank pulley that allows them to spin to 6850 safely.

Kerns

I don't plan to honestly. Well, at least not right now. I'm currently saving for a completely forged bottom end. Was more of a question concerning which route I should go. I have debated keeping my cast iron block and 8 bolt crank but upgrading to Manley forged rods and 18cc dished pistons as it would be about 700$ cheaper then getting a Teksid block and forged crank and still handle 600RWHP just fine. However, I had heard about not being able to take it to 7k rpms because of the crank. Also, valve springs wear down over time so I am going to have to replace them on my heads so it was more of a question as to if I switch out the valve springs + go fully forged would it be able to rev to 6800RPMs without problems.

This isn't a matter of just wanting to take it to 7k RPMs because I think its cool or something. Its simply that I only hit 4psi at 3800RPMS so over the course of 2000RPMs is when I make a majority of my power. If I allow my engine to make power up to 6800RPMs then I am allowing the car to continue to be at max psi for longer along with when I up-shift I'll be at a higher RPM thus farther into my boost.