Safe RPM limit for stock valvetrain?

smedin · Mar 16, 2006

Is 6600 safe for an otherwise stock 4.6?

I noticed that the dyno sheet for Evolution's custom tune showed that the new, higher power peak occured at 6600 RPM. That seems pretty lofty to me, especially considering that I would hate to try to make a warranty claim if I bent a valve...

Any and all insight appreciated.

JTGrant · Mar 16, 2006

i thought the power curve was flat after 6100 on these motors. I don't think that they make anymore power after like 6000 rpm. I spin mine to 6200.

royb · Mar 17, 2006

daily driving i shift a 6,000 When I'm racing I pull mine to 7000. I have a supercharger & some extras but all stock inside. I have about 17k on the clock with no problems & i have 454 RWHP SAE

smedin · Mar 17, 2006

JTGrant said:
i thought the power curve was flat after 6100 on these motors. I don't think that they make anymore power after like 6000 rpm. I spin mine to 6200.

The Evolution Custom tune (and probably the other popular tunes as well) alter the cam timing to raise the power peak. They seem to have done their homework, but I was just a little wary having to hit 6600 RPM to hit that power peak. My god, it has got to sound awesome at that RPM...

I just was afraid that it might float the valves at that speed, but it sounds like it can handle it...

Can anyone confirm whether or not the 4.6 is an "interference" engine? The compression ratio is pretty low, so maybe not?

JTGrant · Mar 19, 2006

i think it is

Siggy33 · Mar 21, 2006

smedin said:
The Evolution Custom tune (and probably the other popular tunes as well) alter the cam timing to raise the power peak. They seem to have done their homework, but I was just a little wary having to hit 6600 RPM to hit that power peak. My god, it has got to sound awesome at that RPM...

I just was afraid that it might float the valves at that speed, but it sounds like it can handle it...

Can anyone confirm whether or not the 4.6 is an "interference" engine? The compression ratio is pretty low, so maybe not?

What is an "interference" engine? I do not see myself as a newbie, but I have never heard of this term...

JTGrant · Mar 21, 2006

where the valve(s) can come into contact with the piston. For example floating valves and then the piston wacks a valve.

fullsagd · Mar 21, 2006

the valves should never touch the piston. if it does, you have or will have some serious problems. i saw a 351W long rod (400 rods) that had seen valve/piston contact. the piston ended up with a surface crack and the valve was pooched.... i'm also curious what an "interference engine" is. i've never heard this term before and i've been around the automotive industry my whole life....

bigcat · Mar 21, 2006

fullsagd said:
the valves should never touch the piston. if it does, you have or will have some serious problems. i saw a 351W long rod (400 rods) that had seen valve/piston contact. the piston ended up with a surface crack and the valve was pooched.... i'm also curious what an "interference engine" is. i've never heard this term before and i've been around the automotive industry my whole life....

interference engine is not a new term. basically it is an engine that if the timing chain jumps, then the pistons interfere with the valves. we see it often in the collision industry. when the pullys get smashed, the cam gear spins and the engine is usually running. bad news, especially since it usually totals the car

fullsagd · Mar 21, 2006

bigcat said:
interference engine is not a new term. basically it is an engine that if the timing chain jumps, then the pistons interfere with the valves. we see it often in the collision industry. when the pullys get smashed, the cam gear spins and the engine is usually running. bad news, especially since it usually totals the car

so it means the lift is greater than the clearance TDC?

bigcat · Mar 21, 2006

that would be a good definition. under normal conditions, this is not a factor, but if the timing is way off, the there is a problem. usually on OHC engines.

JTGrant · Mar 21, 2006

smedin · Mar 21, 2006

You usually hear about it when a timing belt breaks. 944s were notorious for breaking timing belts. If you were idling, you might get lucky. Anything over idle speed and BAM! Instant $3k repair.

The other semi-common occurence is when one over-revs and floats the valves, when the valvesprings are not strong enough to follow the cam to its closed position fast enough, and the piston slaps 'er down. Ouch.

Hence my concern at the top of this thread. Sounds like 6600 is no big deal in our motors, though.

351CJ · Mar 22, 2006

smedin said:
You usually hear about it when a timing belt breaks. 944s were notorious for breaking timing belts.

Ford engines do not use timing belts, they use chains.

I'm not sure on the interference question, but I do know that the VVT adds a level of complexity to the question. Some aftermarket cam companies have said that it is difficult to do more agressive cams and retain VVT due to piston - valve interference.

bigcat · Mar 22, 2006

351CJ said:
Ford engines do not use timing belts, they use chains.

I'm not sure on the interference question, but I do know that the VVT adds a level of complexity to the question. Some aftermarket cam companies have said that it is difficult to do more agressive cams and retain VVT due to piston - valve interference.

sounds like this could be a big problem.....

smedin · Mar 22, 2006

351CJ said:
Ford engines do not use timing belts, they use chains.

I'm not sure on the interference question, but I do know that the VVT adds a level of complexity to the question. Some aftermarket cam companies have said that it is difficult to do more agressive cams and retain VVT due to piston - valve interference.

I know our motors have chains. I was just trying to illustrate the point...

My concern with our motors has to do with valve float at the super-lofty RPMs.

fullsagd · Mar 22, 2006

smedin said:
You usually hear about it when a timing belt breaks. 944s were notorious for breaking timing belts. If you were idling, you might get lucky. Anything over idle speed and BAM! Instant $3k repair.

The other semi-common occurence is when one over-revs and floats the valves, when the valvesprings are not strong enough to follow the cam to its closed position fast enough, and the piston slaps 'er down. Ouch.

Hence my concern at the top of this thread. Sounds like 6600 is no big deal in our motors, though.

that's probably why i haven't come across the term, my exposure to OHC motors has been relatively limited although i've seen what damage it can cause first hand. thanks for the info.

hotdog71 · Mar 22, 2006

Just my 2 cents worth.

Variable Valve Timming (VVT) is not a new technology it has been around in Europe and Japan for quite some time at least since the early 90. These type of timming should not be a cuase for alarm with high reving engines. Honda Civic Type-R, Citreon Saxos, Ford Focus ST (SVT in US) in Europe these car rev to 7000+ rpm and use VVT.
They are OHC belt drven and have been known to break belts through ware and tear and causes plenty of valve damage. But Never heard of a chain breaking by itself.

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