too much RPM?

[Jim Comet]

Hey all, I sent my 9" center to have new 4.71 gears installed in it replacing the 4.11 that I was running. This would put me at about 7400rpms through the traps. Unfortunately they installed 4.86 gears by mistake which would put me at about 7650rpms through the traps. My engine builder is ok with this, but I am low budget and cannot rebuild the motor every year. How much extra wear and tear do you think the extra 250rpms will do? The place is a 4 hour drive from me and I was hoping to get out next weekend. Any thoughts? Jim

[fryedaddy]

it may not be an option but i would be looking at some taller tires

[jayb]

I don't think I'd be worried about anything except possibly valvetrain issues.  If your springs are up to the drill, shouldn't be a problem.  Can't remember, what is the stroke on your engine?  Would be interesting to calculate the piston speed,  but even at a 4.25" stroke it won't be excessive.  You have good connecting rods, right?

[blykins]

Lots of piston speed.  I typically don't like to run that much stroke with that kind of rpm on an engine that won't see yearly freshen ups.  I would expect the rings to take a pretty good beating.  I would also want a steel crank in that scenario.

[Rory428]

What is the diameter of the rear tires and what speed are you going? Unless you have extremely short tires, I would have to think that type of RPM with 4.86 gears must be 140 MPH or so? Can you use taller tires? I would be surprised if a change of 4.71 to 4.86 would gain that much RPM, on my 85 Mustang 5.0 Stock Eliminator car, going from 4.88 to 5.13 picked up about that much, but you difference is only have that amount.

[Falcon67]

What Brent/Jay said - piston speed, rods and valve springs.  For 1/8 mile I have 4:11s in the dragster and 4.56 in the door car.  I might shift a specific engine at 7K or 7.2k but I won't trap at that RPM. The last dragster motor I only shifted at 6800 even though 7200 was recommended based on the cam config.  I build my engines to bracket race about a weekend/month for several years. I just tore down the 351C in the door car, first time it's been open since 2008.

[1968galaxie]

Hello Jim,

Does this car have the 416" Blair engine with the baby roller cam?
What was peak HP rpm?
I would probably not want to go more than 400 or 500 rpm beyond that rpm.
I would also call Blair and ask for his opinion - if the engine was built by him.

Cheers!
Love the car.

[frnkeore]

The difference from 4.71 to 4.86, is 7400 vs 7636 rpm.

Swapping out tires at $250 ea isn't cost effective, either unless you have them already.

As long as the valve train will work with the increase, I don't think it will hurt the longevity of the engine but, I don't really think a 7400 rpm engine, of any type can be expected to run long periods of time.

Blair likes to short shift, then over rev at the lights.

[Jim Comet]

Sorry guys for not responding, busy at work. I appreciate all the comments. I do have a steel crank, Molnar rods and Mahle pistons. The stroke is 3.98. Quick performance contacted me and sent me a call tag on their dime to change the gears to 4.71 like I wanted. With the 29 inch tires (the tallest I can fit without tubbing) and new gears I should run through the traps at 7500 or less (if I don't have too much converter slippage). I am not excited about turning it that high, but Blair says he has guys turning them up to 8000rpms. I am not going to try that.

[Rory428]

Still curious what your MPH is, to have that much finish line RPM? My fairmont has gone 134 MPH, albeit, with 2" taller tires, and with 5.13s, I am only going just over 7000 RPM at the 1/4 mile . I did run 9x29 Hoosiers on my 85 Mustang, and with 4.88s, going 130 MPH (10.2-3 ETs), I again, was right around 7000 RPM at the end of the 1/4. Now, both my cars are/were 4 speeds, so no convertor slippage to deal with.

[Jim Comet]

My finish line mph is 124-127. I am using 8% converter slip. Hopefully it will be less but I wont know until I run it. Jim

[mike7570]

For reference - My 427 in two different combo’s
4.56 gear x 29.5 tire 127-128 mph 6800-6900 rpm (C-6)
4.86 gear x 32 tire 141+ mph 7400 rpm (glide)

[cleandan]

Jim Comet, here is my take, from a non-performance point of view.
1) You did not get what you paid for.
2) The new alteration will cause you to change other things to arrive at a now compromised first aim point.
3) How much second guessing and potential blame will you have on your mind if you don't have what you originally designed to use?

I am the kind of guy who has "been nice" so many times it boggles the mind.
Almost every time I have been nice and accepted the wrong result, because of another persons screw up, I have eventually regretted not having that screw up fixed right away to the original items paid for.

The few hours of drive time used to get this taken care of will be NOTHING compared to the amount of all the other stuff that will, and might, come along because of the compromise when trying to "be nice" with their screw up.

Be polite, and ask if they can put this on todays work list today, but tell them you wanted the other gears, not what they installed by mistake.

The hassles, and other associated troubles, will be done as soon as the correct gears have been installed.

[Stangman]

Being able to handle it is one thing but ringing it out every pass is another. 7700 is a lot
And what happens when it’s 78 and change.

[TomP]

I went from 4.57's to 4.86's and the car picked up even though it would hit the RPM chip about 1100 feet. So I moved the chip from 7200 to 7600 and it crosses about 7400 at 134mph . That's with a IU crank, LeMans rods and TRW pistons.
 Most stock and Super Stock cars are overgeared on purpose, you are faster if all acceleration is finished before the end of the track. Getting up to speed quicker means you are covering that last distance at a higher speed which means lower ET.