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Rev limiter


Truck99

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A question for you engine experts: I noticed a couple of the HMP late models running all over the chip last night. They seem to gear their cars so that they are on the chip for the last 200 feet or so before getting off the gas. Does this do any damage to the motor? Just curious.

 

Thanks

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Prolly would not be that big of a deal on a fuel injected car where it cuts the fuel also. I always here indy cars on the rev limit, but hardly ever a nascar. The rev limit has to be better that the alternative though. Its prolly like getting on and off the gas really quick. But I would not hit the rev limit in your grandma's grocery getter. LOL!

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nascar hits them on the short tracks. will at the end of the strieght. we hit at kyle just before he lets up very short time in it just a miss or two .sas dont even come close with the gears we run only turn 7000 .i need to get some better gears and get that motor to turn 8000 to 8200 .then it would really go .cam would take us to 10000.dont really want to go there thow.your eng rev limiter should come in at 6000 to 6500 i suppose .

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Chase,

 

On my drag car I use a 2 stage chip to launch the car with. Holds the engine at 4400 until I release the transbreak then an upper limit chip at 8000 to keep the rods in the pan.

 

I sit on the chip for just under a second each round. Most are going to an electronic throttle control (Starting Line Controller) instead. In very hi hp cars it seems to disrupt the harmonics of the crank and is blamed for snapping off the cranks at the balancer. But these guys are normally over 1000hp.

 

I am at about 600 and have about 500 passes on my motor with no problems yet. Hammering it all the way down the straightaways may not be the best idea aside from the fact it is cutting your power. It is keeping the engine choked down by droping cylender sparks randomly. You might also notice they start to loose power about 2-300 rpm before the chip so its already doing it, just not banging out so bad you can hear it.

 

I am not an engine builder, just an engine nut... so take it for what it's worth. LOL

 

- ron

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Most rod failures occur at the end of the straight but not under acceleration. They let go when you back out of the gas at the corner entry. This happens because under normal conditions, as the piston comes up it is compressing a wad of air/fuel, kind of like running into a soft bumper. It allows the piston to work against something.. When you close the throttle, what is left is a vacuum, the piston encounters no resistance at all so when it reaches TDC and starts down, that's a sudden stop! Hitting the rev limiter is certainly less force than what you normally see every time you jump off the gas and go for the brake..

 

Now consider where most passing is done, corner entry and corner exit. If you can get a good run on someone coming off the corner, you don't need to worry about running out of steam in the last 200 feet, the pass is done.

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Most rod failures occur at the end of the straight but not under acceleration. They let go when you back out of the gas at the corner entry. This happens because under normal conditions, as the piston comes up it is compressing a wad of air/fuel, kind of like running into a soft bumper. It allows the piston to work against something.. When you close the throttle, what is left is a vacuum,

 

1-crew is correct about this; and I will add that you see a lot of dropped valves at the deceleration point because of the same vacuum factors.

 

I don't know about the higher RPM engines, 8200-8800 or so, but for the 6300 crates, rod stretch has not been a problem. The manufacturing process of the crate engines rods, the so-called "powder metallurgy", is not really prone to stretch; rather, they will probably break or snap in two suddenly, rather than stretch and pull apart.

It may surprise you to know that these rods are manufactured (compacted) in one piece, and the cap is broken off in a fixture that leaves a sharp parting line.

These rods will not stay together at 7000 or so for extended periods of time, thus the 6300 chip limit. This is due to the bonding of the individual grains of metal. While a good process, the bonding lattice, or "Glue", in the PM compaction process is somewhat limited in this type of application.

 

Chase, Something I have heard about bumping the chip from Eddy and a few others is that, by bumping the chip at the end of the straight, it kinda "softens" the transition of the car from acceleration to corner entry.

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48 that was funny and look at those dwarf cars they are in those riv.s limiter for ever .te tom the hardest thing on the little motor i found over the years is letting off full throtle at once .they get alot of grank flex .with 1800 to 2600 pound car depending on the motor 2300 has had a problem snapping in two at number 5 main .so to really keep one together you best spend the money on a good after market crank or get it shot ping balanced and check for cracks or else kaboom sooner than later .

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  • 2 weeks later...

personaly we cannot afford rev limiters so we use the valve springs to control the rev limit.

 

naw just kidding.

 

You NEED the rev limiter so you do not exceed the valve train limit most of all!!!

the valve train needs to work 2000 rpm above the rev limit setting on a V8 for a no load rev.

2nd it,s for the piston and rod weight, strenght limit

and 3rd it's for missimg gears and wheel spin.

 

We have a blown 1.5 liter 430hp motor that we put a stobe on the crank to find real rpm when the rev limiter catches it at 8800 rpm and the strobe showed 14900 rpm after the motor reacted to the electronics. so the more HP the worse it gets.

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dang, i was thinking the same thing that night...... i guess now it's somewhat answered. I thought i was just the "stupid" one for lack of better terms, so i didnt ask a question.. I thought it was more simple than that, but ya, this is interesting stuff, good topic!!!

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