Limiting factor for RPMs?

[moparmaniac07]

I was thinking today about when I eventually rebuild my engine, it won't be for a few years, but I have no idea what my redline is. That got me thinking about how you would even figure out the redline for an engine.

I know the limiting factor for power is how well the flow is in the heads/intake/exhaust. But what about holding together. Is it spring rates, or does that fall under power? I'm sure balance has a lot to do with holding together, but what else. How do you figure out what your max rpm will be without actually blowing it up?

I'd like to have power drop off before the point where stuff would break, but are those two points related, are they even related, and is that even a valid concern. My reasoning behind it is because I'd like to feel the engine sort of cut out letting me know I'm getting close to the redline. I figure if the powers still building at the point where the engine will grenade, I'd be much more likely to stay on too long. Plus, it'll leave me room to up the power levels if I ever start feeling adventurous.

So what areas would I need to concentrate on when I eventually rebuild my engine if I wanted to have power drop off before the engine actually blows up?

[squirrel]

Just put a smaller cam in it. And build a bulletproof bottom end.

[yellomalibu]

Heh. With a smaller cam and a bulletproof bottom end, the valves will float and act as a rev limiter... until eventually the floating valves possibly cause a catastrophic failure.

If you had a Nascar bottom end that could turn 10,000 rpm, chances are the camshaft would be what determines your redline. The cam card or the catalog listings usually have an rpm range that each camshaft is good for. That's a good guideline for a "redline".

I don't know much about Mopar engines, so I don't know what their real world "red lines" are or what their weak links are, but I bet if you tell us what engine you have and what upgraded parts it may have, there will be some people around here who can help you out.

[jcharliem]

So what areas would I need to concentrate on when I eventually rebuild my engine if I wanted to have power drop off before the engine actually blows up?

To me that doesn't seem to be the most efficient approach. Rather, build your mill to operate at a pre-determined maximum RPM... then cap the RPMs at the max. It could be 5,000 or 9,000RPM. There's no benefit when a car "noses over" which is what you're describing.

This is where a rev-limiter comes into play. Build your mill to produce max power, at say, 6,000RPMs - then set your limiter at 6,200RPM. Just ensure the build can support 6,200RPM.

When you start to get into the RPM stratosphere with an overhead valve mill... your limitations will likely be with the valvetrain. As you'll likely already have a forged rotating assembly, etc.

Good luck.

[Barry Donovan]

limit rpms with ignition.

I can't wait to do that.

you know what I would do with a two stage?

switch it on clutch pedal. no more than 3k pressed in, no more than 7k (the second switch) with clutch engaged.

today you got a/fuel gauges from ecms and other gadgets. one could really go as far as the bad or nice day is allowing.

This is assuming your engine is a racer from the build, not a question of ignition too weak or other stuff.

[Caveman Tony]

If you want the 'cut out' at near max rpm, go with one of the ignition boxes that have a 'soft' rev limiter. Cuts ignition at a preset rpm by cutting spark to alternating plug wires.

[DanStokes]

Build the engine.
Put it on a good dyno and take it up in RPM just until the rods appear outside the oil pan.
Back off the throttle slightly.

Hope that's helpful.

But seriously, for the new engine for the Buzz Bomb I'm going with the recommendations from the machinist and the best build book I can find. Basically, I bought the best rods and pistons I could afford, knowing a 250 crank is really stable, using the rod and piston manufacturer's recommendations as MAX. In reality I'll only wind it to 6000/6500 or so and all is expected to be OK at those speeds.

Dan

[squirrel]

Yeah, it's always best to learn from others' mistakes.

[TC]

It's all in the Cam and valvetrain.................. I used to think it was dependent on stroke, but looking at ProStock and them spinning 500ci motors 9500rpm, I tend to not assume that anymore.........

[BigBlockMopar]

Pistonweight comes into play when doing 'high-rpm bolt-stretch tests'...

[Thumpin455]

The weakest link is what determines the max RPM. With a stock B/RB Mopar, the weak links are the stock shaft rocker arms and valve springs. Same with stock Oldsmobile engines, they have an aluminum strap between two rockers that tends to break if revved too high. Pontiac engines stock rods are the weak link, and on a 455 the stock ports will limit RPM too, but the most important part is the rods. 5800 to 6000 rpm with good rod bolts is max in a 455, where the 400 and 428 can handle 6200 with good bolts. Mopars dont have cast rods, so that is a good thing.

I will be honest, I am not entirely sure what the weak link in an LA engine is. I am going to assume valve springs and rod bolts, in that order. The ports are kinda small, and that can limit the max RPM simply because it wont make useful power that high, but good heads, good springs and quality rods with light slugs/pins would allow you to spin it pretty tight.

The 400 Pontiac I stroked to 467 has light pistons and pins, good H beam rods, a solid roller, 330cfm of airflow through the heads, and light valves. It will spin 7500 easily, not chuck any parts out of it, and the valves arent even close to floating. That is pretty high for something with a 4.25" stroke, it makes its peak power at 6500, so I dont really need to wind it to 7500, but I can if I want to, the torque doesnt fall off very fast. Now the weak link is the block itself.

The lighter the parts, the more RPM you can get from it, add strength to it and they will live longer at those RPM. If the valves are too heavy, then the springs needed will deflect the pushrods and bend them at high RPM. If the pistons are too heavy they will snap the rods in half or pop the rod bolts when you wind it too high. If the springs arent strong enough or are too heavy themselves, they wont control the valve and you can drop a valve. All of this is provided you have the cam and flow to make power high enough to be a problem for the engine.

[A/Fuel]

After being around nitro hemis and watching them go to 4700 rpm at the hit and run to 6300 all I think of anymore is torque. Valve springs and rods can last 50 runs. Take away the nitro, add a blower and transmission and it will rev to 10,500. Valve springs get replaced every race, rods get tossed after 20 runs, if your lucky. We can even use used alcohol valve springs. Basically they are almost the same engines, heads, bob weight, rockers, push rods, ect. The price of nitro is a little more than alcohol, but when you add up the parts cost I would have to say the limiting factor in rpms is cost.

[JeffMcKC]

Pick a useable RPM Range for both the power and use of the engine. Then build the motor around that range. With a well thought out engine it wont have a "Peak" Power you can feel it will carry past it and have a long flat ave power in that range. "parts" should not set the RPM limit of the engine. If it does you have not thought it out well enough to start with.

[moparmaniac07]

Thanks guys. I was sitting at work daydreaming and got to thinking how different stuff was related. My 440 currently makes power to 5500, but I try not to take it that high often because I don't know if it's meant to hold together at that speed. A whole lot is unknown because I haven't torn into it yet, and won't as long as it's still running or I have extra money some winter.

So it seems the answer is just about everything

[JeffMcKC]

I doubt 5500 shifts would be a issue in lower gears it should handle it even better