Red Line

[Snakebit]

What determines the red line for an engine? Using top quality parts, forged crank, pistons, and rods with solid lifter cam, stroke, cam, valve springs???

[Silverback51]

E. All of the above.

Recipocation mass is a big part of is. The valve train is another. Then it's also the cam profile as that determines what RPM range it will work in. Combustion chamber design. Intake manifolds and exhaust headers will figure into it also.

The simple answer is that there is no one thing, or even simple combination of things that determine the redline.

[Chaplin]

Yes, all above.

There are really 2 different kind of redlines that are involved, and each is involved in determining the redline of any engine to some degree. One would be a redline imposed by design limitations- i.e., a small cam, dual plane manifold, small carb, small intake and exhaust valves, and the other redline would be what is imposed by the physical limiations of the parts you use, i.e., forged crank v. cast crank, 4 bolt main block v. 2 bolt main block, cross-bolted mains or non-cross bolted mains, etc. In the design limitation example with a small cam, your motor will only make so much power up to a specific rpm and then will fall flat on its face because the motor will run out of air, so for all practical purposes, that is its redline since there is no point of spinning it faster because you are not making more power. Using all forged internals and the highest quality components won't make a difference in that engine for redline purposes.

Then there is the redline imposed by the physical limitations of the actual parts you use. If you use a big solid roller cam with a single plane manifold, huge carb and free flowing heads, you might might be able to make power to 7500 rpm, but how strong is your bottom end? If you have a cast crank, 2-bolt main, non-cross bolted block, etc., you might not want to spin it much past 6000 or 6500 rpm for fear of it grenading, so that might be the redline you use. However, if you had all forged internals, strong block and quality valve train parts, you might be fine spinning that same motor to 7500 rpm.

So when you come down to it, redline is a function of both the design of the engine and the quality of the parts used. Finding the right mix for what you want to do with the motor is always the fun part.

[1ntCobra]

I think that at high rpm your valves start "floating", that is the springs will not close them. And that probably depends on how light your valves are as well as the type of springs.

I think some of the really high revving engines (like formula one) use some mechanism to actually pull the valves closed.

[Snakebit]

So determining the red line is like Russian Roulette? Or should you go by the cam specs recommended rpm range example 2000 - 6000 rpm. So after 6000 rpm the horse power should fall off, so the redline would be 6000 rpm?

[CHANMADD]

The most important thing in determining the "Safe" redline for any particular engine would be determined by the capability of the bottom end ,or reciprocating assembly. If a rod is going to go through the side of the block ot 5000 rpm then a safe limit would be 4000rpm or 4500rpm. There are a lot of factors that decide when a particular component will break.The rod ratio, here we go again, is very important in determining when the rod is likely to break or go through the side of the block.The larger the stroke the longer the rod needs to be. The pistons have to be a better quality the higher the revs. Also the balancing becomes more and more important as the revs get higher.The cam and heads, etc need to be chosen so that they work in the rev range that the bottom end can take!!
John

[Dwight]

You can have you car dyno'ed. The chart will show you were you power starts to drop.
If you know what you want as a redline then you can design the motor for that. But the dyno will tell the true.

Dwight

[Don]

Believe the NASCAR SB engines w/o restrictor plates run at 8,500 to 9,100 RPM , but probably could not last too much longer for additional laps other than the practice sessions, qualifying and the race.

[patrickt]

Originally Posted by Dwight

You can have you car dyno'ed. The chart will show you were you power starts to drop.

Best answer yet. SnakeBit, forget redline, think Shift Points. If you have your engine chassis-dynoed then you can factor your torque curve in with your transmission gearing, wheel sizes, final drive ratio, and find the optimum shift point between different gears. Almost any well built solid-lifter engine should be comfortable going to the 6000-6500 range and, I'm willing to guess, that is where you will find most of your optimum shift points (maybe even lower). Here is a nice little program that will let you factor in all of that data and suggest to you where you should try and shift -- you just might find yourself setting your rev limiter lower than you think. http://www.performancetrends.com/tg.htm

[patrickt]

Here is what the program suggested for me, after giving it my FE's torque curve, the Tremec specs, the rear specs, tire specs, car weight, etc. You can safely ignore the 4-5 shift as I don't hit that too often. You can also ignore the top gear MPH calculation (drag coefficient is not considered). What I do in real life is just try and shift at 6000 and I have my adjustable rev limiter (mounted in the back of the glove box) set at 6400. If I bounce off the rev limiter in a fit of unbridled exhuberance (which does happen) then I quietly scold myself, but I can usually feel what 6000 is without having to look down at the tach. Here's the chart that it spit out for me and a nifty shot of my adjustable rev limiter.