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Redline Question?

When building an engine, how does one find out what their redline is?

Thanks
Old 10-06-2010, 05:41 PM
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Redline is determined by an engine's camshaft profile, thus its effective operating RPM range.

Redline is also affected by its internal mechanical components and their ability to withstand repeated high RPM without failure. Very generally speaking, stock cams and internals are well matched to each other.

Race engines, OTOH, operate at extreme RPM to make power and usually require extensive changes to its components as well as other modifications for durability.
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Old 10-06-2010, 06:10 PM
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or....rev the engine until it blows up....then subtract 500 RPM from that...and dont rev the next engine past that.
Is that called destructive testing?
Just funnin'
Bob
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Old 10-06-2010, 10:17 PM
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haha, that's what a couple of my buddies suggested
Old 10-06-2010, 11:40 PM
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Redline is a somewhat variable concept. For instance, you can rev an SC motor to 7,500 rpm without much fear of failure. But the optimum shift point for a stock SC with a stock transmission is about 6,250 rpm. Waste of time and money (wear items won't last as long) to spin it faster, other than in the occasional autocross or particular straight on a track where you don't want to shift up for just a short distance.

So what you do is figure out where the motor you are building makes its power, and how that fits the gearing you are apt to use, both of which define where you shift. Then you make sure your valve springs and rods (especially bolts) can handle that. And that you have lightened up the rotating and reciprocating masses some to help things work together.

Sounds easy. I'm building a 2.8 motor I want to shift at 8,000. I think I need to do that in order to hit 300 hp around 7,600. The bits and pieces are strong enough for this. But whether I hit this nail right on the head or not remains to be seen. So I think it is not all that easy. There are programs which can help some. I'm still dithering about just the right cam.

What you don't want to do is build a motor and then figure out how fast to spin it. You ought to be able to ball park your torque curve well enough. That's part of cam selection, among other things.

Guys like Steve, who build race motors for a living, have quite an edge on the rest of us here, as they have an extensive mental library of what worked where. And what (usually not a motor they built) didn't.

I bet if you post a "what HP/torque do you think this motor will make, and where?" query, with the specs you are planning, somebody may have a motor like that and can venture an educated guess.

Walt
Old 10-10-2010, 08:57 AM
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At the OEM level, redlines are set by piston speed, which is a good proxy for the stresses involved. In the 1960's, when the 911 was designed, 3000 ft/per/min was one general rule and this works out to around 6500 rpm for the 911. Aircraft engines, that spend much more time at full speed and load, still use this rule. Honda really broke out of this limit and started producing road cars with piston speeds above 4000 ft/per/min that seemed to last forever. They have pushed the envelope to 4800 ft/per/min on a road car. An 8000 rpm 2.8 (70.4) works out to around 3700 ft/per/min. Higher rpms are really only desireable when there is an artificial displacement limit and OEM's are now discovering the benefits of longer strokes, that the steam engine guys knew in the 1800's. A long stroke engine with high piston speed at low rpm, has much better cylinder filling and torque at low speed. And of course, on a road car, the time spent at redline is less than 1% over the life of the engine.
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Old 10-10-2010, 09:44 AM
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Thanks All.

The reason why I am asking is I am building my first engine, and I don't want to take a chance of ruining it. The engine I am building is a 2.7 CIS that I am converting to a 2.7 carburetor, E cam, 9.5 CR and bolting it to a 902 tranny and sticking it in my 66 912. The 902 tranny has a quick gear ratio so basically I am afraid that it would rev up quickly.

What do you think?
Old 10-10-2010, 10:59 PM
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My advice would be to start with the factory recomendation. There was no 2.7 E, but the 2.4 E had a power peak of 6200 rpm, a tach redline of 6500 rpm and an electroniic rev limiter at 7200 rpm. If you are afraid of overreving the engine on the street from some reason, my advice would be to use an MSD box with a limiter.
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Old 10-11-2010, 05:04 AM
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Agree with Paul.

I think you are acting on a false premise: that the car will accelerate so fast that you can't upshift in time. The optimum shift RPM for each upshift will be well below any critical operating RPM. We racers with lightened rotating masses and high squeeze motors are able to keep track of our upshifts.

Build your motor, and then put it on a chassis dyno and get a torque curve for it. Use that and your gearing to calculate where you should shift. Conceptually it is easy: the rear wheel torque in the old gear at upshift RPM should be the same rear wheel torque you get in the new, higher gear. If your gears are decently spaced (whether the gears are quick or not), this will be close to the same RPM for each, other than for first to second.

I calculated that my 2.7 should upshift somewhere around 6,200 or thereabouts (was a while ago). I didn't think I had a rev limiter for a couple of years until finally I ran the motor up past 7,000 and hit it. You can get a rev limiting distributor rotor, if nothing else. That will deal with your forgetting to upshift, but frankly you will know where to shift.

On the track it is usually only at race starts where guys hit their rev limiters.

Walt
Old 10-11-2010, 09:02 PM
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Quote:
Originally Posted by Walt Fricke View Post
Redline is a somewhat variable concept. For instance, you can rev an SC motor to 7,500 rpm without much fear of failure. But the optimum shift point for a stock SC with a stock transmission is about 6,250 rpm. Waste of time and money (wear items won't last as long) to spin it faster, other than in the occasional autocross or particular straight on a track where you don't want to shift up for just a short distance.

So what you do is figure out where the motor you are building makes its power, and how that fits the gearing you are apt to use, both of which define where you shift. Then you make sure your valve springs and rods (especially bolts) can handle that. And that you have lightened up the rotating and reciprocating masses some to help things work together.

Sounds easy. I'm building a 2.8 motor I want to shift at 8,000. I think I need to do that in order to hit 300 hp around 7,600. The bits and pieces are strong enough for this. But whether I hit this nail right on the head or not remains to be seen. So I think it is not all that easy. There are programs which can help some. I'm still dithering about just the right cam.

What you don't want to do is build a motor and then figure out how fast to spin it. You ought to be able to ball park your torque curve well enough. That's part of cam selection, among other things.

Guys like Steve, who build race motors for a living, have quite an edge on the rest of us here, as they have an extensive mental library of what worked where. And what (usually not a motor they built) didn't.

I bet if you post a "what HP/torque do you think this motor will make, and where?" query, with the specs you are planning, somebody may have a motor like that and can venture an educated guess.

Walt
Walt, what are the components necessary to sustain 8000 RPM? I was intrigued by Psalts post. 3700 ft/min is about 18.8 m/sec. Formula 1 engines are supposedly at just under 27 m/sec. These are of course mean values for velocity. The max velocity the piston sees (at mid stroke) has to be higher.
Old 10-13-2010, 02:33 PM
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