How to figure out my redline

[panelbilly]

I have no idea what the redline is for my engine. It started life in a '79 SC. I had it rebuilt last year. New pistons, Cams modified to work with PMO carbs. New valves and such. I didn't do the work, just wrote the check. Only have 300 or so miles on it and I've been shifting around 4500 rpm. On a rare occasion, I run it up to 5500. Hlp me out here. What information is necessary to figure out what I can safely rev the engine up to?

[Steve@Rennsport]

Hi,

Without knowing precisely what cams are in the engine, one cannot offer you an exact number.

Using stock rods & valve springs, you should be safe to 6600 RPM.

I would ask your engine builder for a detailed list of what parts were used in the rebuild.

[wayner]

My engine builder told me my stock rods are good to at least 8000rpm
We set redline to 7500, and I regularly shift at 7.

While engine feel has a role to play in determining is it worth it to rev that high, rods are one of the limiting factors when it comes to engine safety.

(With the cam that I selected, my 3.0 is healthy from 2500 to 5000, but then it really wakes up at 5500 and screams up to 7500.)

[Trackrash]

What Steve said.

There are a number of factors to consider. On SC motors the stock pistons and valve springs are safe to 6,600.

With stock cams CIS and exhaust you don't need to go any higher.

But what pistons and cams and valve springs do you have? Once you know this contact the cam grinder and ask him his opinion.

For example my SC motor with Eibach racing valve springs, Dougherty GT2-102 cams and JE pistons with Webers has a rev limiter set at 7400 rpm.

[panelbilly]

The springs are OEM part number POR90110590
The cams are from WebCam Grind 464/465
Pistons are JE

[Trackrash]

I'm guessing you mean Part #: 901-105-901-51 for the valve springs.
Those are stock SC springs?
I would contact Web cam and ask if those springs are acceptable to be used with your cams.
Those cams are equivalent to a Mod S cam, and you would need, to my knowledge, a race or performance valve spring with those cams.
Ask your engine builder if he measured the seat pressure of your valve springs and if he has verified with Web cam if it is sufficient for the correct operation of those cams.

[Walt Fricke]

Here is a better way to determine when to upshift. Take the car when the work is all done to a chassis dyno, and get a torque curve. You can use that information, and your gear information, to determine where the delivered torque at the rear wheels in, say, 2d gear, matches the delivered torque in 3d gear. If you upshift before or after that point, you are losing some acceleration. Doing this graphically with torque on the vertical axis, and speed on the horizontal, gives a series of curves which ascend to the torque peak, and then descend. This gets flatter and flatter as you go up through the gears. But where these curves cross, there is your optimum acceleration upshift point. With Mod S cams you will find that these points are well below 7,500 RPM.

Stock rods are good for well over 8,000 if you use aftermarket high strength rod bolts. Stock springs will do fine at 8,000 controlling the valves until an inner or an outer gets fatigued and breaks after a lot of time shifting at 8K, but limiting yourself to 7,500 RPM won't cause issues. And breaking an inner, or an outer, won't cause immediate damage - when that happens it is just like a rev limiter at about 7,600 RPM - at least it was on my 2.7 race motor when all six outers finally broke - I finished the race with no particular damage from that and no real loss of time because I should have been shifting at 7,600 all along anyway. And that was with an Elgin 315, which is a much larger cam.

And if only occasionally on a race track do you wind it up in 2d or 3d to 8,000 RPM to stretch a straight and avoid an upshift just before braking, I think you can get away with 8K just fine.

For a street car, there is no reason to forget to upshift, and no reason not to upshift in normal traffic at well below 6K, as you don't need to optimize acceleration. But if you figure out what the torque curve says is where you should upshift, and just follow that when of a mind to get max acceleration, like passing trucks up hills on two lane roads or something, you won't have an issue.

Upshifting earlier just makes the engine's bearings and rings last a bit longer. The car should do just fine driving around at 4,500 max, but it is far from any mechanical limit at that point.

[Trackrash]

Walt. So your experience is that OE valve springs will handle Mod S cams to 7500RPM?

John Dougherty spec'd 80 lbs seat pressure for my cams (GT-2 102 w/ .485" valve lift). Can OE springs be set to that pressure and still handle the .475" lift of those Webcams?

[lvporschepilot]

Redline is all about what the heads will flow, what cam you have and how high it can make power to (equation with static and dynamic compression ratios) and basic math equations involving weight and mass with other rotating components. Good tools are out there to help with this info if you know weights and approximate controlling forces.

[Mothy]

To me the redline is all about engine safety and rev limits - what is the maximum revs my engine could be taken up to. My small port SC shows redline on the tacho at 6200rpm. With heavy duty value springs it could probably safely go to 6500 or even 7000.

However all that is academic since my optimum shift point is much lower. The small heads start running out of breath much lower and on a dyno it shows my peak power at 4,700 after which it drops off. Assuming about a 1000 rev drop between gears, I then shift up to next gear at 5200 and shift down at 4,200, keeping my revs in the peak power band.

[m42racer]

Originally Posted by panelbilly

I have no idea what the redline is for my engine. It started life in a '79 SC. I had it rebuilt last year. New pistons, Cams modified to work with PMO carbs. New valves and such. I didn't do the work, just wrote the check. Only have 300 or so miles on it and I've been shifting around 4500 rpm. On a rare occasion, I run it up to 5500. Hlp me out here. What information is necessary to figure out what I can safely rev the engine up to?

If I had to guess, the "engine builder" didn't rebuild the heads himself and the shop rebuilding the heads didn't ask what cam was going to be used nor what the use was going to be. The springs were probably set up with the typical 5 shims under the bases and shipped.

I'd also guess the seat pressures are all over the place but on average somewhere between 80 and 100 lbs. This is probably way over sprung for the use, but the cam design could require more spring dampening to cancel out the harmonics.

[Walt Fricke]

Mothy - I think you are missing something here about where the optimum shift point is. It has little or nothing to do with an engine's mechanical limits on what basically is a street car. Since you have no idea what sort of cams you have to go along with the conversion to carburetors, everyone of a mind to help you has to do a lot of guessing.

First off, the optimum shift point on most stock SC motors is pretty close to 6,250 RPM. The valve springs handle that with no sweat at all, and as I have pointed out, they can handle up to 8,000 RPM, which most likely is not where you would want to go. So I'd cross that out of your equation - at least until you do what you really need to do if you care to know if you race, do driver's eds or track days, or autocross: put the car on a chassis dyno and get a torque curve.

If this is just used on the street, who cares what you give up in acceleration? You'll never need to optimize it.

If you have a torque curve, you can use graph paper, a pencil, a calculator, and knowing what your ring and pinion and gears are, and construct a graph which has "thrust" (torque at RPM times the gear ratio) on the vertical axis, and speed on the horizontal. You do this for each gear at, say, 500 RPM intervals (on the dyno you ought to spin the motor up to about 7,000 RPM - won't hurt it - to get a complete curve. And you plot the thrust curves for each gear on the graph paper. You will see that in each gear they rise with speed, peak, and fall off. Your optimum shift points are where the curves cross and you need to interpolate what that RPM is by looking at what the dots you used to make the curve on each side of that crossing. Pretty simple, actually.

That is how Porsche did things. Nowadays you can make a spread sheet to do all this for you once you have the curve and the gear data.

Your '79 SC has the larger intake ports (larger than US 80-83s, anyway), your cam and carb may well move the optimum shift points up some - maybe closer to 7,000 as a wild guess. My 2.7 race motor with really big cams had optimum shifting at 7,600 or so.

Note this has nothing to do with horsepower or horsepower peaks. Torque from the engine and gears is what accelerates cars (not that HP isn't important otherwise). Optimum shifting is about always well above the HP peak, and way way above peak torque, so rules of thumb are really not very useful if you actually want performance. Of course, the shape of the torque curve does matter - it can be peaky, so peak HP is much closer to peak torque, or a nice flat "curve", with HP (which is based on torque related to RPM) climbing nicely well after the torque curve is descending.

But it is your car. For highway driving I seldom have run my SC (when it was street legal) up above 5,000 RPM, and 4 or somewhat lower was more usual. But these cars are quick and you don't need to wring them out to get around well on the highway - other traffic, speed limits, police, all that. So why worry about redline at all? Just drive in a way which seems sensible and feels pretty smooth - you won't hurt the engine.

[Mothy]

Walt

I think you might have mistaken me for the first poster - I know exactly what CAMS I have (standard) and my US Spec 83SC engine is definitely a small port unit and only produces 180HP on the dyno. I absolutely know about using the torque of the engine to get me where I have - and it's not by reving it out to the red line.



All good though - your explanation gives a good insight into why knowing you gearing and torque is important - HP is not what punches me out of corners (I don't have much) but I do have bucketloads of torque down low.

[Walt Fricke]

You are right - my bad.