3.0 liter 911 SC redline
 

Hi All,

The attached .jpg file is chassis dyno graph with 3 superimposed pulls. The car is my 1970 Porsche 914-6 with a rebuilt 1983 3.0 liter 911 SC engine. The engine has a Clewett Engineering engine management system with manifold pressure FI and coil at plugs electronic ignition, 1-5/8 Inch ID Billy Boat SS headers with heat, and Patrick Motorsports dual outlet SS sport muffler. The engine management system was running closed feedback [Air]/[Fuel] ratio control with a wide range oxygen sensor. The rebuilt engine had about 1100 break-in miles on it and a 6300 rpm redline was observed. I was a little disappointed with the result. I was expecting something like 180 rear wheel bhp. However, I note that the engine was still pulling strongly at 6300 rpm and would have revved higher if I had allowed it. If this engine could be revved to 7000 rpm, extrapolating the bhp curves would yield ~180 bhp. My question is, what's a safe redline for these 911 SC engines? I've seen everything from 6300 to 7100 rpm. I've heard these 3.0 liter 911 SC engines are nearly indestructible, which is why I went with a 3.0 liter. But, what's the limit?

My other question has to do with the shape of the torque vs rpm curve. I was expecting the classic torque vs rpm peak. Instead, we have what I would describe as a "torque platueau". What's going on here? This is a high revving sports car engine not some kind of tractor engine. Also, does anyone have any insight re what's causing the slight depression in the torque curve just off idle? What does everyone think about increasing the fuel flow to enrich the [Air]/[Fuel] just off idle?http://forums.pelicanparts.com/uploa...1736874568.jpg

What cams did you use, pistons , compression ratio, Ignition curve , cam timing , /
all this matters .

I will run all stock sc's to 7000 if they keep pulling

Ian

Look at post 11 in the link below. It belongs to IanC.
https://forums.pelicanparts.com/porsche-911-technical-forum/1127411-curious-about-how-well-3-0-sc-should-rev.html

Over the last two summers I hit 6500 rpm almost every day. Newish valve springs, stock rod bolts, stock everything. Look at how strong Ian's pulls between 3 and 3.5k rpm, no dip, impressive.

Phil

C3 factory rev limit was 6800+/-200
SC was 6700

the torque and hp curves ought to cross at 5250, a dyno only measures torque and rpm,

the hp is calculated from the torque and rpm data
hp = torque * rpm/5250

Reading the dyno sheet , the fact that the torque never falls off suggests that you are no where near max power . Plenty of 3.0 valve trains have been happy at 7300-7500 rpm .
The limiting factor is how well the top end has been built . All of the components in the engine are rated to 7200-7300 as the components have been used in many engines that have revved to much higher rpms.

Ian

At least 7000.
Not sure what would be the weak link over that, valve train? Its pretty easy to get some stronger springs and lighter retainers if needed.

I ran a modified 2.7 with a big cam and carbs to 8,000 routinely. Eventually the stock springs gave out - most of the inners (or outers, I forget) broke while on track and engine wouldn't pull beyond 7,600. So I finished the race. Even with only one of the springs working on some of the valves there was no noticeable float. With aftermarket better springs I finally dynoed the motor. Turns out 7,600 was the optimal upshift point for my track transmission and that engine.

Conclusion: with a stock cam and stock intake, you'll never intentionally rev it to where things break. The limitation on revs vs power is the intake - how much air you can cram in (or let out, but most good 3 into one exhausts will deal fine with that part of the cycle). If you are a track junkie you probably will use super rod bolts (if not also super rods) when you rebuild the motor, but with stock cams and stock intake diameters you won't exceed strength limits absent a money shift.

The 1983 US SC has the small port heads and intake runners. The lower compression early SCs (and all Euro SCs) have larger intakes and hence breath better at higher RPMs. They all use the same cam. I don't think that EFI is going to overcome the restricting effect of the intake, though it can help some.

Like most who comment I read the dyno graph as showing you can rev a lot higher. Easy to change the rev limiter. Optimum upshifting with stock gears normally has you shifting after the HP peak (figuring optimum upshifting is based on the torque curve, but the HP peak is always higher and you shift on the downside of the torque curve). This ignores track driving where you may want to stretch a gear to avoid a silly upshift.

Take it back to the dyno and don't get off the gas until 7,000 at least. Have the dyno guy tell you to back off when you have passed the hp peak if you are getting up toward 7,500, though I doubt you will.

While I upshift on the track at 6,250 mostly because that is the most efficient for my CIS '82 US engine, I'll stretch to 7,000 every time if that will get me to the braking point.

If you figure a 12% transmission loss, 193 flywheel HP is a pretty stout result for an engine Porsche said had 180 or so. But it is the air flow and not mechanical strength which is your practical rev limit on a largely stock motor.

That flat of a torque curve is highly unusual. It's almost like an electric motor. I would first make sure the dyno is measuring accurately.

Also make sure your tach is calibrated. They can be off quite a bit and make a big difference at higher rpm’s.

I think you need to do more tunning as your HP and TQ are quite low for your specs. As you mentioned you should be closer to 180 HP at the wheels on a Dynojet. The dyno chart below ( also dyno jet )is two different 911 3.0SC. Specs for the two cars are similar. Stock cams, stock CIS Big port, 9.3 stock pistons, Big port heads, one had headers and the other SSI heat exchangers, sport muffler.

http://forums.pelicanparts.com/uploa...1737061173.jpg


john