Safe rev-limit for a 2.7ltr?

[joeblow]

Guys I have a 2.7ltr (76' California) that is now resealed and Webered and headered (sounds cool).

At any rate I would like some consensus on the 'safe' recommended rev limit. I am thinking that my current combo will likely pull a little better up top but I don't want to get to greedy and risk any issues.

The long block is stock but relatively fresh, I got super low leak downs when I was doing the reseal.

Stock is 6200, right?

Thoughts?

[5String43]

I remember reading somewhere that the ideal shift point was where the torque curve crosses the horsepower curve. If indeed that's true it would be good to have access to these two curves for all permutations of 911s (and maybe we already do; I just don't know where). And if it's true, it might not be necessary to rev beyond the stock rev limit. Just thinking aloud, here....

[andrew15]

Unless you have changed the pistons and cams, keep it at 6,200 rpm.

1) Stock CIS pistons were cast - not forged like early E, S, RS, etc pistons

2) Stock CIS cams just won't make more power beyond the 6,200 rpm redline - they were designed to produce top power about 5,700 and rev safely to 6,200 - they don't actually make the most power at redline.

Regards,
Andrew M

[joeblow]

I was not sure if it was going to fall over first or not so that was why I asked. If it starts to fall over at 5700 rpms as you suggest there is no reason to go over stock limits.

Thanks!

Originally Posted by andrew15

Unless you have changed the pistons and cams, keep it at 6,200 rpm.

1) Stock CIS pistons were cast - not forged like early E, S, RS, etc pistons

2) Stock CIS cams just won't make more power beyond the 6,200 rpm redline - they were designed to produce top power about 5,700 and rev safely to 6,200 - they don't actually make the most power at redline.

Regards,
Andrew M

[Evan Fullerton]

Well there are reasons for going over the stock limits if you are tracking or AXing the car to save a shift but for a street car, there isn't. Not sure if the valve springs or cams are much different but I have the limiter on my stock 3.0L set at 6900RPM and it gets used.

[joeblow]

Yep, I'm with you. I doubt the cast pistons would be an issue. I would worry about other things like rods and valve-train but in all the stuff I have done, these bottom ends are stout and the valvetrain is too.

My 3.6 race engine was limited to 9000 RPMs so I do like to buzz them a bit but that engine never did seem to roll over even on a 10K RPM over rev !

Originally Posted by Evan Fullerton

Well there are reasons for going over the stock limits if you are tracking or AXing the car to save a shift but for a street car, there isn't. Not sure if the valve springs or cams are much different but I have the limiter on my stock 3.0L set at 6900RPM and it gets used.

[911st]

Based on something I have read in Bruce Anderson's book and that the 2.7 RS is basically the same crank, rods, springs, and oling, the springs and crank should be safe to 7300rpm and beyond but you will not make power there. Not sure if the CIS pistons are significantly heavyer or not.

Headers, sport muffler, ignition all can help extend the usable power range some as can re-timing the cams to a degree.

The 915 has a significant rpm drop from 1st to 2nd gear (about 2750rpm) so if it is for track work you may want to shift later than earlier for best performance in to second gear. After that short shifts do not seem to hurt much as the rpm drop is closer to 1900, 1300, 1300 rpm with each following shift.

Looking at my HP chart (chipped 3.2) if I come in at 3500rpm I will be at 107whp. If I can come in at 4000rpm I will come in at 127hp. That is almost 20% more power at the beginning of 2nd gear.

The goal is the highest average HP per gear.

If for example I put shorter gears in my car and could lower my 1-2 rpm drop from almost 2750rpm to 2000rpm I would come in with about 50hp more going into second. (My peak HP was 217whp at 6200rpm).

At least that is what I believe so far.

[joeblow]

Thanks good info. I think the shortcoming here will be the low compression and the cams. But this is a standby engine while I build a short stroke combo of some sorts with my box full of 3.2 Carrera goodies.

I just have to figure out what to build now that my 3.2 block is no longer. I have 3.2ltr heads, Elgin Big 'S' cams, Forged high compression deep pocket pistons and stock cylinders.

On the gearing note I am pretty happy there. I have a 2000 pound car with a 901/16. I do get the 2700 RPM drop 1st-2nd but in the higher gears I am at around 1100rpm drop! I have a tall 1st for it when I decide to freshen it and put in an LSD.

Originally Posted by 911st

Based on something I have read in Bruce Anderson's book and that the 2.7 RS is basically the same crank, rods, springs, and oling, the springs and crank should be safe to 7300rpm and beyond but you will not make power there. Not sure if the CIS pistons are significantly heavyer or not.

Headers, sport muffler, ignition all can help extend the usable power range some as can re-timing the cams to a degree.

The 915 has a significant rpm drop from 1st to 2nd gear (about 2750rpm) so if it is for track work you may want to shift later than earlier for best performance in to second gear. After that short shifts do not seem to hurt much as the rpm drop is closer to 1900, 1300, 1300 rpm with each following shift.

Looking at my HP chart (chipped 3.2) if I come in at 3500rpm I will be at 107whp. If I can come in at 4000rpm I will come in at 127hp. That is almost 20% more power at the beginning of 2nd gear.

The goal is the highest average HP per gear.

If for example I put shorter gears in my car and could lower my 1-2 rpm drop from almost 2750rpm to 2000rpm I would come in with about 50hp more going into second. (My peak HP was 217whp at 6200rpm).

At least that is what I believe so far.

[Amorag59]

Originally Posted by 5String43

I remember reading somewhere that the ideal shift point was where the torque curve crosses the horsepower curve. If indeed that's true it would be good to have access to these two curves for all permutations of 911s (and maybe we already do; I just don't know where). And if it's true, it might not be necessary to rev beyond the stock rev limit. Just thinking aloud, here....

5String43,

To calculate Horsepower you multiply torque by the RPM and divide by 5252. 5252 is a constant so no matter what hp or torque values they will cross at 5252.

Andrew