What's a good rev limit for my 79 3.0?
 

Hi all,

Took my 72 911 (3.0 conversion) to it's first dry autocross today. Much to my chagrin, the rev limiter was kicking in at 5850. (Doh!!!) Shifting to 3rd all over the course was making me quite a busy driver!

So, I just read that the CD box is actually the limiter for the 79 SC motor. Where does it kick in? I'm thinking of going with an MSD anyway, so I'm curious as to where you performance-minded folks are setting your rev limiter with this box? Obviously, staying in 2nd gear is a good thing for autocross, but I'm geared short due to the 731 R&P in the 72 tranny. So, if I can stay in the revs somewhere in the 73-7500 range, that would be cool. I just don't know if that's too high for this motor or not. Anyone?

(yes, I know I have the wrong rotor in the distributor...thanks. ;) )

MSD-6420 CD System w/adjustable rev limiter, 911 All [url]http://www.pelicanparts.com/catalog/shopcart/911E/POR_911E_igniti_pg3.htm#item11 MS-6420

Thanks Svandamme...I know the part I need...just curious about the rev limit. Do you have one? What's your rev limit set for?

Hell! Be brave! Run her up to 8 grand! :D Seriously, if it's a stock 3.0 with stock cams? I'd suggest shifting at around 750 rpm past max torque, to drop you back into the torque band while auto-x ing. Max torque revs on a 3.0? I don't have a clue...

(edit) This comment of yours I have a problem with: "So, I just read that the CD box is actually the limiter for the 79 SC motor" Perhaps for SC's? My (mostly) stock '72S has no problem revving to it's 7300 redline with a stock CD box.

mine's not installed yet, UPS should deliver tomorrow(i hope)
the factory limit for my engine is 7300; dunno what yours is.
i kinda misread your post , thought you meant where to set the limit = where in the setup.. getting late here. should go to sleep instead.. :)

S'okay...you can go to bed now...;)

i suffer from terminal insomnia
except when i'm at work

I have an '83 3.0 with a 7.31, and can rev it to about 6500. I've heard 3.0s can rev to 7,000 with little problem. But why do it if it's not making anymore power?

If racing, I wouldn't shift past 5,500, because it doesn't feel the engine is making that much more power after that point.

My rev limiter kicks in at 6800, I try to check it when ever I take it out for a Sunday drive. :D

7300-7500 on a 3.0L stock is just asking for trouble if you ask me...

Will it take it a few times, probably. If you slightly miss a downshift, "BLAM!" new motor.

The factory rev-limit is sufficient and well within the power band of your cams. I revved my 3.0L stock motor in the racecar to about 7000 when needed, but with those cams, there is little advantage and much too lose...

Just my 2 cents, but an engine rebuild is more expensive than .01 seconds on course...

Ya, BTDT, but it was an upshift that I missed. :mad:
I agree with Chris. The CIS cams in the 3.0 run out of breath at about 6500, so I would usually shift there. They can be run briefly up to 7,000 to avoid shifting, but the power actually falls off. Max Torque is at about 4200 and max HP at about 6,000. Here's a dyno sheet from two runs I made with my stock 3.0 (pre- and post-tuning) a few years ago.
http://members.rennlist.com/tweedt/dyno.jpg

TT

TT, what kind of 'tuning' resulted in 8 extra RWHP?

Don't mean to highjack this thread. However, on a 78 US 911, is the rev limiter in the tach or in the CD box and what is the limit?

Yeah..Tom, what tune-up was performed to go from 160.5RWHP to 168.2RWHP?

You mention "stock" SC with CIS. What year 3.0?

If you consider a 15% drive train loss, that engine makes 197.9 HP and 213.9 T at the crank! :eek:

I want that!!!

Lothar...78-79 SC's have a fuel cut off at 6800+/-250 RPM. (I believe that's right..check the spec book..my memory is a bit unreliable)

Wayull, I could tell you, but then I'd have to kill you, as my mechanic swore me to secrecy. All I can say is it involved pushing the timing advance and enriching the mixture in a simple but unconventional way.

Quote:

You mention "stock" SC with CIS. What year 3.0?

This was a 1980 SC motor (small valve, US Spec) in a '73 911E, so it was smog free in CA. It had a backdated exhaust (SSI's and sport muffler) and no A/C compressor, but the only difference between the baseline run and the second one was that we put new plugs and wires on, performed the above mentioned timing and fueling mods, and turned 8 more HP on the same dyno, but a different day.

TT

We'll watch for you on the next episode of Secrets of Magic Revealed, Mr. "Black Mask." :p

Rev it till the valves float!!

I did a little test because I was thinking along the same lines. For me there's a favorite corner that I apex at about 85 mph and fully track at at around 95 or so. I then very soon there after shift into 4th at about 6500+ RPM. My dillemma was whether to go into the corner in 4th and not shift after the turn.

My test was 60 mph to 80mph acceleration in 3rd versus same speeds in 4th.

Accel in 3rd was much faster than 4th. I assumed it would be but wanted to know by how much.

I think in my test 3rd was like 5+ seconds and 4th was like 8-9+ seconds. I don't recall the exact details but I do remember the overwhelming conclusion.

So I guess I am saying that I would rather approach and hit the rev limiter in 3rd before I shift to 4th if I want max exit speed.

Yes, for autocross it's definitely better to hit the rev-limiter for a second or two rather then have to shift to 3rd. Anytime you can 'quiet' the work you have to do in the car, it's better.