Car seems down on power. Any ideas

[5:04]

Hey guys,

I've got a '76 911S with a warmed up 2.7. I've noticed that most track days my car is pretty slow on the straights. If it was a stock 2.7 I really wouldn't think much of it. However it's a rebuilt 2.7 with JE 9.5:1 pistons, Weber 40 IDA's, 73 headers, pertronix, solex cams, and a stock distributor.
When I say slow I don't mean I'm unrealistic and upset because getting passed by Vettes. I'm thrown off by it because I'm getting pulled on the straights by cars with around 200 crank horse power. From what I've seen from other cars my car should be in the 190-200whp range with the parts it has.

The car weighs about 2200 lbs, has full aero including a large wing, is wide bodied, and is on 285 wide R6's. Around technical tracks it is very fast, and is quick up to about 70mph. Once I get passed that the car feels like it hits a wall .

My current jetting is very rich with afr's of 10.5-11.5. Would this be enough to cause my power loss? I plan on rejetting this week. I wonder if the Solex cams are running out of breath? I'm just trying to figure out if I potentially have a bigger underlying problem, if my set up just needs to be tuned, or if this is normal and I'm expecting too much.

Thanks in advance.

[Steve@Rennsport]

Hi,

Lots of good data here that provide the keys to your dilemma,....

I have some Q's for you:

1) What kind of oil temps are you getting?

2) What size venturies do you have in the carbs?

Your jetting sounds rich and needs some fine-tuning. Don't go leaner than 12.8:1 at WOT.

3) How much total timing are you running?

4) Pump gas or race gas?

Remember, your high-drag, wide-body car affords lots of mechanical grip using big tires but that slows the upper speeds down dramatically. Drag goes up with the square of the speed and a high-downforce wing adds to that drag. Flatten it out for more speed, but don't forget that compromise between grip & top MPH.

[5:04]

Steve you're always so helpful lol!

1) I was running a max of 190
2) 34mm Venturies
I agree it is way too rich. I'll be reducing both the idle and main jets. I verified that the float levels are correct a few months ago.
3) Total timing is 35 at 6k iirc.
4) Pump gas

I definitely agree on the affect of the high drag wide body, and the rolling resistance of the large tires.

Do you think I'd notice a big improvement if I were to go to a more aggressive cam? I know the Solex is pretty mild.

To give you an idea. At Buttonwillow Raceway Park most cars at the end of the front straight are at 115-120mph. I'm hitting about 99mph, yet I'm also 2-5 seconds faster per lap than most these cars.

[Steve@Rennsport]

Solex cams are well matched to your 34mm venturies so I don't see any problems there. I'd certainly recommend getting the car on a Dynojet to fine-tune the jetting and see what RWHP you have.

I think you need more cam for what you are doing at the very least.

Beyond that, more motor is needed for pushing a high-drag car down a long straight. Still,....if you are 2-5 secs faster than most other cars so I'd say you were doing a decent job,...

[Fixer]

Sounds like DRAG.

You have a wide body and large wing, if the car is running right it's got to be drag.

Normal 911 bodies are more slippery thru the air than wide bodies, i suspect your wing isn't helping much too, is it adjustable? Pic?

[5:04]

Originally Posted by Steve@Rennsport

Solex cams are well matched to your 34mm venturies so I don't see any problems there. I'd certainly recommend getting the car on a Dynojet to fine-tune the jetting and see what RWHP you have.

I think you need more cam for what you are doing at the very least.

Beyond that, more motor is needed for pushing a high-drag car down a long straight. Still,....if you are 2-5 secs faster than most other cars so I'd say you were doing a decent job,...

Thanks again for your help Steve! I'm going to do a little more tuning to the jets with my wideband this week, and then I'll be heading to a dyno this Saturday so I can have a real idea of what's going on. Maybe I'm there is nothing wrong and it is just drag. We'll find out soon enough.

I also agree that I'm running out of torque with the mild cam. I think part of my high speed problems are related to that.

Originally Posted by The Fixer

Sounds like DRAG.

You have a wide body and large wing, if the car is running right it's got to be drag.

Normal 911 bodies are more slippery thru the air than wide bodies, i suspect your wing isn't helping much too, is it adjustable? Pic?

You could be right.

Here is a picture of the car. I converted the bodywork to 964 turbo, have a splitter mounted to the chassis, and run no rear bumper to reduce drag and save weight.