79 911 sc. couple questions

[FinallyGotOne]

I assume running too leaned out is certainly not going to make my car faster, running richer would give me more power, correct?

My car seems a lot slower since the mixture was adjusted. I am still getting idle bounce./stall at startup. This weekend i plan on getting in to the mixture adjustment and see if I cannot just turn the mixture while the car is doing the idle bounce until it runs steady. What size is that allen nut in the mixture adjustment area? which direction goes rich vs lean? counter clockwise for Lean?

What has Pelicans done here to increase the range of 1st gear on these SC's. I still have been debating doing something that gives my car more speed in first gear. I still am finding that I need to shift out at about 4,000 RPM to keep it smooth and the power range drops off so quick. What is a typical speed in MPH for common shifting out of first gear. I can't be going more than 20 if that max.

I have heard of changing gears to give 1st gear a longer pull, but that it cuts significantly into top end performance and speed, and was told I'd find myself on the freeway in 5th revving annoyingly high.

Should I pull out my glass 1/4 windows to replace with Lexan for a daily driver, that occasionally sees the cones? The PO painted the rubber black when he attempted to black out trim. It bothers me, and I wanted to replace the windows with black trim pieces and unpainted rubber.

[Ricks911s]

Uhhhhhhh . . . I'd make sure you REALLY know what you're doing before playing with the mixture. There's a TON of info here.

But really, do yourself a favor and do the due diligence first.

[Gunter]

A long, LONG 3 mm Allen key will fit into the adjustment screw but.......................
how will you determine the correct mix?
Richer doesn't mean more power.
What you call idle bounce may be hunting due to the mix being too rich.
There is a lot more to this and would do a lot more reading and searching.
Keep up maintenance dilligently: valve clearance, plugs, cap, rotor, wires, oil, good octane etc

1st and 2nd is the weak-spot. The 915 is not a fast-shifting transmission.

[scottrx7tt]

an engine makes more power a little on the lean side.

[kidrock]

I would be more concerned about running the proper air/fuel mixture, as opposed to leaning or richening the system to gain a horse or 2. Seriously.

[mca]

Originally Posted by scottrx7tt

an engine makes more power a little on the lean side.

hmmm... i thought it was the opposite.

[scottrx7tt]

Originally Posted by mca

hmmm... i thought it was the opposite.

that is what the Honda guys believe. Thats why you see their cars pouring fuel out the tailpipe when they richen their fuel mixtures up with their SAFCs.

Ive been into building/tuning 3rd gen rx7s for years. you make the most power right before detonation, actually. Most people tune their cars a little richer and make less power to make it a little safer on the motor. Ive seen many rx7s tuned to around 10.5:1 to be safe. You could make about 15hp more running in the 12s-13s, but with a rotary, detonation is a death warrant for the motor. With naturally aspirated porsche engines, you would be able to be a lot leaner than that, and be safe.

[FinallyGotOne]

Whoa, I learned something! ...again

[SteveinTO]

Problem is you can dyno tune a car to the threshold of detonation on a humid, damp cold day at sea level and then show up at Willow Springs (2900 feet above sea level) in August with the ambient at 115 degrees. The heat and declining air density, and suddenly you have a problem.

Uh, which direction do you re-jet?

Careful, it's a trick question. Not at all intuitive. Relates to horsepower, not detonation.

[mca]

Originally Posted by scottrx7tt

an engine makes more power a little on the lean side.

Can you provide some support for this claim? Everything I have heard / read is contrary to this statement.

For example, I did a dyno run at Autometrics a few months ago. My AFR at WOT was around 14.0 - 14.4. The owner and tech said that I would get more power by richening my mixture.

Autometrics has their own racing team. Obviously they have vast experience tuning their race cars on the dyno. Why would they tell me to richen it instead of lean it out some?

Not trying to be confrontational. I just want some supporting evidence from your side of the argument.

Thanks,
Craig

[Gunter]

Hm........ there is a big difference between racing engines vs. street engines.

Completely different dynamics.

Do what makes you feel good.
Set it rich and when it starts to hunt, go ccw until it stops hunting.

[mca]

Originally Posted by Gunter

Hm........ there is a big difference between racing engines vs. street engines.

Completely different dynamics.

Do what makes you feel good.
Set it rich and when it starts to hunt, go ccw until it stops hunting.

I don't have a mixture issue. It is fine as it is.

However, I am going back to the dyno in order to do some further tuning and want to make the most of it. They will want to richen it for power while scottrx7tt says to lean it.

[scottrx7tt]

here is part of an article in car craft magazine about AFRs. BTW, im not saying that if you run 15:1, you are going to be making more power. Ive seen most naturally aspirated vehicles make the most power around 13.5:1, any richer, and you are losing HP, and wasting fuel.


Best Power




We mentioned the stoichiometric air-fuel ratio (14.7:1) that is the ideal ratio for lowest emissions, but this isn't the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1. This is roughly 13 parts of air to one part fuel. It's what combustion engineers call an excess fuel ratio and is intended to ensure that all the air is used to support the combustion process. This is because air is the oxidizer in combustion. Too many enthusiasts think that adding additional fuel beyond the ideal to create a richer mixture will make more power. This doesn't work because you can only burn the fuel when you have enough air to support combustion. That's why engines make more power when you add a supercharger or nitrous--you're shoving more air in the cylinder so that you can burn more fuel. Regardless of the amount of air in the cylinder, it still requires a given ratio of fuel to burn. Add too much extra fuel, and power will decrease.




When it comes to fuel mileage and increased fuel efficiency, this ratio changes again. All new cars run at 14.7:1 air-fuel ratio at part throttle because this is the lowest emission point. But depending upon the engine, it's possible to run an engine at leaner mixtures like 16:1 or more at part throttle to gain mileage. The difficulty with this is that driveability and throttle response suffers at these ratios. Engine response is lazy and stumbles are commonplace. Each engine will be different, but there is fuel mileage to be gained by fine-tuning your carburetor. Don't be intimidated by these lean mixtures at part throttle. You won't burn the engine up since it is making very little horsepower at part throttle cruise--often less than 30 hp.

[mca]

Originally Posted by scottrx7tt

here is part of an article in car craft magazine about AFRs. BTW, im not saying that if you run 15:1, you are going to be making more power. Ive seen most naturally aspirated vehicles make the most power around 13.5:1, any richer, and you are losing HP, and wasting fuel.

That is at WOT right? I am in that area now b/c I made some adjustments since my last dyno.

Thanks for the info and clarifying what you meant by "leaner" side. When you said that I was thinking north of 14.7.