911racer

Hi,

I have a 2.7L race 911. High compression, ge80 cams. very nice motor.

I had a 2.2L and have upgraded a year ago. I have 40mm weber carbs on it. They have 36mm venturies.

My question is. Would there be an advantage in going to 46mm carbs, Either pmo or webber. I am looking for ways to get more power.

I run 1-5/8" headers. I found that my lap times decreased significantly by using the 1-5/8" vs the 1-1/2. I do not want to spend the big bucks to find out that the new carbs would be a wash with the old ones.

Also. Has anyone run a cross over pipe between the headers.

Thanks

Ed

0396

911 racer,

Very interesting question. I can't answer, but maybe some one else can chim in.
I would like to know too.

Please allow me to tag on to your question.

I have a 3.4 ..will be running 46s..what should my target jetting
be - ie. air correction/ venturies etc.

Any suggestions?

Thanks

Steve Weiner-Rennsport Systems

Ed:

I answered your question on the Racing List as well as directly to you.


0396:

Jetting has many many variables and I'd need a LOT more information about your engine to make any suggestions.

Displacement
Compression ratio
Camshafts
Exhaust system

0396

Steve,

Come to think of it, your correct.
I was just searching for a 'base' line to start with.
With that said, here's my specs:
* twin plug
* 10 1/2 compression
* 46 webers
* 's' cam
* ported heads
* 1 3/4 headers / monty
* 3.4 - 98mm on a 3.2 crank

That's it..


Hey I met you at Ventura meet, love that '6' that you put together!
Did you say hi to Dick E. for me?

Best Regards,

911racer

Steve,

Here is some more info on my setup too.

Displacement 2.7 L using 2.7L crank and cylinders
Compression ratio 12.5:1 using pistons custom built by WISCO
Camshafts GE80 cams
Exhaust system 1-5/8 headers with megaphones. I did trials at the track to see that the 1-5/8's were faster than the 1-1/2's. Also could see that open flow "header" mufflers took away lap times significantly.

What else to tell you. Some head work complete. I did not twin plug this motor (bad move) although I do run electromotive. The motor has about 1.5 seasons on it. Case has usual tricks done to it.

The car weighs in at about 2k without me. It is very light and very quick. I am able to turn competitive lap times with others in my class even though they catch me about half way down the straights. I seem to be able to slip around the corners very well.

Background on track tuning. This was done on a track that I am very familiar with. I am able to run very consistant track times. On that very day I ran 3 laps in a row within a tenth of a second of one another.

Thanks

Ed

por9146gt

Hi there,

I beleive that with the right combo of larger headers, ports, valves, cams, the 46 will make more power but will move it in the upper rpm bands, that said , if you are willing to rev your engine up, especially at sea level, you will probably dyno more power , keep in mind the new torque and power characteristics may require a change in gearing. Good luck.

Steve Weiner-Rennsport Systems

The VERY first thing I did was call Dick on Monday AM (per usual) and pass along the greetings,...

Wasn't that Sixer awesome? Too bad we couldn't give some rides as its rather scary.

OK,...on to your question,..............

First thing I will tell you is that those 1.75" headers are too big for your S-cammed 3.4. All you are doing is giving up low and mid range torque without any payback at the upper end. If you were using a race cam of some kind that made power to 8000 RPM, that would be a different story. 1 5/8" is a better choice for that configuration; bigger is not always better,...

A good start for jetting:

38mm or 40mm main venturies
Tall aux venturies (very hard to get, now)
165-170 mains
175-180 air correctors
60-65 idles

Your muffler is great,.......!

Finish that jetting on a chassis dyno with a good A/F meter so you can accurately see your A/F ratios.

0396

Steve,


Thanks for the help / suggestion.