1 5/8 headers down to 1 1/2 muffler

Shaun @ Tru6 · 07-08-2011, 06:24 AM

I recently put a 3.2 into my 73 and part of the project was installing a set of George's 1 5/8 inch headers and 69E muffler with a matching Steve Wong chip.

First, what effect does the abrupt step-down create?

The car is fast, but I now think I'm leaving some power on the table. Is welding on 1 5/8 mounts on the muffler a good idea? Will gutting the muffler help? or a new muffler like an M&K?

I drove a friend's SC/Carrera with heavy RUF 17 inch wheels and it seemed to have more torque.

Any ideas?

175K911 · 07-08-2011, 06:55 AM

What exhaust did your friend's SC have? So many variables could contribute to this difference.

Shaun @ Tru6 · 07:17 AM

Hi Ed,

his car has an 86 3.2 like mine, stock Carrera exhaust, no chip. I have a 75 915 w/ 7:31, he's got an 86 915 BTW.

r_towle · 07-08-2011, 07:18 AM

torque is going to be created by the backpressure of your friends stock exhaust.
1 5/8 headers and an open muffle will reduce your backpressure and you will gain more top end HP, but lose bottom end torque.

What is the purpose of the car? Is this a dedicated, trailered track car?

Rich

175K911 · 07-08-2011, 08:22 AM

Shaun-
I've had two different '86 911s over the years and had installed George's 1-5/8 headers on both.

My previous '86 had SSI's when I converted to the headers. Yup I lost torque but the SSI's only made torque and not much above 6000.

Current car I converted from stock exhaust to the headers you now have, but I used an M&K 911R muffler at the same time. I felt like I lost a bit below about 3000, but not much. The difference from 3000-7000 was far better especially in the 6000+ range. However the stock exhaust I took off included a gutted cat and a 1-in and 2-out modified muffler that flowed pretty well.

So I would expect that your car as it's set up should feel a lot stronger across the power band. Is it possible that the car just feels like it has less torque because the power curve with the headers and the Steve Wong chip is so smoothe and linear compared to the stock setup?

gtc · gtc's Garage

My regular B&B muffler is being repaired, so I am temporarily running an early muffler that has had the "sport muffler conversion" performed on it.
My 3.2 doesn't seem to mind it, but it does sound REALLY AWFUL. So I would keep the muffler stock if I were you.

175K911 · 07-08-2011, 10:30 AM

Hey Shaun-
I've got a '74 2-in muffler that I'd modified internally per instructions from the good folks at SSI (they'd given me their flow data something like 20 years ago) and added a 3.5" stainless top on. Flows quite well, isn't loud, and no resonance. Lent it to a friend who didn't like how loud his Monty was, and he said he had a lot more driveability with mine compared to the Monty. He's since sold the car so I need to get it back from him anyway. You're welcome to try it out if you'd like. I don't want to sell it because it's quiet enough behind the headers to meet the noise restrictions at some track.

Shaun @ Tru6 · 07-08-2011, 11:18 AM

Thank you Ed, that's extremely kind of you. Guys like you is what makes this board so great! I may take you up on that, will send a tomorrow on it.

Quote:

Originally Posted by 175K911

Hey Shaun-
I've got a '74 2-in muffler that I'd modified internally per instructions from the good folks at SSI (they'd given me their flow data something like 20 years ago) and added a 3.5" stainless top on. Flows quite well, isn't loud, and no resonance. Lent it to a friend who didn't like how loud his Monty was, and he said he had a lot more driveability with mine compared to the Monty. He's since sold the car so I need to get it back from him anyway. You're welcome to try it out if you'd like. I don't want to sell it because it's quiet enough behind the headers to meet the noise restrictions at some track.

Shaun @ Tru6 · 07-08-2011, 11:19 AM

Quote:

Originally Posted by r_towle

torque is going to be created by the backpressure of your friends stock exhaust.
1 5/8 headers and an open muffle will reduce your backpressure and you will gain more top end HP, but lose bottom end torque.

What is the purpose of the car? Is this a dedicated, trailered track car?

Rich

Thanks Rich, good to know. Car is simply sports-purpose this year, DE next.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by r_towle

torque is going to be created by the backpressure of your friends stock exhaust.
Rich

nonsense, engines are tuned for a particular set of circumstances, changing the circumstances will require alternate tuning is all.

The best exhaust as long as an engine is tuned for it is an open free flowing exhaust.

The design of the exhaust dies influence the rpm where best torque is obtained, larger diameter pipes will generally have best torque at a higher rpm than smaller diameter pipes
here is an example of what is typically going on

this assumes of course that the system is in the ball park of being correctly sized

as to the original ?, the geometry of the transition is as important as the change in sizes

you want to avoid steps w/ the exception that an extended in let is good, do use transition pieces where possible

Extended inlet & transition piece on the out let

Shaun @ Tru6 · 07-10-2011, 05:40 PM

Thank you Bill, much appreciated. I think in September I'll have the time to weld on proper 1 5/8 receivers/pipes to the muffler.

We did find the main culprit today. What's wrong with this picture? Step down or not, I can only say how thrilled I am with this motor package now.