Tom W

I've now completed the sequence for the dyno results as a result of my misguided venture looking for more hp in all the wrong places...

Stock headers, cat, primary, secondary bypass: 230 rwhp
Stock headers, fabspeed cat bypass, primary, secondary bypass: 244 rwhp
Fabspeed equal length headers (no heat exchangers), Fabspeed muffler: 239 rwhp

At tests on the same dyno with very similar weather (temp and humidity).

So, now that I've passed my semi-annual smog test, the cat comes back off and I add the cat bypass back. Headers and muffler have been returned to Fabspeed. The added benefit of this configuration is it easily passes sound requirements at Laguna Seca (at 89 db) while the fancy headers and muffler were way too loud (96 db).

Dunasso

Did you get the Fabspeed parts on a loan to return basis, Or did you have to purchase then return?

Tom W

The Fabspeed parts were purchased by my shop/mechanic (Devek/Bob Brooks, "viperbob") with an agreement to return if 20+ hp was not obtained. Bob does a significant business (from what I can tell) with Fabspeed (Joe Fabiani).

joey bagadonuts

Excellent info, Tom!

Monique

Interesting results.. thanks

Bill Verburg

The headers would work a lot better w/ upgraded cams to feed them

Tom W

Bill: That may be true, but I do have a budget and my wife makes noise when I get to 3x or more of budget. I figure I'll worry about cams when I need a rebuild because of value guide wear (probably next year's budget given the current oil consumption and smoke on start-up). I plan to spend the rest of them year's budget on track time and improving the nut behind the wheel (and, hey, I now have $3k more to spend!).

MarcMF

Did you check your AFR before and after the swap ? A free flow exhaust will give better results if you fine tune the AFR .

Tom W

If you mean Air/Fuel Ratio, it was examined as part of the dyno run. It wasn't bad and I didn't want to get into the time and expense of spending a day on the dyno trying to tweek a chip. The engine ran a bit lean with the stock components/cat bypass and a bit more rich (more optimized) with the new exhaust. You can search for a previous post where I have the dyno results plotted and included in the thread. The system was advertised as a bolt on solution (it included a chip change to adjust for the exhaust).

My point in this post is simply to show how well you can do with stock or minor modifications to stock. I learned that throwing money at a simple bolt on solution doesn't always work as some might claim or you might think. It's a bit of data that I thought I'd share for others to consider in their plans.

Red rooster

Tom,
Its good to see someone going through the process.I have run 911 motors on engine dynos and what you have seen is IMO spot on.
The worse part of the exhaust is the transverse / first silencer- if ever you get the chance ,cut one open and you will see why!
Big cams ,headers +++ rod bolts -yes they work but the cost to do it properly !

Geoff

garrett376

Hey Geoff,
I'm not sure what to look for, but the primary muffler doesn't look all that bad inside - what makes it so bad?

pzull

Baffles Galore!! That's why the primary is bad bad bad. The secondary is better but still not great as they placed restrictors in them

This is the design of the Primary;

garrett376

The plates are all perforated (the red in your diagram below)... I bet if you flowed this muffler, it would not be that bad - has anyone every flow rated this muffler?

In early 911 mufflers, the areas shown in red are actually solid/sealed so opening them up improves things, but with it perforated like that, and with a straight through design, it does not seem all that bad... just speculation here... I'd love to know if someone ever flowed this thing!

pzull

Despite the perforations there is increased resistance compared to a real straight through muffler like the magnaflow. Air molecules slam into a perforated wall some "bounce" back some pass through the perforations and end up as eddy current on the other end of the perforations and need to regain momentum again. It is not a streamlined flow but a really turbulent one.

If you smoke you can do a simple test which I did. Blow a puff of smoke from one end of the magnaflow and it appears at the other end (you don't need to seal your mouth on the pipe (it would look obscene with a 2.75' pipe!). Blow a puff into the stock primary - not only do you see ltlle to no smoke appearing at the other end, some of the smoke comes back out where it went in.

Test above not very scientific but the true test was actually fitting a magnaflow (or any straight through muffler) in place of the primary. A more responsive throttle can be felt revving at neutral gear and also driving

pzull

The secondary is more of a straight through muffler. Remove the restrictors and replace the u-bend with large holes (this part is also smaller pipe size that the rest of muffler) with a solid Mandrell U-bend and it will be optimised. Improvement is not as great as replacing primary with a real straight through muffler though.