raffaplymouth wrote:
this is a good idea .. i can buy another header flange ant tube long 12" and drill for put another evac and lambda nipples , only for races ... you think will make power ??? or only much noise ??
It's my understanding that what you want is the right amount of back pressure for the best bottom end torque. Part of the benefit to tweaking back pressure is to help increase exhaust scavenge at the heads. You get back pressure from your street exhaust, but maybe too much and you most certainly get the weight penalty of all those tubes and mufflers.
Mine seems to run best with about 12" of extension - it just has more grunt and pulls better over the curve. An easy test for you would be to fab up some 18" extensions and make one run with full exhaust, one with the 18" extensions and one with just open tubes. Aside from the noise aspect, you'll be able to feel the difference. If the extensions are an improvement, you’ll need to determine how long they need to be.
The trick I used on mine was to start with 18" extensions and use a heavy crayon to mark the extension all the way down its length. Then, make a run. The point at which the crayon stops burning is where you cut it. There may be better ways to get the right length (on a dyno, for instance) but the crayon trick is supposedly the best way to figure it out on the track. Mine doesn't perform as well with straight open headers, and, the induced harmonics makes all the screws in the interior back out and drop on the floor
Also: You're running 2" primaries, 3 1/2" collectors and 3 1/2" tubes. I have close to the same, with 2 1/8" primaries and 3 1/2" collectors. With mine however, I neck my extensions down to 3" out the back. That ensures I get good back pressure and doing the crayon thing lets me figure out the right extension length to level it out. If you can afford it, you might want to try both 3" and 3 1/2" extensions. Again, as I understand it, the idea is to get the right scavenge effect at the exhaust ports - the amount of back pressure needed to help that along will be purely dependent on your intake air flow, intake/exhaust runner size, cam profile and compression ratio. You're running enough displacement to maybe need the bigger tube size all the way back, but it might be worth trying the slightly smaller tube size just to check.
Note that I'm running a lot smaller motor (434 ci / .030 over 427) but I'm also running a bunch of boost out of the blower, so that changes the mix, when looking at overall tube size and flow, etc. With your much larger displacement you would think you'd need much larger tubes in back but (in my opinion) you're using relatively small primary tubes (2") for your displacement. Add to that the fact that (according to your first post) you're pushing a healthy shot of nitrous into the motor. That increases cylinder pressures and delivers a corresponding increase in exhaust flow and volume. I'd prefer you had bigger primaries (2 1/8" to 2 1/4") but again, that’s a personal opinion (they may be just perfect as is
). I think tho, the 2” primary size makes tuning your exhaust even more important. Getting the right level of back pressure to balance primary tube flow and the resulting scavenge effect at your exhaust ports will help a lot overall.
I'm also running a TH400 with a brake. I launch at 4000 and shift at 7400 so I have a nice long power band to play with and only 3 gears to worry about. You don't have the benefit of the transbrake and all that raw grunt at launch (I launch with full boost out of the hole
)
This aspect of tuning is as much voodoo as science so you'll just need to play with it a little. Like I said tho, same day, same track, same tune - I made one pass with my street exhaust and the next with my extensions. 2 tenths difference at the stripe and a big diff with my 60’s.
If / when you get to the right length of the extensions let us know the weight difference between the extensions and the full exhaust. I'll bet you're looking at least 35 pounds difference, weight wise.
However, like everything else in this business, your mileage will vary
I tend to build all this stuff in my head while I’m puzzling out what I want to put together and it all seems to make sense to me
. If you asked any number of the guys on the forum here you might get a completely different view of things.
It'll be interesting to find out what you find in playing with this part of your build.