I'm up to 170whp Power peaks early and then falls off like a rock.



I think it's time to graduate to a full 2 1/2" system, instead of this 2 1/4" monstrosity:



feeding into the stock 12A midpipe (1 7/8" I think?) and then into I think a Mind-Train rear muffler (quiet!)

But before that, need to switch out the 680x2 for 440x4

 

same ol' crazy, peejay.

yeah, it's time to step up, man. with all the expansion chamber talk i've been reading recently, i have to ask, have you considered designing one for your setup?

 

it actually should be 'how much power can YOU make on the stock midpipe'

you ever try to measure exhaust backpressure? the hard part, it turns out, is that people drive 40mph on the freeway, so i know the ABS works, but not backpressure past 2nd gear

 

It would be really easy to check, actually. My system is tuned to run open-loop, so the nice expensive wideband is just sitting in the pipe getting clogged, so I really could just yank it out and thread in the pressure tap we use for checking converters.

All it is is an old spark plug shell with a pipe fitting and a short length of tube brazed in. Stick any common pressure/vacuum gauge on it and take a spin around the block...

All the talk of expansion chambers DOES get me thinking, but I don't have the math to do it right. I'd only be able to do it cargo cult style: it would look like one, but how well would it function? Or would it actually be hurting things?

I do have a pair of collector flanges for my header - the Mind-Train uses a 3 bolt flange like most "real car" headers. (IE headers that are not direct-fit style) This would make it a pain in the *** to do, since you really can't access one of the bolts due to the way the collector is positioned. And, from what I see, a chamber would need to be mounted directly at the collector, so that the merge is kind of a venturi in the greater scope of things. Which, to me, seems backwards, feeding a high pressure area (the slow expansion chamber) with a low pressure area (the fast moving gases in the collector) but again, the math kinda escapes me.

All the same, I have an empty converter shell lying around here that will get ONE of the flanges welded directly to its inlet. Because of the way the M-T header is shaped, the converter shell will *be* the collector extension. I will probably mount the shell rotated 90 degrees because the header is over/under where the two pipes merge.

On the other hand, maybe this would also be a good time to switch to a 13B header instead of a modded 12A header... Nah! I may be going back to a 12A after a while. I still have lots of 12A goodies (like the half assembled ITB manifold) waiting for me to find a good pair of rotorhousings so I can build another engine.

 

hmm thats a neat tool, you should use it... according to the graph in the book, 4psi of backpressure on a stock 12a = 10hp. the more overlap you have the more it looses

 

14.360 at 98.01 with a 2.25 short.

I don't think I'm going to eke much more from this exhaust system

I noticed that the car always gets slower as the day goes on, best run is always the one straight off of the highway. Turns out, my clutch is starting to slip. (ACT something or other, not the 6 puck Death Clutch) So I am going to hold off on 1/4mi excursions for a while, given that best launches involve quite a lot of clutch slip thanks to the tall gearing.

 

so what all do you have done to the car anyways?

 

what mods/porting do you have on your 12a? whats the most hp you have seen on a stockport 12a with everything you can think of to give it power, except NOS of course lol

 

See sig. Turbo II block with street port, basically. Other than that it's really stock, nothing serious done with the car. Aside from the heavily rolled-out fenders to clear the front tires, and the fact that some jackass hacked up the wiring when he installed the Holley, it would be an easy return-to-stock if I was able to find a 12A.

 

ok peejay, since you seem to be the master of not spending money, i am about to drop in a 12a with a large streetport and was wondering what i should do as far as the exhaust.

car currently has an RB dual header, and then 2" off the flange back through 2 glasspacks and then collected before the axle, and then it is 2 1/2" out the back.don't know what the collected length is, and probably no where near equal length.

I was thinking, i could merge it at the flange, into then 2" all the way out the back?

 

do work!!!

 

All the talk of expansion chambers DOES get me thinking, but I don't have the math to do it right. I'd only be able to do it cargo cult style: it would look like one, but how well would it function? Or would it actually be hurting things?

{snip excues out} And, from what I see, a chamber would need to be mounted directly at the collector, so that the merge is kind of a venturi in the greater scope of things. Which, to me, seems backwards, feeding a high pressure area (the slow expansion chamber) with a low pressure area (the fast moving gases in the collector) but again, the math kinda escapes me.

Perhaps the expansion chamber is really a combustion chamber and that is why it works better with more rich mixtures and retarded timing.

How would that change how you see things working out?

 

well, to me that actually makes some sort of sense.

anyone know where i can find the math for an expansion chamber?

 

With a given combustion chamber shape, the better your VE and/or the lower the amount of residual exhaust gases in the chamber, the less timing you need.

In short, the more efficient your exhaust the less timing the engine will like.

BTW - 14.07 at 99.76. Still through 2x680s...

 

that sounds like it should be ok, depending on how long the primaries end up.

the factory race setup was 45mm pipes, 230cm long into a collector into a megaphone. with a street port, going to 2.5" should work just fine, and not be too loud

 

ok, thanks, i guess i'll measure it out and see where they need to collect according to the RB website.


o and btw, its pretty damn loud