Today, at Thunder Racing, we were putting the finishing touches on my new 383 / AFR / Extreme 224/236 / T-trim combination and found that the driver's side AS&M header interferes badly with the new BMR K-member.

I've read that Patvis(?) found that under 900 hp, a shorty header made more power (sometimes 50 hp more) than some LT's. However, all of the fast guys I read about (LJ in particular) have Hooker LT headers.

Any thoughts on how much this will help or hurt me?

Thanks,
Mike

 

I doubt there will be a huge difference. The AS&M's I have on mine seem to work fine, but so do the LT's that I see.

Rich Krause

 

I can't see any reason why in a well setup vehicle a shorty would be better. Now if for some reason you had far too much overlap and a little backpressure in the exhaust actually helped keep the charge in the cylinder you might benefit - but you would make even more power to properly setup the cam timing.

Regarding that specific test I can't comment as I don't know all that was involved.


Chris

 

I tuned a car with a solid roller 396 and hooker long tubes. Made 452rwhp.

Ended up putting the engine into a different car with AS&M's and it made 455rwhp. (I was suprised)


Not forced induction, but still may be helpful.

 

Well, it's not just the headers, it is the rest of the exhaust system which is equally important. With most shorties, a y-pipe leads to a single cat converter and single exhaust pipe. With most longtube installations, a cat converter is installed on each side and then a y-pipe installed.

With edelbrock shorties, single cat, and flowmaster muffler my engine made 480 rwhp.
With flp longtubes, the supplied 2 cats, custom single 3" exhaust with 2 magnaflow mufflers, the engine made nearly 525 rwhp, a 45 rwhp difference. The torque curve was raised everywhere above 3000 rpm, but I lost a little torque below 3000 rpms.
I guess you can do the same thing with shorties, but why? I would just get the flp's or hooker longtubes.

 

Mike,
I remember George Baxter and Injuneer (Fred) was telling me that George made more HP with the AS&M Headers compared to long tubes upto (I belive it was) 800HP but then George changed to a different cam w/ tuning and made more power over 900HP with the long tubes. I dont remember the specifics but you can probably PM Injuneer to see what combo George was actually running at the time and see what he recommends.
I know that Fred was running AS&M on his car for a Long time too (actually) it was the same set that was on Georges car but recently (I believe) Fred changed over to Long Tubes.
Hopefully Fred will remember the specifics and can recommend what would be best for you.

Hope that helps somewhat.
Claude

 

Thanks, 95SilverTA. I summoned Injuneer to this thread.

FYI: Combo is 8.3/1 compression 383, AFR 210 heads (280i/218e cfm peak), 224/236 Extreme high-lift cam (.605"intake/.593"exhaust with 1.6/1.52 rockers) on 114 LSA, 0 deg advance, Port-matched LT4 intake, T-trim with 6.66/2.75 pullies (probably around 12 psi boost), Vortech aftercooler, 3" exhaust with Hooker Aerochamber muffler, TH350/2800Vigilante.

Thanks,
Mike

 

Fred's ASM headers were modified for a dual cat installation, they did not retain the original y-pipe.

 

It seems to me that the problem here is that everyone only looks at "peak HP".... the largest # they got on the dyno. In reality, what you need to look at is "area under the curve". If you read the theory of header primary length, at least as expounded by David Vizzard, the longer tube moves the torque peak down to lower RPM, and may hamper peak flow at max HP. The shorter primary tunes the exhaust for higher RPM, and provides less resistance to flow at peak HP.

So, what you need to look at is not peak HP, which seems to be the holy grail of the dyno testers, and look instead at how you plan to use the car. A street car is probably going to respond best with LT's. It needs a strong moderate RPM performance. On the other hand, a very specific race application - like George's - would not need much of anything on the low RPM range, but need to respond with max flow, max scavenging at very high RPM.

The test that I documented was for a 383 running maybe 18# boost, and designed to operate of the range of 5,000-7,500 RPM. The exhaust system consisted of a custom Mufflex 3" Y-pipe, incorporating a Borla XR1 shorty in each branch of the Y, and a Mufflex 4" catback with the Flowmaster straight-thru muffler. The engine was making 850 HP with the AS&M's. The AS&M's were "stock" to the header collector flange, then had "extensions" bolted to the collectors to route them straight back to the Borla XR1 on each side. Only the AS&M's, extensions and Borlas were installed on the engine dyno. The borlas connected to the two large diameter (5"?) test cell exhaust pipes, that went up to the roof.

They swapped out the AS&M's for the Hooker LT's, and bolted up the Borlas. The engine lost approx 50 flywheel HP.

Then they changed the cam, and made some other minor changes, and the engine, with the AS&M's made approx 925HP. Again they swapped to the LT's and they made a small amount of extra HP... and I don't remember the number, but I think it was less than 20HP.

At that point, the LT's were installed on the engine when it went back into the car. Eventually, the engine got new heads and was pushed up to 1,125HP, with the LT's. At one point, with the car running low to mid-9's, they pulled off the Mufflex catback. There was no change in engine performance measured. But the weight savings (~50#) along with other weight saving changes, allowed the car to run 9.04 on a very soft launch.

I'm only relating this info to point out that there is no "certainty". The headers need to be matched to the application. Somietimes the results fly in the face of "theory". But LT's are not the slam-dunk for everyone. I bought the AS&M's from George and used them on my setup. Last year I bought a set of Hooker LT's, had the O2 bungs and an EGT bung welded on. Ran a little low on $$$ and never did anything with them. They need to be coated and installed still.

 

CarCraft did a really good article on this a few months ago. Fred gave a good explanation but I will add my expeirence to this. I have two blown F-bodys one has AS&M shortys with a Random Tech Y-Pipe the other has LTs with a a muflex Y-pipe. I make good power for my mods on the AS&Ms This is a good set-up.

My other car had JBA shortys before the Longtubes were installed the car was on the Dyno with both cominations. With JBAs it made 594RWHP with LTs it made 556RWHP both peaks were very close in RPMs but I had big increases with the LTs under peak. I was very suprised to see a power loss but it sure happened. we did several calculations based on the new and old graphs and the gains below the peak out weighed the more impressive peak of the JBAs. Bottom line if it was possible to run my car with JBAs against my car with LTs the LT car would win the 1/4 mile race. My LT car would pull out in front and continue to pull untill near the end of the 1/4 when the JBA car would start gaining but would run out of track way before it could catch the LT car. I hope this makes sense because now I'm confused.

One thing I should add is that the JBA dyno was with a stock rear the LT was with a 12 Bolt. The 12 bolt uses up about 10 RWHP more than stock.

Jeff
96 SS BLOWN
96 TA 396 D1

 

That all then begs the question - what would be the peak/average hp gains for a larger diameter primary longtube - say 1 7/8 or 2" ...

Chris

 

AS@M's are mid length not shortys

 

Mike,

Wouldn't it have been nice to know the AS&M headers would interfer with the tubular k-member?

Guess it would have been to much for BMR to mention such an issue. Maybe it was "don't ask don't tell".

I have nothing intelligent to contribute to this thread, so I'll let you guys take it back over. Please resume.

Hoping to hear only good things from this point on Mike. Best of luck.

Ben T.

 

After looking closer at the K-member, I really don't think any shory header, with the y-pipe passing under the oil pan, would fit.

Not that it helps me any (I've already spent about $1400 on the K-member and LT headers that I wouldn't have bought if I had known about this problem), but BMR called Thunder Racing to ask questions about what is interfering. Maybe their future K-members will fit better.

Mike