Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - small exhaust diameter up front and big out back??
View Single Post
  #7 (permalink)  
Old 12-08-2003, 05:46 PM
Dubz Dubz is offline
Muscle car fan
Last photo:
Join Date: Mar 2003
Location: Manitoba, Canada
Posts: 864
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Originally posted by TurboS10
Really any dyno sheets on that? From everything I have seen, the step headers have been proven to be pretty much worthless. I have also never seen any information that supports the fact that large pipes will "kill" power. I have seen dyno sheets that do prove that a small amount of backpressure will produce favorable mid range torque advantages. When I say small, we are talking less than 5 percent. Of course mufflers on 3 inch pipes will usually provide a small amount of backpressure just because of the slowed flow.

So, I would like some supporting data(read dyno sheets) that large pipes will "KILL" low end torque. I did not say it would not be effected. Hot Rod magazine did a series of small block dyno pulls to test open, straight, H pipe, and cross over a little while back. They used 3 inch pipe on a fairly mild build, with good results. This or another also tested the step headers with no advantage over a header of the same size as the smallest tube. The theory is that the step size increases scavenging ability of the headers if I remember correctly. This would supposedly help in the top end of the power range, but the dyno did not support this.

I guess we should define "kill" here, huh.....
Must have hit a sore spot, you got pretty defensive....however if you ask just about any rodder they will tell you that big pipes on a small stock engine will KILL power down low.

do you have any dyno sheets to support your side of the argument??

And you said "small amount of backpressure will produce favorable mid range torque advantages", but we were talking about low end, not midrange.

Also you mentioned that the sb responded well to 3" on a farily mild build. What were the results?? more peak hp? (expected) or more low end torque? (or do you know)

The race boys have been running tuned step headers, and the idea behind them is proven. Off the shelf stepped headers may not boost anything however (such as Hedmans tork-step) because thier not TUNED to the exact engine and rpm range, althouh since you mention hotrod look in March 2002 in the 500lb-ft article. "....we tried a swap to regular 1 5/8" headers and ended up gaining a few horsepower while losing a few ft-lb of torque" which if you think about a graph of torque and hp, meens that if you gain torque and lose hp it must make the torque down low.

Dyno proven by a NHRA/IHRA/SRA member and licensed Superstock driver "We did some header testing over the weekend on the dyno. I'll get into specifics later.....1 3/4 lost 15 ft lbs of torque and 10 HP over 1 5/8 and that's on a 9000 RPM engine! A good step header will always out perform any single diameter tube header on a race engine. (not top fuel or big blown stuff you smart a$$es lol)"

Exhast backpressure changes the torque curve, lots of backpressure gives lots of low end torque, and no backpressure (open pipes) give great mid range and high range power (moves the torque band higher in rpms).

from another thread on another forum "A 306 with stock heads, street gears, in a heavy car with a 4 spd will lose some valuable lowend torque with mandrel 2 1/2 pipes, but if you have future head and stroker plans for the motor, go ahead and do it so you won't have to pay again later. Otherwise, a standard 2 1/4" job will be a big improvement. I'm still running 2 1/4" on my little 331 and it's nice and torquey!"

That is with a difference between a cross section of 4.9 sq inches and 4.0 sq inches, if you were to put a set of 3" pipes (7.07 sq inches) you would loose alot of the down low torque and on a small motor that isn't good.

It all depends on the application, sure if you have a BBC such as a 502 you can stand to loose 20-30 ft lbs by using an exhaust that gives you more peak hp, but if you are running something much smaller, say a 289 that kind of loss would change alot.
'74 Gran Torino Project
351W, c6 trans, 9"
Reply With Quote