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Old 12-08-2003, 10:07 PM
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Originally posted by DoubleVision
I remember years ago on a subject like this, it was said that exhaust gasses cool the further back they travel in the pipe, so the tail pipe section or sections after the muffler could be slightly smaller in diameter.
Very true, but you still have to be carful IMHO. Really depends on how far back you want to downsize.

Originally posted by Dubz
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.
It was not a sore spot, I just dont like being called a liar. Most dont......appology accepted. I Like I said, I guess you have to define kill. Is it 5% or 25%?

I dont doubt that alot of rodders agree, but many hotorodders also think the chevy 302 was the hottest engine ever built. Exhaust upgrades are also almost always the first and most benifitial upgrades(more later) My experience has been the same. Typically .1-.3 tenths better open than when I ran 2.5 inch exhaust and super turbo mufflers on my 57. Have not run the same setup since I installed 3 inch flowmasters and H pipe. I will add that you can not just uncork the headers and not retune the engine. Typically fatten it a bit on the primaries.

Originally posted by Dubz
do you have any dyno sheets to support your side of the argument??
Only the HRM article that dispelled the header theory. I have a hard time believing they would run 3 inch pipes if there was no advantage. Most of the time those guys do their homework.

We actually upgraded from 2.25 inch duals and turbo mufflers to 3 inch and dynomax race mufflers with H pipe on my brothers 383 powered Nova. It is kindof hard to tell how much it gained since it did and still does roast the tires in anything other than high gear over 50 the burp of the throttle. What it did help alot was top end. It is pushing 450-500 ponie range and was seeing serious restriction. He is running 1.75 primary headers BTW.

Originally posted by Dubz
And you said "small amount of backpressure will produce favorable mid range torque advantages", but we were talking about low end, not midrange.
Okay, exactly how low? I guess I have never delt with a bone stock engine other than my daily driver 2000 Chevy. I upgraded to 3 inch single over 2.5 and changed to dynomax race muffler and it picked up considerably through the whole range by the seat-of-the-pants-ometer. I always run at least a little stall in a hotrod and really dont need anything less than 2000 RPM so I have no experience there. The area under the curve at that point is so little it really does not matter IMO. What really counts is how much total work you are getting under the curve. Most of this is mid to top end which benifit from large diameters.

Originally posted by Dubz
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)
Will have to try to look it up......have the last twleve or so years of HRM.

Originally posted by Dubz
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.
Well all the "race boys" I know and run around with run regular 1.75 to 2.25 inch header tubes. The 1.75 for NA SB and larger for real nasty SB and BB. The idea is that hot expanding gases will always move to an area of lower temp and pressure. The step provides a lower pressure area in the larger tube. The idea is that is sucks the gas out of the chamber inproving scavenging. This is a good theory, but what hurts it is the pumping losses if the small tube section becomes a restriction. I dont doubt that when tuned on a race engine there are benifits, but they aint runnin' 1.5 inch primaries unless it is a small cid. As for the curve, all it means is it made more torque somewhere below 5252 RPM where the curves always cross and are equal. They are usually talking peak torque which is somewhere in the last 1/2 of the curve before the curves cross. This is midrange to me.

Originally posted by Dubz
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)".
If that is truely a 9000 RPM engine, it aint no big block and it is probably an itty bitty smallblock or an import. If we are talking import, we are talking turbocharged. At this point we can throw this conversation out the window because it all changes.

Originally posted by Dubz
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!"

Originally posted by Dubz
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.
Obviously engine size has alot to do with this. You would never run smaller than 1.75 on a BB I would hope, and usually it is larger. 1.5 is plenty good on a 289 unless it is a real screamer.

Point my original post was that his idea would just not work. He may as well run small the whole way. One thing that I have heard but not seen proven is that if the pipe is severly oversized for the application, you to not get a laminar(believe that is the right word), but rather a sort of tumble effect. Smaller pipes keep velocity high enough to keep this from happening. Point is that small and large are all relaitive to displacent, rpm range and VE which all effect flow characteristics.

I guess, it is also relative to what low end is for an engine. For a race engine, below 5K is usually not important and this leads to the gargantuous headers. For a rock crawler, where you need 500-1500 to be optimized the little pipes have a place. For the average 2000-6000 RPM street bruiser, I like big pipe and header tubes without going overboard. I believe you get the most usuable power through the range. I will point out again, a simple swap without retuning is not a good indicator.


Last edited by TurboS10; 12-08-2003 at 10:46 PM.
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