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-   -   small exhaust diameter up front and big out back?? (http://www.hotrodders.com/forum/small-exhaust-diameter-up-front-big-out-back-29689.html)

smlblcks10 12-08-2003 03:48 PM

small exhaust diameter up front and big out back??
 
i was just thinking about something and let me know if im right or wrong. the general consencus seems to be that smaller exhaust pipe diameter gives better low end torque and big pipes give better high end and revving capability due to the increased flow. now why not build an exhaust system that has both? it seems that smaller tubes closer towards the motor such as the headers and first bit of pipe coming off of them is the most crutial to low end torque (smaller tube headers=more torque, etc.) i mean look at high performance motorcycles and such. they usually have a very skinny header pipe which tapers out into a large diameter muffler. so would it make sense to build an exhaust system that starts out out with say 2" or 2 1/4" piping from the headers to the mufflers then uses 2 1/2" or 3" piping for the tailpipes?? wouldnt this give you the benefit of having smaller pipes close to the motor for low end and torque and then have the bigger pipes towards the back for less high-end restriction, plus the added bonus of having a deeper sound due to the bigger tailpipes?? or am i just talking out my a** and what you would actually have is a system with flow characteristics somewhere inbetween the two??

TurboS10 12-08-2003 04:16 PM

You are confused. Smaller pipes are generally used on engines that need less air flow and are generally lower end type engines. Engines that make power consume more air and need the larger exhaust tubes. Air flow is related to engine capacity and output more than power band. You are not going to kill low end torque with larger than needed pipes, but you will kill high end power by putting a section of small pipe. The pipe will only flow as much as the smallest section.

Chris

Dubz 12-08-2003 04:26 PM

Quote:

Originally posted by TurboS10
You are confused. Smaller pipes are generally used on engines that need less air flow and are generally lower end type engines. Engines that make power consume more air and need the larger exhaust tubes. Air flow is related to engine capacity and output more than power band. You are not going to kill low end torque with larger than needed pipes, but you will kill high end power by putting a section of small pipe. The pipe will only flow as much as the smallest section.

Chris

stepped headers work in this way to boost low end power and still maintain midrange and upper. Backpressure is important for torque production TurboS10, very important. By going with bigger tube headers and big exhaust you can loose alot of getup and go from a small motor, so saying that you are not going to kill low end torque is a lie. It may be more evident if you use smaller than needed pipes that restrict flow and kill top end, but finding the balance is the key.

smlblcks10 12-08-2003 04:27 PM

ok i see what your saying, but im pretty sure im correct in saying that if you just throw some dual 3" pipes on a small block like my 283 for example that your going to kill all your low end and it will be bad for overall performance. so what if you want the better performance of the smaller pipes but the big, deep sound of large diameter pipes. would there be anything wrong with using smaller pipes up front and large tailpies after the mufflers so you get the sound of big pipes without the performance loss that would occur with slapping them on a smaller engine???

TurboS10 12-08-2003 04:52 PM

Really Dubz.....got 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.....


Chris

DoubleVision 12-08-2003 05:03 PM

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.

Dubz 12-08-2003 05:46 PM

Quote:

Originally posted by TurboS10
Really Dubz.....got 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.....
Chris

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.

troy-curt 12-08-2003 05:59 PM

Most oem stuff comes with smaller tailpipe than the header pipe.

I have always used the same size all the way , with good result.

Dubz, telling a member on this board that his opinion is a lie is a little strong. I think that would make a sore spot for me.

Troy

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Dubz 12-08-2003 06:13 PM

Quote:

Originally posted by troy-curt
Most oem stuff comes with smaller tailpipe than the header pipe.

I have always used the same size all the way , with good result.

Dubz, telling a member on this board that his opinion is a lie is a little strong. I think that would make a sore spot for me.

Troy

I re-read that and it does sound a little strong, but to me it wasn't an opinion type question, it was a fact based question (where there is a right and a wrong)

Sorry Turbo :(

OddRodder 12-08-2003 06:14 PM

Being a king of low budget-- low buck, I have had numerous cars that prove this backpressure issue in real world situations with seat of the pants information. ( I don't have any Dyno proof and probably will never be able to "dyno" anything!) But, I did have a 455" buick electra that the exhaust got torn off of (gravel road racin') With only two feet of lead pipe on the car, it was no longer the tire shredding monster it was with full exhaust and actually labored to pull away from a dead stop--- replaced the exhaust--TaDah!--back came the lowend torque! Another time, I put a very mild 305" in a 65 chevy pickup but I ran out of money so I welded the 305's leadpipe to the 6cyl. exhaust (1-3/4, 2"?). WOW! gobs of lowend power! A month or so later, when I had the money, I had a complete dual exhaust put on the truck (2-1/4") Guess what? lowend torque was nowhere near what it had been with the single system! I have used 2-1/2" ( 18"long ) collectors at the dragstrip on a small block car and picked up a tenth over open headers! Anyway, my answer to the question would be, on your 283, I would run a 2-1/4" dual exhaust through the mufflers and then step up on the tailpipes if you want the big sound and look!

TurboS10 12-08-2003 10:07 PM

Quote:

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.

Quote:

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.

Quote:

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 mph....at 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.

Quote:

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.


Quote:

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.

Quote:

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.


Quote:

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.


Quote:

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!"
Opinion......



Quote:

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.

Chris

85CHEVY 12-08-2003 10:08 PM

Before I put the new exhaust system on my 85' blazer with a 355 dynomax 1 5/8 headers, 268XE comp, I ran open headers for a day just to see what it would be like. It was loud but the low end was absolutely horrible, and the top end also seemed to lack. I dont know if running open headers is even near the same as running too large diameter of pipe, but I thought I would mention it anyway. The 2.25 inch dual with H-pipe really brought up the power by alot.

adryan16 12-08-2003 10:16 PM

I just read a book that outlined the advantages of stepped headers, or even stepped exhaust systems. They gave percentages for length that each step should be of your total header length should be. Lengthening certain steps affects different RPM ranges, enabling tuning. I wish I remembered more, but the book was just loaned to me for a while. If I get it, I'll try to retain a bit more of that. My bad:rolleyes:

Dubz 12-08-2003 10:55 PM

Quote:

Originally posted by TurboS10
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.

a 289 or 302 sbf

smlblcks10 12-09-2003 12:46 AM

so if its true that hot expanding exhaust gasses will always go to the place of lower temperature and pressure, wouldnt using small pipes from the header-mufflers and big tailpipes "suck" the exhaust away from the motor better than one smaller size all the way through?? the bigger tailpies are areas of lower pressure and temperature are so wouldnt the exhaust gasses rush to this area more??


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