I recently put a new 330 hp GM crate engine in my '69 and was all set to put a set of plated headers and probably Flowmasters on it when I ran into a guy that told me something that really set me back. I would have ignored the guy, because what he told me is inconsistent with everthing else I've heard. But I'm hesitant to dismiss what he said because he seems to know more about Holley carbs than the techs at Holly. He tore mine down in about five minutes, diagnosed the problem, made a couple of modifications to stock design features, totally cleaned it out, set all the adjustments (all on the bench). I took it home, put it back on my truck and it ran perfectly from the minute I put it on. I figure a guy like that deserves a good measure of credibility. So now that you have some background this is what the guy told me: He said that a high flow exhaust actually hurts horsepower at lower rpm's. He said this is not an issue with a manual transmission, because you can build 3 to 4 thousand rpm's before releasing the clutch, but with an auto transm, you actually lose power from zero to 3000 or 4000 rpm's by putting headers, dual exhaust and high flow mufflers on. He confirms that once you get above 4000 to 5000 rpms that the high flow exhaust gives you more horse power, but that for low end power, which is what most of us want for rodding around town with an automatic transm, you get more power (as much as 40 or 50 horse power) with a stock exhaust manifold, a crossover pipe, and normal mufflers. He even had a technical explanation for why this is true, although I didn't understand a word of it. By the way, part of his opinion on this was that anyone in the business of selling exhaust systems doesn't want you to know this, because they obviously make money selling headers and expensive mufflers. I think most of us novices believe the more flow, the more power, but does anyone out there have the true expertise and/or experience to dispute what he told me?
RE: What's the truth about exhaust I'n no exhaust expert, I don't even play on on TV ! BUT, What your friend said is sort of true. I say sort of, because this is how I understand it. An engine requires some back pressure at lower RPMs to maintain torque. A way to overcome this is to use a crossover pipe before the mufflers that will provide some back pressure while equalizing the exhaust flow. I also believe that the "name brand" high flow mufflers provide some back pressure at lower RPMs and kick in the lower restriction as the exhaust flow increases.\ OK, That being said, Unless you are running a full-on bracket racer, I doubt that you would notice anything on a street driven truck other than a slight improvement in gas milage, and a more pleasing tone. I feel the same about those that say that shorty headers will cause you to lose HP over full length. First of all, that is a myth and does not jive with the facts. Second, Who could possibly detect a small HP loss on a street driven vehicle ? OK, I'm climbing down off the soap box now... Dennis
RE: What's the truth about exhaust I do not claim to be an expert on cars or exhaust, but from everything I have read in magazines, as far as tech articles, I would be inclined to side with the statements in the previous replies. Unless you are running high horse, high rpm, I personally do not think that the headaches with headers are worth it. I think a good 21/2"-3" dual exhaust system would be fine for the street. In most of the articles I have read the torque and power always increases at the top of the rpm band, while hurting low end torque when headers are used. Motors need backpressure, without it you lose torque. I think the guy knows his stuff. ( any one who can make a holly work must be good) any one else? eugene68
RE: What's the truth about exhaust http://www.sandersonheaders.com/letsgettech3.htm Everything you wanted to know about headers.....
RE: What's the truth about exhaust I'am not an expert or anything but I did read that if you want lotsa torque, use headers that are made of smaller diameter tubing.just my two cents worth.Karl ________ MICHIGAN MEDICAL MARIJUANA DISPENSARY
RE: What's the truth about exhaust Yes sir, the first guy is right on. There is a lot of math to figure out the flow characteristics of a matched exhaust sys. Smokey Yunic has a book out about chevy small block motors and he goes into Great detail about exhaust systems. Find that book and you will find all you need to know about small blocks.
RE: What's the truth about exhaust Having been in the exhaust industry for tOOOOOOOOO many years I feel that I can awnser your question and offer a suggestion. First, yes high flow headers can and do hurt low end performance, they are made for the 1/4 mile cars with lots of carb and cam that operate best at high rpm. These headers have large primary tubes, usually around 2 inches and more. Small block street headers are around 1 and 5/8 inches. Awhile ago the old "Try Y" headers were the best compromise by "DESIGN". So buy the STREET type header of your choice. Second, as mentioned before, a good dual system works well. If you do install a balance tube between the two pipes it will balance the flow in both tubes and greatly help low end torque. This was stated in Chevy Power magazine. Now for my own modification of an existing system, X pipes. X pipes balance flow as well as blend both sides helping low and mid range rpm torque. The problem with existing X pipes is that the cross (X) is usually the same diameter as the pipe it is made from. So I made my own, from 2 1/4 tubing to look like a Y, I made two of them, cut them to look like a V and welded a 3 1/2 pipe between the two V's. The old 2.8 s-15 never sounded or ran so good. I used aeroflow mufflers. good luck JIm
RE: What's the truth about exhaust [updated:LAST EDITED ON Mar-28-04 AT 08:07 AM (CST)]And for the disagreement reply... I disagree. A high-flow exhaust will have zero effect on bottom end torque. A poorly designed high-flow exhaust system, however, can have an effect. The flow rate of the exhaust is not what hurts the bottom-end torque, it is the scavenging ability. In a sense he is right, but it is not the fact that it is high flowing that is a problem, it is the fact that the scavenging can be affected if the system is not designed properly. Things to prevent problems with scavenging are proper length headers, cross-pipes or crossover pipes, etc. In one sentence: If the system is designed properly, there is no such thing as too much flow. BTW, this information is supported by David Vizard. EDIT: I just wanted to add some more information as to what is really happening... At low RPMs, if your exhaust system is not scavenging well, then the engine is actually choking on it's own exhaust that it is not able to effectively push away from the exhaust valves. Think of filling a piece of 2" pipe with smoke and then try blowing all of the smoke out with your mouth. Nothing is creating difficulty with you breathing out and nothing is affecting the power you made when you took that breath in. Where the power loss occurs is when you go to take another breath and there is still some of that smoke lingering around. If you had a friend next to you doing the exact same thing, you would both have the exact same results, but if you connected those pipes together in a way that would allow the alternating puffs to scavenge from the other pipe, you would successfully push all of the smoke away and not inhale any on your next breath.
RE: What's the truth about exhaust The guy is perfectly right, for high RPM's a big bore exhaust is necessary but it will hurt flow velocity and thus torque at low RPM. The engineering principle involved here is this: " There is no such thing as a free lunch." I am building up a 350 small block to use in an airplane in a direct drive conversion (i.e. the prop is driven directly off the crank as opposed through a gear of belt drive) and every common hot rod 'go faster' trick would be wrong to apply in my case because I need low RPM torque.
RE: What's the truth about exhaust >I am building up a 350 small block to use in an airplane in >a direct drive conversion (i.e. the prop is driven directly >off the crank as opposed through a gear of belt drive) and >every common hot rod 'go faster' trick would be wrong to >apply in my case because I need low RPM torque. Now HERE is a guy with confidence in Chevys !! Thumbs Up !! Dennis
RE: What's the truth about exhaust I feel that I agree. I have stock manifolds with 2" downpipes welded to 2.5" pipes and mufflers (dynomax super turbos). The reason for this is I was planning for headers and a 2.5" pipe will not mate to the ram horn manifold. I have it "true-dual" with out a crossover, balance tube, x, y, ect or a catalytic converter . At low RPM I have noticed a loss of torque, it will actually bog a little if you floor it from a standstill. But once it gets to 2000-2500 RPM it pulls like a tractor. Since I have low gears and run 3000 RPM @ 65 MPH it makes for one hell of a ride on the interstate. I notice that the passing power is greater, but the bottom end sucks. This is a very interesting topic... I would like to hear more. Also, this makes me debate headers with these mufflers. I may go ahead and install sm. primary headers, but replace the dynomax with stock type mufflers and a catalytic on each side to increase back pressure. The dynomax is a little loud for everyday driving at 3000+ RPM. 79 C20 Bonanza 350/400 4bbl Q-Jet 3.73 14-Bolt Corp.
RE: What's the truth about exhaust hello a engine is a air pump.. plain and simp.. if you reduce the pumping losses you gain ft/lb's at the rear wheels.. people get in trouble when they over do it.. nascar engines rev. to 8-9 grand with a 390 cfm carb.. people but a 750 or 850 on a small block and add huge headers loss torque... a small block with a 600 cfm carb and 5/8 headers will out run anything(set up for street) small headers and small duals 2" or 2&a quarter are more than enough..add bigger tail pipes for that "look"
RE: What's the truth about exhaust let me start off by saying that i don't beleive dual or high flowing exhaust systems were created until high flowing motors with a higher intake/compression were created. the more air you have coming in, the more exhaust relief you will need. if you take a 350 sb chevy and put a blower on it and try to run a stock single muffler type exhaust on it, you will have big problems. you will be putting amazingly high stress levels on your motor and the exhaust. that being said, most people driving their cars on the street probably aren't losing enough power to even notice a difference by sticking with a stock exhaust. on the other hand, i do have a friend that works in a machine shop and he told me of several combinations they have tried on the dyno. the key to it is finding the right combination. so you can gain low end HP/TQ with a bigger exhaust, if your motor warrants it.
