Some people will tell you (a) a smaller pipe will produce more torque (b) SBCs need a bit of back pressure. Are you using dual exhaust? For what it may be worth, I had a 283 power pack/4bbl with twin 2 inch pipes and fairly quiet mufflers and never had a bit of trouble with it. Ran good and sounded nice. Good luck.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Every engine needs back pressure. You'll be fine with 2 inch pipes. Have an H Pipe, X Pipe, or Balance/Crossover Pipe installed if going duals, it'll improve scavenging and the engine will thank you for it.
Shane
Shane's Toys... 2007 Forester XT Limited (2nd Owner) 1991 Cherokee Laredo (2nd Owner) 1981 Chevy 3/4 Ton Fleetside 8,600 Camper Special (3rd Owner) 1965 Chevy 3/4 Ton Fleetside (3rd Owner)
Engines don’t “NEED” back pressure. Said the 12000hp dragster tuner!
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
Unfortunately, the use of an exhaust restriction to "cover" a not good enough choice of engine parameters (there are dozens) is effective in many cases. I can repeat myself, too: no engine needs backpressure, it's near the top of the list of "things to do when you can't fix the problem". Read Vizard, Smith, Blair, Vincent, McFarlane.
Panic, you need to fill in a signature that contains this ---" Full list of engines that need backpressure:"
That's a classic statement.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
I'm the one who mentioned this and please note...I was careful to say "some people" and careful not to take a position. The whole idea of back pressure (in a naturally aspirated vehicle) brings along with it some complexities. Like you folks, I've heard about the need for some back pressure for a long time...over 50 years. So let's see what some eggheads think.
Here is a guy who I think breaks much of it down pretty well:
A change in diameter, restriction, baffle, or anything other change in the exhaust path that causes a pressure change (reflected wave) may "cancel" an unfortunate (and accidental) flat spot, hesitation, etc. in even a docile engine. In some cases the factory exhaust system has been designed to eliminate such a hiccup. This always reduces exhaust gas volume at peak power, although there may be a small improvement (partial or complete cure of the flat spot) in torque especially at part throttle and mid RPM.
Anticipating a problem and adding a restriction where it was not needed may appear harmless, since the only effective test is... to remove the restriction.
An easy, cheap, and reversible test is to add a short section to the final tailpipe at least 1/4" smaller ID. Your FLAPS has these as adapters. If this size change is not available, simply crush one end into a "stadium" shape. The circumference will not change but the X-area will shrink.
It's more fun to poke a potato into the tailpipe- - - - -instant back pressure! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I had a problem with the engine stalling after putting in neutral from 45mph+ with the old side pipe and thought it was stalling from back pressure but it still doing it. Idles fine. Just coming down it wants to stall Any ideas what could be going on?
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
Any other hints, symptoms, etc? Which carburetor? Fuel pump...mechanical or electric? I can think of about 20 things off the bat that could cause you to stall at that speed. I take it you're stalling when you put it in neutral and take your foot off the gas?
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
To prevent more confusion, let me define "back pressure". This is when the volume of exhaust gas from the engine is significantly larger than the outlet pipe can flow with minimal resistance, resulting in a pressure rise in the pipe. If it's high enough, the engine will run more slowly until it stops (potato in the tailpipe). When the engine speed drops to idle, the exhaust pipe is many, many times larger than necessary to permit the exhaust to exit. The internal pressure may be very close to atmospheric.
I believe you have a vacuum leak somewhere and I'll explain. Could be a worn out carburetor. I've seen that happen many, many times and it usually means the throttle shaft needs new bushings...more about that in a moment. If at 45mph your engine stalls or dies when you put it in neutral and take your foot off the gas it tells me your engine is suddenly running too lean at that point. What you've done: taking foot off gas closes the throttle valve and this means the available fuel just went from richer to idle-speed lean pretty rapidly. I imagine the same thing would occur in your driveway if you were running the engine at say...2000 rpm and suddenly closed the throttle. Something just happened which caused your engine to get more air than fuel...too much more and enough air that your spark plugs can't fire the mix well. When your engine goes from higher rpm to idle, you get a vacuum spike of ~50% (say from 18 to 27 or 30)...if the engine is not in gear, this spike will be more temporary for certain, but it will be present just the same. If you have a vacuum leak, your engine (which is really just an air pump) will try hard to suck air from anywhere at all when you cause that vacuum spike. If a vacuum leak is present, that will be your culprit. When you have the throttle open, you're providing air and fuel. Even with a vacuum leak, you may still be running somewhat lean, but you probably won't notice a problem during normal operation. Vacuum leaks can be like that. When that vacuum spike occurs and you're denying fuel to the engine you force a very lean condition to happen. If your throttle shaft is loose, air can and will enter through the gap when your vacuum goes high. It won't have fuel to mix with (since your throttle is closed) and you'll get a stall. A vacuum leak could also come from power brakes and a leaky hose/bad power unit, vacuum windshield wipers and a leaky hose, a warped carburetor base where it attaches to the manifold, if your vacuum advance works on manifold vacuum that could be a possibility, leaky manifold gaskets can be a problem on SBC engines...etc, etc. Lots of things to consider. Good luck.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
And I should add one way to help chase down a vacuum leak (won't work in some cases) is to get a propane torch, don't light it but open the valve slightly and pass it over possible areas of concern. Throttle shaft exits, where carb attaches to manifold, where manifold attaches to head...places like that. Hear the idle speed increase? There's your leak. Or at least there's one of them. There could be more.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Confusion over "back pressure" and Wave harmonics. Look at a 2 stroke pipe for an explanation. No back pressure. Standing pressure wave caused by the tapered section, sends a shock wave back to stop incoming charge escaping the combustion chamber. Now look at those old fashioned tapered mufflers on Brit racing bikes. Same thing. What does matter? The timing is critical. Only works at a narrow RPM range and is dispensed with on a Dragster for the simple fact that it runs at a very limited RPM range and pipe length does that for them. So no back pressure. Why is the idle unstable? Something is variable. Air fuel mix(vacuum leak) or, something is moving when it should not, or holding still when it should. Stuck power piston, and loose throttle shaft come to mind. OK now Panic will hit me where I am wrong.
I have a vacuum to the power booster brake but of course its the last thing I put on and first check was the EGR valve. Put on a vent cover and plug for carb. Instant fix. I did not put a hose clamp around the rubber hose there at carb. Could it have been leaking there ??
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
Do you have (or did you have) a CEC solenoid on the carb? These were designed to keep the throttle plate from shutting fully when you take your foot off the gas; this reduced emissions. Some people took them off since they hindered engine braking.
I'm curious as to why you want to shift into neutral and coast at 45 mph. Our motor vehicle act prohibits the practice of coasting, but I have never hears of anyone getting a ticket for it. Probably some relic from the past...
Could have been leaking there. Hard to say from this distance. The more I've thought of your situation, the more it sounds like a classic vacuum leak. So from your posting I can't be certain, but is it now fixed? Hopefully so.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
It idles better now. I knew right away driving down the road. Far as EGR , the port at the carb rubber felt tight, not so much at the valve itself sitting in rubber gasket at valve cover. I`ll run it with the 2 vented covers for awhile. My apologies for the posting lack thereof.
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
