I am looking into doing the exhaust on my pickup and i was wondering how much backpressure the stovebolt needs? I have been looking at running a 2 1/4 in pipe off the manifold and then spliting it just under the cab and running 2 1/4in straight pipes with 3in tips out under the step on either side. also how will this sound? the engine is all stock except for a 60,000v coil and a rochester 2gc carb.
Your engine is just a big fancy air pump, the more air comes in the more it's gotta flow out. I'd suguest a 3 inch exhaust all the way back, and a hi flow air system. If that's what your goin for.
What sound are you looking for? why not split the manifold if yoou are going split exhaust? dont know what it would sound like with single manifold and split pipes with 3 in tips split manifold with 2 in pipes all the way back sound pretty good, at least from experience
I'd say 2 1/4 is way too much pipe for a Stovebolt, not to mention splitting it into two 2 1/4 pipes.
Early six Chevy's sound great with split exhaust manifolds, tail pipes should be no larger than 1 7/8 inch. At least in 1963 they sounded great!
No engine needs back pressure. Although there are some situations where some backpressure could help under certian conditions, general rule, the less backpressure the better. There is the issue of exhaust velocity and exhaust tuning. There are cases where a smaller pipe works out better overall than a bigger one, but we aren't talking about a lot. In this case and most others, coming up with a system that optimizes exhaust tuning isn't worth the time and effort.
Wasn't that back pressure bit, just a hustle to sell high dollar, shitty sounding, "performance" exhaust systems to the unknowing masses?
Backpressure in exhaust = bad Scavenging the cylinders = good For a 235: 1-1/2" to 1-3/4" pipes with dual exhaust and dual carbs and the later 235 cams gives good breathing. 2" pipes will give a lower exhaust note. More diameter will slow acceleration at low rpm.
Crashnburns, You are right !!! I seem to remember a company that built anti-reversion headers and promised that they would prevent back pressure in the exhaust system. I remember Guys older that me saying that if you did not have back pressure, that you could actually burn your exhaust valves due to the rapid escape of exhaust gas. This was usually one of the thoughts regarding straight pipes. Jeff
3" Where's that one with the smiley face rolling on the floor laughing his ass off? What's the scavenge factor of a stock manifold ?
spray some rattle can paint on your exhaust, then drive the heck out of it. then jack it up and check where the paint is not burned off. this is where you cur off your exhaust or where your should have enoughf for back pressure. something an old round track racer told me anyway.
Ran a 389 Pontiac stock car asphalt circle car back in the day I dumped the collected exhaust into wood stove pipe and ran it back . I sounded like a herd of bumble bees !
im going to run a 235 with duel carbs and headers. the oldtimers i talk to advised not to exceed 2 inch pipe.I think these motors sound great split with no mufflers
I'd say negligible. Need a tuned length exhaust tube and collector, and a cam with enough valve overlap. At a specific rpm, there will be negative pressure in the exhaust port while both valves are open. This pulls the intake mixture through the chamber, cleaning out any residual exhaust. The tuned length only works in a narrow rpm range, and will hurt power at another spot in the rpm band. Typically near half the tuned rpm. If a header is length tuned for a helpful negative pulse at 6000 rpm, there will be a harmful positive pulse somewhere around 3000 rpm, reducing power at that rpm. I think this is part of where the back pressure myth comes from. Somebody puts "Super Race" headers on their street car, and they reduce power in the rpm range that they normally drive. Then they claim the reduced back pressure hurt torque. Actually, they are just not using the headers at the rpm they are designed for. Tri-Y headers pair up exhaust ports to reduce the harmful effects at half tuned speed. A negative pulse from the paired cylinder overlaps the positive pulse, and they effectively cancel each other out. At tuned length rpm, the effectiveness is somewhat reduced due to the added volume from the paired cylinder primary tube.
Why does the back pressure myth persist? Is it a misinterpretation of negative-pressure, in other words, low-pressure, created behind a high pressure exhaust pulse traveling through the tube? You know, SCAVENGING!
Start with a 6" pipe and plug it with potatoes - If you plug the pipe with a potato you will get back pressure - Put a 3/8 hole in the potatoe and you still have back pressure but reduced. Make it 1/2 hole - still back pressure but now less Keep increasing the hole until you have no pressure . If you want scavange, or a 5th cycle on the engine you need lots more than just a bigger pipe. You need velocity in the exhaust flow. Big pipes kill velocity . This velocity creates negative pressure behind the pulse and actually pulls the remnant combustion gasses out of the combustion chamber. Get the cam and pulses working correctly and it will suck fresh fuel and air in at greater than atmospheric pressure .
im also kinda looking for a cracklin and poppin noise on decell. I have a for speed sm420 backing the motor with factory 3.70 or 3.90 gears
In regard to backpressure, pressure is measured in positive and negative numbers.(not vaccuum) Backpressure can be negative and still be backpressure. Yes positive backpressure is bad..but too much negative backpressure can be just as bad. When you hear of needing back pressure it is not a positive number that engineers are referring to. On a small slowly rotating/poorly pumping/low volume exhaust/low compression engine a big exhaust tube or short tube out the side is not a wise move. The statement about splitting the exhaust and making the tubes smaller is valid. Too big of a tube cause exhaust gases slows down too much creating less negative pressure, cools the exhaust too quickly and hurts engine breathing. If one is going to drive the car on the street and the engine is stock, then a smaller tube after the split is a great idea. How much smaller is a guess. After the exhaust cools the size tends to not really matter much. Sonic pressure sine waves also become a factor too. Now I made my brain hurt. I have to stop now. Your results may vary.
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