Re: Muffler back pressure question

Posted by Steve - IN on June 24, 2003 at 21:39:03 from (12.223.205.149):

In Reply to: Muffler back pressure question posted by KX on June 24, 2003 at 20:12:33:

KX,
There are no hard and fast rules about how much -- if any --back pressure an engine needs, but like most things in engines, it's a series of trade-offs.

Most cams create some valve overlap. That is, the exhaust is just closing while the intake is opening. Creating a bit of a low pressure area on the exhaust side with less backpressure will help suck in more mixture on the intake side, making a bigger explosion next time around --BUT as RPM's increase and backpressure stays the same, there's less time to do that, so more of the burnt stuff stays around for the next explosion. If you reduce backpressure - that's the equivalent to increasing valve overlap, like a longer duration cam, making the engine happier at higher RPM's. If you increase backpressure, you've effectively decreased valve overlap and the engine is happier at lower RPM's.

Here's more trickery -- you can change effective backpressure by making the intake velocity greater or lesser. You've seen velocity stacks on carb intakes, or those block risers bolted between carbs and manifolds, and high rise manifolds -- that's what they do. In other words, the faster the air comes in, the faster it has to go out, or hit a wall created by backpressure. Of course, you can change your whole setup by getting the head ported to the nth degree; which requires you to rethink and re-do the intake and exhaust diameters and velocities. See, -- trade-offs.

The basic results of all the above are: More backpressure shifts the torque peak down lower in the RPM range and tilts the torque curve to the lower end. Less backpressure shifts the torque peak upward in RPM, tilts the torque curve upward in RPM like a seesaw, also can lessen peak torque, but adds horsepower at the higher RPM range. Mufflers are to reduce noise. If you really want to tune the exhaust, i.e. it's backpressure against a fixed volume and velocity of input on the intake side and head ports; the way to do it is by varying the diameter and the length of the header pipe tubes (not a cast manifold) and the length of pipe that's between the head and the juncture of the individual pipes - or no juncture at all; like zoomie pipes on dragsters for better low end torque. Punching holes in mufflers or straight pipes on a cast manifold makes small changes compared to what you can get by varying the diameter and length to juncture of tubular headers.

The practical bent of all the above is that if you're not racing, or pulling, but simply using the cast manifold on your tractor with a stock intake and head port job -- replacing the muffler with a straight pipe will slightly increase high RPM horsepower at the expense of low end torque. In my experience it's not pressure or the lack of it kills exhaust valves -- it's heat. You vary the heat by changing the mixture and timing. But that's a whole 'nother story.

Ask me all this in person and I'll say: "Huh, what"? That's because I spent many of my younger years setting max advance on a mid engine race car at 4500 to 5000 RPM at the flywheel -- which was real close to where the straight pipe exhaust dumped out. It was real "trick" to learn we could and should change the header to match the track (tight tracks - more torque, longer tracks - more HP), then change the gear ratios to match the changes in torque and HP peaks the header had shifted -- but never did we see an engine blow up 'cause of lack of back pressure. The big downside to less backpressure, more noise, is irritating people by saying "what?" all the time. All this may be more than you might have wanted to hear, but the important thing is to hear, so whatever you do; wear some hearing protection.

Follow Ups:

• Re: Re: Muffler back pressure question KX 08:31:26 06/25/03 (1)
  • Re: Re: Re: Muffler back pressure question Steve - IN 21:17:22 06/25/03 (0)

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