Exhaust Systems 101?

I am interested in learning the basics of exhaust system design. Is the idea to vent the gasses as quickly as possible? Is the the most efficient engine one which has no pipes or cats or mufflers at all? What if an engine had the proper fuel/air input but was in a vacuum which would actually "pull" the
exhaust gasses out from the exhaust valves, would it be very much more efficient?

Since we need exhaust systems for safety and sound suppression what are we giving up as far as engine efficiency? What does back pressure from the exhaust system do to HP and Torque? Are different exhaust system designs just a trade-off between Hp/Torque at different RPM? I would really appreciate
answers from the knowledgeable people on this board. Any engineers out there? Thankyou in advance.

wmcnally

 

i will throw my 2 cents in, first off, running and engine with no manifolds or headers or anyything is extremely bad because as soon as you shut it off, cold air will rush in and hit the hot exhause valves causing them to warp and or crack, the object in an exhaust system is to have a full scavenging effect with no backpressure. Scavenging of exhaust gases is is when each pulse of exhaust pulls the next one along, think of each pulse of exhaust coming through each tub in a header and creating a vacume behind it which pulls the next exhaust gas pulse, thats the best i can explain it, oh the more built and tweaked the motor is, the more sensitive& responsive to exhaust mods it will be

1985 F-150/351Ho/c6/4wd~94,000 miles
1979 F-150extcab4x4/460/c6/6inchlift w/60rear44frnt (250 axles and springs)
37 inch boggers

 

The idea is to get the exhaust gasses out of the cylinder. Backpressure in the exhaust system will prevent complete clearing of the cylinder. In the worst case a poorly designed manifold will have pressure from one cylinder beginning its exhaust stroke backing up into another cylinder that is just finishing its exhaust stroke. Any back pressure will push against the crank, reducing power output, and will contaminate the intake charge with what is left in the cylinder, again reducing power output. Torque (and consequently horsepower) are strongly related to volumetric efficency, which is how well the cylinder is filled with fresh fuel-air mixture. Any residual exhaust gasses trapped in the cylinder reduce volumetric efficency. The later Ford 300 cid I6 engines with fuel injection have two exhaust manifolds so that you should never have one cylinder's exhaust contaminating anothers. A vaccum on the tailpipe likely would be a great help since normally the exhaust has to fight atmospheric pressure (about 15 pounds per sq inch?).

There is also a tuning effect of sound waves that cause pressure fluctuations in the pipe and manifold. Often someone will remove the exhaust system leaving only the header pipe and maybe the downpipe and find the truck loses power. Then the assumption is that some backpressure is needed to run right. But that is wrong. Backpressure never helps. What happened is they destroyed the tuning effect by removing the pipe. An engine with higher average backpressure, but properly tuned exhaust where there is a below atmospheric pressure condition at the end of the exhaust stroke, will perform better than an engine with lower average backpressure that happens to be poorly tuned in the exhaust so that the pressure is high at the end of the exhaust stroke, thereby causing residuals to remain in the cylinder.

There are other problems that can occur with radical race camshafts, like fuel air mix passing through to the exhaust manifold and vice versa because of the huge valve overlap. But these engines are meant to run at very high rpms and run like a motor with burnt valves at low speed, which is why they lope at idle speed.

Many folks put headers on their trucks. A website for four wheeling Broncos (http://performanceunlimited.com/) has a good discussion of exhaust (select Tech Info Library, Tech Info Docs, Exhaust Selection Chart and go to second page (http://performanceunlimited.com/documents/exhaustguide.html). These folks are one of the few that actually understand that backpressure never helps. In their writeup they say headers don't do much good below 4000 rpm.

The easiest (read cheapest) way to quiet an engine is to muffle it with backpressure, but there are systems designed for low backpressure and sound reduction. Flowmaster 70 series mufflers are supposed to be pretty quiet, yet fairly free flowing.

Also, a tuned exhaust tends to work best at a certain rpm range, as do tuned intakes. Generally the larger the pipe diameter, the higher rpm for the torque peak and the longer the pipe (within reason) the more torque below the peak, the shorter the pipe, the more torque above the peak. See lecture on exhaust systems at http://n2performance.com/index.html

The two sites I provided above seem to have it together on exhaust systems. Outside of that, there is a lot of misinformation out there on exhaust systems.

 

Get a copy of:
Smith, Philip H. and Morrison, John C., "Scientific Design of Intake and Exhaust Systems, 3rd Edition", Cambridge, MA: Robert Bentley Publishers, 1971. LOC#72-86569 ISBN 0-8376-0309-9.

It's more than a bit dry, but it has all the info you could ever want on the subject.

 

Excellent book Strange Ranger. My Ford-engineer neighbor loaned it to me and then I bought my own copy. Have read it three times except for the intake chapters at the end and the sound wave chapter. Tons of good information, graphs and charts etc.

 

Hey TallPaul, i looked up those sites you gave in your reply and it talks about having a siphoning effect on exhaust pipes to help scavenge exhaust. I was just wondering how those big tips effect that siphoning effect. Whether they're just for sound and looks or add performance? I thought about constructing a stainless steel impeller connercted to electrical motors in the pipes to help the air flow through, what do you think on that?

 

tx83Bronco: Since my last post I found more exhaust discussion at PerformanceUnlimited.com. From your message you may already have found these discussions, but here they are: Select "Tech Info Library," then click on "Illustrations and Real Life..." Then click on "Header Basics," where they say "the optimum exhaust would include a vacuum fan sucking out every last bit of burned fuel." Also can click on the "Flowmaster Muffler" to read about their vacuum effect. So yes a suction fan would be great, but there must be a reason they are not mass produced. Either, it would take a massive amount of power to produce enough suction to do the job (as opposed to a weak fan actually blocking your exhaust), or there are cheaper and more reliable ways to obtain exhaust performance. I suspect it is pretty hard to keep a fan going very long with the heat of the exhaust system blowing through it all the time. Also, if the fan is not powerful enough and fast enough it would become a hinderence fo flow. You may want to try Flowmaster mufflers, it their ad claims are true, they create a suction effect to help scavenge the system. As for exhaust tips, I don't know. I have heard all kinds of things like "turned down tips help scavenge by the partial vacuum created by the air rushing by" (sort of like the old road draft tube before PCV valves), that "smaller tips are quieter and larger louder," that "a big tip helps horsepower," that "a big tip does not help horsepower, but only looks good." I suspect the sound level is most likely the tip effect--that and looks. I do know that my F150 had about a 2-inch tail pipe with a 2.5 or larger tip on the last 6 or 8 inches. There may be something on this in the exhaust book referenced above. If I get a chance I will look. BTW the part of my discussion above about loss of power when the exhaust is removed, the perceived need for backpressure, and the actual problem of loss of tuning is drawn from my readings of that book. If you want the book, I believe is is only about $22 from I think Robert Bently Publishing which is on the net. I don't recall anything about "siphoning effect" in the sites I referenced. I have heard from several sources that for a stock 300 I6 engine, most of the exhaust imrpovement benefits can be realized by sticking a Flowmaster Muffler in the stock piping as the stock muffler is a major plug in the system. Let us know what you do and how it works.--Paul

 

Thanks for the info, the suction fan was just some idea i thought up in my sleep. Performance unlimited was where i heard about the "siphoning effect" in the exhaust selection guide. Thanks for posting those sites and your infor really cleared some things up. I'll check into that book and explore those sites some more. Thanks