exhaust

[willowfarm]

There is a point at which larger exhaust is doing no good and also the point to where too restrictive is detrimental to performance. In my experience with the Porsche 911 the free flow exhausts built for performance generally cannot be improved on except for sound. Also in my experience with the internal combustion engine in general the exhaust needs to be balanced with the valve train duration and lift and also the intake type and amount. NA motors are going to run differently than injected since they seem to dump more raw fuel into the motor and don't atomize as well as injected, also NAmotors are in my opinion more inclined to load up, not breathe as well and require a tad more tinkering to run well. Any motor if run too lean is going to be proned to burn exhaust valves with todays fuel of low lead types. the ideal situation would be to put a restricted pipe on with a given length and gradually open it up to the best size and over some time and find out where it runs best and make notes. I think you will find they will run just fine with a straight pipe of say 1 7/8 or 2" but the local cops and neighbors are going to notice. I have also done lots of vw air cooled and I have to laugh at these guys that put dual carbs on with tuned exhaust and make no other changes. The most HP from a flat 4 vw with stock pistons and valve train is from the stock 34 pict carb with tuned exhaust, and that is dyno HP. duals look great and do nothing but dump more gas.

[drddan]

^ What is an NA motor?

[GenevaDirt]

^ What is an NA motor?

Normally Aspirated, (carbed)

[Pu241]

^^^ Uh, not boosted!
EFI and Carbies are both NA unless boosted via turbo or supercharger.

[perucho]

NA motors are going to run differently than injected since they seem to dump more raw fuel into the motor and don't atomize as well as injected...

Sorry, but unless it has forced induction, it is a naturally aspirated (NA) engine. That goes for carburetted and fuel injected.

[perucho]

Gotta put in my 2 cents! I have the stock 1.5 inch exhaust on my festy.
Here is an interesting post on what I have experienced as of late:

My muffler just blew out and guess what? With muffler blown out my festy suddenly got more HP and more gas mileage! I posted about my gas mileage run a week ago and didnt know it at the time, but muffler had already rusted and was leaking, didnt make more noise til about 1-2 days later. Well it was flyin up hiils, breaking tires loose on takeoff, and gas was 46 mpg, which i've never gotten before. My gas post states that this was due to all the work/components I put on, but now... I wonder...
This weekend I put on new muffler and gee... now it wont hardly climb a hill, lost most of its power, and generally is slow. Feels like i've lost 20 HP.
This I do know: engines need to breathe! In and Out. Obviously something is restricting about the festy exhaust system and maybe timing needs to be corrected to account for the different size exhaust's/different flows of different mufflers.
I know I WILL be changing to a bigger, less restrictive exhaust soon.
After thought: I still have the cat on there, and maybe its gettin plugged up, but the muffler blowing out shouldnt have had an effect if the cat is plugged up? Right? I'll be doing more research on whats happening with the exhaust.

Well, here are my 2 cents. First, I'm not an expert in fluid mechanics; however, I've taken several classes in physics, fluid mechanics, and thermodynamics when I was in college. The flow in your exhaust pipe goes at a certain speed depending on the engine's rpms. When it enters the muffler, the cross sectional area increases dramatically very abruptly. This causes two things, it slows down the flow (which cools down exhaust fumes and further slows them) and allows the flow to follow the muffler walls. When this happens it creates a low pressure side and atmospheric pressure builds up against the incoming gases at the entrance of the muffler. It is basically the same principle of airplane wings and lift (when air flows faster over the wing it creates a low pressure situation and atmospheric pressure "lifts" the wing). Same thing with main and head sails on a sailboat.
Anyway, the same principle applies to large exhaust pipes. When the smaller primaries collect on a poorly-designed very large exhaust pipe, you will have the same issues (flow will slow down and cool, and the low pressure scenario will greatly restrict flow.
Try something fun: grab a funnel and a ping-pong ball. Blow into the funnel's small end and place the ping-pong ball in the large opening. You won't be able to blow the ping-pong ball off the funnel, even if you blow very hard. Atmospheric pressure is pushing on the ball when you blow. You can even blow with the large opening facing down and the ball will stay in place until you stop blowing!

[drddan]

^


Ok, i'm goin' to have to try that!

[drddan]

Well in my case, right now its muffler on= less hp+ less mileage
and muffler off= more hp+ more mileage.
So I would like to have less restriction and a quiet exhaust.

Anybody know of a less restrictive muffler, that is quiet as a stock muffler?
Right now i'd like to just get a muffler that is better, until I change to new pipe. The cat will have to go and either 1 3/4" or 2" exhaust with a good flowing muffler is the plan. I cant imagine using the 1 1/2" pipe that is on there again. Unless, of course, someone can show me that 1 1/2" pipe with less a restrictve miffler is the way to go.

[hofficergrover]

i didn't get a pic of the newer or the old but it allot better then the old has no leaks. i used the 2" just because it's all i had so i will deal with the loss of power and millage.