how to determine backpressure for exhaust system

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alfonso
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how to determine backpressure for exhaust system

Post by alfonso »

i have a 1.6 liter 78 chevette scooter and i need a muffler for it
with open pipe it loses all the bottom end tq
would like to dial in a glasspack length due to clearance
it has a 2" od pipe and how far from collector would you put it for optimum backpressure?
if from now on you blame anything on anybody
it might be the alien spores from the meteors...
http://www.wallaceracing.com/Calculators.htm
justahoby
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Re: how to determine backpressure for exhaust system

Post by justahoby »

You want back pressure? :lol:
Is this a stock cam deal? You may want to advance the cam 8degrees somehow.... Build cylinder pressure that way..
Them engineers in the smog era did a lot with the cam seriously retarded
As I'm approaching 40,I still think I'm 20. What the hell is wrong with me?
induction apprentice
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Re: how to determine backpressure for exhaust system

Post by induction apprentice »

It is almost an urban legend that an engine needs back pressure. I am sure there is some freak situation where it helps. But that is one in a million. If you lowered back pressure and it noticeably lost torque. It probably needs to be re tuned around the better breathing engine. It is more likely it went lean giving you the sense that it lost torque.
alfonso
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Re: how to determine backpressure for exhaust system

Post by alfonso »

car is stock cam and want to buy the right muffler to get the best response for it
and was wondering of any tips for exhaust system choices
and yes she was a dog from idle but i decided since i was not going open exhaust not to fiddle with timing till muffler installed
so the question is how long of a tube length for a glasspack will work?
do not want that coffee can racer sound
but to pick up some minimal hp
if from now on you blame anything on anybody
it might be the alien spores from the meteors...
http://www.wallaceracing.com/Calculators.htm
adam728
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Re: how to determine backpressure for exhaust system

Post by adam728 »

Why not wait on the exhaust until you get your other mods underway. Weren't you going to cam this engine and run individual carbs?

Being that there something in the current setup is compromised enough that less backpressure hurts I'd say any exhaust tuning theory on here gets thrown out the window.
alfonso
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Re: how to determine backpressure for exhaust system

Post by alfonso »

my daily driver blew some rings and this only thing that is driveable
so intake manifold went back on tuned it and my gravel driveway ate the muffler
so i am between rock and hard place for moment.and the exhaust took priority
if from now on you blame anything on anybody
it might be the alien spores from the meteors...
http://www.wallaceracing.com/Calculators.htm
Barry Burch
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Re: how to determine backpressure for exhaust system

Post by Barry Burch »

Without the muffler backpressure did not down it went up.
Atmospheric pressure is 14.7 psi at sea level standard.
What happened is reverse flow at overlap is leaning out the engine
You would have seen that with a tap at an exhaust port.
Dan Timberlake
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Re: how to determine backpressure for exhaust system

Post by Dan Timberlake »

I never played with a muffler/no muffler setup, but would think extra wave activity at low rpm might cause double or triple carburetion resulting in big richness. Driving around doing repeated heavy throttle from low rpm test runs with no muffler and the hood and air cleaner top off watching for reversion impresses the babes and REALLY impresses the cops. An air fuel ratio meter might help identify what's going on too.
alfonso
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Re: how to determine backpressure for exhaust system

Post by alfonso »

sometimes is as easy as finding that the timing belt is off a tooth
hopefully that will remedy situation
so as soon as the weather breaks again one more time into the breech
if from now on you blame anything on anybody
it might be the alien spores from the meteors...
http://www.wallaceracing.com/Calculators.htm
peejay
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Re: how to determine backpressure for exhaust system

Post by peejay »

I've usually found that "lost bottom end" means the engine now makes a lot more noise than it used to, for the same amount of go, so it feels slower.

The corollary is that quiet cars always feel a lot faster than they really are.
induction apprentice
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Re: how to determine backpressure for exhaust system

Post by induction apprentice »

peejay wrote:I've usually found that "lost bottom end" means the engine now makes a lot more noise than it used to, for the same amount of go, so it feels slower.

The corollary is that quiet cars always feel a lot faster than they really are.



There is a lot of truth to that idea. Exhaust aside. Sound energy is "Energy". If you cam an engine and it gets louder at WOT. It in some cases can be an indication of less power as there is less available to become mechanical energy. I always related to that idea with how quiet a Pro stock idles. But That does not really apply in the case of the muffler has been removed. I would say 99.9 percent of the time. Less back pressure and a good tune-up is a better deal.
Dan Timberlake
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Re: how to determine backpressure for exhaust system

Post by Dan Timberlake »

noise vs perceived power.
A young friend had a peppy small bore 2 stroke dirt bike with a "silencer" made from a VW muffler tip. He was a pretty good and only moderately showy rider. He enjoyed ripping around the track with varied terrain we had in the yard.
I had a bunch of different silencers available in the 70s, and talked him into trying a particularly effective one by Skyway with a silicon rubber flap blocking the end. A few blasts up and down the driveway and he reported back that the bike was now pretty slow. But When he headed out on the track he lost the rear end and lowsided for the first several corners, maybe even for the first few laps. I don't think that traction was lost because the bike was MORE powerful with the better silencer, although I suppose if he was cornering with max throttle it MAY have been. I figured he was adding throttle to get the sound he was used to, so was applying twice as much throttle as he really wanted.
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Re: how to determine backpressure for exhaust system

Post by pdq67 »

Back years ago, I installed a little-bitty glass pack on my 1987 Chevy Sprint ER 1000 cc 3-banger el-strippo 5-speed 3-dr hatch car.

Had to put it way at the rear and it only had a 6" tailpipe on it.

Well the little bugger was only 48hp and I think that I gained 2 to MAYBE 3hp, but after a while I took it off because the DRONE liked to wore me slick.

pdq67
alfonso
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Re: how to determine backpressure for exhaust system

Post by alfonso »

i am going to use a insert from a harley drag pipe to tone down the exhaust
on the glass pack tail pipe and see what it does.
the car has a hell of a mid range power change
45-75 mph
the gearing 4.11 has a lot to do with it ,but it does feel quicker
if from now on you blame anything on anybody
it might be the alien spores from the meteors...
http://www.wallaceracing.com/Calculators.htm
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