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Now, this is an interesting thread! What would happen if you tee'd in a turbo wastegate just before the main cat and set it to open at whatever the psi is occuring @ 4-5k?  ...Ed
2000 Silver Frost SVT
#1625 14 Jan 2000
Dual Mode Dampener
A/C switched
K&N Dropin
Magnecor
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Simple dyno test-run with stock exhaust in place, then drop it at Y/cat junction and re-dyno. Care to add anything Tom?
Technical Director/Co-Owner Performance Fords-check out our new throttle body service
95 SE with lots of custom 1 off mods. All design, fabrication and installation by owner.
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So, Terry. What's your view on this subject that you stir up? I'm interested to know what you say more than anyone else because obviously you took part in designing our car.
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Terry? Wanna share your thoughts?
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VOLUMETERIC EFFICIENCY: If you've forgotten is the volume of the intake charge-inhaled during the intake stroke vs. the actual displacement of the cylinder. Excess backpressure causes what is known as reversion. Reversion is when hot exhaust gas gets pumped backwards into the engine during the overlap period. This can cause the engine to get excessivly hot as cross flow of the cool intake charge-during overlap is one of the ways the engine cools itself internally. This can cause detonation if the backpressure is to much.
Something to think about.
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On a Sunday morning on TNN was a show about exhaust. They were talking about the more you free up the rate of flow the more horsepower you are going to gain. They were talking about This is why race cars use a straight through exhaust. Can't remember the name of the show though. I have heard from many sources though that you do need the backpressure to keep the torque. I have not dynoed my car to prove this, but i did take off the exhaust system to see with so much free flow would actually do. I did see a noticable difference in low end torque loss but I did feel an increase in top end horsepower. I dont have any proof though, but I thought I would share what I have noticed and heard about in this area.
Sam
1998 Contour SVT #180
1/4=14.2 @ 101.17MPH
AOL users = PhatSVT
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1986 Mustang GT
12 second club
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backpressure is bad all it does is restrict flow which ultimately reduces power.
The proper term is scavenging
The idea is to have the exhaust gasses moving as fast as possible to make a low pressure in the pipe which helps to more efficently "suck" the gasses from the cylinders.
There is a trade off, a small pipe will yeild good power at a low RPM but will be restrictive at high RPM, and a large pipe will do the opposite.
Another aspect that is often overlooked in designing an exhaust system is that the noise actually helps move the gasses. The sound waves moving through the pipe actually help to keep the gasses moving. Why do you think race cars are so damn loud?
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lets not forget heat.
the hotter the gasses are the less dense they are and the less dense the easier they are to move.
thats why you see guys putting blankets on their headers.
also the reason why some of the Ford F- and E- series exhaust systems are heat shielded.
that does 2 things, 1 lowers the ammount of energy radiated to sourrounding objects by the hot pipe, and 2 keeps the pipes hot so the gasses stay hot so they flow better.
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Originally posted by JSmith:
lets not forget heat.
the hotter the gasses are the less dense they are and the less dense the easier they are to move.
So...can we call our cats "afterburners"?  Would that partially explain why a cat can actually improve performance as noted by certain dyno tests posted? ...Ed
2000 Silver Frost SVT
#1625 14 Jan 2000
Dual Mode Dampener
A/C switched
K&N Dropin
Magnecor
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Posts: 148
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Dont know with anyone here but I went with a custom y-pipe that is supposed to be more free flowing and went with a free flowing cat and can say that yes it does rev better on the top end but I've lost power below 4K. My 2-3 shift is now soft. 3rd does not pull the same in its lower ranges. I think if the car was a 3L then it would be fine but I think its probably just better if one sticks with the stock exhaust if they dont have a lot of mods. My only choice now is to do stuff that can get me some of the torque back.
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