egr pressure
Posting Guru
Joined: Jan 2006
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From: The dark carnival
I have never heard of a published spec for that, but some would not need any and others would need a couple pounds.....I would guess.
My motor had the EGR pipe connecting to the intake manifold and for this I would think that VERY little to none would be fine. Other set ups have the EGR pipe that goes into the exhaust pipe and for that you may need a pound or two. Just don't do long tubes w/ 3 inch or bigger true duals on a stock motor and you should be OK. If you do true duals y-pipe it into a single 2 1/2 or 3. Rule of thumb, make sure the exhaust is tuned properly to the motor.
My motor had the EGR pipe connecting to the intake manifold and for this I would think that VERY little to none would be fine. Other set ups have the EGR pipe that goes into the exhaust pipe and for that you may need a pound or two. Just don't do long tubes w/ 3 inch or bigger true duals on a stock motor and you should be OK. If you do true duals y-pipe it into a single 2 1/2 or 3. Rule of thumb, make sure the exhaust is tuned properly to the motor.
Elder User
Joined: Jun 2006
Posts: 805
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Umm, first of all, ANY back pressure, when the EGR opens, is gonna be getting dumped into a vacuum in the intake. At idle, the EGR should be closed, doesn't matter what kinda pressure in the exhaust then. At WOT, EGR is closed, so again, back-pressure doesn't matter. At mid throttle, or cruising speed, EGR will get modulated to provide the best bang for the buck on fuel and help keep the engine cool. So, even if you didn't have much back pressure, at mid throttle there will be some vacuum in the intake, making any back-pressure in the exhaust add on to that. As long as there isn't a vacuum in the exhaust caused by scavenging, especially if it isn't greater than the vacuum in the intake, it should work OK. The objective of the EGR is to pull in exhaust gas to displace fuel/air mix, and make the engine run more efficiently, and also helps keep it cooler by not having as much fuel/air mix to burn. Since the exhaust is already spent, it doesn't cause lean-miss or pinging problems when mixed with the intake stroke. Unless you're worried about exhaust pulses (again the scavenging) causing some sort of weird pulsing in your intake vacuum, all should go good. I don't think you'll ever get an exhaust large enough to produce that effect, though. As far as the EGR performance is concerned, I don't think you'll see much difference. Another effect of lowering the combustion temperature, is that it lowers the NOS emissions. With lower back pressure, you may have less exhaust recycled into the burn, meaning a slightly higher combustion temperature, and possibly issues with emissions. You mentioned a system with no cats, so obviously, emissions are not your top priority.
HOWEVER, what will happen, is you're probably going to lose mid-range to top-end torque pretty quick, and worse yet, I have heard of insufficient back-pressure causing major headaches with exhaust valves. In fact, I've experienced this with air-cooled engines. I ran four short open headers on my VW for a while. After two head jobs, I decided I didn't like them any more, despite it sounding like a whole pen full of pissed off "Hawgs" when idling, and like an uncapped small block V8 when engine braking. It is EXTREMELY loud with that setup, and runs like a scalded dog, until I burn the seat on an exhaust valve. Just a tiny change, with an extractor type header, and it still runs as if the water poured on the dog was 211 deg, and the exhaust valves last a lot longer. But, with that setup, the sound changes to that of a supercharged bumblebee.
If you want low-end power, shoot for lots of flow with little back-pressure. If you want torque more evenly distributed across the RPM range, well, gonna have to play with the restriction a little. NASCAR has a whole science devoted to exhaust design to maximize the engine performance curves based on what type of track they're running and what kind of abuse the engine's gonna get. On the super speedways, the engine's wound up tighter than a fiddle string for the entire race, so what difference does low-end grunt make? (Besides that, the engine runs out of intake long before it runs out of exhaust). On a short track or road course, now we need a strong bottom end, and less at top end because we're geared lower. Changing the shape/flow rate of the exhaust for each application is part of the setup of the car.
HOWEVER, what will happen, is you're probably going to lose mid-range to top-end torque pretty quick, and worse yet, I have heard of insufficient back-pressure causing major headaches with exhaust valves. In fact, I've experienced this with air-cooled engines. I ran four short open headers on my VW for a while. After two head jobs, I decided I didn't like them any more, despite it sounding like a whole pen full of pissed off "Hawgs" when idling, and like an uncapped small block V8 when engine braking. It is EXTREMELY loud with that setup, and runs like a scalded dog, until I burn the seat on an exhaust valve. Just a tiny change, with an extractor type header, and it still runs as if the water poured on the dog was 211 deg, and the exhaust valves last a lot longer. But, with that setup, the sound changes to that of a supercharged bumblebee.
If you want low-end power, shoot for lots of flow with little back-pressure. If you want torque more evenly distributed across the RPM range, well, gonna have to play with the restriction a little. NASCAR has a whole science devoted to exhaust design to maximize the engine performance curves based on what type of track they're running and what kind of abuse the engine's gonna get. On the super speedways, the engine's wound up tighter than a fiddle string for the entire race, so what difference does low-end grunt make? (Besides that, the engine runs out of intake long before it runs out of exhaust). On a short track or road course, now we need a strong bottom end, and less at top end because we're geared lower. Changing the shape/flow rate of the exhaust for each application is part of the setup of the car.
