Backpressure
Backpressure Hey guys, whats the real word on backpressure. I've heard it is a myth, i've heard not enough backpressure will blow up your motor. I've heard so much BS and gotten in so many debates that i wanna know the real technical factual information, or atleast a breakdown of backpressure and what role it really serves...
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From: The South
You don't want backpressure.
What you want is exhaust gas velocity. The larger your exhaust is, the more air it holds, the more mass there is to get moving. Thus it tends to soften up the buttom end just a wee bit. However, once you get all that mass moving you now have momentum on your side, and lower restriction so the top end is good.
Something like that is what I have read and it makes perfect sense.
What you want is exhaust gas velocity. The larger your exhaust is, the more air it holds, the more mass there is to get moving. Thus it tends to soften up the buttom end just a wee bit. However, once you get all that mass moving you now have momentum on your side, and lower restriction so the top end is good.
Something like that is what I have read and it makes perfect sense.
Originally Posted by blkZ28spt
You don't want backpressure.
What you want is exhaust gas velocity.
What you want is exhaust gas velocity.
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From: The South
What I have gathered is that it's complicated. 
There is such a thing as "too little backpressure" or too large of an exhaust. If you have a very low stall speed converter (like stock) or a manual with regular gears and a larger exhaust than your engine needs to breathe you will lose a little much down low and could hurt your overal ET. If you are like me and run a 4k stall converter straight up open headers would be fine. It's all about, once again, your setup/combination and making everything work in concert.
At least this is my take on it. I'm sure others will chime in.
There is such a thing as "too little backpressure" or too large of an exhaust. If you have a very low stall speed converter (like stock) or a manual with regular gears and a larger exhaust than your engine needs to breathe you will lose a little much down low and could hurt your overal ET. If you are like me and run a 4k stall converter straight up open headers would be fine. It's all about, once again, your setup/combination and making everything work in concert.
At least this is my take on it. I'm sure others will chime in.
i figure exhaust gas velocity has a direct correlation with backpressure i.e. more backpressure = more exhaust velocity but also increases restriction, but less backpressure = less exhaust gas velocity but decreases restriction which i guess would help wxhaust velocity at higher RPMs, if thats the case, backpressure would help out at low RPMs, but doesnt really help much in either case.
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Verdad Gallardo 11 Second Club
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From: The South
Originally Posted by spy2520
is it possible to blow a motor up if you have no backpressure at all?? i dont think so, but i want the real story.
The only possible problem with a very short/open exhaust is that cold outside air gets up into the exhaust port and contacts a very hot valve, possible causing damage. Even open headers shouldn't allow this because the air will be warmed by the exhaust before it gets up that far. If you ran no exhaust at all you might have problems but it wouldn't be "back pressure" related.
You want absolutely 0 backpressure, none, ziltch, nada, period!!! You do, however, want velocity. You have to find the right setup for your combination. You're not going to blow your engine or burn a valve or any of those old wives tales you hear about either. Seriously, think about it. Why would you ever want a restriction in your exhaust system making your engine work harder to push out the spent exhaust gasses?
hey guys i believe you, i'm just trying to show some "other people" that i didnt just make **** up. i ran open headers for a few weeks as well, it was fun while it lasted.
Has anyone read the mod's. sticky in this section (third one down) relating to intake & exhaust modifications? Basically, it says that the exhaust flow we're looking for creates 28 inches of water back pressure. So, if the parts on your exhaust are sized to create 28 inches of water back pressure based on the max. air flowing through the part, you're set-up will be optimized. Further it says that it is possible to have too big of an exhaust.
Nothing's gonna blow up. HP & torque are lost as a result of too much or too little pressure drop according to the information.
Regarding cold air intakes, Install University article says that an intake flows best when the parts are sized for 10 inches of water or less of back pressure. Don't recall what the sticky says about it, but, have made good power following both of these intake & exhaust recommendations. To clarify; this is related to naturally aspirated engines
Nothing's gonna blow up. HP & torque are lost as a result of too much or too little pressure drop according to the information.
Regarding cold air intakes, Install University article says that an intake flows best when the parts are sized for 10 inches of water or less of back pressure. Don't recall what the sticky says about it, but, have made good power following both of these intake & exhaust recommendations. To clarify; this is related to naturally aspirated engines
Last edited by LS1-450; Oct 11, 2005 at 10:27 PM. Reason: added naturally aspirated comment
Originally Posted by LS1-450
Has anyone read the mod's. sticky in this section (third one down) relating to intake & exhaust modifications? Basically, it says that the exhaust flow we're looking for creates 28 inches of water back pressure. So, if the parts on your exhaust are sized to create 28 inches of water back pressure based on the max. air flowing through the part, you're set-up will be optimized. Further it says that it is possible to have too big of an exhaust.
Nothing's gonna blow up. HP & torque are lost as a result of too much or too little pressure drop according to the information.
Regarding cold air intakes, Install University article says that an intake flows best when the parts are sized for 10 inches of water or less of back pressure. Don't recall what the sticky says about it, but, have made good power following both of these intake & exhaust recommendations. To clarify; this is related to naturally aspirated engines
Nothing's gonna blow up. HP & torque are lost as a result of too much or too little pressure drop according to the information.
Regarding cold air intakes, Install University article says that an intake flows best when the parts are sized for 10 inches of water or less of back pressure. Don't recall what the sticky says about it, but, have made good power following both of these intake & exhaust recommendations. To clarify; this is related to naturally aspirated engines
I think you are confused. 28 inches is just the standard for measuring flow on a flowbench. 11 Second Club
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From: The South
1. An X pipe will help through exhaust gas scavenging. Scavenging is when the exhaust gases flowing out of one cylinder help pull the exhaust out of the following cylinder when it's exhasut valve opens. With an equal length exhasut and a crossover (X pipe little better than H) your engine can utilize the flow from both banks so you get a nice increase in scavenging and thus power. Usually shows up in the low-mid range, aka power under the curve. Not at peak.
2) When you measure pressure in inches it's just pressure, not backpressure. It's inches of mercury. Same thing as measuring in inches Hg, Torr, atmospheres, pascals, etc. Although now that I say that I am only 95% certain...
2) When you measure pressure in inches it's just pressure, not backpressure. It's inches of mercury. Same thing as measuring in inches Hg, Torr, atmospheres, pascals, etc. Although now that I say that I am only 95% certain...
Originally Posted by LS1-450
So, if the parts on your exhaust are sized to create 28 inches of water back pressure based on the max. air flowing through the part, you're set-up will be optimized.
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From: The South
Originally Posted by 01 LS1 SS
I donno if i'm off topic here, but what role do cut-outs play w/ backpressure and all. Are they good or bad for the car?
Treat cutouts as less backpressure/higher velocity. Might lose a little down low but gain more up top. Generally speaking.