why would you want backpressure in a turbo application? backpressure is a restriction. if the exhaust is restricted, this means it isn't flowing. when the exhaust isn't flowing, the turbo doesn't spin, as it relies on flowing exhaust.

back·pres·sure (bāk'prěsh'ər) Pronunciation Key
n. Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system.

why do we want to oppose the free flow of the exhaust???

 

because you need backpressure in a turbo(in FRONT of the turbo, not behind) setup to spool the turbo. There must be a pressure differential for the turbo to spool.

 

well, i'm not the smartest when it comes to turbos, but the exhaust is what 'spools' the turbo. the psi rating of turbos is nothing more than how much resistance there is in front of it ( the outlet side ). if there is a resistance behind the turbo ( the inlet side ) that's called a vacuum.

again, why would we want to oppose the free flow of any part of the engine?

this is one of those never ending debates.

old saying, if you can't get it out, you can't get it in.

 

Thats correct. Measuring the BP ahead of the turbo. Or what the engine actually "sees".
There should be no back pressure in a NA motor. Period.
I've seen 8 psi of BP kill 30 rwhp in a NA car (measuring with a BP gauge in an 02 bung). Drop the exhaust, and it pick up 30. Weld it back up and loose 30. Send the kid home to install his own exhaust, bring it back, dyno again, and pick up 30 with the new exhaust.

 

There will be significant BP AHEAD of a turbo. I've talked to some of the big name turbo guys and they have told me that there is sometimes higher BP than boost pressure. Never could grasp that. But, been told by more than one big name company. I'm definitely NOT a turbo expert, but thats never really made sense to me.

 

when the turbo is doing it's job, sure there's pressure inside the intake. because you are trying to compress more air into a given space. you are raising atmospheric pressure. you could call this backpressure because there is opposition from the valves that are closed restricting the free flow of the air. in reality, it's still raising the atmosheric pressure inside the intake above 29.92 Hg.

(just using numbers here)
you can have a motor with 5 psi make as much power as a motor that is running 15 psi. this is because the motor with 5 psi is less restrictive than the 15 psi motor.

boost pressure is just a way of showing how much faster the turbo is pushing air into the motor versus what the motor can take in/out. the lower the boost pressure, the more the motor is keeping up with what the turbo is pushing in at that given time.

of course, there are always those rules about this kinda stuff. but this is off the path the original poster started. he was refering to exhaust pressure and we got onto intake pressure.

 

I have a stock engine internally.....[see sig]...have Edelbrock LT'S ...had an ORY..had to go to emissions and installed 2 Magnaflow Cats right off the Headers...I KNOW...That I have more low end power now... ..[less noise though..o well].... [got a elec. cutout to cure that.[not as loud as before though]

 

^^^^You're readin me wrong. mrr23
Theres exhaust backpressure between the exhaust valve, and the turbo exhaust impeller. If you put a BP gauge ahead of the turbo, on the exhaust side, you will see a significant amount of BP.
Sorry to the OP for getting side tracked..........

 

because of the impeller blades and housing restricting the flow of the exhaust. i understand that. i don't know turbo sizes, but would the impeller housing match the size of the exhaust pipe? if the impeller housing is smaller, that would account for most of the backpressure.

 

my guess is the ORY was 3" piping. if so, then a 2.5" piping would have brought your low end up as well.

 

I think just the impeller being in the way, impeding the flow is the cause of the back pressure.

 

If you do have more power, its not from back pressure. You can screw a back pressure gauge into your upstream 02 bung, and in the lower RPMs it will have NO BACK PRESSURE. We make this check all the time. Under WOT, you will have very, very little BP, if your exhaust is any good at all.
The only way you'll end up with any amount of significant BP without a backed up exhaust, is if you have a big stroker or something of that sort with a stock exhaust.

 

What is the relationship between backpressure and velocity? For example, on headers with High Velocity Merge Collectors, the collector narrows down, which might be thought to create a restriction, and thus backpressure. Yet the HVMC's have been shown to create more power. Why?

 

I guess what I said got blown up and misunderstood.

If your car has no backpressure driving around normal and backpressure when hotrodding it around then it is properly designed for normal driving and not loud as hell/ticket worthy.

That is all I was orginally trying to say.

 

it's rocket science, literally. the HVMC is designed just like the thruster on rockets. the necking down accelerates the exhaust gasses.

 

again, backpressure is not needed. not in normal driving or hotrodding it.

a properly designed exhaust system will have very little backpressure throughout the entire rpm range, without being too large to make the low speed exhaust gasses move too slow.

when someone puts too large of an exhaust on their car, it slows the low rpm exhaust pulses. this is where the magical 'needs more backpressure' myth comes into play.

another thing that slows exhaust down is cold pipes. with the pipes being cold, the heat starts exchanging into the pipe thus slowing the pulse down. with the pipe being at least the same temp as the exhaust, the heat has nowhere to go. this keeps the velocity up, until it gets to a lower temp spot and/or exits the system. this is where ceramic coating the exhaust can make a little more power.

 

No, I understand that, but my question is, does the restriction in the neck-down create backpressure? Why or why not?

 

I agree with the above post. Well said.

 

I can understand that it is not needed. I get that it isn't differential pressure with the outside atmosphere that is pushing the air out.

So. In summary, if you have no backpressure at any point than the system could be perfect or overdone. If you have backpressure than the system is too small.

From my frame of mind, building at mass manufacture, I was thinking why overdo it. If the car has a little backpressure at peak power and no where else then I didn't waste money. Yes, on my personal car I put a free flowing exhaust. I was just thinking about money.

Thanks guys. See what you are saying. I like to play around with my car but I will never live up to the drag racing guys I personally know.


By the way. Can anyone confirm that exhuast restrictions and the cam's specs were used to replace EGR?

 

no, it creates an acceleration of the gasses with the neck down. with it suddenly opening back up, it creates a scavenging effect. what scavenging does is speed up the exhaust pulse and create a vacuum behind it. this vacuum helps to pull the next exhaust pulse coming out and speed it up as well. and so on and so on.

if it stayed necked down, this would create a restriction because of the distance needing to travel.