Backpressure and the Engine

[habitatguy187]

I searched and couldn't find what I was looking for so here goes.

I've been learning a lot about the engine and valvetrain and such lately, and I have a question about backpressure. Basically what I want to know is....

1. How much backpressure our stock motors can handle?

2. What parts get hurt/how do they get hurt if we run too much backpressure?

3. I've learned the simple stuff about backpressure so please don't start talking about exhaust and low-end torque and all that, that's for other threads. This thread is just for how it affects the engine and it's parts.

 

[romeen]

I searched and couldn't find what I was looking for so here goes.

I've been learning a lot about the engine and valvetrain and such lately, and I have a question about backpressure. Basically what I want to know is....

1. How much backpressure our stock motors can handle?

2. What parts get hurt/how do they get hurt if we run too much backpressure?

3. I've learned the simple stuff about backpressure so please don't start talking about exhaust and low-end torque and all that, that's for other threads. This thread is just for how it affects the engine and it's parts.

If you search I'm sure this discussion has been covered. But the brief answers to your questions are:

1) I don't know if this can be quantified since it depends on several variables.

2) That's a loaded question. NO amount of back pressure is desirable. Read this for a better understanding:

Turbo Exhaust Theory

In addition to negatively affecting turbo performance, increased back pressure will also increase heat in the cylinder head making the motor more susceptible to knock.

It can also cause reversion of the exhaust gases. Remember, to make power you want to evacuate as much exhaust gas out of the combustion chamber as possible and draw in a maximal amount of fresh air.

I'm sure there is a lot more that can be said but you get the idea and I am no expert.

 

[knochgoon24]

Are you talking along the lines of back pressure from a potato in the tail-pipe? Back pressure is created from the resistance to flow, kinda like a big dookie clogging your toilet. Too much back pressure and it stops flow. (And the car runs like dookie.)

 

[bpestilence]

I see some confusion here.

"backpressure" as the engine is concerned is the same as the pressure on the high side of the turbine. If you had no backpressure your turbo wouldn't have any power to extract.

In an N/A engine, some backpressure is necessary. Power is lost by running too free-flowing of an exhaust due to the loss of cylinder scavenging. While it is not the actual backpressure that causes this effect, the backpressure is an unavoidable consequence of developing the necessary exhaust velocities.

In a turbocharged motor, scavenging is handled by the exhaust manifold entirely, and you want as large of a pressure drop across the turbo as possible. Power extraced is proportional to this value. So as free-flowing a exhaust as possible is your goal, with a simple flared cone being the ideal exhaust.

Backpressure will not damage your engine. You will, however, see that it takes longer to make boost due to a smaller pressure drop across the turbine.

 

[habitatguy187]

In addition to negatively affecting turbo performance, increased back pressure will also increase heat in the cylinder head making the motor more susceptible to knock.

That's something good to know, how often does this happen?

It can also cause reversion of the exhaust gases. Remember, to make power you want to evacuate as much exhaust gas out of the combustion chamber as possible and draw in a maximal amount of fresh air.

Are you saying that back pressure can prevent new air from entering the combustion chamber? How common is this?

Sorry if this was a newb question guys it's just stuff that I've never thought/read about until now.

 

[romeen]

That's something good to know, how often does this happen?

Hopefully, it doesn't happen at all.

Are you saying that back pressure can prevent new air from entering the combustion chamber? How common is this?

Yes. It can affect the scavenging of the exhaust gases out of the combustion chamber. In addition to back pressure a number of variables affect this including but not limited to the amount of cam overlap. I can't say how common it is because each set up is different. But I think you get the general idea. You want to evacuate as much exhaust gas out of the combustion chamber during each exhaust stroke and pull in as much fresh air as possible during the intake stroke.

Search around on the topic. There is some good discussions about it with input from members who are more knowledgeable than myself on it.

 

[pboglio]

Backpressure creates more cylinder heat, more chance of knock, and higher pumping losses. If your car is prone to overheat, backpressure will heatload the engine more. If your car is near its knock limit, it'll knock more possibly causing engine structural damage. If your car is at WOT, it'll rob the car of more high rpm engine power. It's not rocket science.

No one can put a backpressure "psi" threshold limit cause of the difference(s) in cam specifications, intercooler(s), turbine wheels, turbine housings, turboback exhaust construction, & individual car's knock resistance.

Backpressure is a necessary evil for a turbo car. In fact, turbo diesels take this to the extreme and actually use exhaust restriction to physically slow the engine down (i.e. exhaust brake). Not saying a gas engine could handle this but it is interesting to note.

From a practical standpoint, improving your engines cooling system capacity and running higher octane gasoline or other knock resistant fuel takes alot of the risks away from running higher backpressure. What's left is the higher pumping losses.

 

[4pistons]

Backpressure is NEVER desired on any internal combustion engine, whether turbocharged, supercharged, N/A, Injected, Carbureted, gasoline, diesel, E85, whatever.

The power and efficiency of an internal combustion engine is determined by the volume of air and fuel you can efficiently get through it. ANY backpressure slows that down. ANY engine that has ANY backpressure can be more efficient with none.

Rick - '91 GSX