post #1 of Old 07-18-2011, 12:45 PM Thread Starter
child9
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Manifold design: Does boost mean you can get lazy?

So I've heard a lot of people say in various communities that when you turbo a vehicle and are building the manifold for it, things like the collector smoothness and runner lengths don't matter as much as they do for an NA vehicle.

This contradicts what I believe, but I was curious if anyone on here strongly agrees with that and why.

I think that runner length, smoothness of merges and collectors, and other factors all mean much much more when you start adding boost. For example, friction and drag caused by wind resistance increases as velocity squared. I think this also applies to the gasses in the manifold. If you are doubling the amount of air molecules being pushed through the engine by running 1 bar of boost, any disturbances in the flow of these gasses through the manifold will be exponentially increased...I suspect as velocity squared also. The density of the fluid has been doubled and I believe the relationship there to be exponential, not linear.

Does anyone know any different? Thoughts?


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post #2 of Old 07-18-2011, 02:14 PM
cruiser54
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You're in Texas. Contact Corky Bell at Bell Engineering. He knows his stuff.
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post #3 of Old 07-18-2011, 02:25 PM
MONSooNmonkey
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Quote:
Originally Posted by child9 View Post
So I've heard a lot of people say in various communities that when you turbo a vehicle and are building the manifold for it, things like the collector smoothness and runner lengths don't matter as much as they do for an NA vehicle.

This contradicts what I believe, but I was curious if anyone on here strongly agrees with that and why.

I think that runner length, smoothness of merges and collectors, and other factors all mean much much more when you start adding boost. For example, friction and drag caused by wind resistance increases as velocity squared. I think this also applies to the gasses in the manifold. If you are doubling the amount of air molecules being pushed through the engine by running 1 bar of boost, any disturbances in the flow of these gasses through the manifold will be exponentially increased...I suspect as velocity squared also. The density of the fluid has been doubled and I believe the relationship there to be exponential, not linear.

Does anyone know any different? Thoughts?
Pressure is pressure, the roughness makes turbulance that helps mix the air\fuel, which is why they went from making the smoothest intake possible to 'roughing' up the surface of the intake slightly.

Just like the surface of shark skin is rough and makes it possible to sharks move quickly.

Sadface :(
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post #4 of Old 07-18-2011, 02:40 PM
Boodyrider
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Actual experience with a modified Dodge Daytona at 26 psi boost on pump gas:

Manifold design on the intake side can be lazy, because boost trumps trying to pull air into the motor with vac. However... remember you're not always in boost with a turbo. Good design still matters for efficiency and VE during all those times you are cruising without being in boost.

Good design on the turbo and exhaust side though... that's critical. A rough rule of thumb was you'd see about 3 times as much backpressure on the upstream side of the turbo at a given boost pressure... meaning that the more you can do to get air past the turbo and out the exhaust, the better, since that backpressure in the cylinder is part of what causes excess EGTS, and means boosted vehicles hate cam overlap....

HTH

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post #5 of Old 07-18-2011, 07:41 PM Thread Starter
child9
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Yeah, I probably should have specified that I was actually referring to the exhaust side of things.

Speaking of the intake side, fuel injectors are usually pretty close to the intake valve openings, so I can't imagine that turbulence is helping much with breaking up the liquid molecules at that point. Doubly so for a direct-injection vehicle. It does seem like it could help matters somewhat in a TBI setup though, since the air/fuel mixture actually travels through the intake in that case (get a "Tornado", LOFL). I bet it may be difficult for the intake manifold to be limiting the overall flow through the entire system BEFORE the exhaust system would, given back pressure at the turbine, mufflers, cats and all that.

If you are trying to eek every last horse out of a specialized setup, at some point would it be safe to say that characteristics that are conducive to velocity and smooth, laminar flow will outweigh any benefits that a rough wall may give to the a/f mixture? I mean, at it's lowest common denominator, the power an IC engine can produce is based on how much air it can pump through it. Meh, maybe I just answered my own question.

"We exist to do great things." -CT Kassem
. '96 Jeep Cherokee XJ "R2" .
. Bring on the apocalypse .
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post #6 of Old 07-18-2011, 07:55 PM
flatlander757
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Quote:
Originally Posted by Boodyrider View Post
Actual experience with a modified Dodge Daytona at 26 psi boost on pump gas:

Manifold design on the intake side can be lazy, because boost trumps trying to pull air into the motor with vac. However... remember you're not always in boost with a turbo. Good design still matters for efficiency and VE during all those times you are cruising without being in boost.

Good design on the turbo and exhaust side though... that's critical. A rough rule of thumb was you'd see about 3 times as much backpressure on the upstream side of the turbo at a given boost pressure... meaning that the more you can do to get air past the turbo and out the exhaust, the better, since that backpressure in the cylinder is part of what causes excess EGTS, and means boosted vehicles hate cam overlap....

HTH
Ha! You were into the Turbo Dodges too I see!

I was going to mention those... they're the most horribly designed motors for boost, the solution that most have is to just throw more at it and deal with a head gasket popping every so often... it takes all of 40 min to replace

OP: Check out www.thedodgegarage.com and look at the Reliant, the Caravan(yes turbo Caravan), and the Daytona in particular... that is some old school turbo car building and stuff like aftermarket intake manifolds simply don't exist or didn't exist at the time... and the stock ones are pretty bad.... yet with the right turbo and some slicks they can put a FWD car in the 10s...




Also, click on "turbo database" on the left, then click on "intake manifolds" and stuff... that is the exact info you're after there... restrictive intake manifolds and how to modify them and then compensate with boost.

SOLDThe lumbering steel-laden pig - 2003 TJ - 40" LTBs - D60/D70HD - 5.86s - Detroit lockers - 110" wheelbase
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