Plastic Manifold and Forced Induction

FastDEW · 11-01-2009, 01:32 AM

Hi all,

I am wondering about all those running either turbos or centrifugal superchargers (Paxton or Vortex) if there is a problem with splitting the plastic intake manifolds on the 4.6? It seems that the manifold would not be designed for the pressure of forced induction. Has this been a problem for anyone? Does anyone know how much boost the manifold will take and for how long?

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mac10chap · 11-01-2009, 01:53 AM

i'm running 10lbs of boost through my procharger and i have no problems. in fact, i cant remember hearing of anybody splitting their intake manifolds under boost.

bobby1110 · 11-01-2009, 10:33 AM

i make 18psi and about to go to 25-30psi no problem here

candymanjl · 11-01-2009, 11:33 AM

Quote:

Originally Posted by mac10chap

i'm running 10lbs of boost through my procharger and i have no problems. in fact, i cant remember hearing of anybody splitting their intake manifolds under boost.

I second that, I've never heard of anyone breaking them on boost pressure alone. I have however seen a few videos of nitrous backfires disintegrating plastic manifolds on mustangs and other cars. they just disappear, it's crazy! pretty funny when it's not your car

FastDEW · 11-01-2009, 08:02 PM

Thanks for the replies. This makes me feel more confident. I am thinking about the Paxton setup when my car arrives but I was worried about pressure on the plastic manifold.

Norm Peterson · 11-02-2009, 08:18 AM

Quote:

Originally Posted by FastDEW

It seems that the manifold would not be designed for the pressure of forced induction. Has this been a problem for anyone? Does anyone know how much boost the manifold will take and for how long?

It already has to be able to withstand having somewhat more than 20" Hg vacuum, possibly 30", just to withstand sudden closed-throttle conditions as NA. That tells you that it's going to be good for 10 - 15 psi boost for at least as long as the warranteed life of the car/engine without investigating any further. As far as the magnitude of the material stresses is concerned, it's the difference from atmospheric pressure that matters. IOW, one atmosphere of boost is mostly the same as one atmosphere of manifold vacuum, just that boost gives mostly tension stresses instead of compression and any bending bends the other way.

Norm

Z06 Killer · 11-07-2009, 11:51 PM

14 psi for over 40,000 miles now and no manifold issues.

FastDEW · 11-08-2009, 12:34 AM

Quote:

Originally Posted by Norm Peterson

It already has to be able to withstand having somewhat more than 20" Hg vacuum, possibly 30", just to withstand sudden closed-throttle conditions as NA. That tells you that it's going to be good for 10 - 15 psi boost for at least as long as the warranteed life of the car/engine without investigating any further. As far as the magnitude of the material stresses is concerned, it's the difference from atmospheric pressure that matters. IOW, one atmosphere of boost is mostly the same as one atmosphere of manifold vacuum, just that boost gives mostly tension stresses instead of compression and any bending bends the other way.

Norm

Hi Norm,

I sort of agree with you and disagree with you. As we both know (it sounds like you are an engineer as well - I am an ME) the compression and tension stresses are not really equal for equal depending upon the part. However your point is valid to some unknown pressure and is understood.

I am glad that the manifolds are holding up so well. Ford (and others) had problems early on in the plastic manifold era, of cracking under normal operating conditions. There was a class action on the early Ford 4.6 manifolds in cars over manifold collant leak issues as well. I want to run a Paxton setup and was a little concerned about the pressure.

Thanks for all of the input from everyone.

forensicsteve · 11-08-2009, 08:14 PM

22 lbs of boost, all last season and up to 28 lbs one time on the dyno....did not crack. The problem with the stocker is that it is not designed for a mixture...like gas + methanol. We found out, that it did not evenly distribute the mixture and some cylinders ran rich while others lean...while all the time showing a very safe a/f.

We went with a custom intake manifold that promised a design that addressed this issue. At the same time, if the IATs behave, just as soon not have to play with methanol at all.

Norm Peterson · 11-09-2009, 07:33 AM

Quote:

Originally Posted by FastDEW

I am glad that the manifolds are holding up so well. Ford (and others) had problems early on in the plastic manifold era, of cracking under normal operating conditions. There was a class action on the early Ford 4.6 manifolds in cars over manifold collant leak issues as well. I want to run a Paxton setup and was a little concerned about the pressure.

Up to today, I wasn't aware of an unusual number of difficulties with the plastic manifolds, or what years might have been involved (I've had no problems with the manifold on my wife's 2001 Maxima, and I believe that it's a plastic piece).

I suspect that other issues were involved (transient thermal expansion conditions during warm-up?). The pressure stress term would only be a hundred psi or two at most, but relative thermal displacement stresses could get considerably higher. A little research turns up information that leads me to believe that the 4.6's problem happened with the iron block, and was more or less solved by the time the aluminum 4.6 was produced.

Just a side thought - I'm wondering if perhaps this problem is behind the aluminum 4.6 being such a notoriously cold-blooded beast. It's by far the slowest to reach operating temperature of any engine I can remember.

Norm
(Civil/Structural guy working in pipe stress)