Patrick L

I search the forum, seems most people suggest plastic manifold and later throttle are better. My question is, is it worth to spend $300~$400 for a used set (manifold and throttle) to replace my aluminium manifold? Will it make any difference in performacne?

Thanks!

garrett376

By itself: not worth it!
If you're going to spend around $8000 to make your engine really fancy, then another $300 is nothing so it makes it worth it then...

For a street machine, not really worth it. The C4 RS lightweight had the aluminum intake...

Steve Weiner-Rennsport Systems

Hi Patrick:

I'd concur with Garrett376 although you do not need to spend $8K to see the difference.

The plastic intake manifold has much better branch-to-branch airflow than the aluminum one and that is one of the prime reasons they did that.

Porsche runs each and every engine on an engine dyno to ensure they all make rated power and the aluminum intake created inconsistencies in HP. The injection molded plastic ones are much more consistent and deliver the same amount of air to each cylinder.

That said, the aluminum ones respond very well to the Extrude-Hone process.

The plastic intake works really well as we found when we installed one of them onto a bone-stock early 3.0 SC motor to make 252 HP with DTA engine management and headers.

garrett376

Thanks for the info as always, Steve! That is interesting about the inconsistencies in the Al intake!

Patrick L

Thanks! Looks like I can save some money...

springer3

Stay with the metal. You will be very glad if you have an engine fire.

IanR

You only need the plastic intake manifold if you're planning mods such as a hot wire MAF or supercharging. Otherwise you can just stay with the alu one.

tonytaylor

You don't need a plastic manifold to fit a MAF. I have the aluminium intake with a MAF. Similarly there is no reason why a supercharger requires the use of a plastic manifold but I am lead to believe the TPC kit is designed to fit the plastic and not the aluminium manifold. To a large extent the use of forced induction neutralises any deficiences in airflow that the aluminium manifold may have.
9M have a flow bench and are on record as stating the plastic manifold flows more air. However the manifold is not the bottleneck as far as airflow restriction is concerned.
9M are quoted as having tested 2 MAF fitted 964s; one with the plastic manifold and one with the aluminium manifold; They both produced 298bhp. There is a school of thought that suggests the 964 fuel injectors will not support power outputs of over 300bhp as they will not provide enough fuel at standard fuel pressures. So it would appear the plastic manifold is unnessacery to reach the maximium power outputs available using the standard injectors with standard fuel pressure.
With Motec fitted the plastic manifold cars have produced more power - in the region of 6-10bhp so in this case a change would be worthwhile.

IanR

I've been thinking about the 9M supercharger kit, which does need the later manifold - mine's a '90 with the alu intake. This would potentially involve higher flow injectors as well.

tonytaylor

So have I

AFAIK the 9M conversion is the same as the TPC kit and, as you say, apparently needs the plastic manifold. ( for reasons that are not entirely clear to me)

Have you seen this thread http://www.9mracing.co.uk/Forum/viewtopic.php?t=53

Robert Linton

Assuming a street engine it is not worth your money.

IanR

Thanks Tony, I hadn't seen that. I'm looking at a full rebuild soon (leaking through bolts etc.), so investigating the options available. In an ideal world I'd go for 3.8 / Motec / supercharger but the budget won't stretch to that. I suppose the sensible choice is to start with the 3.8 as I need the rebuild anyway, but I like superchargers!

Gavin

I have had a direct comparison of aluminium and plastic inlet manifolds on my engine. After fitting the plastic manifolds, a new dyno run showed I had gained a maximum gain of about 6bhp at the flywheel. The engine had been rebuilt with different cams, raised compression ratio etc, before the dyno runs, so some of that gain,may well have been the engine loosening up between dyno runs.
The plastic inlet manifold does allow a larger throttle body to be fitted, although I do not know how this compares with the old twin inlet throttle body for the aluminium system.

So the answer is, at a cost of about £350 for the second hand parts plus the labour to fit the inlet system, plus the enlarged throttle body ( can't remember cost but not cheap, the engine was out of the car at the time, quite alot if in the car) , on balance don't bother........On a standard car, I would imagine it gives the square root of diddly squat.

One point, on my set up, with different cams, and a cup airbox and K & N, after fitting the palstic inlet manifolds, the induction note became even more evil, a really deep resonant hard edged howl !!!

So perhaps it was worth it after all !!!!

BLACK3.2

I just came upon this thread while researching intakes. It's a good reminder Steve has forgotten more than most will ever know about these cars. Steve, if you see this, thanks for all the advice over the years!