Manifolds - Replacement Options & Performance

crash7 · 11 Apr 2017

My manifold studs are looking a little worst for wear and will eventually require changing, I will go Ti, as the manifolds are original I will also look to replace these, the question is with what?

What are the proven stainless options that are on the market? Although I would like a little more bhp, I am more concerned with not losing any if I go away from the OEM item, I read previous posts by a user whom dyno'd his car before and after the fitment of a performance manifold and bhp had dropped (I do not want to get into the accuracy of dyno's)

So what are the options? who produces a decent quality item, that fits and offers some kind of proven performance gain, be it via bhp increase or Kg decrease.

Your experiences are welcome!

infrasilver · 11 Apr 2017

You have already read up on the pro's and con's already so it's just a case of making your mind up. I just kept my standard manifolds as they are tried and tested by Porsche for good performance. I think I know who did the manifold change only to find less power, he used the same dyno for both tests IIRC.

Harv · 11 Apr 2017

I have a set of the eBay 3into1 tubular manifolds. Quality looks good but haven't run them yet.

Others here have run them and found them to be perfectly acceptable.

Not sure if they make more or less power but would suggest having each exhaust pulsing into same length of pipe run will optimise things. May not give increased max bhp but may help torque.

My OEM ones were dissolving so something had to be done :thumb:

Martin996RSR · 11 Apr 2017

OZ951(A noted 996 experimentalist and forum member) did back to back tests of the OEM manifolds vs ebay ones with the three into one collectors. He found a drop in torque across nearly all the rev range, barring the top end which was no different.

The best swap would be to find some less crusty OEM manifolds and put those in.

crash7 · 11 Apr 2017

Yes it was Oz951, used to have a great web-site full of useful info, however I can no longer find it.

The crust factor is what puts me off going OEM again, I hate rust!! - Although that said going to Ti studs and nuts would mean changing them in the future would be less painful.

Dansk do a stainless header, but whether it is worth it is what I am trying to ascertain.

infrasilver · 11 Apr 2017

The username for all things 996 was oz996, oz951 was for the 944 Turbo days. The 996 was sold when back in Oz but has still got the 951 and is using on track again.

EDIT, link to new site. http://www.kellyanneporsche.com/emmy.htm

crash7 · 11 Apr 2017

I just found this

Harv · 11 Apr 2017

Here is that thread

http://911uk.com/viewtopic.php?t=62769&highlight=

maldren · 11 Apr 2017

infrasilver said:
The username for all things 996 was oz996, oz951 was for the 944 Turbo days. The 996 was sold when back in Oz but has still got the 951 and is using on track again.

EDIT, link to new site. http://www.kellyanneporsche.com/emmy.htm

An excellent site, well worth a read, thanks

Mike

Alex · 11 Apr 2017

The test he did was full of holes. I had a good read and study of it and wasn't convinced. I've fit the £100 stainless ones 2.5 years ago. Just a few points from me:

The original Porsche headers are Stainless........but go manky over time as you've witnessed.
Aftermarket Stainless ones also go manky over time and look in the same condition whatever you buy.
The £100 Chinese ones I fitted, although not dyno'd by myself, felt like they altered the following characteristics:

More low end torque - the car now drives up hills in 6th gear at (low) speeds it could never achieve before.
Louder than the OEM - not massive but deffo louder.
No noticeable diffference on top end power.
No problems with cracked welds, etc.

:cap:

Jamesx19 · 11 Apr 2017

Equal length primary tubes in exhaust manifolds are usually designed to improve the scavenging of exhaust gases from inside the cylinder during the exhaust stroke, and help pull in inlet charge at the start of the inlet stroke. (there is a valve overlap during this phase) This in turn leads to the engine producing more torque as the inlet charge is denser, and the engine is achieving a higher volumetric efficiency. (Percentage to cylinder that is full of air/fuel charge prior to ignition) Maximum volumetric efficiency will correspond with maximum torque (not power) on a naturally aspirated engine.

The way this works is:

The hot, and expanding exhaust gases travel down the manifold primary, when they get to a sudden change of diameter of pipe ie where the primary tube meets the secondary pipe a pressure wave is created which is reflected back up toward the back of the exhaust valve, this (Negative) pressure wave creates a depression at the back of the valve, sucking gases out of the cylinder.

The gases are of course moving very fast, the pressure wave moves at the local speed of sound. I say local, because the heat and pressure in the pipes is extremely high and this affects the speed of sound.

Depending on the engine characteristics ie cylinder capacity, bore, stroke, camshaft, inlet design, etc etc. AND PRIMARY PIPE LENGTH, the optimum RPM that this scavenging occurs will vary. The one variable that can be changed easily to alter the RPM that this scavenging takes place is the primary pipe length.

It can be changed so that the scavenging occurs at a useful RPM. On a road car a useful rpm might be 4,500 rpm. Where as a racecar might be a much higher RPM say 6,000 or even 10,000 RPM for a current F1 car???

A rough guide to this is longer primary pipes equals more torque at a lower RPM, shorter pipe, higher RPM.

If you have an equal length header then all three cylinders will have a volumetric efficiency at approximately the same RPM, producing a higher peak torque for the whole engine at that RPM.
An unequal length system tends to produce a spread or torque at different RPM's, but a lower peak for the whole engine.

Ignoring all this......

The ebay manifolds I have seen appear to have terrible exhaust port to manifold pipe mismatch upstream, which may be causing them to flow less gas as it has to negotiate tight turns and sharp edges as it enters the manifold, rather than smooth aerodynamic forms.....

I stand by to be corrected! :nooo:

poppopbangbang · 11 Apr 2017

crash7 said:
I just found this

Ambient air temp 17 degrees but Inlet Air Temp 47 degrees? With DD's compensations for air temp in shootout mode unless both runs were performed with the inlet air temp probe in the engine bay yielding similar inlet air temp readings then which ever one was done without the probe still on the stand/hotter would show many BHPs up!

Years ago when we still had a need for a chassis dyno I got shot of our DD chassis dyno and purchased a SuperFlow instead, mainly because of the shootout modes leading to odd readings with varying inlet air temps etc. plus there was no way to correct easily to an SAE standard, only DD's versions of them.

FWIW the Chinese equal length manifolds are a copy of the early FVD manifolds for 3.4L cars (which gained around 10ftlbs from 3000RPM to 4750RPM). They are absolutely fine to run, the weld quality isn't even that bad and all the ones I have seen have been flap wheeled down the inside of the flanges to take the weld intrusion out. Plus they are cheap enough you don't care about grinding them off the floor! When I finally go through my FVD ones again I have a set of cheapy chinese ones ready to go on.

Manifolds - Replacement Options & Performance

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