ported and polished intake manifold

im looking at a ported and polished intake manifold for a Renix 4.0. my buddy wants $75 for it. i want to put it on my 89. he sold his Renix Rig and hes doing the 99+ switch on his HO. im wondering if its a worthwhile investment. obviously i can change it out myself. what do you guys think for hp and mpg gains? also, this might be a stupid question, but are there different degrees of porting or is it all the same? can the manifold ported a littel or alot? is what i mean.

my rig has an air intake currently and im adding a freeflow exhaust and probably boring the throttle body. :eeks1:

"Polished" would mean that the sand-cast surface has been smoothed. You don't really want a glass-smooth surface, but that's less important with port fuel injection (turbulence is necessary with 'wet' manifolds to keep the fuel droplets in suspension.)

If it were 'extrude honed,' I'd want a look at it myself. Extrude honing involves running a mild abrasive slurry through the manifold as if it were air - this not only polishes the surface, but it also helps to smooth out and round off projections that are in the way of airflow (since the theory goes that the slurry follows the same pattern as the air charge.)

"Porting" a manifold usually refers to "port-matching" - where the gasket is laid on the manifold face in its "as installed" position, and the manifold (and, usually, cylinder head) ports are opened up and tapered out to remove any "steps" at that point that would generate turbulence harmful to airflow.

"Port-matched" manifolds tend to want to stay with the heads they're matched to, since "port-matching" usually entails work on the cylinder head as well (you're matching the manifold port to the gasket port to the cylinder head port.) However, that just might mean that you'll only need to do a little work to the manifold to make it match up with your head.

Will it help? If the job was done well, it will probably help a bit. Airflow mods usually help - but they also tend to complement each other, and a number of small airflow mods can have a greater effect that just one big mod...

I'd probably get it, but I wouldn't use it raw for a project. I'd want to match it to my other parts first. Or, match my other parts to it.

As mentioned, the Extrude honing process would have smoothed out the runners to the point where I would consider it port/polished.

I seriously doubt he did that, the cost is pretty high and there aren't a whole lot of shops that will/can do the work.

If all he did was gasket match the thing and smooth what he could, I'd wonder if it is even worth it.

thanks guys. yeah he must have just port matched it. like i said im going to add a freeflow exhaust and a bored out throttle body so those should compliment the manifold. he's a buddy, so ill get the price down a little. thanks for your input.

jk333 said:

thanks guys. yeah he must have just port matched it. like i said im going to add a freeflow exhaust and a bored out throttle body so those should compliment the manifold. he's a buddy, so ill get the price down a little. thanks for your input.

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"Port-matching" the intake should be done with an eye toward smoothing out the flow path. Ideally, the intake runner, intake gasket, and intake port entry in the head would all be one smooth line (it isn't usually, but a little work can at least make it close. Practically, you end up with gentle tapers going into the head and coming out of the manifold, and a slight groove where the gasket sits. This prevents the gasket becoming a "baffle" to airflow.)

"Port-matching" isn't properly done on an exhaust tho - the small step that is usually left there is beneficial to exhaust scavenging, by reducing the potential for flow reversion.

I've always been a little leery of work like that when I didn't do it myself, or when I didn't know the guy who did the work by reputation (there are damned few mechanics and machinists in the country I trust.) Also, any work done to improve flow - when it involves reshaping the flow channel - is best done over a flowbench so you know when to stop.

The benefits of porting and polishing become more beneficial as RPM increases, and if the runners are opened substantially will actually hurt lower RPM performance. The 4.0 isn't a high reving engine so the gains would be minimal in the main operating range of the 4.0. I think you'd be way ahead putting the $75 towards the better designed 99+ intake instead. Like 5-90 said, I'd be very leary of someone else's port work.