RX-7Impreza

i was thinking about getting either ceramic or powder coat on the inside of my intake. if it was slick would this cause worse or better airflow

justin

NZConvertible

Wow, now you're getting serious!
In the bad old days old carbs, manufacturers deliberately left the inside of cast manifolds rough to provide some turbulence to aid the fuel mixing with the air. With EFI this isn't necessary due to the injectors doing a much better job of fuel atomisation than carbs, and being mounted so close to the ports.
In theory there are gains to be made, but they wouldn't be huge. An increase in port diameter would have a bigger effect. It's bloody expensive, but the ExtrudeHone process does an amazing job of enlarging ports and smoothing casting dags and bumps, and also leaves the surface much smoother. I think you'd have trouble getting complete coverage inside the runners with a spray process, but you could always give it a go!

Evil Aviator

Actually, you would have a slight fuel adhesion problem with a slick manifold like NZConvertible mentioned, even with EFI. Removing casting burrs, flashing, sharp edges, etc. is good, but you don't want a mirror-smooth finish, either.

mazdaspeed7

I was trying to explain it to him, but I forgot about the fuel adhesion problems. Thanks, he did not believe me that a perfectly smooth manifold would hurt power.

E6KT2

My father told me that he and his friends would polish thier manifolds to "mirror-like" before they knew better.


The "surface adhesion" creates a slow-moving boundry layer at the walls of the manifold. This slows down velocity. Leaving a slight finish on the walls disturbs this boundry layer and increases efficiency.

Merovign

So, for the sake of argument, would the best thing to do be to Extrude Hone or Port & Polish, THEN rough up the surface a bit? Then you get rid of the major restrictions, increase diameter, and avoid the adhesion...

If so, about how much roughness do you need? A quick pass with 600-grit? Shot-peening? A quick pass with a stiff wire bottle brush on a flex shaft drill?

I'm doing a manifold soon...

RX-7Impreza

yeah, were lies the fine line between good and bad airlfow???\

Justin

Evil Aviator

The most common way to modify the manifold is to cut it in half, grind down all the sharp edges, fill the voids with epoxy, and then weld it back together. This is really more for those with advanced mechanical skills, though. I guess you could extrude hone the manifold, and then coat it with something like FloKote, but I think that would be a lot of work and expense for the small performance increase that you would get from it. I think the manifold would still give a small performance increase if you simply extrude honed it, but once again, you are looking at about $400 USD.

Merovign

Okay, problem #2: Half of y'all are saying "small performance increase," the other half are saying "huge performance increase."

Paul Yaw ('Da Man'), who admittedly does his proting with a flowbench, does claim significant increases.

Has anybody done a Before/After dyno on their project here?

If I get my engine going this summer, I might do just that, in fact I'm thinking:

Stock 86 manfiold dyno (w/headers, cold air intake, pineapple ports)
Stock 89 VDI intakePorted 89 VDI intake

The engine _might_ be mildly ported & have 89 rotors/balance kit.

Anyone want to donate a VDI intake complete or sell it cheap?

FloKote info: http://www.sonic.net/~sc7500/

Check out the Technical Data

NZConvertible

Be careful how you interpret these kinds of statements. If you port a manifold and measure "significant" improvements in its flow, say 15%, this will probably only improve an engines power by a few percent. Unless you're doing it yourself or it's a particularly bad manifold, manifold porting is not big on hp/$ returns.

Merovign

I meant significant increases in HP - he does the testing on an engine dyno. I don't recall any numbers from that - when we chatted a couple of years ago I think he was talking 10-15 RWHP.

IIRC he was talking about airflow increases of around 30%... but unfortunately he's so busy as far as I know he's not taking much in the way of new projects now...
Still, we need fresh dyno numbers. I need to find a good dyno locally...