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Has anyone polished the inside of their intake manifold?

ManyFacesofSchwinn

Active member
Member
City
Rathdrum
State
ID
Hello,

Has anyone polished the inside of their own intake manifold. I tried to find an after market intake for my Gm 2.5L 151 4 cylinder engine without any luck. So I thought maybe I could polish the stock one to make it a little more efficient. Any advice would be greatly appreciated. Thank you. Barry
 

kgt0001

Jeep Restorer
BENEFACTOR
Gold Member
Lifetime Member
City
San Antonio
State
TX
I have polished intakes before- never dyno’d so do not know if the effort was worth the end result. I am not sure on a 2.5L that the effort would provide an increase you would feel. How rough is the surface?
 

93_Fummins

CJ-8 Member
City
Edmond
State
OK
According to an interesting article in a hotrodding rag years ago, porting is worth the effort, but polishing is not. I believe it was Edlebrock that did a comparison of ported/rough versus ported/polished, and they found that a certain degree of roughness actually aided air flow. It worked on the same principal as golf ball dimples; the "roughness" created tiny air pockets, and air has less friction against itself than against polished metal. That being said, there is a broad spectrum of "rough". I would say give porting a shot, aiming for "smooth rough" and not necessarily polished, BUT do it on a spare manifold just in case the porting goes south. And definitely gasket match the manifold ports and head ports; that seems to be one of the biggest gains without getting totally carried away.
 

ManyFacesofSchwinn

Active member
Member
City
Rathdrum
State
ID
I have polished intakes before- never dyno’d so do not know if the effort was worth the end result. I am not sure on a 2.5L that the effort would provide an increase you would feel. How rough is the surface?
I don't know how ruff the inside is, I haven't taken it apart yet.
 

cbford

Legacy Registered User
City
CLT
State
NC
I would agree with Fummins...
Gasket matching or porting is worth the time, polishing is not.
Every little bit helps on a 2.5.
 

Ron84cj

Engine nerd
Lifetime Member
SOA Member
City
West Bend
State
WI
Polishing does nothing. And as far as porting the manifold goes be careful with that one. How the air flows is more important than just the amount. A huge port will lose low rpm torque and gas mileage. The fuel doesn't atomize as well. Look up single vs duel plane intake manifolds. Although they aren't for your engine it will help you understand what I'm talking about.
 

Randyzzz

Blown Budget
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Redmond
State
OR
Mercruiser used the Iron Duke in their stern drives for many years. They had decent power. The cam from one of them may be an improvement. Just guessing here.
Wasn’t that a 3.0 iron duke? We had one back in the early 80’s.
 

sxer

Legacy Registered User
Member
City
Long Prairie
State
Mn
Wasn’t that a 3.0 iron duke? We had one back in the early 80’s.
The 3.0 was Mercruiser’s version. They did use the Iron Duke before that. The Iron Duke was the 120/140. Mercruiser’s was the 3.0. Mercruiser also made a 3.7L 4 cylinder. It had a 460 Ford head. The charging system was behind the harmonic balancer like an outboard.
 

sxer

Legacy Registered User
Member
City
Long Prairie
State
Mn
Wasn’t that a 3.0 iron duke? We had one back in the early 80’s.
I should correct myself. The 120 and 2.5L were the Iron Duke. The 140 and 3.0L were the Mercruiser version. Since you mentioned the 3.0 I've been thinking that may be a good swap. I think it would be more powerful than my 4.2. I think the bell housing is the same as the 2.5, GM. There are a couple things you would have to figure out. Intake and exhaust for one. The marine version runs lake water through a water jacket around the exhaust. You might still be able to use it without the water if you could fab an exhaust connection. You can safely run the engine 4800 RPM. It has to have good low end because other than the 2:1 reduction in the gearcase, there is no shifting. You could gear it pretty aggressively. They make a power steering version and had an EFI before they discontinued the engine. They are plentiful. Something to think about for guys switching to EFI. Mercruiser uses a self contained "cool fuel module". It has the high pressure pump and regulator. They return the excess fuel to the water separating fuel filter mounted on the engine. It would eliminate mounting a pump in the tank and a return line to the tank. The marine version runs lake water through a heat exchanger in the module but would probably work without that.
 

CBRogers

Perpetual Parts Collector
Lifetime Member
SOA Member
City
Yorktown
State
Va
I went to a Class with David Vizzard who has done more dyno testing and porting than most anyone out there.

His opinion is that you can go in with an old very rough carbide and get ride of casting flash from the seams and things that will cause intruptions with the wet flow but stay away from polishing. The golf ball treatment is even better. But a rough sand cast finish is usually best to keep fuel from dropping out of suspension.

Always good to check that the gasket will not cause an issue between intake and head. You can also round the lip of the head if there is an issue there. Basically you do not want any step to really screw with the flow. He has a pretty good series on You tube and several good books on the subject, but beware there is a lot of issues with getting the most out of what you have. And with porting the heads and intake runners a lot to consider.

First things are to look for big flow restrictions, then go slow.


Carl
 
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