# porting and polishing a forced induction head ?????



## golfturbo95 (Jul 11, 2005)

Ok Im wondering about porting and polishing a head that will be receiving boost. 
Ive already ported and polished my head. I ported them all to match the gasket and polished with 120 grit sand paper. 
I have heard that I prolly just waisted my time doing this because ported and polishing your head wont help out on a forced induction motor. Is this true. 
I have also heard that porting will help out but polishing wont do a dam thing on a forced induction motor. Is this true? I was thinking that this theory is probably true. But I was wondering, if the air is forced in....is it forced out? I would think not. So wouldnt it be the right thing to port the intakes and port and polish (until its like glass) the exhaust ports? That is a theory of mine. 
Some one help me, fill me in. Im lost.


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## PhReE (Sep 16, 2004)

*Re: porting and polishing a forced induction head ????? (golfturbo95)*

Every bit helps (on intake and exhaust) but with FI since it is already 'forced' you get less of a gain than P&P in N/A but more than none.


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## golfturbo95 (Jul 11, 2005)

*Re: porting and polishing a forced induction head ????? (PhReE)*


_Quote, originally posted by *PhReE* »_Every bit helps (on intake and exhaust) but with FI since it is already 'forced' you get less of a gain than P&P in N/A but more than none.

ok great so it will help. 
The my main question is this. NA port and polish job is critical, everybit helps. That I know is true. Whatever you can sqeeze in you want to be able to get out. But with forced induction the air is forced in.....but is it forced out? In my little head im thinking that the air is not forced out so therfore it would not be a bad idea that after you port the exhaust ports to not only polish the exhaust ports but to polish the sh** out of them. Now is my theory some bullsh** or should I give it a try?


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## TBT-Syncro (Apr 28, 2001)

*Re: porting and polishing a forced induction head ????? (PhReE)*


_Quote, originally posted by *PhReE* »_Every bit helps (on intake and exhaust) but with FI since it is already 'forced' you get less of a gain than P&P in N/A but more than none.

not true. in fact, the exact opposite is usually true.
'holes' in the system will not only show up with forced induction, but they're usually magnified.


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## GTTechnics (Apr 2, 2003)

*Re: porting and polishing a forced induction head ????? (TBT-PassatG60)*

the easier it is for the air to get into the cylinder the less of a pressure drop you will see. What this means for us is that the turbo will have to turn at a slower RPM to reach your desired boost level. Since it takes less time for the turbo to get up to a lower RPM the engine should then see full boost at a lower engine RPM. Also, when the turbo has to move faster it creates more heat...so a good port/polish has the potential to effectively lower the intake charge temperatures slightly, which is always nice!
All in all it isn't a great first mod, but it certainly isn't pointless!


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## fast_a2_20v (Jun 25, 2004)

yea except boost is almost always reverenced in the intake manifold- not at the valve- so pressure drop in the port / intake runner is unknown and likewise not compensated for. 
FI simply magnifies the SCALE of everything your doing- on an N/a motor you might gain 5 hp on a FI motor you might gain 25- or 50 the larger the scale you are operating on (airflow) the larger this effect is. 
the notion that induction principles do not apply because it is "forced" induction cannot be further from the truth.


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## mechsoldier (Aug 14, 2004)

*Re: porting and polishing a forced induction head ????? (golfturbo95)*


_Quote, originally posted by *golfturbo95* »_
ok great so it will help. 
The my main question is this. NA port and polish job is critical, everybit helps. That I know is true. Whatever you can sqeeze in you want to be able to get out. But with forced induction the air is forced in.....but is it forced out? In my little head im thinking that the air is not forced out so therfore it would not be a bad idea that after you port the exhaust ports to not only polish the exhaust ports but to polish the sh** out of them. Now is my theory some bullsh** or should I give it a try? 

You don't want to go over what you've already done polishing wise on the intake. A lack of turbulence will cause the fuel to fall out of suspension and you wont get a proper mixture. The air IS forced out because during overlap (the small period of time at the end of the exhaust cycle) both valves are open and there's still boost being made. Don't port match the exhaust side though, if the head ports aren't smaller than the intake manifold, the air will force itself through the cylinder, get backed up at the turbo, and try to go BACK into the head (reversion)....You can mirror polish the combustion chambers and exhaust ports, it'll help stop carbon buildup which can cause preignition by getting so hot it ignites the fuel on its own. Also it can raise the compression ratio.


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## golfturbo95 (Jul 11, 2005)

*Re: porting and polishing a forced induction head ????? (mechsoldier)*

Ok so basically what Im getting now is that a ported and polished head does matter on a FI engine. It will help, it could add 5 hp or in some cases 40 to 50.
Right now everything is sanded with 120 grit. Someone said mirror finish the exhaust prots and combustion chambers. So if I do that and leave the intakes sanded at 120 grit. Think that will be good enough?
thanks for all the help everyone, very helpfull.


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## PBWB (Sep 12, 2004)

*Re: porting and polishing a forced induction head ????? (golfturbo95)*


_Quote, originally posted by *golfturbo95* »_
Right now everything is sanded with 120 grit. Someone said mirror finish the exhaust prots and combustion chambers. So if I do that and leave the intakes sanded at 120 grit. Think that will be good enough?
thanks for all the help everyone, very helpfull.

yep, same thing that was explained to me.


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## Geoff Rood (Apr 30, 2001)

*Re: porting and polishing a forced induction head ????? (PBWB)*

Just some tidbits from me on porting & polishing:
Polishing the intake side only accounts for 1-2% of any flow increase, which will probably not yeild any power increases due to fuel sticking to the walls fo the port. A rough finish promotes turbulence in the air and atomization of the fuel.
Polishing the exhaust side yeilds a surface that is harder for carbon to stick to
Leaving the exhaust port on the head smaller htan the exhaust port on the manifold (IE a small step when transitioning into the manifold) helps with anti-reversion. That's basically where exhaust gasses reverse and flow back into your cylinder. That leads to less power








Hogging out the ports is not adviseable. The best ports are usually pretty small. Having small ports increases gas velocity at low engine speeds and gives you a more driveable car. A common mistake is people grinding away material at the inside edge of where the port meets the valve seat, essentially destroying any radius that was there. This interupts the smooth flow and will actually decrease the amount of air flowing into your cylinder. Less material is not always better!!!
I'd go into swirl but that's an entirely more complex topic that needs a little more visual help


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## Butcher (Jan 31, 2001)

*Re: porting and polishing a forced induction head ????? (mechsoldier)*


_Quote, originally posted by *mechsoldier* »_
You don't want to go over what you've already done polishing wise on the intake. A lack of turbulence will cause the fuel to fall out of suspension and you wont get a proper mixture. The air IS forced out because during overlap (the small period of time at the end of the exhaust cycle) both valves are open and there's still boost being made. Don't port match the exhaust side though, if the head ports aren't smaller than the intake manifold, the air will force itself through the cylinder, get backed up at the turbo, and try to go BACK into the head (reversion)....You can mirror polish the combustion chambers and exhaust ports, it'll help stop carbon buildup which can cause preignition by getting so hot it ignites the fuel on its own. Also it can raise the compression ratio. 

Not to be picky but the fuel dropping out is something that works more with carbed motors and less with EFI. 
As for the boost pressure going out of the exhaust valve, most likely not unless it is supercharged. 
Turbocharged engines have as much and most likely more pressure in the exhaust manifold than the intake port so the exhaust would come back in the combustion chamber and not the boost going out of the exhaust manifold. There might be a few exceptions but none on a street engine.
I have seen carbon stick on mirror polished ports so I have come to the conclusion that polishing does not do much except for your ego.


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## GTijoejoe (Oct 6, 2001)

*Re: porting and polishing a forced induction head ????? (Geoff Rood)*


_Quote, originally posted by *Geoff Rood* »_Just some tidbits from me on porting & polishing:
Polishing the intake side only accounts for 1-2% of any flow increase, which will probably not yeild any power increases due to fuel sticking to the walls fo the port. A rough finish promotes turbulence in the air and atomization of the fuel.
Polishing the exhaust side yeilds a surface that is harder for carbon to stick to
Leaving the exhaust port on the head smaller htan the exhaust port on the manifold (IE a small step when transitioning into the manifold) helps with anti-reversion. That's basically where exhaust gasses reverse and flow back into your cylinder. That leads to less power








Hogging out the ports is not adviseable. The best ports are usually pretty small. Having small ports increases gas velocity at low engine speeds and gives you a more driveable car. A common mistake is people grinding away material at the inside edge of where the port meets the valve seat, essentially destroying any radius that was there. This interupts the smooth flow and will actually decrease the amount of air flowing into your cylinder. Less material is not always better!!!
I'd go into swirl but that's an entirely more complex topic that needs a little more visual help









sounds like someone has took a bit of engineering classes








Listen to this man http://****************.com/smile/emthup.gif


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## Geoff Rood (Apr 30, 2001)

*Re: porting and polishing a forced induction head ????? (GTijoejoe)*

Long family history in auto racing, taught by the best, not engineering classes. I havent' started college yet LOL


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## GTijoejoe (Oct 6, 2001)

*Re: porting and polishing a forced induction head ????? (Geoff Rood)*

well thats funny, kinda sounded like my IC engineering perfessor.


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