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Registered User
Join Date: Mar 2003
Location: Huntington New York
Posts: 13
Port Size

I am building a 2.4 engine and have what is probably an elementary question regarding the port size of the heads. I am using 85mm pistons, an E-cam and carbs. The heads are standard ‘T’ heads (30mm intake / 33mm exhaust). The car is basically a street machine and I am simply looking for some additional punch.

What will I gain in terms of performance from increasing the size of the intake and or exhaust ports. Perhaps a more general question is with little or no change in the size of the combustion chamber, what is the effect of increasing the size of the ports.
Ps: Wayne’s book only touches on this very briefly.
Mike R
Old 11-01-2004, 05:54 PM
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Bill Verburg's Avatar
Join Date: Dec 2000
Posts: 22,278
In general and all other things being equal, larger ports will have gases moving w/ less velocity through them. In an engine less velocity usually translates to less throttle response at low rpms, but better throttle response and power at at high rpm. Of course you need the cams, valves and compression to match.

There are many variables besides absolute size.
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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Old 11-01-2004, 07:53 PM
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Warren Hall Student
Bobboloo's Avatar
Join Date: Aug 2000
Location: Los Angeles Ca.USA
Posts: 3,959
Since your goal is for a punchy street car then I think you would better off leaving the ports alone. As Bill stated enlarging the ports will give you more top end but at the expense of the bottom end.

The 2.4T is a faster car than the 2.4E to 3500 RPM at which point the E becomes faster. So it all depends on your driving style. For me, my choice for a 2.4 motor would be T cams and ports for a street car on a daily basis.

Since you've already got the E cams it's kind of a waste not to let them see their full potential. So it's a tough call.

Do you think you'll be reving the motor above 4k on each shift? If so then port the heads to 32mm. If not then you might be best to leave them alone.

Keep in mind that if you port the heads you also have to port the intake manis to match. Once you see how much all this porting costs it should help you make your decision. I would at least consider driving it with the T ports before I decide to port it to larger ports. That way you'll know if you feel like there's not enough top end.

Warren Hall 1950 - 2008

Last edited by Bobboloo; 11-02-2004 at 01:24 AM..
Old 11-02-2004, 01:19 AM
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jluetjen's Avatar
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Hi Mike;
Bill and Bobby are making sound points. I'd start by asking what is the 2000 RPM rev range that is most important to you when driving? In other words, where in the engine's rev range do you spend the most time? That is where you want to optimise your engine. As Bill mentions, it has a lot to do with the gas speeds, the problem is that there is about a 20 meters / second range (between peak torque and peak HP) where an engine is happy, but across the engine's entire 6500 +/- RPM rev range the gas speed will range from virtually 0 m/s to over 100 m/s at peak RPM. So you have to pick your best compromise.

Where do you want your engine's sweet spot to be?
'69 911E

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Old 11-02-2004, 05:16 AM
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Registered User
Join Date: Mar 2003
Location: Huntington New York
Posts: 13
Thanks guys, you make some very good points. Since it will be a street car and I will not be spending any real time in the higher revs, I think I will leave the ports alone. I guess my thinking was if porting the intake and exhaust paths would make the heads more efficient then larger port sizes would be even better. Guess not.

Well you just saved me about $800, and probably some disappointments with the finished product.

Mike R
Old 11-02-2004, 06:16 AM
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