Large Increase In Intake/exhaust Ports

Don888 · 10-20-2005, 11:15 PM

hi,
I am in the process of rebuilding a 3.3 turbo and have changed it to a 3.5 with 7.5 to 1 compression. I am running C2 cams. In addition I am trying to decide to open up the exhaust and intake ports to either 40MM or to 38MM. I am running the stock CIS system with an Andial fuel enrichment system. I am using the standard ceramic blocks between the intake manifold and the engine case to cut down on heat conduction as the factory did. I just am going to drill them out to the correct new intake size. I understand the aluminum blocks just conduct heat and boil the gas. I have been told that if I go to 40MM on the intake manifold the gas will boil when the engine is turned off and then is hard to start. However, 38MM will not create this problem? This modification should make a big difference in power and the mid range. The 935s were running 40MM and over?

Does any one have experience with this. This will be a totally rebuilt engine with all new parts, including pauer rods, and the entire system balanced, along with the the Porsche racing oil pump, etc. --plus many other items. Before I started the car was running 390HP at the rear wheels. --lg intercooler, custom turbo, headers, and plumbing.

Thanks,
Don888

asphaltgambler · 10-21-2005, 05:49 AM

I think that size alone is not the answer in porting cyilnder heads. The shape, contour and the transition under the valve seat have to all work together to achieve increased velocity which = HP.

My understanding is that the turbo guys use 3.2 SC heads where the port size and configuration is a lot better to start with. Maybe someone here will chime in with their experience.

aturboman · 07:40 AM

Don, I in the middle of the same 3.5/3.6 project with the same cams etc.. I have had my heads ported to 3.2 Carrera specs and went with matched aluminum injector blocks. The factory blocks do not have enough material to reliably bore them out, they'll crack. The billet blocks come with a nice insulator plate between the head and block so you won't have any head sink issues. I went the Carrera port route for a few reasons, including the fact that I plan on eventually going to EFI. I will then be able to just bolt up a Carrera manifold without having to take the motor apart. The stock manifold must also be ported and it gets a bit tricky when you go above 38mm. You will need to add some material to a couple runners or you'll break through. I've also increased my capacity to 3.6 by installing a 964 crankshaft in my original 3.3 case. I've increased my comp ratio but mine is 8.0:1. The increase in displacement & compression should help offset any torque loss due to port velocity changes from the bigger ports. You also need to open the exhaust ports as you mentioned to help increase flow to the turbo. 38mm is what I went to. I've done alot of reasearch and caculations on this engine. I hope it helps.

KobaltBlau · 10-21-2005, 08:38 AM

Quote:

Originally posted by asphaltgambler
My understanding is that the turbo guys use 3.2 SC heads where the port size and configuration is a lot better to start with.

not sure I understand, "big port" SC, "small port" SC, and carrera 3.2 heads are all different port sizes.

hobieboy · 10-21-2005, 09:23 AM

With stock intake, the biggest I have heard of is 38mm. As asphaltgambler said, I think there's more to drilling out the port. You'll also need to port the intake as well.

Many folks who go EFI are also using 3.2 intake hence the head will be ported to match that intake size; which is 41mm.

Henry Schmidt · Henry Schmidt's Garage

38 mm can be achieved but the sides get so thin that you can squeeze them with you fingers. We bore them on a lathe to 37.5 and have not bad experiences.
With the 37.5 intake and a 38 mm port we have achieved 480 RWHP. With CIS.
It is true that the new aluminum blocks have insulators but the aluminum still pull heat from the engine at hot rest.

The insulators improve the cold start problem but they don't cure it.

If you are concerned with performance before boost port size should remain small to maintain port velocity.
If all you care about is boost, boost, boost then big, big, big is your chant.

934 and 935 had big ports 41 to 43 and they didn't run under 5000 rpm.

asphaltgambler · 10-21-2005, 10:16 AM

[SC] as 3.2 Carrera heads

Don888 · 10-21-2005, 07:28 PM

Guys,

Thanks a lot for the great input. Your information and help is fantastic. What can you use if you do not want to use the aluminum blocks. Can I fabricate a set of heat resistant blocks made from a different material?

Don