39mm intake ports too big for a 3.2 street motor?

[Shuie]

Here is how my motor is shaping up:

3.2 Max Moritz short stroke, single plug, 9.8:1, GE40 cams, and MFI from a 2.7 RS. This engine was previously setup to run with 40mm Weber’s instead of the MFI.

Grady has lined me out on how to make the MFI work.

My current issue is that the throttle bodies on my MFI system are bored to 39mm and I have a completely refreshed set of small port (34mm intake ports) SC heads that were used before with carbs.

This will be at least a 90% street motor. Are 39mm intake ports too big for this engine? Should I look for a smaller set of MFI stacks and throttle bodies? I’m not entertaining the idea of changing cams or pistons at this point.

I know that drivability and streetability are relative terms and that you must look at the entire engine as a system instead of trying to make decisions based on individual components.

What am I going to lose if I go to the bigger 39mm intake ports? How big of a difference do the bigger ports make on a '78-'79 SC compared with a '80-'83 SC?

My guess is that I will lose some low end grunt and I will have a little more breathing room in a rev range that a street motor will probably never see.

Any thoughts or advice is appreciated.

[jluetjen]

I wouldn't lose sleep over the 39 mm ports being too big. Given that you are going to be using MFI, I think that the driveability will be fine. The 34 mm intake ports on the other hand will tend to choke off the peak HP a bit, but I doubt that you'd notice without a back-to-back comparison -- if even then. It's something that would most likely only be noticable on a dyno and in competitive situations -- neither of which seem to apply in your case.

It wasn't clear to me from your post if you currently have heads with 39 mm ports? 34 mm ports? or both? Either way I'd go with the 39 mm ports if you can get your hands on them for a reasonable investment since they will match the throttle bodies that you already have.

[Henry Schmidt]

39 mm intake ports will work great with the engine specs you have quoted. Build it and enjoy the performance.

[Shuie]

Thanks for the help John. I was hoping Id hear from you on this.

My current heads are 34mm intake SC heads ('80-'83) with all new valves, springs, and retainers. Everything is stock since I never expected the engine to rev past 6700RPM with the GE40s and the 40mm Webers. Do you think of running the GE40s with MFI and the bigger ports will change anything drastically? Where do you think this engine will stop making power now?

I kind of fell into this MFI setup after I had already sent my heads off to be rebuilt and I'm re-engineering the top end as a result. Now the heads need to be disassembled, ported, and tapped for MFI (~$800), or I need to sell them and start over with a set of early SC heads that already have the 39mm intakes (~$???).

[Shuie]

Thanks Henry! Thats good news!

[jluetjen]

Sherman;
I think that your 3.2 with GE40's and MFI will be an exciting engine -- don't turn back now!!! Here's my $0.02, but pay attention to Henry's input. He's executed a lot more of these engines then I have -- I've just plugged a bunch of numbers. Here's a few thoughts.

1) GE40's are kind of an "improvement" on the early S cams in so much as they use faster valve accelerations to provide better flow without increasing the overlap. So for a "larger" engine then the original S cams were designed for -- such as yours -- this should be a great cam. As a result of this increased rev range you'll need to plan some things into your project.
a) If everything works out, the engine should pull just like an S up to 7500 RPM. But your engine has larger valves with increased inertia which means that you really should be getting competition valve springs and light weight retainers.
b) I'd check around on if your rod bolts will be strong enough to rev to 7500 RPM. I'm not as tuned into the 3.0 and larger engine issues, but rod bolts always seem to come up in the conversation.

2) Taking apart the heads when they are off the engine is really a trivial matter. Now's the time to get the intake ports opened up rather then a year from now when the engine is in the car and you have second thoughts.

Good luck! This sounds like a great engine, especially for a lightweight early car!

- John

[Henry Schmidt]

Although we make our own racing valve springs, there really is no need for racing valve springs or titanium retainers on an engine of this type. I agree that the engine could occasionally see 7500 rpm most likely it will be rare.
BTW: 962 two valve engines, as well as all previous factory two valve Porsche racing engines used stock valve springs sorted to higher QC.

[Shuie]

Thanks Henry, I didnt want to have to buy new springs and retainers. More good news.

Oh, the sound of MFI at + 7000RPM .

I know Im a long way away from this, but where should set the redline? 7000? 7200? or lower? Rod bolts are ARP, BTW.

[emcon5]

I would be more worried about the 34mm ports being too small.

I have a CIS 3.2 with less cam than you (964) and I believe my small ports hurt me up top.



Tom

[jluetjen]

Henry;
The only reason that I was recommending that he consider aftermarket valve springs is because he's running a GE40 camshaft which has more lift and less duration then a factory S camshaft -- which suggests faster accelerations. I understand that the factory used stock springs with their S or 906 cams at least through the 2.8 race engines. My concern was that the GE series of camshafts tend to have higher valve accelerations then the factory camshafts and thus if you wanted to rev it safely up to 7500 RPM it would be a good idea to go with the stronger springs.

As far as your redline, I guess I'd see how the torque and power curve looks on the dyno. Ideally you want your red-line to be at the point where further rev's will result in less torque then if you shifted to the next gear (assuming that the engine doesn't have any rev-limiting factors like valve float or rod bolts.)

[mpdevelopment]

How did the mfi manifolds get 39mm holes in them? The stock mfi street car throttle plates are 38mm and the bore steps down to the port size (no larger than 36mm) . The mfi manifolds will flow about the same amount of air with the 2mm step as they will with no step , due to the shaft and plate. I have found that 38mm throttles are too small for a good flowing port, they restrict the flow quite a bit. I would bore out the manifolds to 43-44mm if possible and go with the 39mm port and maybe a GE-60 profile. You would have to also bore out and taper the stacks.

[mpdevelopment]

Also will need to fatten the injection pump curve quite a bit.

[350HP930]

Concidering that a stock carrera has 40 mm ports I would say that 39 mm on your motor should be a safe bet.

I too am in the middle of boring out my intake ports from 34 to 39 mm to mate up with the carrera manifold I acquired.

Welcome to the club.

[Wayne 962]

Originally posted by Shuie
I know Im a long way away from this, but where should set the redline? 7000? 7200? or lower? Rod bolts are ARP, BTW.

Set at 6500 and then see how much you come up against it. I've got mine at 6500 - rarely hits it.

As for the ports, you'll be fine on this engine. Anything bigger would be too big, but this should be just about right.

-Wayne

[emcon5]

Originally posted by Wayne at Pelican Parts
As for the ports, you'll be fine on this engine. Anything bigger would be too big, but this should be just about right.

You mean 39mm right? Right now he has 34mm.

Tom

[Shuie]

Thanks guys, I appreciate the help.

I separated my stacks from the throttle bodies for a closer look. Looks like my throttle bodies are bored 39mm all the way through, just as was suspected in a different thread. Not good. The good news is they can be made better. The bad news is that I wont be able to use the plastic RS stacks (which are perfect) once the throttle bodies are fixed.

[Wayne 962]

You'll have to get yourself a set of the early magnesium stacks and bore them out then.

Yes, that's what I meant, the 34mm ports might be a bit small...

-Wayne

[Shuie]

Yep, they need to be 70-71 E or S stacks. I have a set of '69 E mag stacks and they wont bolt up to my throttle bodies.

[TonyG]

I don't know if the 39mm ports are too big, but I do know that the 34mm ports are not too small.

I got 200RWHP on a 3.0SC with 34mm ports, 20/21 cams, and 40mm PMO carbs with SSI's.

Since then I've advanced the timing, and played with the jetting, and the car definitely feels faster, but I've not had time to get it onto the dyno again yet...

TonyG

[TonyG]

emcon5

The 34mm ports are not the cause of your problem (not that you have a problem...).

TonyG