ITB size for 3.2ss?
 

I'm thinking of doing the ITB/MS thing. Just trying to decide what size tb's to use. Choices are 38's or 42's.
Engine is going to be 3.2ss, 10.5-1cr, GE 20 or 40 cams. Throttle plates will be mounted about 4" from valves.
This will be mostly street, redline set at 6500 or so.
Any suggestions?

I don't have an exact answer, but as a reference point Rothsport is using 48mm throttles on stock 3.0s for their ITB + 3.6 intake plenum builds. It would be a good idea to talk to an expert on the subject.

I use 40 mm PMO... why 38 or 42? and not 40? Id probably go 42 if those where my only choices. Runs great through the power band with the 40's. I use a GT2 108 cam that is similar to a 40 cam (in between a 20 and a 40 but closer to the 40). You will love that set up. Make sure you get your Dizzy recurved. Even with early SC heads that are large port they will only be 39mm so wouldnt go bigger than 42 unless this is a race motor.. Actually the PMO 40's should be added to your list. they are probably better sized for a street motor. Could talk to Steve or Henry.. Id think low end torque vs high end HP is a consideration...

For the street I'd pick the ones that are about the same size as the ports.

That redline sounds pretty low for a SS with ITBs. I don't think you're taking advantage of the breathing ITBs and the short stroke geometry give you with it set there- why so low?

Also the 6500 redline is safe and a good target, but i believe the stock rods and bolts are good up to 7500, but why risk it.

Before you jump into getting a distributor recurve conisder that the 78 and 79 distrubutors had a better curve then the 80 and up SC's. Dont drink the kool aid.

I would use 46mm. I use 50mm on my normal 3.2.
Move the redline up to 7000, that's no problem at all.
But redline is decided by the power curve, so you will decide after the dyno.

These are stock SC heads. there is no use for 50 mm ITBs with 34 mm or 39 mm ports. also, a 3.2 SS has different characteristics than a straight 3.2.
IMHO, these need to be matched to piston choice, cam choice, etc and not just pick a number because someone tried it and it worked. My 40's were recommended by 3 engine builders that had been there and done that... doubt any engine builders would recommend 50's on a street motor with 34 mm or 39 mm ports.

Hi Safe,

I think this is some risky advice, most of the advice I have received when building my engine involved following manufacturer recommendations of the mechanical limits of the parts I was using.

Your screen name is misleading.

I didn't recommend 50s, I said 46mm.

If you do the move to ITBs you should not go halfway, thats just throwing away money and hp.
Edit: As you said 6500 is safe, 5500 is even safer....

I have not taked to ANY builder that claims 7000 as "unsafe" on a 70.4mm crank.

I run my 3.2 up to 6800 and have done so for a while, that might not be as safe.

Even 46's are overkill unless you CNC machine/port/polish those stock heads and use the right cam. 46's on stock heads will likely result in poor low end torque. these are 3.0 heads on a street car where low end torque matters most.

The stock redline for a 70.4 stroked S was 7300rpm with stock rods and bolts.

Throttle plate size should be a minimum of 2mm over the port size.

I like 40mm ports with 42/44 mm plates.

I am planning to use motorcycle throttles as many here have done. I've just been looking at the gsxr600/750 sizes, 38/42 respectively. The ignition will be crank fired and controlled by the MS using EDIS. As for heads, I was planning on the later ones so the ports can be sized accordingly. So far it sounds like the 42mm TB's are a good bet.
They will be mounted atop some weber or equivalent manifolds.
As for the redline, I just threw that out there. I have no issues going higher. I might cap it off around 7200?

MSD has a limiter that is 7k. Thats what I use. Something about going over 7k on stock internals scares me. if 7300 is the limit then thats only 4% from the limit.

Also, is there a big difference in fun factor between GE20/40? I can have the CIS cam reground to GE20. I was also thinking easier to tune.

No need to repeat above, but will add the size will influence where in the RPM band you will make power. Smaller will be lower, bigger will move power up the RPM range. So it will effect driving characteristics. For the street I would opt for smaller within the range for the motor configuration. FWIW a 3.2ss can run 8K with the correct bits.

I think the engine sounds a bit confused.

In general:
ITBs and a short stroke are better for high rpm to run race cams and have durability.

Yet, you plan to have a relatively low redline to use GE20 or 40 cams. Pretty mild.

If you are doing a 3.2 displacement, I like Aaron's recommendation of a 40mm port and 42/44mm throttle plates.

However, a street motor with a 6500 redline would be a better 3.2 as a long-stroke 3.2 in my opinion. Stroke, rod length-to-stroke ratio all indicate the long-stroke 3.2 is going to fill the cylinders better at lower r's, and that is what power is all about. If you already have the pistons and cylinders, you might play with the rods to get this to work, and yield a 3.4L. (98X74.4) But I think you will have more fun with your engine as a long-stroke than short. Dynamic compression ratio is a bit high with a GE20 cam , I'd go for the GE40 or drop the compression down a tad, 10.3:1 would probably be OK on pump gas. Other than that a nice GE40 long-stroke 3.2 @10.5:1 will make a fantastic street motor. You will have torque all over the place

If you do indeed want to trade off some low-end torque and build a 3.2SS for more top-end power and durability, it is a whole 'nother flavor. But then you want to rev it, to 7300-7500 (or even up to 8k with good rod bolts, balancing and valve springs), get a cam to suit, like a DC60, DC70 or RSR. This would be probably what I would do personally, because I like big cams and revs in a hooligan car. A 40mm port is still probably adequate for this..maybe 41. This motor is best served with close-ratio gears. If you want to 'just' drive around on the street the above long-stroke (standard) 3.2 is a better choice

Best of luck.
Scott

7300 isn't the limit, it's where Porsche put the redline on the 2.4S (same stroke), I bet they put it there with a good safety margin.

Basically, I'm shooting for a streetable engine/car I can autoX and the wife can drive... maybe.
:D