opening venturis
 

I am investigating opening my venturi's on the center and end carbs. Bigger cam and heads going in and I think a bit more CFM will be a good thing.

I assume to do this I need a cone shaped grinding stone, but can't really seem to find a lot of options on diameters out there. I I think a 1 3/8 for the end carbs, and maybe a 1 1/4 will be a small adjustment for each based on current measurements. So two questions:

Does anyone know where I can source these size stones? and

Any tips on mirroring the venturi proportions once the narrowest part is opened up?

If original TriPower carbs i would save for the restorer.

I would consider a set of Holley carbs for flow on a modified engine.

JMHO

not original to the motor or the car, but they are real end carbs and a large center carb. been reading some more online about this procedure and I have to agree I will undoubtedly ruin these carbs if I try this myself. So I'll be looking for some parts at norwalk...

What year Tripower ? 64/65 or a 66 ?

its a 66

1-1/4" venturi is stock size on a 1964 center carb.

Tom V.

The stock '66 center carb venturi's are 1.180" vs the earlier center carbs at 1.250". The '66 carb probably flows more air due to the larger airhorn and throttle plates. There are Rochester 2 barrel carbs that have the same 1.310" venturi's as the end carbs. Using a float bowl from one of those would make sense to me rather than ruin a "575" carb.

As I said, the end carbs' venturi's are at 1.310" stock. I've successfully opened them up to 1.44", using a sanding drum, beginning with #80 and finishing with #340 sandpaper. This is about the maximum you can enlarge the venturi's on the end carbs with out breaking out of the casting. Some use epoxy to go beyond what I've done. I see no need to use a cone-shaped tool, as the area below the venturi small diameter is tapered to the throttle plate size.

I can send pictures if you want, but they look just like stock end carbs with a .130" larger hole. I'd love to know how much this increases flow. As I've stated in previous posts, Dan Whitmore built a 455 engine for the '65 GTO ,Glory Days, with a modified '66 Tri-Power that ran very low 10's before he switched to a Dominator. This was in the 1980 time frame. I have the manifold he used that was extensively modified, but he only removed the air cleaner webs from the carbs, did not enlarge the venturi's.

Thanks dick. The measurements are dead on to what I am showing. So you are suggesting I buy the center carb 65 float bowl that has a bigger 1.25 Venturi? And that will bolt to my 66 throttle plate and air horn?

No. It’s not hard to find a float bowl with larger venturi’s for the center carb. The ‘65 and earlier smaller carb float bowl will not fit.

End carb venturis
 

End carb venturis

Thanks for the feedback and links. from the table on Mike's site, it seems I have the smallest venturi center carb, as well as smallest for end carbs too.

If anyone is selling the larger floatbowl/venturi for center of end carbs, that would fit a 66, please IM me, I'm interested.

chrisp did you open that up?

The stock size is 1.310". Did you enlarge the ones in the picture?

Yes/No.
Jim Lehart at CVMS actually did the work for me, about 15-20 years ago.
I left the center carb venturi alone.
About the same time, I had Dave Wilcox modify/max port my 1st run 66 aluminum intake into emulating a single plane.
This N/A setup replaced a cast iron 65 intake/carb set up with an NOS fogger I had set on kill.

The 59 to 66 tripower end carbs have a 1 3/8" venturi = 1.375" from the link posted , Is this info. wrong ?

mine are not 1 3/8. They are the 1 5/16 or 1.31 which is also on that table. And they are true end carbs - no adjustments screws etc.

That is what i thought , center carb venturi is avail in 1-3/8" , i may have some . Currently have 1 on my 461 & works excellent.

Agreed. The '59-'66 end carbs were all 1.31", or 1 5/16". There are regular Rochester 2G carbs that are larger for use in the center, as chrisp pointed out.

This thread has some interesting information. I only wish we could check out performance numbers with a stock vs. modified Tri-Power on a dyno or drag strip--like was done in 1067 when the Quadrajet comparison to a '66 Tri-Power was published. My experience with Quadrajet vs. '66 Tri-Power on the drag strip is similar--about .2-.3 seconds better in the quarter mile with Tri-Power on my GTO with a nearly stock 400 with 670 heads. The only four barrel I tried that matched the Tri-Power was a 850 Holley. But, street drivability with the Holley was terrible compared to the Tri-Power, including mpg.

Main fix for the poorer driveability on the 850 #4781 carb is to open up the Primary Idle Air Bleeds from .070" to .074". What Cliff and I did on his carb.

Tom V.

Back when I was autocrossing the V8 Sunbird (thutty yar ago) I was limited to one Rochester 2G. Found there had been some large ones for big block trailer towing packages think 1 3/8 flange. Closing on 400 cfm. Good tripower could flow over 1,000 cfm with proper stacks - little factory AC93s were only about 200 cfm (big cars got a big air cleaner, think I have one in the attic). Was also a 500 cfm Holly 2bb but that was right out.

Also do not forget that for the '67 QJ vs Trips test, Milt was under strict instructions which would be faster