[ejgoat]

A question came up at norwalk about carb Venturi size between 65/66 tri power center carb size someone said he read here that 65 has a larger venturi size than a 66 can someone shed some light on this need to settle an argument

[Doug]

65 larger than 66

check this discussion:

http://forums.maxperformanceinc.com/...t=venturi+size

[pfilean]

As I remember the story 57 & 58 all three were a smaller 1 11/16 bore. Then in 59 the end carbs were increased to 1 11/16. Then in 66 the center was also increased to 1 11/16 at a time that coincided with the change in the choke design. There were some 1 1/4 bore used for MT back in about 59 and earlier and you will find some Chevy carbs that look like the same 2GC Pontiac had that also had 1 1/4.

[ejgoat]

Are we talking center carb or all three

[carbking]

While the 1965 center carburetor venturi WAS larger than the 1966 center carburetor venturi, the 1966 tripower was rated at a slightly higher figure than the 1965 unit. A matter of some 45 CFM. Unfortunately for tripower enthusiasts, the maximum rating of either tripower was not obtained at the maximum RPM of the engines. Due to flow issues from the "join" of multiple sources feeding the same cylinder, the CFM rating of the tripower decreases as the RPM increases.

I wish I had talked John S. out of the factory tests on these, rather than just verbal conversation. It would have either settled an argument that will never be settled, or just created more controversy, depending on the beliefs of the reader.

Jon.

[Tom Vaught]

Originally Posted by carbking

Due to flow issues from the "join" of multiple sources feeding the same cylinder, the CFM rating of the tripower decreases as the RPM increases. Jon.

The unfortunate thing is that a Twin Turbo System does the same thing to some extent.
When you feed two discharge outlets into a common air inlet the air tends to pulsate back and forth between the two sources of the air, basically argue with each other on who gets to fill the cylinder (just like the "join" of multiple sources feeding the same cylinder,). A fix for that is to have the turbo discharges point at each other which disperses the air everywhere in a plenum. Unfortunately you cannot do that on a factory Tri-Power intake manifold.
The old Offy Tri-Power Intakes had "Windows" in the center divider to try to balance out that pulsation effect.

Dave Wilcox I believe had a design for a Tri-Power Intake that made it more of a open plenum 360 design vs a factory 180 design intake.

So I agree with you on that one Jon.

And yes the 65 Carb had a slightly bigger venturi on the CENTER Carb.
Tom V.

[carbking]

Tom - John S. did mention the "6-barrel" in our conversations, but apparently it was never put in production. Wished I had asked about the intake planned. If all 6 barrels fed a common plenum, then it would probably perform well, but might lose the "sexiness" of three carburetors.

Jon.

[Dick Boneske]

Originally Posted by pfilean

As I remember the story 57 & 58 all three were a smaller 1 11/16 bore. Then in 59 the end carbs were increased to 1 11/16. Then in 66 the center was also increased to 1 11/16 at a time that coincided with the change in the choke design. There were some 1 1/4 bore used for MT back in about 59 and earlier and you will find some Chevy carbs that look like the same 2GC Pontiac had that also had 1 1/4.

I think you mean 1 5/16" for the end carb venturis from '59-'66. They measure 1.315" with a snap gage.

The OEM center carb for '66 (bowl casting 7031575) had venturis measuring 1.190" or about 1 3/16 vs. the '61-'65 center carb venturi at 1.250".

The largest venturi I have found on the large 2G carbs is 1.307", which I think is the '57 Pontiac single 2 bbl. carb. This is almost a large as the '59-'66 end carb venturis at 1.315".

On a side note, I have modified a pair of end carb float bowls by enlarging the venturis to 1.410", which is a large as you can go without breaking through the casting walls. I'm anxious to try them on one of my Tripower setups. Jetting will have to be larger than the .070"-.073" sizes I use in stock end carbs. I'll start out with .075" jets and go up or down from there.

[pfilean]

Yes Dick - that is a typo. And maybe a little confusion. What I meant was 1 7/16 for all three carbs in 57-58. And on closer rereading the manual says that is the bore size. In this manual the venturi size is never stated and it is easy to end up confusing one for the other. And it sounds like when people have heard that the 66 was a larger center carb than 65 that may be true of the bore only and not the venturi.

[Tom Vaught]

Originally Posted by pfilean

Yes Dick - that is a typo. And maybe a little confusion. What I meant was 1 7/16 for all three carbs in 57-58. And on closer rereading the manual says that is the bore size. In this manual the venturi size is never stated and it is easy to end up confusing one for the other. And it sounds like when people have heard that the 66 was a larger center carb than 65 that may be true of the bore only and not the venturi.

X2 Exactly right. They see the throttles are bigger and assume the venturis were also bigger. Between the larger throttles and the slightly smaller venturi the signal strength to the booster was probably a measureable improvement or the Rochester Engineers would not have done it.
Carb Engineers do stuff for a reason

Tom V.