[67GP428]

Is there much difference in drivability between the 2 years? I know the 65 primary carb has a smaller bore than 66. Would the feel a difference starting from a dead stop and driving a lower speed? What about at WOT? I have a complete 66 tri-power for my 65 421HO. Have my sights on a complete 65 tri-power for my car. Don't know much about either. Scott

[Tom Vaught]

The 1965 "Small" Tri-Power center carb has the following:

a) Small Airhorn inlet vs the end carbs "2G" inlet
b) Small Throttle Blades vs the end carbs larger 1-11/16" throttle blades
c) The 1965 "Small" Tri-Power center carb does have a slightly larger venturi vs the 2G (1966) center carb. That very small increase in Venturi airflow does not match the disadvantage of the 65 smaller airhorn and smaller throttle blades which really control the airflow thru the carb.
So the 65 Tri-Power Guys will brag about their "venturi being larger" just like the 428 Pontiac guys used to brag about their engine being 1 cubic inch larger vs the 427 chebby guys engine.

It is the WHOLE SYSTEM DESIGN not one small part of the design.

Put on the 66 Tri-Power system on your 65 421 HO and the engine will be happy. 421 has more than enough low speed torque to offset the very slight venturi effect of the 65 small carb.

Always run the larger carb vs the smaller carb if you can get the same basic drivability for most parts of the drive cycle. 600 cfm holley vs 780 holley being a good example.

Tom V.

[Dick Boneske]

I agree with Tom. The small center carb was used with larger end carbs from '59 to '65 with engines from 389 to 421 ci. Drivability with all of them is very good.

The center carb used in '66 was had smaller venturis than the large 2 bbl. Rochesters used on single two barrel 400's (1.190" vs. 1.250"). That carb was picked by Pontiac engineers. I believe, to give better drivability than a larger carb while offering decent performance.

For more extreme performance requirements, there are many larger Rochester 2 barrel carbs to choose from for the center carb on a '66 Tripower. IIRC, the single two barrel used in '57 had venturis equal to the '59-'66 end carbs. I often wonder what the venturi size is of the single two barrel carb used on the '71/'72 455's.

I don't think any of the small base Rochester two barrel carbs had venturis larger than 1.250", the size of the '59-'65 center carb.

[67GP428]

Great info!!! Thanks Scott

[Tom Vaught]

Dick, I have seen 2G center carbs with 1.375" venturis like a 750/780 Holley and the same 1-11/16 throttle blades.

Unfortunately a lot of those carbs disappeared to wind up on 2-bbl dirt track engines. They were put on 455 cid 2-bbl engines I believe.

You see one occasionally on E-**Y.

Tom V.

[Dick Boneske]

As we know, the end carbs have venturi's of 1.310" and several other Rochester carbs I have also are at 1.310". Although I've modified several by grinding material off the venturi area (some to 1.500"), I've never found a stock carb at 1.375". I agree, Tom, they were probably the ones used on the 455 2 bbl. engines in '71 & '72.