I have a 71 qjet that I had rebuilt by Ken at everyday performance and I believe the 71 had 750 sfm. I am putting the carb and intake onto my 72 GS/GSX clone and want to know if it is worth changing to the 800 cfm? I am not sure how much diference it realy makes. If anyone has had or driven both please let me know. Thanks in advance
I've back to back dyno and drag strip tested the smaller "750" cfm carbs against the larger castings. On a 455 Pontiac engine making right at 500hp, the smaller carb lost 11hp (peak). Both carburetors were dead even to apprx 4400rpm's. At the track the larger carb ran .02 seconds quicker and .30mph faster. Not really much difference in actual vehicle performance, simply because the engine doesn't spend a lot of time up near peak power unless it's a full race car with a loose converter and pretty tall gearing. The biggest difference is having more cfm available when you drive the car on the street. It allows a higher vehicle speed to be reached with less throttle opening when you aren't on the secondaries. Heavy part throttle response is improved some as well. In the big scheme of things it's not really worth the swap, unless your goals for the vehicle are to run the absolute fastest that it can at the track, and you are making pretty big power with the engine.....Cliff
The 71 Qjets that came on 455 cars were 800 cfm. For 350 car Q jets were 750 CFM. So the correct answer depends on the carb number that you have.
Thanks for the detailed answer. I am not a speed demon so I think I am fine right where I am with the carb. Now if I can get it on the car and get the car on the street.
Just to add a tidbit to my post above. When I did the back to back dyno testing, the engine was a 455 with 10.4 to 1 compression using Edelbrock heads. The larger carburetor made 11 more HP. The dragstrip testing was done with my vehicle, powered by a 455 with 10.48 to 1 compression, small HR cam, 10" converter and 3.42 gears. The engine made around 514hp on the dyno, and pushed my car to mid-11's at 117-118mph. So I would say, going by the testing we did, that the smaller "750" cfm q-jets are plenty of carb to support a 455 engine making at least 500hp. The fastest runs I know of from someone using a "750" cfm Q-jet are mid 9's in the 1/4 mile, in an early Camaro powered by a 355cid SBC engine......Cliff
I am sure the 750 cfm carb will work great for you. I just want to point out that the extra CFM is on the primary side so when driving around in the primaries the car has more power and better take off. I only use the 800 cfm Q jets now, after finding out how much better they are on the street even with the 350 engine. More power for the street. :3gears:
In 1971, ALL carburetors that went on Buick 455's were 800 CFM. If your carburetor is a Buick carburetor and it is a 71, the only way it could be a 750 CFM carb is if it is for a 350. The 350 carburetors had different idle calibrations and passages for the smaller engine. Post the carburetor number.
Here is as best as I can tell the # on my 71 carb. 7041544 MH 0851 That is how it is setup on the carb.
That's a 350 carburetor, so it is 750 CFM. Ken may have modified the idle system if you told him it was going on a 455.
I've found that the 800 qjet's idle system works just right on the 350 motors. On the 455s it couldn't be made rich enough especially with some mods. The 800 on a stock 350 can be made to idle perfectly. Remember they were starting to tighten emissions in 71, they didn't want idle circuits to be able to be set rich....
