750 or 800 cfm Q-jet on mild street 455? I've got a really nice John Osborne - built 750 Q-jet on my 350, and am in the middle of building a mild street 455 (maybe TA212 cam, but no bigger). My question is: do I *need* an 800 cfm Q-jet? Buick put the smaller one on the high-zoot '70 engines, and they apparently did just fine. I'm not going to rev over 5500rpm, and rarely at that. Something about the smaller (and more sensitive/efficient) primaries makes me want to stick with the 'little' 750cfm carb. Opinions? Comments? Snide remarks?
Might run too lean if it's calibrated specifically for a 350 and then run on a warmed up 455. Best talk to John Osborne for his advise.
I was figuring on possible re-jetting - but I'm asking: do I *need* the extra cfm of the bigger carb? Has anybody swapped from one to the other? What were the differences?
If you look up CFM calculator on google, and type in 455cu. in. and 5500 RPM, it will tell you you do not need any more than the 750. Honestly you dont need any more than that. If the fastest Buick to roll off the showroom had a 750, you dont need any bigger right? I know a guy who runs low 12s in the quarter with a 750. Im sure there are plenty who have gone faster. Call Osborne and ask him, im sure he can help you out there. I swapped from a 800 to a 750 and saw a world of difference in driveability, but this was on a 350 car and two totally different jet settings. Nothing comparable to your setup. Either way if you get your carb done by him its gonna turn out just fine.
Hi James, Hi James, To my understanding the 800 CFM carb is better suited for "performance"on the big block Buick. GSX Eric :3gears:
When jetted properly you will not see much difference between the 2 carbs. I'm running low 12's with a 750 cfm on my Pure Stock car. The 750 will give slightly better street manners and driveability due to the smaller primaries while the 800 will give slightly better high rpm potential.
The jetting and metering rods are the easier things to address; It's the internal passages and air bleeds that can cause a world of problems if not set up correctly for your application. Not that it can't be done, but it may be easier to start with a Q-jet that is already calibrated for a 455. My understanding was the 800's kept the larger engines running on the primaries a little longer during cruise conditions; it's the constant transition to the secondaries that can kill gas mileage. But, it you're hot rodding your engine, what's a little more gas....
I agree with Tommy. It's not the 750 vs. 800 difference to worry about, but the internal passages being 350 vs. 455 specific. If I were you I'd hunt for a 70 455 carb. Like Dave said, once set up right you won't see a difference based on cfm. And since you are concerned with economy, go 750.
My 71 455 had a 72 350 carb on it when I first got it and I thought it ran fairly well. I have since swapped in a 73 455 carb that I put a rebuild kit in, what a difference it made. Really woke up the 455! I can break the tires loose by just punching it from 20MPH now. Just my personal experience, yours may vary..
One of the reasons the 800 cfm unit appeared on the scene is due to emissions issues. The relationship between the throttle blade and the idle, off-idle and transition slots is critical to the smooth operation of this carb. The larger 800cfm unit allowed a more closed throttle, and therefore more precise metering of these critical circuits. The opening necessary to make the 750 idle correctly MAY cause idle and off idle metering problems due to the signals required by the carb. Ray
The 800 will provide better street performance, and driveability. No, you probably do not NEED 800 CFM; but consider the CFM in a different light. The 750 is 150 CFM on the primary, with a variable 600 CFM on the secondary. The 800 is 200 CFM on the primary, with the same variable 600 CFM on the secondary. On the 455 CID engine, the 200 CFM on the primary will give crisper performance on the primary side, and the transition from primary to secondary. Jon.
Guys, You can check out Jon's stuff on the following link. http://www.thecarburetorshop.com/ He does know a thing or two about carburetors....
Thanks for all the replies, there's lots of food for thought. I never looked at the bigger Q-jets as operating with less throttle opening... Jon, good to see you here, I've browsed your website quite a bit regarding old Ford (holley) 1-bbls.
James - glad it was useful for you. While that website represents thousands of hours of research (it began in 1967 using IBM puch-cards as a storage media), it is still incomplete. We invite any corrections or additons with original factory documentation. Jon.
Is the 750 carb from a 350 or a 455 originally? I'd say ask J. Osborne about waht he thinks and go from there. Keep us posted.
Is that a fact? Wow!!! No wonder my Q Jet drives so differently than my Holley 750. 150 CFM primaries Vs 375 CFM primaries! More than double!
Norm Dihle who races Buicks and builds Q jet carbs for many Board members has told me more than once that he has run the 750 and the 800 back to back on the same car at the strip with no difference in performance between the two.
It is very easy to get into a condition of comparing "apples" and "oranges" in these threads. An engine, if one neglects the spark and fuel, is an airpump. Most enthusiasts are aware of the equation: CFM = (RPM x CID) / 3456 Few understand the equation, even fewer correctly apply the equation. First, the equation is valid for multi-cylinder (4 or more) 4-stroke engines with a common plenum (or at least good equalization between multiple plenums) used for normal street use up through engines modified for amateur to semi-professional racing. It does not apply to 2 stroke, 6 stroke or 8 stroke engines, 4 stroke engines of 3 or fewer cylinders, engines without a common plenum, or professional race engines. The equation would be more meaningful to some if the constant 3456 were divided into its factors (12 x 12 x 12 x 2). The equation has cubic feet (CFM) on the left side, and cubic inches (CID) on the right. Therefore the (3) 12's are necessary to convert different units. The 2 is necessary because there is only 1 intake stroke for each 2 revolutions of the crankshaft (now it is obvious why the equation does not apply to other than 4 stroke engines). Most enthusiasts attempt to apply the equation to WOT (wide open throttle) conditions only. Many lie to themselves and end up with a carburetor that is too large (especially those who convert to a square-bore) carburetor. However, the factor RPM can be varied in the equation. Thus it is possible to determine approximate airflow requirements for ANY RPM in the engine's range. Most four barrel carburetor SHOULD be thought of as TWO carburetors; the CFM delivered by the primary side, and the CFM delivered by the secondary side. The spread-bore design came about in order to increase velocity in the primary side for street engines. It is quite possible for an engine to be undercarbed on the primary, and overcarbed on the secondary (a very mild 455 would be a good example here). But remember that the Q-Jet secondary is variable based on engine demand. Therefore the 750 CFM can be thought of as a 150 CFM carburetor with 600 CFM in reserve if needed; while the 800 CFM can be thought of as a 200 CFM carburetor with 600 CFM in reserve. Thus an 800 CFM square-bore with mechanical secondary might be grossly over-carburetion while an 800 CFM spread-bore might be perfect. If the engine does not require 750 CFM at WOT, then installing an 800 is NOT going to improve the maximum horsepower, but MAY significantly improve street driveability at less than WOT, because of the extra CFM on the primary. Testing that we have done indicates that a perfectly stock 455 may see little or no gain at the track with the 800 over the 750; however a change of cam and exhaust will see a gain of the 800 over the 750, and a fairly radical street engine will see an additional increase if the 800 is swapped out for an 850 Carter TQ. Contrary to what I have read, our testing indicates that, except for full-blown race engines (trailer race cars), no additional benefit is derived from the Carter 1000 TQ; although there is a benefit to using this unit on a professional race engine in a trailered race car. And while my own high performance street car is not a Buick, I have an 800 CFM Carter TQ on a Pontiac 350 that did improve performance at WOT. But this engine is pretty radical. Very few 350's will need 800 CFM. It has been our experience that the 455 on the street will benefit from the 800; while the 350 generally will not (the 350 probably will not suffer from the 800, it just won't benefit). The key is always to size the carburetor(s) to the needs of both the engine and the operating environment. Sorry to be so verbose, however sometimes it may (or may not) be necessary. Jon.
I think Norm was dealing with 11-12 second quarter mile cars and I would assume that the original poster is not going to have a car in that speed class since he used the world "mild'. Under these conditions I think the chances of improvement between the two ranks right up there with using bigger caliber spark plug wires and going from points to an HEI distrib. Your mind and the placebo effect are in the only things showing an improvement in performance. I do agree that with the more radical engines a bigger carb will show actual performance gains on the dyno while bigger caliber wires and an HEI still will not show gains on the dyno. Whenever JW has time to make those dyno runs I would like to see those combinations changed to demonstrate this once and for all. (To give you a sneek preview the May 2005 MCE, page 49 covered an Olds 455 W30 dyno run. The regular points distributor made 3.5 horsepower more than the HEI.)
72 and newer Buick 350 Q-jet carbs have some internal passages that the 455 does not like. These passages affect the idle and part throttle operation of the carb.
