Does anyone rebuild a quadrajet to flow more CFM i have L88 valve ported heads i dont want to change to a holley the hood wont clear. Its a 68 427. Thanks for any help

 

I seem to recall that the Buick Gran Sports, circa 1970-71, with the 'Stage 1' package, came with a Q-Jet rated at 800 CFM.........

 

All the large casting 800 CFM Quads will flow to right around 890ish based on Cliff Ruggle's testing.
Has to be set up right to take advantage of it.
Get ahold of Lars here or Cliff and I am sure they can help with a 'recipe' for success.

 

On second thought, I think the early 70s Trans Ams, with the Pontiac 455SD engines, may also have had the 800 CFM Q-Jets.

 

'79 and '80 L-82s have 800 CFM Qjets. Maybe other years? Not sure.

 

Holley and Q-Jet carbs are exactly the same height: 3.27" You gain absolutely no hood clearance advantage by using one or the other.

That's a good idea if you can find one and if you don't mind paying the current going rates for those carbs at $2500-$3000.

If you want an 800 cfm Q-Jet, there are several models of the M4M Chevy carbs that are 800 cfm and that typically sell for under $200 for a good core. For a performance application, they need to have the Main Air Bleeds and idle circuit reworked to be responsive to the manifold vacuum and fuel flow requirements typical on a high performance engine.

However, your heads and valves have nothing to do with your hood clearance problems, and the two carbs are both exactly the same height. That being the case, you don't have a carb problem or problems caused by the valves in your heads (which is irrelevant). You have an intake manifold problem. Use the correct manifold for the engine (the LS6 manifold is the GM manifold for a Holley carb) and it will clear the stock hood just fine.

 

What is "L88 valve ported heads"?

If you want to go this direction I'm using the LS6 Intake manifold with a 4150, Fit's just fine under the BBC hood.

 

The heads have L88 valves and have been ported. Ive always heard that with a holley it wont clear the stock hood

 

your intake manifold choice and the type of hood you have will be the main consideration as to whether the engine clears the hood.
Pat

 

That's completely irrelevant to carb choice and hood clearance.
Versions of the 427 came from the factory with Holley carbs, and they clear the stock hood just fine when used with the correct intake manifold.

 

I don't know anything about your car, but from the above statement I would assume that you have the low powered L36 390 hp and you had some head shop install the larger diameter valves in your oval port heads.

So my next question would be: What has led you to the conclusion that you are lacking carburator CFM? There are online CFM calculators where you plug in your 427 ci, then mild cam probably sub 80% volumetric efficiency times maximum rpm. I just ran one version and if you are spinning your 427 to a red line of 5800 rpm you only need a whopping 609 cfm carb. But for pure dual planes a 750 cfm is more than enough to feed even 6500 rpm.

The cam is the real heart beat of a engine. A roller cam will make your motor run like a champ with no increase in carb size.

 

Before I spent any money on a new/bigger carb, I'd plumb a sensitive differential pressure gauge across your carb, and see what the actual restriction (pressure drop) is.

 

Here’s the biggest Q-Jet , a 850 Edelbrock, I agree with the others but this answer the original question

https://www.ebay.com/itm/11594557654...Bk9SR4ibrfzvYw

 

I believe that these photos are of a 1979 Buick Q-Jet, 17059247, dated 0969) that is 800 cfm. According to Cliff Ruggles, "it makes a nice HP carb and converts easily to electric choke." If you're interested in having it rebuilt, I'd be willing to get it to you for the price of shipping. You'd need to check with Lars or another knowledgeable carb person to make sure it would work in your application. Just a thought.

 

Lots of 800 CFM Qjets , even plain Jane smogger 77-78 Pontiac motors, Not sure if the 850 Edlebrock is really any bigger or just difference in flow pressures-like a Holley 850 4781 has the same size butterflies and venturis as an HP 1000. They used a different flow bench pressure. There are pictures out there comparing 750 Qjet primaries to 800 cfm venturis.
Any smogger carb will need idle tube and CRO tube increased in size to idle on idle circuit if you have any cam in it

 

I have a 1968 Corvette set up as a L88 Clone. I have many parts used on the factory L88 Corvettes including "L88 Pistons" that match the valve pockets in the L88 heads. No really I have a factory 427 re-built to be a L88 clone and I have lots of experience with different carburetors fit and which don't. I have a L88 factory Hood with the air box in it. The stock L88 Hood will fit perfectly with the 427 engine. You will need a really low base plate for the air filter. The L88 parts are commonly available out there at ZIP Products and others.

Having an engine that wants more air, my last real carburetor was a AED Custom built carburetor that would flow over 1000 cfm all day long. A.E.D. builds custom carburetors starting with Holley Casting's. I used that Holley for years as it fit and worked with my cold air intake Hood and helped my high compression engine run on pump gas most of the year. I spoke with an engineer and we spent almost an hour and reviewed every detail of my 427 just so they could make me an awesome carburetor. When they delivered it the carburetor was completely set up, I didn't have to change anything, not even a single jet. Incredible products with Awesome Service if you need a carburetor.

If you are interested, the Holley EFI systems are great and they fit under the hood moderately well. I have a Holley Stealth Sniper under my L88 hood and it works. The foam gaskets get crunched but it seals the hood to the carburetor. Efisystempro.com is the name of the Holley EFI dealer that gives you 2 years of POST-SALE Support to get your car working right. A Great place to shop or to learn about the issues with the Holley EFI systems. I bought mine and installed it without any help from Efisystempro.com. The guys at Efisystempro have great prices and fast delivery.