Quadrajet limitations?
 

I'm wanting to replace the old Holley carbs on my 500 hp 454's.
I'm a big fan on Quadrajets but I've heard that at higher power settings they don't distribute the fuel/air mixture evenly across all the cyls due the the small primaries and large secondaries.

I've also heard the the float bowls can run dry when ran at WOT for extended periods.

Any thoughts on these issues?
What size Q-jet would work best on a heavy breathing 454? I was considering 850's. The current carbs are 760cfm.

Where are you planning on buying a quadrajet marine carb ,of any size

Didn't mercruiser use quadrajets?

They DID. It's not possible today to buy a new one

Not sure if Merc ever ran the Q-jets on anything with more than around 400 HP. I think there were some 800 cfm carbs on some early pontiacs, but they are getting hard to find, and might not work on a marine application anyway. Then you have to figure out what jets and rods to run with them, along with a plethora of other adjustments. All for probably little gain. If your Holley's are running fine, I would leave them. I am a fan of Q-jets on stock or near stock applications, as they are trouble free once dialed in properly, but they have their limitations. I do know that there are plenty of healthy drag cars running them in class racing, and they have a lot of tweaks that will make them perform, but by the time you spend that money, you would probably be a long way towards going with one of the aftermarket FI setups.

Mercruiser used them on everything back in the 80's 235 hp , 260 hp, 330hp trs, 365 hp trs, 4 cylinders , etc... Good carb. economical at part throttle. Now Hard to find. And yes Holleys are a better for race applications, but are not as economical.

After a bit more looking around, the biggest marine versions I can find are 750's. There's a number of them available on the web, of course they are used/rebuilt.
I found upgraded needle/seat assemblies that allow faster bowl fill, but still not sure they wouldn't run dry during extended WOT runs.

The Holleys I have now are the old 4011's. The idle quality is terrible. They're either idling too high or they're stalling the eng. Once on plane they are OK, but around the docks, they are a real pain.
I put rebuild kits in them, replaced the power valves and adj'ed the idle mixture numerous times, but no help. They gotta go.

I think some of the drag and circle track guys would modify or eliminate the anti-slosh baffle in the float bowl to give a little more capacity. I was running a regular marine Q-jet on a mildly modified 454 (making maybe 375 - 390 HP) and just changed the jets and rods to richen it up some, and never ran out of fuel. I think as long as your fuel pump and lines can keep up, you will probably be OK for an occasional full throttle burst. Now if you are running poker runs or your only speeds are idle and WFO, then you probably need to stick with a Holley anyway. What kind of intake are you running - does it even have the correct flange to accept a Q-jet spreadbore arrangement?

I used to have a spec sheet that had the rods and jets that were used on a 502 application. If I can find it I will pass it along.

If you stay Holley your best bet might be to try to find someone selling a pair of the HP500 Holley 800's. That would probably work well for your application. Merc used to build a blue racing version of the 454 that used the same heads, cam, intake, cam and exhaust as the HP500.

The Super Stock guys are running qjets up to about 900hp I highly doubt you will run out of fuel at 500 if you do it has to do with fuel delivery and not the carb. They made 750's and 800's the 800's are hard to find.

For simplicity I'd stick with the Holley base or maybe a BG.The problem with a lot of the alleged reman marine quadrajets is they aren't really marine carbs .They are modified automotive .A lot of reman "junk" out there in internet land.

"I think some of the drag and circle track guys would modify or eliminate the anti-slosh baffle in the float bowl to give a little more capacity. I was running a regular marine Q-jet on a mildly modified 454 (making maybe 375 - 390 HP) and just changed the jets and rods to richen it up some, and never ran out of fuel. I think as long as your fuel pump and lines can keep up, you will probably be OK for an occasional full throttle burst. Now if you are running poker runs or your only speeds are idle and WFO, then you probably need to stick with a Holley anyway. What kind of intake are you running - does it even have the correct flange to accept a Q-jet spreadbore arrangement?"

I do a lot of mid-power cruising, so part throttle performance is a must and fuel economy would be nice too. I actually do very little full-power runs, but when I do, I don't want the bowls going dry. The fuel delivery is no problem.
The intake is a world castings model that matchs my Merlin rectangle port heads. I'm pretty sure the carb flange is a 4150 so I would need an adapter plate if I went with a Q-jet.

There's still the question of equal fuel/air charge reaching the cyls with the Q-jet. I don't believe the mixture would be an issue but maybe the rear cyls may be receiving more charge than the front cyls at WOT?


"I used to have a spec sheet that had the rods and jets that were used on a 502 application. If I can find it I will pass it along."
That would be great! Thx..


"If you stay Holley your best bet might be to try to find someone selling a pair of the HP500 Holley 800's. That would probably work well for your application. Merc used to build a blue racing version of the 454 that used the same heads, cam, intake, cam and exhaust as the HP500. "

That sounds like a good choice if I do wind up with Holleys.


"For simplicity I'd stick with the Holley base or maybe a BG.The problem with a lot of the alleged remain marine quadrajets is they aren't really marine carbs .They are modified automotive .A lot of remain "junk" out there in internet land. "

What is a BG? I know the marine versions have the fuel bowl vent dumping into the throat and the throttle shaft bushings have a tighter fit around the throttle shaft. Any other differences? You're right abt finding good carbs. That will take some digging.



"The Super Stock guys are running qjets up to about 900hp I highly doubt you will run out of fuel at 500 if you do it has to do with fuel delivery and not the carb. They made 750's and 800's the 800's are hard to find."

I had a 800cfm Q-jet on my old 67 GTO.. It was a great carb. Set it up once and be done with it. All my friends with their Holleys were constantly tinkering, fixing fuel leaks and never did get them to run right.
I've been through 3 power valves in one shortened season with my current set-up.

Super Stock Guys have many mods done to the inside of their Quadrajets.

Your buddies with the Holley's did not know what they are doing. Not being confrontational. Just stating facts.

Same reason why Quadrajets got a bad rap.....ie: from people that didn't know how to work on them.

No offense taken abt my buddies mechanical expertise, that was the funniest part..

The majority of the complaints I've heard abt Q-jets is that they fall a little short when it comes to WOT performance compared to Holleys, which seems to be true, but the Q-jet's idle quality and mid-throttle economy are far superior from what I've experienced.

To be fair, I haven't messed with a Holley in over 30 years (other than the 4011's which are close to being that old). I'm sure they have made some improvements since then.

I don't like the 4010/4011's (especially the later 2 piece one's) and the Spreadbore 3875's I think they where. But, that's probably because I didn't know what I was doing with them, even though I thought I did. LOL. Much easier on performance motors to use the 4150/4160 and of course King of all, the 4500's.

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"What is a BG? I know the marine versions have the fuel bowl vent dumping into the throat and the throttle shaft bushings have a tighter fit around the throttle shaft. Any other differences? You're right abt finding good carbs. That will take some digging."

BG= Barry Grant...Marine carbs are designed to overflow into motor ,not all over intake .I've been told different things about other differences but in reality, Not sure!...But if i'm bolting a carb on a boat motor ,I'm going to C.M.A. ,just in case. If I've done everything right and to abyc spec ,I have no apologies in any event

thought Edelbrock made a nice spread bore carb with an easy tune 750-800cfm

There is an ad every month in the back of Chevy HiProformance mag selling spreadbore carbs. Maybe they could make a marine version.

500HP 454, I'd go with these

http://www.cpperformance.com/p-16704...arburetor.aspx

bingo!!

Pretty sure the Eddy's are based on the old Carter AFB's. Center fuel bowl like the Q-jets. Someone please correct me if I'm wrong about this. Have not fooled with one first hand.

Two bowls .one on each side

looks like a great carb at a fair price.you can,t go wrong with a quickfuel carb.

What? No ditch the carbs and go EFI Mike. I'm proud of u:lolhit:

Call SMI Induction at 714-843-9169. They are the newest QJet experts.

I've been looking at those for a while. I was wondering abt trying to tune it on the water. Jet changes on the water would be tough but air bleed screws would be easy. Just don't know how much the afr can be changed by swapping air bleeds only.

Changing jets on the water isnt that difficult....

No, it's not that difficult but air bleeds would be much easier and faster, plus you have to dispose of 4 float bowls of fuel somewhere every time you change jets.

Yes but they are squarebore. The Carter Thermoquad was the spread bore version. I have a shelf full of Competition series 1,000cfm Thermoquads along with more than a few complete Carter Strip Kits that I've accumulated over the years.