cp5899

iTrader: (1)

I'm just finishing new 468ci build. I had a 330 hp motor to start. New motor has 781 heads ported and gasket matched along with 2.19 and 1.88 valves. Also, going with a high rise dual plane intake ported and gasket matched to the heads. Cam is .507 intake and .510 exhaust with 224 duration on intake and exhaust. I have stock manifolds as of right now, but I'm in the middle of buying a set of full length headers (not that it will make much difference as I've read, but I do plan on another motor were they will come in handy).

My question is about the carb. I have a q-jet to run as of now and want to go with a bigger holley style carb, but I have friends telling me its not going to help and will just kill my fuel mileage. Well, I'm not so much concerned with fuel mileage (obviously, I own a boat), but I also don't have unlimited funds and want to add money where it will help. By no means am I trying to be cheap, just want to know if there are real benefits to be gained with a larger carb. I was under the impression its a must have being I'm adding performance to the motor. That is until friends said don't bother. I'm learning the ins and outs of performance boating and this is were I come for real world advise.

Thanks in advance!

cp

mcollinstn

QJet is a good carb. Easy to set up, and can make good power with it.
Carter (Edelbrock) is a good carb. Some think it's even easier to setup.
Holley is a good carb, too.

Your QJet is already a marine carb and is in the 750 cfm size range.
It's a spreadbore which gives arguably better low speed mixing accuracy.
It also easily supports 450hp.

Your motor will be limited to under 5000 rpm due to the cast internals.
Your cam choice is suitable for that as well.
A good dual plane intake (GM aluminum hi rise, Performer RPM, etc) will be much better than the cast iron low rise you just ditched.

Carb? My thoughts are that you already have a good carb that will support what you wish to do, and is already a marine carb with the appropriate accomodations.

If I had "no carb" and was buying one brand new, I would buy a Holley.
If I had a good QJet and was wondering what to run? I would run the QJet.

As far as which carb will give you the MOST HP?
If you leave the choke horns on, and leave the chokes intact, and run a standard marine flame arrestor, and it's on the motor you described......
Then whichever carb you get tuned best to your motor will make the most hp.

If you want to start modifying and cutting choke horns, polishing parts, and messing with all of it, you have much more flexibility with the Holley.

MC

cp5899

iTrader: (1)

Thanks for the highly detailed reply. I guess I will stay with the q-jet for now until I can find a deal on a nice holley that I can modify.

Thanks again

fireboatpilot

+1 to everything Mcollinstn said. My 2 cents is that if you really want a new carb on your new engine then I would look at the Edelbrocks, easy peasy. Chances are if you build a bigger more powerful engine the 750 cfm will not be enough and above that your only choice is a Holley style carb. Even if you bought a Holley now you'd have to buy another bigger one for the bigger engine. Think ahead, it sometimes can save you a step and maybe a few bucks.

Budman II

iTrader: (1)

I ran a nearly identical setup on a warmed over 454 + .030. I went with the Performer RPM with the dual pattern flange that will accept either a spreadbore (as in Q-jet) of the square bore Holley. The original QJet that was on the boat was set up for a 350 mag, so I had to change out the jets and hangers. If you were already running a 454 with your current QJet, then all you probably need to do would be to just go with a slightly richer (smaller) secondary metering rod. Very easy to change - all you need is the appropriate torx head, and you can change it while the engine is running! (just don't drop that screw down the carb throat!) FWIW, the QJet ran great on my setup for 10 years with nary a single tweak or adjustment. Just put the hatch down and go. Thing would happily idle forever. The Holley that I am running now seems much more prone to loading up after a long idle. Wants to "clear its throat" on occasion.

cp5899

iTrader: (1)

The q-jet that is on it ran flawlessly for the 95 hours I put on the boat with the last motor. Idles good, starts good. I jussi didn't know if I needed more cfm for the motor. If there isn't much of a gain I'll just put the q-jet back on and call it good. At the point next his winter when I start building a 540 I'll plan on buying a carb for that motor.

bunky1957

I freshend up a 454 mag over the winter. Used a little over stock cam and 9.0 compression. It made 473 hp with dry dyno headers and the stock Q-jet and dual plane. Put on the stock 6 1/2 x 2 inch 454 mag flame arrester on and made only 446 hp on back to back pulls.

cp5899

iTrader: (1)

That's good to know! What are some better performing options over these style flame arresters? I'm running one off of a 330 motor so it must be garbage as well.

trex232

I had basically the same setup as you and ran the Qjet for a year, with a Gaffrig flame arrestor, then changed to a 800cfm holley it woke the motor up actually gained a few rpm. You can always sell the 800 they never last long in the swap and shop section. I think a 750 would have been fine though. That is just my 2 cents and I have always been a Qjet man