Budman

OK, since my previous thread got blown away with the "Apache Thread Resurrection Debacle", I'll try this question again and see if I get any responses.

Can an HP500 (Carb) single-plane intake (made by Dart, I think) be bolted to a set of large oval-port heads? Will this combo work, or am I better off getting another manifold? I have heard some people say that it will work, while others say that the ports will mismatch. I know that rectangular-port intakes are different, but are the marine Darts the same port configuration as automotive, or are the runners smaller to help out with midrange?

Thanks for any replies - hopefully this time they will be more permanent!

Budman

OK - 17 reads and no replies.

C'mon guys, throw a dog a bone! Surely someone has tried this. Jim V? McCollinstn? Anyone?

Tinkerer

If it is the same as the rectangular port dart intake it will have a mismatch with the heads. make a pattern of the intake ports on the head and match it to the intake and you eill see that the corners on the intake are larger than on the heads. What I would do is weld up the intake to match the head ports - dont grind the head ports to match the intake. Yes this will be more work but I think that it will flow better in the midrange. I am running the same dart intake on stock ported GM rec. port heads and have the same problem but not as bad. I intend to go with different heads next year so that will solve my problem.

blown1500

OK, Budman,
You can just bolt on the big port intake and it will work pretty good. It is a mismatch, but the power loss is not too much. It would be cheaper to replace the manifold than to try to match the one you have, but unless I was after every horsepower, I would just bolt on and go. Use the square port gaskets.

Budman

Thanks for the replies! I have a Performer/RPM on there right now with a Q-jet. Baddog has the 800cfm Holley and Dart intake available off of his 502. I will probably start with just the carb - curious to see if this will make any difference on top end. Thought of trying the single plane to see if I can shift the powerband up in the RPM range a little. Right now I have more than enough mid-low power to get on plane and cruise. Besides, I'm sick of the Q-jet leaking down every time I start the boat after sitting for a while. I epoxied the well plugs in the float bowls, but that didn't solve the problem.

I think Waterfoul has an oval single plane that he said I could try out. Would that be a better choice?

Truckin

The fuel is not leaking out of your carburetor, it is evaporating.
The hot engine is heating the fuel in the small Quadrajet float bowl. A Holley has much larger float bowls and the fuel will not evaporate out so fast (or completely). Keep the Quadrajet but install a carb/heat insultor gasket between the intake manifold and carb to keep the fuel in the bowl cooler. Block off the exhaust crossover passage. Cool the engine compartment with more vents and an additional bilge blower.
The dual plane manifold is the hot ticket, you will loose power with the single plane!
Dennis Moore
Tech Editor
Wet Toys Section
Truckin' Magazine

Lee

iTrader: (4)

"The dual plane manifold is the hot ticket, you will loose power with the single plane!
Dennis Moore
Tech Editor
Wet Toys Section
Truckin' Magazine"

Dennis, Can you please explain.

Budman

Dennis,

Thanks for the reply! I notice the hard starting only after the boat has sat without running for a day or more. Doesn't seem to be a problem on hot days. I suspected the float bowl plugs because it is a noted problem with the Q-jets.

This engine has never really run up to my expectations. It should be a good combination in this boat, but it does not run any better than stock 7.4's in other boats of the same vintage and make. I am suspicious of a number of possibilities for this. Maybe you can provide some ideas.


For one, I am running a 1.36 ratio Bravo instead of a 1.5. I have seen a lot of discussion stating that the 1.5 will allow me to run a taller prop, which is more efficient.

I am running a set of early '70's open chamber oval port heads. They have had some minor bowl porting done on them, and have had the larger 2.19 intake valves installed. Regretfully, I took my engine builder's advice and stayed with the 1.72's instead of the larger 1.88's on the exhaust side. I am suspicious that this is another bottleneck.

Cam is a Comp Cam's Xtreme Marine grind - 226*/236* @ .050, .540/.550 (I think), 112 LSA. I have head that this should be a pretty good cam for this application. Also running Crane full roller rockers - 1.7 ratio.

Forged TRW pistons, small dome, around 9.2:1 static CR. I had to find a head with at least 122 cc chambers to get away with these pistons because of the dome. Wanted to stay below 9.5:1 - probably would have been better off going to a flat-top design with 119 cc chambers.

As mentioned before, marine Q-jet (750 CFM) jetted up with rods and hangers from a 502. Might be a little rich for a 454, but I would rather play it safe with the 9+:1 CR. Stock flame arrestor.

Stock Merc exhaust with Silent Choice (yes, I am sure that it is in "thru-hull" mode when making top speed passes.

Not sure where to go next. I have a number of things I want to try - carb is one of them. Basically, I am going to have to analyze one component at a time to see what makes a positive difference. Better exhaust would probably make it breathe better, but that is an expensive upgrade for me.

I feel I should be easily outrunning stock 7.4's. Sounds like some other folks on the board have had less improvement than expected out of 7.4L upgrades - Whiteknuckle and Jayl13 come to mind. Must be something all of us did wrong - like not buildind a 502!

Thanks for all the help, guys!

BOB ONEIL

I think the exhaust is the next thing I would try.

I've got a simular set-up with a 502 and it helped allot. I'm going to put the carb (800 of an HP)on the stock dual plane manifold with a plastic (phenolic) open spacer this year. I blocked off the heat transfer with a new set of intake gaskets from Felpro #1211.

You need to rejet that carb if your going to use it. It's set-up for the single plane manifold.

Bob

Budman

Bob,

I had planned to rejet if I use the carb on my dual-plane. Thought about just swapping manifold and carb together, in which case it would probably just be a little rich.

If I tried to run the carb on the dual-plane as-is, what problems would it cause? Lean condition in one or more cylinders, rich in the others?