Mercruiser 420's single plane or dual plane??
 

I am betting this has been asked, but I am not really finding anything on this to tell me much, I have mercruiser 420's and when I built them I had free parts on hand that I used..

4 years ago, I built.....stock shoprtblock, Merlin heads that were comparable to stock 420 heads, used 420 cams Holley single plane intakes, and 800CFM HP500 carbs..

Never got a good baseline since I bough boat, oil line broke and smoked one engine, pulled both and rebuilt..

With 23P props the boat spins 45-4600 @65mph WOT with 21P it spun 5000..@63mph. put on 24P cleaver 3 blades it spun 5000@63mph

Best ever was 66.9@4700 in rouhger water with a set of repitched quicksilver 24P 3 blades I had ...

I asked about 420 specs the other day, some guys said heads are too big, even though they are close to the stock merc ones I don't know if that is it, but I am wondering if the single planes are killin me all around??? I knew I would lose low end with them and it still planes fine and all that, but can I also lose higher rpm hp too? should I try dual planes??

Has anyone tried both on same engines??

Seems like your getting your best speed at the lower rpm's but is that good for the motor?? I have a similar set up spinning it around 5000 wot, i never tried next prop size up bc i didnt want to bog down motor bob mandera told me max hp was at 5200 rpm

The merc 420's came with dual planes. Those Holley single planes, probably strip dominators, are recommended for 45-8500 rpm. Most popular dual plane out there the edelbrock RPM air gap has a operating range of 1500-6500 rpm. I think I would give them a try if all your getting is 45-4600 prm's.

I think you need to prop the boat down and get into the RPM range. My 420's ran the best around 5200RPM. I don't believe that you are leaving that much torque on the table with the single planes. Whether you have single or dual plane you will not gain 400-600RPM. Depending on year I believe the 420 came with a Wieand X-cellerator single plane intake. Get the RPM's to 5200-5400, 35* timing and I think you will be doing good.

Brand Crane Cams
Manufacturer's Part Number 132561
Cam Style Hydraulic flat tappet
Basic Operating RPM Range 2,800-5,800
Intake Duration at 050 inch Lift 228
Exhaust Duration at 050 inch Lift 236
Duration at 050 inch Lift 228 int./236 exh.
Advertised Intake Duration 298
Advertised Exhaust Duration 306
Advertised Duration 298 int./306 exh.

It is so odd that I dont make the same power as other guys with 420's, all other examples I see with similar boats running 23P props at 5000-5200... thats why I am stumped, I have asked other 320 Baja guys so I am even comparing same hull and trs drives.. even a friends stock 420 357 Formula can spin 23P mirage at 5000-5100 and that boat is way heavier... I am so lost with theis at this point..unless there are issues somewhere else I cannot find, engines are not tired and are identical ..

If you're interested I have a factory duel plane I'll sell you cheap.. I had two but I'm down to one now, but I'm sure you could get a mate within a day or so. If the intake that you're running is really rated from 4500-8500 I'd say its safe to say that is at least part of your problem.

The 420s I had, had single plane weiand intakes, and they use the same Holley 9022 800cfm carbs as hp 500s

Some were duals, some were the X-celerators but none ran an intake designed for all out ultra high RPM racing. I'd be willing to guess he's losing a lot of torque where he needs to be running his engine, not to mention those merlin heads are not helping matters any. Basically he has no port velocity, the intake and heads would be fine on a 540 but would stink on a 454. The best thing would be to have wedges installed in the intake runners of the heads and change to a dual plane intake. Unless your over 6000 rpms on a 454 there is just not need for a single plane! I think this is a classic case of bigger is not always better.

The 420's in my 311 came with the Weiand single planes which rotted out after two years. I replaced them with dual planes with a brass insert in the thermostat housing and never noticed a bit of difference in performance. Boat ran at 5200 with both.

Are you absolutely positive that your tach is reading correctly?