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Mercruiser 420's single plane or dual plane??

Old 05-10-2012, 10:06 PM
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Originally Posted by Full Force
Found my paperwork....

what I wrote down....

Merlin 91

1-043

D25


thats the markings I found...

Some say 345CC runner...some say smaller 320cc... still not sure.. UGH!! if the stock Merc stuff is around 320 would 25CC kill it that bad??
It would hurt a bit, as the 325CC GM heads were too large for a 454. 345's would be way to big for a 454. They are even large for a marine 540CI. If you wanna swap heads, and gain a bit of power on a budget, get yourself some Dart 308 Iron Eagles. They will wake up. I just read a article where they dynoed a N/A 454 with a set of 049 GM ovals, well prepped, vs a stock set of Dart 308 iron eagles. 80HP gain over the 049's at 5500. I believe summit sells their "summit" brand 308 iron head, which is made by dart, but at a cheaper price.

My heads are basically the iron eagles, but aluminum. They dont make mine anymore, in the 308cc high velocity runner. I just dynoed my 468, 420 blower and 6psi of boost. No chillers. 804HP. I've seen other builds similar to mine, using the GM rect ports. If they made 700-725HP with equal boost they were lucky. The GM rect ports exhaust just doesnt flow well. And the ports are too big for a 454 on the intake side.

I wouldnt bother with going to a dual plane intake. If you wanna wake them up big time, do the heads and a roller cam.
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Old 05-10-2012, 10:20 PM
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There are two guys that I suggest you talk to that will set you straight with no BS. Bob Madara and Jim Valako. If your even thinking about new heads buy the AFR Oval 265, IMO the best 454 head made. I ran it on my last engine and the throttle response was insane! It will out flow a merlin 345 runner but its only 265! Valako can wedge your merlins to work as well. Either way call one or both of them and get some straight answers. If you want to contact them PM me and I'll send you their numbers, both are great guys and I'm sure everyone that everyone that has contributed here will agree! I'm building a pair of 420's with bigger cams that I bought from Mild Thunder and asked Scott from CFM Technologies what intake I should run and he said I should run and he said the Edelbrock RPM. I still think your heads and intake are killing you!

Last edited by endeavour32; 05-10-2012 at 10:29 PM.
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Old 05-10-2012, 10:48 PM
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heres some good reading about BBC cylinder heads. Enjoy!

http://www.popularhotrodding.com/tec...k/viewall.html
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Old 05-10-2012, 11:01 PM
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Default gm dual plane vs. single plane

I did a dyno comparison on the GM marine dual plane vs.the Merlin single plane intake.The dual plane only gave up 5 horsepower to the Merlin single plane intake, 599 vs 604 hp.This was on a 516 inch motor at 5200 rpm.The midrange was as much as 50 ft/ lbs more torque with the dual plane.
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Old 05-10-2012, 11:24 PM
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That article makes some good points. I see soooo many threads, posts, and talk about 454CI upgrades. All I ever read is small oval port heads, 750cfm carbs, dual plane intakes, and so on. If i read one more recommendation on how the 049 and 781 GM ovals are so outstanding on a 454, i might puke. lol

While the 454CI by todays standard, is small compared to the ever so popular 500+ cubic inch big blocks, it by no means is a small engine. While yes, there is no substitute for cubic inches, a proper combination of parts, is where its at. A well thought out 454 build, can easily outrun a poorly thought out 502 build.

20 years ago, small ovals, dual planes, small carbs, were the hot ticket on 454 marine engines, and even street car engines. Mainly becuase they make good low end torque, and back then, you weren't doing more than about 5200RPM with a flat tappet hydraulic. There just wasn't a need for big single plane intakes, dominator carbs, etc, if you were only turining 5000rpm. Back then, dominators and single planes were strictly for drag racers.

Times have changed. In this day and age, I see no good reason to build a tow rig engine for a offshore boat. In my boating, I can care less if the engine makes an extra 30HP at 5000, if it can make an extra 100HP at 5500. To go fast, you need the HP at your wot rpm, not at cruise speed, if thats your goal(who isnt looking for top GPS numbers).

What Im getting at, is to make good power out of a 454, it can be done. You just need to be willing to spin the RPM to take advantage of the modern cam profiles with the added lift, and the added airflow capabilities of say a dominator and a single plane intake . If 4800-5000RPM tickles your fancy, by all means stick with the dual planes, small carbs, and small heads.

Get yourself a bob madera roller cam, a nice set of afr, brodix, or dart heads properly sized, a good sized carb, single plane intake, and I think you just might be PLEASANTLY surprised!
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Old 05-11-2012, 05:14 AM
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The biggest thing is, lets face it.... I have a 32 foot 86 Baja, #1... it's never gonna be "fast" #2.... I am not sinking a million dollars in a boat worth 10,000 in this market, I used the parts I got because I bought a huge pile of parts 5 years ago and it seemed like a good combo, I was wrong not knowing BBC stuff then too well, I seriously think I might get stock 420 heads and intakes and keep em stock 420, I thought one day of doing aluminum heads and roller cams... but thats a lot of money to spend when the boat will barely crest 70-72 mph, I am not looking for a fast boat, just want to run what the boat should run which is 68-71 average for this combo and hull... I can get stock heads and intakes way cheaper then more aftermarket stuff and still run within 2-4 mph, if I wanna go fast I got friends with fast boats to go on... thats outta my league...lol

I can sell off Merlin heads and intakes to pay for my stock stuff and probably make some money too...I know it seems like a step back but will probably be a step forward in performance in my case...
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Old 05-11-2012, 05:26 AM
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Starting over may be the way to go here....
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Old 05-11-2012, 06:55 AM
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I also have a stock '90 454 MAG dual plane aluminum intake if you need one. i know someone else said they had one also so that would get your intakes sorted out at least!
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Old 05-11-2012, 08:05 AM
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Call the guys that I mentioned they will set you up on the right path. A agree with Mild Thunder, 049 Ovals have there place but your not going to make big power with them. Rectangle port heads are also not great for a 454, they won't come to life until 4500 rpms or so but they can be modified to work well with wedges and some porting. I'm pretty sure Rookie did with with some merlins on his old 468's.

The AFR 265 oval
Lift .40/.50/.60
CFM 295/326/342
Dart 308 Rectangle Port
CFM 275/318/345
Dart Pro 1 310
CFM 275/318/345
Merlin 320
CFM 272/308/321

So as you can see here the little afr's are equal to the big rectangle heads but with a much smaller runner which is going to give you much better throttle response as well as power in the lower RPM's. You really get the best of both worlds with this head on a 454. I know some guys love single plane intake but the same applies. New dual plane intakes like the RPM or Stealth will make as much power as a single up to about 6000 rpms and that difference might only be 10 to 15 hp more but below that the dual will make a lot more power from 2500 to 5000 rpms above the 5000 mark, they will be similar at 6000 the single plane will begin to pull ahead. Unless your going to spend big bucks and run a shaft rocker system which you don't need because you're not spinning your engine above 6000! When I built my last engine I spent a lot of time talking with both Marine Kinetics as well as my builder which was Wesco Racing. That engine was disigned to run at 6-6100 rpms. At that point the Victor Jr was an option but when I asked Dave at Wesco if I should ditch the Stealth and go for a Vic Jr., his comment was, you're splitting hairs and you will see little to no gain with the Vic Jr.

To lump all single planes together and all dual planes together is silly it would be like saying all oval ports and rectangle port heads are the same. Each is designed to work well for a particular engine combo. I don't know much about the holley you're running but it sounds like its designed for an ultra high RPM engine which a marine engine is not. Throttle response is very important, if your coming off a wave and nail the sticks you want your engine to respond, not hesitate, catch its breath and take off once the RPM's increase. Or if you're cruising along and someone comes along side you and you nail it, the boat with the better port velocity will get the jump by as much as 3-5 boat lengths and even if the other boat makes more power up top it won't be much and most likely will not pass you. Its pretty rare that you can hold the throttle wide open, usually you're going to be on an off the throttles, unless you're boating in an inland lake. Either way, call Bob @ Marine Kinetics his number is 585-654-8583. He will basically tell you what I just wrote but be able to expain everything in more detail. He's a nice guy and will get you on the right path! I would also get in touch with Valako, he can modify your heads to work, which will most likely be cheaper than trying to find a nice set of 188 or 990 heads and having them rebuilt which you will need to do.

Last edited by endeavour32; 05-11-2012 at 09:37 PM.
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Old 05-11-2012, 08:09 AM
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Just be careful with the GM heads. You may be able to pick up a set for cheap, but by the time you go thru them, it adds up. My buddy just sunk a ton of money in a pair of GM rect ports, only reason he did is he had to match the other engine in the boat. As did i years ago, i had over 1000 bucks rebuilding one pair of heads.

If, new heads are out of the budget, i'd probably just keep the merlins. Jim V can install the wedges and freshen them up. He will get them flowing better. I think Jim would probably tell you to run the merlins over the GM rectangles.

I remember a guy around here had a cool old 320 Force Baja. He had built some 600'ish horsepower 540's for it. I was running down the river in my old 402 Formula at about 65mph, and he walked past me. He had to be doing 80. But, he didnt have hydraulic steering, and once he got past me, the thing was chine walking badly, and he almost lost it going into a turn. Scared the crap out of me, and IM sure it scared the crap out of his pasengers!!
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