454/420Hp build up
 

Right now in my excalibur Hawk I have twin 454/420 mercs. Actually, one is a crate 425HP, and the other is a 400 cylone that was supposedly rebuilt to 420 specs. The cylone engine was down on rpm this past season. It ran well, but not that much power.

The engines currently have dual plane aluminum merc intakes, holley 800 double pumpers, mallory breakerless distributors, and 3" log maifolds. My plans are to remove both engines. Regasket and paint the 425 motor. It runs very well.

The cylone engine had good compression on all 8, but I would like to install a new cam(stock crane 425Hp cam), valve springs, and regasket. I also want to get rid of the old crappy manifolds and go to some revolution marine manifolds.

My question is, should I change the intakes? They are the ones that accept a spreadbore or a square bore. Also, I hear there are better cams available. Which ones? I would like a reliable set of 450-475Hp motors. Im thinking between upgrading the intakes, exhaust, and possibly cams, I shouldnt be too far off my goal. It would be great to get another 5mph out of her

Re: 454/420Hp build up
 

The 420 and 400 Cyclone are the same engine with the exception of the carb, intake and exhaust. The Cyclone came with a Quadrajet and probably the dual plane intake you are using now. It also used the 3" exhaust. The 420 came with a Holley 800 and a Wieand X-cellerator intake. It had it's own cast iron individual runner exhaust manifold that was prone to leaks and reversion. Most were replaced with Gil manifolds. The heads, cams and shortblocks are the same.

The 420 was designed for max rpm of 5100. If you put a longer duration cam in it, it'll have to spin faster. I would go to a hyd. roller with more lift and about the same duration. The 420 had very low compression from the factory. Advertised at 8.8 to 1 but probably more like 8.5 to 1 with the deck heights you normally see. Having the heads milled .020 while their off would be a good idea. The Air Gap intake will probably make a few more hp and a good aftermarket exhaust will be good for about 20 hp and take a hundred pounds off each engine.

Will all that get you 5 mph? I'm not sure, that's a big heavy boat. To be sure I would reccomend a 1/4 in. stroker in addition to the above. Your going to have the engines out anyway. Nothing beats cubic inches. Good luck.

Re: 454/420Hp build up
 

You have two engines that are quite different. Your 420/454 is an old Gen IV with a flat tappet cam. Your 425HP crate engine is a Gen VI roller cam engine.
I agree with PYacht, rebuild your Gen IV with a retrofit hydraulic roller cam and the associated parts, link bar lifters, new valve springs...
Your 425HP Gen VI has a rather wimpy cam that can easily be replaced.
Most cam companies have the same grinds for retrofitting Gen IV/V engines as well as replacing the OEM roller cam in your GenVI.
You will want to spin those 454s to at least 5200rpm and probably more like 5600rpm to make 475HP. The "off the shelf" cam that most suggest is the Crane 731.
The RPM Air Gap is a good suggestion for an intake, but many say the Merc/GM dual plane with an additional 1" open spacer is pretty good. If your going to buy new intakes a single plane may work better if your going to prop for higher rpm. Your 800cfm carbs should do the trick.
Good exhaust is a must, those old logs will choke the engines. I would suggest Stainless Marine or similar high flow aluminum exhaust manifolds.

Re: 454/420Hp build up
 

I missed the crate motor part. My bad. Later versions of the 420 came with Gil manifolds and were called 425's. I'm easily confused.

Re: 454/420Hp build up
 

Be careful how big of an intake you use. In the 5500 rpm and under, Dart, Merlin, Victor will probably lose power to the Airgap. They're better on the 500+ cubic inch engines. The E-xcellerator is a good one for the higher rpm's. It has smaller, longer runner cross section than the manifold above.

Re: 454/420Hp build up
 

According to mercruiserparts.com, they list the same part # for the 400HP, 420Hp and 425HP cam. Weird.

The funny thing is, my starboard engine (425) has powder coated valve covers with the 425 sticker. The intake, which is a dual plane, identical to the 365hp intake, still has the factory merc lock tag on the intake bolt. I guess I really dont know what I have here. I coulnt find the serail number tag on either engine. I know for sure the cyclone engine was rebuilt at one point, because of the timing cover bolts, roller rocker(not Crane golds like the other motor) and it has flat tappets.

So far, I found that the intake part # for the 400Hp is 72608T, and the 420/425 Intakes are 814167. The rockers, cam, are the same. They do however list two diffent fuel pumps for the 420 and 425 depending on serial number. Mine have the block mounted pumps.

Here are some numbers off the engines
Starboard 5716174
Port 6627089

Re: 454/420Hp build up
 

I thought you said that you had a "crate" engine not a Merc 425. I believe the Merc 425 is a Gen V with the same Crane hydraulic flat cam as the old Gen IV Merc 400 & 420...

Either way, rollerizing is a great way to build more power... :drink:

454 420 exhaust manifolds and risers
 

ISO 1 pair of exhaust manifolds and risers for Mercruiser 454 420 horse motor

MT back when he was asking the questions :)! It's good to see that we all had to start somewhere, lol.

Or you can use your stock manifolds for boat anchors and upgrade to something a little more reliable! :lolhit:

The big mercruiser manifolds with the 6 bolt riser? They no longer exist. The wet joint was too big and leaked. This manifold is the reason merc started using gils. They support a lot of power, but will wreck your engine. They make good wall hangings. You need 2 modern sets of exhausts. And please, don't resurrect old threads. Start a new one.

MT you going to stay dual plane? Something tells me you likely will. Those stock intakes work well and one of the most even EGTs dual planes I've tried. This goes bac ka few years but I don't see any drastic changes since either. Something to be said about the original look also. Just my opinion. When you acquire the Excalibur anyway?

2005ish :)

nothing like bringing back a 12 yo thread,lol.

Tks Mike. That would explain the Excalibur. I never even looked at the OT. I was beginning to think I missed something in the last few months I haven't logged in much. Wtf. How did this happen??

By the way. The 425hp I was speaking of, was neither a GEN 6, nor a GEN V. It was a Mark IV mercruiser high performance crate long block. Basically the same engine as the 454 /420hp Merc.

You still run those short blocks right?

Nah. Not those . But Similar though . Same blocks, same cranks, but Manley H beam rods with arp2000 bolts, Arp main stud kits, JE blower Pistons.

Simple. A guy looking for a set of those extinct manifolds found this thread by Google searching 420 exhaust. So he joined OSO and resurrected a 12 year old thread not realizing it was 12 tears old to try to find a set. I bought a set at a boat swap meet in 1993. 30 bucks. Already extinct. Went to Trenton marine to buy tails. They said 750. I said nobody wants them. 100. They said 750. I said throw them out and threw the manifolds out.

My 311 had those manifolds when I purchased it over 10 years ago. Bought the imcos from Teague at the time, I notice now that they don't list the TRS tails available anymore.....I can't believe i'm saying this but the TRS stuff is getting more obsolete every day. Doesn't surprise me that he would reach out to OSO to find something.

BTW, forceofhabit, welcome aboard. Sorry to b!tch about your first post, but those manifolds are bad news. Float test the 1 set you have. (Throw off boat and see if they float.) Now, there are a bunch of Chicago area guys on here. Mild Thunder and Mike Tkach among them. Season is about done. You need 2 sets of exhausts. The gil mercruiser are pretty good and will support more than those 420's need. 2k will get you 4 new manifolds, and the risers can be found used with a little patience and ebay-craigslist surfing. There may even be a setup you can use those tails on, but I doubt it.