85486

I just bought a truck and the kid told me the motor is a mercruiser 486 with 67 427 oval port heads. My question is has anyone run or heard of the 486 mercruiser motor and where could I find info on it?. He told me he was running a big cam with a set of rectangle heads and turning the motor over 8000 rpm in the mud pit. Is there any way of believing this as a big block spinning like that is unheard of on the street? The only reason I question it is because it being a marine motor and there known to spin.
I'm going to be buying a new carb for it as well as the carb on it is a full race carb with mechanical sec. and no choke. My question is will a demon 650 be enough or should I go with a 750?. I am running a turbo 400 with a small stall and I do want vacuum seconds and electric choke. thanks in advance!

MrOverkill

iTrader: (1)

Dont recall a 486 other than in reference to the Bravo series drive. As for a BBC spinning over 8k rpm not a problem at all for a short stroke engine like a 427, L-88 cars had a 4400-8800rpm powerband from the factory. Chances are you bought a 330hp 454 with oval ports and he was telling stories. Lastly the 650 is pretty small to feed even a very mild BBC 750 would be the very smallest carb I'd even consider unless you have low compression, a tiny cam and dont plan on reving it over 5500rpm.

If the motor was used in a mud truck I'd tear it apart and freshen everything as those guys rag the **** out of their stuff I've built a few of them for people.....

1 FMF

iTrader: (1)

http://www.mercurymarine.com/engines...1sho_specs.php

you typo'd an 8 instead of a 9,
its the 496 cid 8.1L engine which is the replacement for the 502 cid big block.

the 496 has a 4.25" bore and 4.38" stroke according to mercury, I am not familiar with the newer big blocks from merc but it sounds like the info you were told was incorrect. It sounds like you're friend heard merc sells a 496 engine, so he equates his 496 engine as being a mercruiser.

Mercruiser just marinizes the chevy 496 big block to sell as a marine engine, just because it's a 496 doesn't make it a mercruiser or marine engine necessarily. It is first and foremost a chevy block. Whatever happened to it after it left the chevy engine facility is what you have to find out. It's possible it's a motor that went to mercruiser to be finished as a marine motor then ended up in a truck, the block and the internals are the same.

MrOverkill

iTrader: (1)

496 Mag HO is a Gen 6 and you cant install Gen 4 heads on a Gen 6 so I doubt thats the same engine he's refering to. You can run Gen 5&6 heads on a Gen 4 with special gaskets though.....

85486

I apologize, Its a 482 not a 486. I have been reading too much about the 496 lately. I did find a 482 under the mercruiser parts website. Its 460 trs GM 482 (83-86). A buddy talked to his friend who races big block boats and he says the marine motors are build good due to running at high rpm's at a long period of time. I will try to find some numbers on the block to find out for sure?. I also think I will be buying a 750 demon with vacuum secondaries and a choke as the carb on there is a built Holley with no choke housing or anything. thanks guys.

LS1Z28-00

iTrader: (8)

i dont know alot about big blocks or this l88 engine you speak but a factory BBC from the factory in the old days spining 8800 RPM sounds a little WTF to me especially since that no v8s rev that out of the factory. thats like a vtec civic powerband someone correct me if im wrong.

MrOverkill

iTrader: (1)

Roll your eyes and say WTF all you want, do some research my friend they were factory race cars with 12.5:1 compression and VERY long duration solid lifter camshafts. So I have just corrected you because you are wrong

85486

I didn't mean to start a ordeal here. My understanding is that Mercruiser motors were meant to spin high R's for a long period of time. I guess I will have to get in touch with a dealer and find out some specs and facts. I need to pull apart the motor and find out what exactly is in there?. I know the cam is wimpy along with the offehauser intake. Sooo, that will be the first to go. I'm thinking a Voodoo cam(227/233 .542/.554) and a perfromer RPM or a stealth intake. I am looking for around 450-500hp or so but streetable. I will be running in the mud with a heavy truck, I'm thinking close to 4500-5000. I will also need a new carb as the one on my truck now is a Holley HP clone. I think it was tuned up by someone in the racing field as there is no choke housing what so ever, mechanical secondaries. Too much for the motor now and I need Vacumm secondaries for the street with a choke.

MrOverkill

iTrader: (1)

Comp Cams 280 or 292 Magnum cam is a great choice and I have recommended it to many guys for similar applications.

Marine engines do run sustained higher RPM but most not usally dont see more than around 5200-6000rpm unless its a race deal. The only real difference is greater piston to wall clearance on a marine package.....

85486

I learned some new info today abot the motor. It is a 482 with GM forged 2.25 dome pistons. This should put it at 11.1 or so, mabye a snot higher due to the heads. The heads are either 68 or 70 cc chambers with big oval port and closed chambers. Valve springs are 390#. This motor had a .600 lift cam in it with rectangle port heads and a open intake. It ran over 7000rpm in the mud with that combo and a 900 carb. I will be buying a new cam and intake for now and run a 3000 stall. I'm hoping that will get me to around 450-500hp?. Any more input would be great! thanks

MrOverkill

iTrader: (1)

Pull a valve cover and tell me the casting #'s on the heads, I'm concearned about the closed chamber heads as the 482 (.125" over 454) should have come with open chambers from Mercury. Did you hear it run? If not you may want to pull the heads real quick and double check what pistons are actually in it. Cheap insurance and you'll know for sure what your working with.

Its should make 500hp w/o even breaking a sweat w/ a mild cam and a decent intake&carb. I'm not a huge vacuum secondary fan but they can run good dont get me wrong, its just hard to beat a properly sized double pumper. I've never felt the need for a choke myself but I have no problem warming it up with my foot, and if its all dialed in you can give it a pump or two and it should fire and run.

Get me the casting numbers off the heads and the one on the back of the bloock as well and I'll tell you exactly what you have and where to go from there....

xx_ED_xx

iTrader: (17)

496 is a gen 7 and yes with a few new head bolts holes you can put early heads on a gen 7 496. and you can also put them on a gen 6 which is what I have in my 04 silverado. 1996 454 with gmpp heads