5.7l mercruiser gen 1 alpha 1 to vortec head swap problem please help...

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Mr.Ferrari

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I put vortec heads on my gen 1 alpha one mercruiser 5.7l and it ran great until it didn't. I had no compression in cylinder 4 and 8. The head gasket had blown between cylinder 4 and 6 (burn though the middle )and the valves were tuliped in cylinder 4 and 8. After a trip to the machine shop I reinstalled and now it has detonation after 3800 rpm. Timimg set to 8° btdc. Stock cam, stock pistons. I have a edelbrok performer intake and a rochester quadrajet. Why is this happening? Should I rejet the qjet? Timing? Please help me
 
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Scott Danforth

Scott Danforth

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first, welcome aboard

second, what year motor did you start with?what style of ignition? did you read your plugs after your conversion?

was the motor seriously overheated?

did you re-jet your carburetor? tuliped valves come from overheat and running lean.

to blow a head gasket means that either the deck is warped, or the head is warped from an overheat.

was the block decked?

detonation with vortec heads means that you are running very very hot, or your timing is really off (the whole design of a vortec head is to minimize detonation) you should be able to run well over 12 degrees of base timing and a full 22 degrees of advance (34 total) with vortec heads if you have enough fuel going to the motor on regular 89 pump swill.
 
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Mr.Ferrari

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Scott Danforth said:
first, welcome aboard

second, what year motor did you start with?what style of ignition? did you read your plugs after your conversion?

was the motor seriously overheated?

did you re-jet your carburetor? tuliped valves come from overheat and running lean.

to blow a head gasket means that either the deck is warped, or the head is warped from an overheat.

was the block decked?

detonation with vortec heads means that you are running very very hot, or your timing is really off (the whole design of a vortec head is to minimize detonation) you should be able to run well over 12 degrees of base timing and a full 22 degrees of advance (34 total) with vortec heads if you have enough fuel going to the motor on regular 89 pump swill.
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Thank you.
The motor is a 1991 mercruiser mcm 350 i believe with a thunderbolt 4 ignition.
I read the plugs which looked to be ok but were still mostly clean, the plugs are acdelco mr43lts. The engine itself never overheated but I am assuming I had excessive cylinder temp .
I did not rejet the quadrajet carburetor(but thinking that is my problem). The block was not decked but the heads were. I had base timing set to 8° btdc and don't have a advance timing light. I run premium fuel but the first tank was last seasons stabilized. After the valve and head gasket failure the valves were replaced head was machined true with fresh fuel.
 
alldodge

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Plugs should have a cream color. If they remain white then your running lean
 
Scott Danforth

Scott Danforth

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agreed, plugs should be caramel brown and not "still mostly clean"

if you did not re-jet, that is one part of the problem.

to loose the head gasket, you would have to have either lots of piston eating detonation or you havent got sufficient clamping.
 
alldodge

alldodge

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Depends on what your comfortable with. Even with a new carb I would check what the plugs look like after a few runs. Slow quickly and pull some plugs. Don't want to idle much because that can change the reading

Either that or have to install exhaust bungs and use a air fuel ratio meter
 
AKJohne

AKJohne

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Did you check the compression with the new heads? vortex heads have a smaller combustion chamber and often the pistons should be a reverse dome rather than dished. Suspect your compression is too high for the fuel you are using. I believe they make head spacers for this issue and not saying that is the way to go, but could be a option. I would check the compression, that wont necessarily tell you what the comp ratio is, but if its high it could be a factor.
 
Bondo

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Ayuh,.... Welcome Aboard,..... That, ^^^, is where I was goin' with my comments,....
Vortec heads have tiny combustion chambers, 'n really jack up the compression ratio,....
 
Scott Danforth

Scott Danforth

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with the flat top pistons that are in the engine, and with vortec heads, the compression ratio is 9.4:1

the heads on the 91 should have been 64cc heads just like the vortec, however the vortec heads have the LT3 combustion chamber

on my AQ271, I had run stock GM block, comp cam XM262 and vortec heads with 14 degree timing (total of 36) running 91 pump gas. max I ran on 89 pump gas was 12 BTDC. and I treated the boat like a rental, usually doing WOT for about an hour to get to the anchor hole. that is between 4600 and 5000 rpm (prop dependent) pushing about 4700# for an hour.
 
Scott Danforth

Scott Danforth

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alldodge said:
Said he's using premium gas
Click to expand...
correct, however premium gas in montana is 89 octane, premium in Wisconsin is 91, and premium gas in florida is 93 octane.

OP also stated running old fuel with stabilizer. as fuel gets past 6 months, it starts to break down

however octane doesnt matter if you are running lean
 
nola mike

nola mike

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I didn't think the pistons changed between the vortec and non. Actually, I know that's the case. At least the vortec truck pistons were the same as the merc non vortec...
Also skeptical that these engines care what octane fuel you're running under most conditions. 9.4 isn't exactly supercar territory.
 
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Mr.Ferrari

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I am in southern Ontario and I feel the gas quality is good as I have never had an issue running premium in my modified forced induction vehicles. I was told by the machine shop the 193 swirlport heads that were on the boat are the same 64cc as the 906 vortec heads, they just would bolt on and go with a intake manifold and my rochester quadrajet would do the job. If I am running lean which I belive what should I start at for jetting?
 
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