cstraub

mtgbiker,
I would stick to around 9 to 1 with iron heads. How much rpm are you going to run with these engines? If this build is targeted to around 5200 to 5500 max, then your induction system is pretty big and port velocity is not going to be good. This can be crutched with a small cam to improve velocity but you would be better suited with going with a set of merlin oval ports on your mills and an air gap intake.

Chris

rmbuilder

Sutphen 30,
Must be missing something here, nowhere in my post did I make any reference to or suggest a "low compression" motor?

mtgbiker

I spoke to the engine builder yesterday and he still wants to go up to 10:1 compression on these mills. He has assured me that with a CUSTOM GRIND cam and re-worked heads that the motors will not detonate. But he also wants me to run 93 octane!!

He actually got defensive when I suggested no higher than 9.5:1. Of course "I'll do what you want me to do" was his final position. I do trust him since he's had his shop for 28 yrs but 9:5 to 1 is where it will be !!

mtgbiker

Where would you suggest I back down to?

Dan

PatriYacht

I'm running 9.5 with aluminum heads and I find I have to use 92 octane. I wouldn't run 10 with iron heads. With today's crap gas, all you need is one tank of bad gas and you're done.

cstraub

Parti nailed it. Say you get caught at a marina that has 89 only. I would run no more then 9 to 1. Your .5 a point will not yield anymore then roughly 20 more HP per engine and is not worth the chance of popping an engine.

Chris

formula31

Yea, I have friends who have to have someone follow them around with a pickup full of barrels of race fuel so they can siphon it in when they need it. Build for 89 octane unless you are full race. 9-1 as was said.

vette131

low 9's

Phazer

I've been keeping an eye on these posts, hoping I'm not digging myself a hole. I'm ordering parts to rebuild my '98 Mercruiser
7.4 MPI (L29)...thus far, I've got the Edelbrock aluminum oval port 110cc heads and HP rods for full floating pins (3/8 ARP bolts). I'm looking at Speed Pro H-426CP pistons that have a .100 dome...that should give me a 9.19 cr.

I live at the end of the earth, and the gas is 87 octane. I was thinking the aluminum head would give me a cushion, but now I'm wondering if I should order a flat top piston.

I have a 6000lb, 25' cabin cruiser. I'm looking for mid 40's at around 4800 while cruising at around 3200. I couldn't imagine WOT for more than 60 seconds.

At this point I'm going to keep my stock cam and thru-prop exhaust. I got a Bravo-3 drive to replace my Bravo-2, but haven't installed it yet. In the future I'd like to upgrade to the GLM aluminum exhaust, or maybe the stock 496 exhaust.

I'd really appreciate some advice on piston choice. I've already been advised to go with a steel crank, but that's probably not going to happen. I know I'm talking to the go-fast crowd, but my engine doesn't.

PatriYacht

Merc built most of their old engines with 8.8 to 1 compression with iron heads. They ran just fine with 87 octane. You should be ok provided you engineer the rest of the system properly. Make sure the engine runs cool: 140 dgree thermostat, large oil cooler. Be conservative with the ignition timing: 34 dgrees or less total. Make sure the carb is jetted right. Lean mixture in a modified engine is certain death. Under prop the boat a little to take a bit of the load off.

My engines are more highly modified 540's putting out 650+ hp so I think they're generating more combustion chamber pressure even though the comp ratios are close to the same. I wouldn't expect you to need as much octane as I do.