View Full Version : Max Compression, Cam Specs for Small Block Chevy

07-11-2012, 02:26 PM
My dad and I are heavy into early Camaros and also have a Corvette...we may either build a motor for the 16 we have, or maybe even pick up another 16 or 18 and do a swap. We are not concerned with the amount of gas it will use or how practical it will be.

What is a realistic compression ratio to run in a boat? I hear that when you creep up compression, you can be apt to suck in water, or is that hooplah? Also what would be ideal as far as cam specs, likely we will use a hydraulic flat tappet.

The boat will just be something to whip around with on the lake, but if we are going to build it...we want it to be fast...like 75 ish range. Also any guys running heavily ported heads? or does the higher rpms that work well with a boat simply not match up well with the rpm range that a boat will see, say in the 4000 - 5500 range.

I have countless motors available, maybe build a 383? I also have various complete motors built right now.

Thanks, Tim

07-11-2012, 03:12 PM
reversion is the water sucking back into the exhaust, it's caused by cam overlap. Compression is a different beast. you can bump up over stock... Mr. Fixxall has some good specs for a build, i'd ask him for help.

07-11-2012, 03:16 PM
I am in the very early stages of considering a build, the 16 we have runs great, so maybe get another to build...maybe an earlier hull and swap in a SBC.

So maybe a 10.5 to 1 compression motor would be fine? given cam specs were ideal?...

07-11-2012, 03:19 PM
Vette beat me to it. I'd add that exhaust design is the other big factor in reversion of water back into the motor. (How far from the manifold before water mixes with the exhaust, slopes, vacuum breaks, etc.)

Some highly recommended reading: http://www.michel-christen.com/ExhaustElbows.pdf

07-11-2012, 07:46 PM
iron or alum heads?

what gas is available?

10.5?? no way...

07-11-2012, 10:40 PM
not that i disagree that 10.5 to 1 may be too much, but if people are running blowers and superchargers, they are inherently raising the compression ratio

what do you think would be a happy medium? build a 9 to 1 motor? I have the stuff to build a 383...I would likely run iron heads, in case I did want to bring it in and around the harbor at some point

thanks, tim

07-12-2012, 08:49 AM
If you roll back to stock numbers, the stock 350 Mag I think is 9.4; 6.2 is 9. These are 87 octane engines so there is room to play. 10.5 is high but I think upper 9's if you plan on running premium is doable.

07-13-2012, 12:59 PM
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anyone running a 406 Chevy in their boat?

I have seen some with 383s....

07-13-2012, 11:17 PM
Mike (Maddad) is the 400 king!

07-14-2012, 08:23 AM
Running sbc385 with 9.7:1 comp in 18c. Burn prem. fuel.

07-14-2012, 10:22 AM
With the way I wreck them, I feel a bit more like the court jester.
406 with 9.3/1 CR, Comps HR276 cam, iron Dart heads, good exhaust, intake and ignition. It make 440 hp, almost 500 lbs of torque, all before 5500 rpm, and will do it all day long on whatever they've got at the pump. Well, all day or until it breaks. With this latest one, which I just took apart and put back together, running it a bit on the rich side and keeping the timing at 32 seems to help the pistons and valves live even though I run it at higher rpms(4000-4500)for 15 or 20 minutes at a time.

Speed Junkies
07-14-2012, 10:56 AM
Boston, I'm new here and to boating. BUT have been doing the race car thing for years. SO:

"the way I see it, these boats are just a water break dyno. I would go for a motor with big tq and long stroke. 420-434-440 inch. There low on the RPM range compared to a typical drag motor. With that being said, the low end grunt of a big inch SB and say 5000RPM redline, I myself do not see an issue. I wouldn't think twice about 10.5-1 BUT with alum. heads. Alum will displace the heat better then iron, which helps with ping and detonation. I would look for a "vortec" designed aftermarket alum head. Or a late model 'Vette alum head. Better yet, if you can swing it $$$ wise, drop in an LS series motor at 427 inches. There designed to run on pump piss for fuel and make huge Hp with those new design heads. THEN if you want more, just call up Procharger and put a 'charger on the LS"

07-14-2012, 10:15 PM
I build a lot of race builds, worked at speed shop for years, new at building a nice set up for the water

I posted a lot of car pics in the 'whatever you want to talk' section, take a look