Wobble

FROM my bad gas thread.

Here's an update,

The gas was undoubtably a factor but the real problem is compression. This motor came out of my 19' jetboat it was 9.8-1 compression and ran another 170 hours in my 29 Stryker before I had problems.

At that time we went .030 over for 509CI and went to full hyd marine roller setup, single plane intake, porting etc. The engine builder was supposed to change pistons to get us below 9.5-1 CR. The pistons installed actually took us up to 10.5-1

It is amazing that I got 34 hours out of it running Shell super. The offbrand gas was the final straw causing it to burn the side of the piston in #2, the ring probably stuck peeling up a piece of the ring land that landed in the quench area punching a hole in the top of the piston.

using smokemup's online calculator this is what I had

You Entered:
Bore 4.5 (inches)
Stroke 4.0 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 116 (cc)
Deck Height 0.0 (inches)
Head Gasket Bore 4.540 (inches)
Head Gasket Thickness 0.039 (inches)
Piston to Cyl. Wall Clearance .004 (inches)
Top Ring Land Height .310 (inches)
Piston Dome Vol. 17 (cc)


Compression Ratio:
10.525 : 1

Bore / Stroke Ratio:
1.125 : 1

Rod / Stroke Ratio:
1.534 : 1

Total volume:
1152.157 cc's

Compressed volume:
109.471 cc's

The heads have been sent off to unshroud the valves and gain whatever cc's we can there. The SRP piston domes are solid so once we get the chamber cc's we will cut the domes to get the CR down to about 9.3-1. something like this.

You Entered:
Bore 4.5 (inches)
Stroke 4 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 122 (cc)
Deck Height 0 (inches)
Head Gasket Bore 4.54 (inches)
Head Gasket Thickness 0.039 (inches)
Piston to Cyl. Wall Clearance .004 (inches)
Top Ring Land Height .310 (inches)
Piston Dome Vol. 6 (cc)


Compression Ratio:
9.245 : 1

Bore / Stroke Ratio:
1.125 : 1

Rod / Stroke Ratio:
1.534 : 1

Total volume:
1169.154 cc's

Compressed volume:
126.467 cc's

We are doing the heads now, what do you guys think about cutting the dome off forged pistons?

Wobble

Sutphen30,

are you running iron heads? mine are gen4 iron heads. My pistons have ceramic coating on top, I just called polydyn and they say add .25 to .4cc for the coating which would put my actual CR at 10.56-1. It ran great until I put in an offbrand gas. Probably didn't see the octane needed and that was all she wrote.

I never intended to be that high. 9.8-1 worked fine, but all the advice I have so far says less than 9.5-1 for super and reliable with iron head BBC

BBB

just out of curiosity, what was youre total timing on the moter, one time at band camp I was running 36 deg total on a 10.5 to 1 moter and ran into detination but couldent hear it over the exhaust and... a.... it was ugly, anyway I heard 34 deg total was what I should have been running.

Wobble

BBB,

we ended up at 36* based on multiple dyno runs.

BBB

sometimes best power settings on the dyno are not the best for continuous reliable marine applications, are you using any type of knock sensor ignition retard

Linster

I thought I commented on the compression. I would not cut 80 off the piston. You will need to re-balance. Also you need to make sure you keep the piston thickness at least .200 thick. Most pop up pistons average about .240. Cutting 80off will make it too weak. I would change pistons to get down to 9.5 or hog out the heads and buy some thick Cometic gaskets.

Wobble

Thanks for the help guys, the pistons are SRP 212140 we decided to get some chamber cc's to help flow and minimize piston cut. Also working the chambers should somewhat offset the HP loss I was at 580HP estimate 560 with just the CR drop.

I have heard a lot of different opinions on compression ratios for boat motors. I would like to be at or just under 9.5-1. Some say never more than 9-1.

What is your opinion of a safe CR for an iron head gen4 BBC.
Willing to run super gas.

Linster

All I know is what JE tells me. They want at least .200 thick. I not saying it won't work, I just try to follow manufacturers directions as much as possible. With thinner metal, maybe it will not hurt now but may show up later. All the heat can fatigue metal over time. I hope you do not have a problem.

Linster

My motors are 9.4CR. That way you can cruise with 87 in a pinch.

Lmarth

Linster- You DID comment on the compression. I had the same reaction the first split second I saw that short block. I would have sworn I was looking at some 11:0 to 1:0's from a prior life. I've been meaning to get back to you on that point. Don't you miss the days of fly cutting those suckers?