Engine Tech Guys I need advice
 

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?

Re: Engine Tech Guys I need advice
 

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

Re: Engine Tech Guys I need advice
 

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.

Re: Engine Tech Guys I need advice
 

BBB,

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

Re: Engine Tech Guys I need advice
 

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

Re: Engine Tech Guys I need advice
 

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.

Re: Engine Tech Guys I need advice
 

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.

Re: Engine Tech Guys I need advice
 

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.

Re: Engine Tech Guys I need advice
 

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

Re: Engine Tech Guys I need advice
 

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?

Re: Engine Tech Guys I need advice
 

You guys were spot on on the compression. I have been dealing with this guy so long I couldn't believe he made a mistake of that order. He is working with me.

I am trying to make sure I don't need to go down this road again. Too soon.

more pictures

Re: Engine Tech Guys I need advice
 

Make sure your timing marks are CORRECT!!!
Had same problem 10.5-1 92 plus 2 gal 110 per tank.
now run 32 deg. It took twice to find that marks were not correct though. Had 2 1/2 yrs or 1200 hrs on Pfaff motor,
its fresh again this yr.
GEO

Re: Engine Tech Guys I need advice
 

Got with the cylinder head guy today, we can get 3cc's out of the head without getting too radical.

That gets us to this:

Bore 4.5 (inches)
Stroke 4 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 119 (cc)
Deck Height .005 (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. 17 (cc)

Compression Ratio:
10.165 : 1
Bore / Stroke Ratio:
1.125 : 1
Rod / Stroke Ratio:
1.534 : 1
Total volume:
1156.46 cc's
Compressed volume:
113.773 cc's
my previous calcs used 0" for a deck figure, it is actualy .005"

they are going to machine the pistons to get 5cc off them if that still leaves .200" thickness.

that should get me
Bore 4.5 (inches)
Stroke 4 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 119 (cc)
Deck Height .005 (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. 12 (cc)

Compression Ratio:
9.779 : 1
Bore / Stroke Ratio:
1.125 : 1
Rod / Stroke Ratio:
1.534 : 1
Total volume:
1161.459 cc's
Compressed volume:
118.772 cc's

everything will be cc'd prior to actual assembly to make sure we have got to around 9.8-1

only one chamber was finished the others wer roughed out.

Re: Engine Tech Guys I need advice
 

With that much compression, you're still in the danger zone. Try never to run less than 92 octane, don't run more than 34 degrees timing. Run the carb jets a size or two rich. Run a 140 degree thermostat max, if you run one at all. Good luck.

Re: Engine Tech Guys I need advice
 

PatriYacht,

thanks for the input, what would you consider a safe CR for 92 octane? We don't know exactly what cc's we can take off the pistons and still leave .200" thickness. We are going to try one and mock it up and cc everything before final assembly.

Re: Engine Tech Guys I need advice
 

With alum. heads 10.0 to 1. With iron heads 9.5 to 1. With care, retarded timing, rich mixture etc you can probably run another half point but one tank of bad gas and you're right back where you started.

Re: Engine Tech Guys I need advice
 

With where you are at now how would your CR look with a head gasket in the range of .057? Look into the thicker Cometics. I think you may make more power with a slightly lower CR and more advance.

Re: Engine Tech Guys I need advice
 

I would check with JE/SRP tech and see if that is a solid dome piston. If so I would remove the dome then coat the tops of the pistons. For better atomization of the the fuel and air I would rough up the chamber around the intake valve side only.

Max Compression Ratio:
Remember some guys on this board are very capable of working on their own stuff and have access to machine shops to get work done very quickly. If you are a pleasure boater and don't enjoy "wrenching" then I would sacrifice a point of compression and 50HP to have a reliable mill that you don't have to tweak all the time. I would run conservative on the high 8 range to a max of 9 to 1. If you don't mind the tweaking then run 9.5 and through some octane booster in for good measure.

Chris

Re: Engine Tech Guys I need advice
 

I am running the same pistons in a pair of 509s. With the chambers opened up it was still over 9.5. Decided to use cometic extra thick head gaskets to remedy the whole problem and have a superior gasket.

Re: Engine Tech Guys I need advice
 

Mark,
That piston has .350" deck thickness, so you will be OK with the material removal. As I mentioned stay away from the plug relief and radius the edges after the cut.
Bob

Re: Engine Tech Guys I need advice
 

BadDog,

if my info is correct, a .057gasket and all else the same would yield about 9.8-1.

at this point going to go for the extra 3cc in the chamber + cut the pistons to get as close to, or below 9.5-1 as possible.

Actualy the engine guy just called, they have just got done cutting one piston using the specs RMbuilder supplied.

He cc'd it in the hole and came up with 9.38-1. This is what we will go with. Not 9-1 but it saves the pistons and should be ok for 92 octane.

I would love to re-coat the pistons, however Polydyn is running 2-3 weeks turnaround at the moment and I have been boatless since Good Friday already.

Thanks for everyones help I'll try and get a picture of the cut pistons in the morning.

Re: Engine Tech Guys I need advice
 

Get a hold of Techline Coatings. You can do it yourself for about 40 bucks. All that is required is a covential oven.

Chris

Re: Engine Tech Guys I need advice
 

Thanks for the info Chris, I am checking into it http://www.techlinecoatings.com/Engine.htm As you can see from the above picture the pistons are coated top and sides with Polydyn. The only area not coated will be where we remove material from the dome.

Re: Engine Tech Guys I need advice
 

Piston cut to safe min thickness with edges of cut chamferred.

Should have engine back today. Was going to thrash all day tommorrow and get it installed, but wife says kids don't recognize me anymore so I'd better spend some quality time :rolleyes:

Re: Engine Tech Guys I need advice
 

You might want to be careful with the ceramic top coat on the pistons with iron heads. Its been our expierence that the coating can raise the cylinder temperature in a big block iron head motor much higher than it will be in an aluminum head bbc motor. Your piston detonation point and damage is in the quench area not in the combustion chamber indicating you had possible pre-ignition in that area. You could be having a detonation issue with 34 to 36 degree timing and high cylinder temperatures with todays rot gut fuel. I would suggest not using the ceramic top coat in this motor. Just my opinion.

Ray @ Raylar

Re: Engine Tech Guys I need advice
 

I took the stock J & E pistons out of my crate 502 and had them cut to flat tops for a blower motor. J & E told my mechanic how much he could take off safetly. There was just a slight valve relief left after cutting. I lowered my CR from 8:75 to 8:00 and I ran merlin cast iron heads with 8 lbs of boost with 32 degrees of timing and never had a detonation problem or a problem with the pistons.
Just have to be careful with poor gas

Re: Engine Tech Guys I need advice
 

Bob,

You were spot on, I found the air leak, talk about a 10 cent part.

Apparently when starting the motor that morning I had a slight backfire which pushed the cap off the vacuum port that goes to the butterfly closest to #2 cylinder.

With the engine already on the edge because of the high compression that was the final straw.

I have now tie-wraped all vacuum port caps on the carb.

Took the boat out last night and ran 72mph on glass smooth river water at 80* air temp, which is as good as it ran before in those conditions.

Again, thanks to everyone for their input on this thread.

Re: Engine Tech Guys I need advice
 

Mark,
Glad you found the lean condition, otherwise the engine would have failed again. All the signs pointed away from detonation, gas quality, and timing. The (1) piston crown being uncolored, the whole being in the quench area, the fault area being melted not shattered, and the other 7 showing no signs of failure. The SRP’s have less elasticity than the JE’s and when the crown lost integrity the land area shattered. Considering the C/R of the engine the other pistons looked quite good. Don’t know if any valid observations can be made from the other seven regarding the effectiveness of the thermal barrier coatings or not.
Bob