Back4More

14 psi...racegas...6700rpm...could be the problem, how long do you lean on it when you do? Other than that I would gues your washin the cylinders out with too much fuel or to high a float levels.

Lmarth

Keith--I apologize in advance for beating this to death but I've read this thread at least a dozen times and I keep trippin' on those horrendous leakdown numbers. For all intents and purposes, it's happened twice in roughly 20 hours each. When you had these cylinders losing 24-32% , I assume the valves were not a contributor and it was blowing down in the pan. With that much crankcase pressure I would have expected it to have pushed oil out the breathers. Inpost #6 you indicate an oil residue on the top of a few pistons. Were those the cylinders with the high leakdown numbers? Forgetting what the tops of the pistons looked like for a second, did your builder say anything about the condition of the sides of the piston between the deck of the piston down to the first ring groove and beyond? I'm pretty well tapped out unless new info becomes available. If I think of anything else, I'll let you know.--Lou

Robbie Racer

Strip Poker and Lou, I got the new leak down numbers back today that you asked about and as suspected they aren't good. The worst cylinder (#5) was 30%, #6 was 28%, the best cylinder was 17% and the rest were in the 20 to 23% range. These numbers are about the same as last year before we pulled it apart. Do you think that the lateral gas porting in itself can cause these high leak down numbers? I did talk to Roger at Competition Products as Articfriends suggested. He asked the rod length and when I told him it had a 6.635" rod with a 4.625" stroke, he said that rod angle ratio was better than stock. I find that hard to believe. I wonder if he didn't hear me tell him the motor had a 4.625" stroke. Can anyone else compute this ratio and tell me if it's better or worse than the stock rod angle ratio? Roger agrees with MC and believes the premature ring failure is due to the lateral gas porting on the pistons. That makes sense to me if the rod angle ratio is no worse than stock. If on the other hand the rod angle ratio is worse than stock, then it still might be caused by too much side loading of the piston causing rocking and premature ring failure.

Lou, to answer your other question about blow by. I have -8 braided lines coming off the valve cover breathers going into a puke tank and when I checked it yesterday, there was very little oil in the puke tank.

Any thoughts?

Lmarth

Keith-- I think the root cause of your problems is the gas ported pistons being incompatible with sustained WOT running with 8-14# of boost at up to 6500 rpm. I think you're blowing the rings out of it as evidenced by the leakdown numbers and oil consumption. As you know, drag racers are fanatical about friction losses. They don't gas port to get good ring sealing. They do it to get good ring sealing with zero tension rings. I'm sure the CP pistons are great,though I'd be willing to bet that there are more blown marine monster motors out there running JE pistons with chrome moly rings. No matter, the important thing is that your piston and ring manufacturer know the details of your motor and how you plan to run it. Again, I don't have the luxury of seeing the pieces and verifying bore clearance, cylinder hone, etc. I'd take a good look at the top/sides of your pistons on teardown. I suspect your going to see burned oil on the tops of the pistons. You had the same serious problem twice now. A lot of us boaters that were hot rodders had to unlearn a lot of things that worked on the street. Keep us posted.--Lou

Strip Poker 388

iTrader: (6)

Was the motor warm when you did a leak down?It would make a diff. The #s your get are high .I highest I usally see is 12-15 % time to rebuild . Burned a piston down to the ring land was 50 % :eek I my last blower motor it was at 12 psi and there was a a little blowby when you ran it hard for a while .any thing above 12 when we dynoed it ran out of blower. . just built heat then

At those # you are not getting a ring seal. I see a few motor that are square on the bore and stroke and if its not buzzed it is usally ok . Most of the drag racers dont like it sq but in the big cub motors you cant help it.

I would double ck bore size on piston clearance and replase the pistons order a blower piston . The blower piston is like the nos piston we use with the top ring land down 275 -300 thousands. Childs and alsbert makes a good blower ring , i have even used there stainless top ring on some high nitros motors . They should have a blower ring package .We had trouble with the crome moly rings blowing the chrome off .

What was the ring gap?Ck and see if the rings are butting or too much gap . This might be the cause of the high leak down ?

I dont think anybody uses the gas ported pistions anymore.

as far as rod ratio look at the skirts . You will have some scuffing on th thrush side but it should not be severe.


Keek us informed ??

Rob

Lmarth

Rob-- Good input as usual. After all, if you have a Slingshot you've gotta be OK, right? This is for the "whatever it's worth " category. Remenber the thread last week about Chief engines? I was on their website this morning ( chiefengines.com). Every blower motor between 750 and 1400 hp uses je pistons and chrome moly rings. I respect your experience and it's our experiences that gets us through this stuff, right?--Lou

Robbie Racer

Lou and Rob, thanks for staying with me on this. Other than the high oil consumption (and the ugly leak down numbers), I wouldn't even think I had a problem. I ran it several hours over this last weekend and made a couple of passes at 120 mph at 6400 RPM or so (at 8 lbs. boost on 91 octane gas). The motor sounds and runs great except I am getting a little movement on my knock meter once it gets above about 6300 or so. I'm not sure if the oil consumption is starting to cause a little detonation at the upper rpm's or not yet. I pulled a couple of plugs which look okay by me but I'm going to find someone that is better at plug reading this week to get their opinion. Rob, to answer your question about if the leak down was done hot or cold, it was done hot. Would that be your recommendation too? Would the lateral gas porting cause excessive leak down numbers in itself? I do remember we did see a little scuffing on the pistons when we pulled it apart last winter but as you said it wasn't severe. My motor builder won't tell me the ring gap setting so I don't know about that. I think your recommendation of going with a good blower piston that isn't gas ported with a good set of C & A rings is a good idea. I have used J & E pistons with C & A rings in normally aspirated motors in the past and have always had good luck with them. Now, if I just make it through the summer so I don't have to tear it down again until this winter...........

Thanks againk guys.

Keith

Lmarth

Keith--Glad to hear you're having a little fun. At the beginning of this season, I took my boat out, came back and had this sensation that there other things to do with a boat other than work on it all the time. It's always nice to get out on the water and make some noise. Yes, I do believe the boost/rpm's and gas porting are wearing out the rings. Keith, you've gotta expect a little scuffing with that Queen Mary rod stroke! Doesn't sound like a problem. It's tough getting a good plug reading other than on a dyno. Suggest changing them out on the water, make a long hard run, cut it clean without having water run back up your pipes. There have been a couple of good plug reading threads on this site in the last six weeks or so. Keep us posted. Rob's been keeping a low profile lately (probably having too much fun). Good luck.--Lou

rmbuilder

Robbieracer,
Are the gas ports vertical or lateral? How far down is the top ring land?
Bob

cuda

I don't know squat about blower motors, but in a regular engine, I thinking along with B4M. Too much gas may be washing the oil off the cylinder walls. Sometimes we are looking for Zebras, when the problem is a Horse.