Detonation Issues - "Why is the Gen 7 496 such a bad platform to build"
11-10-2015, 08:00 PM
#31
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I do agree, but remember, it's a 625-650hp motor. May seem 'mild' to today's standard, but it's still 625-650hp. Any tune issues, and these pistons are less forgiving.
Anyhow, we want to make sure ring gap spec was chosen wisely, and to make note that there are a lot of Mahle 4032 pistons that have broke this way on other type motor, and many in the same spot. Yes, that relief is a pretty normal spot for any piston if detonation and/or too much fuel if.....but, again, just seams more Mahle 4032' have done this.
My IMHO, and repeating myuself, we still have to respect 625-650 as a lot of N/A horsepower. Because, quite frankly, it is.
Anyhow, we want to make sure ring gap spec was chosen wisely, and to make note that there are a lot of Mahle 4032 pistons that have broke this way on other type motor, and many in the same spot. Yes, that relief is a pretty normal spot for any piston if detonation and/or too much fuel if.....but, again, just seams more Mahle 4032' have done this.
My IMHO, and repeating myuself, we still have to respect 625-650 as a lot of N/A horsepower. Because, quite frankly, it is.
11-10-2015, 08:10 PM
#32
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To take a step back from the whole EFI tuning thing for a second, what are we looking at for cam specs on this build? Is there a cam card available? I know you mentioned you added lift. Also, what is the static compression, and cranking compression?
11-10-2015, 08:15 PM
#33
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I have always been paranoid about ring gap so I did a lot of reading on it. the general thought from those that had done a lot of dyno work with different gaps was that being too big did not hurt the power noticeably but being too tight could cause butting and break something. after that I always run them bigger than recommended and just take the slight loss rather than risk damage.
One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.
11-10-2015, 08:34 PM
#34
From Reher Morrison
One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.
One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.
Last edited by donzi matt; 11-10-2015 at 08:40 PM.
11-10-2015, 09:45 PM
#35
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What was the cylinder bore diameter that these ring gaps were assigned to?
Total Seal gapless rings sounds like a goo option.
I looked up KB pistons. All I could find available from KB were 4032 alloy for other than motor cycles or ATV's which the latter could be had in 2618. Is 4032 what you used?
Total Seal gapless rings sounds like a goo option.
I looked up KB pistons. All I could find available from KB were 4032 alloy for other than motor cycles or ATV's which the latter could be had in 2618. Is 4032 what you used?
I like the gapless top ring. I get no blow by at all now. Even if I plug the PCV, I get only a small amount. Used to get a huge amount that you could smell after a run.
EDit : they are pricey, but as a % of what you are spending, the increase is small.
Last edited by apollard; 11-10-2015 at 09:55 PM.
11-10-2015, 09:54 PM
#36
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When I talked with Total Seal, they recommended higher gaps than what was listed for any application on the sheet that came in the box. The reason was the sustained nature of the WOT runs vs the intermittent nature of circle track and the short nature of drags. IIRC, for blown drags the sheet recommended almost 0.010 LESS than they told me to run on a marine engine.
11-10-2015, 09:56 PM
#37
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This thread actually makes me wonder if some of the stock 496 piston failures when hopped up aren't just random piston failures or detonation, but rings butting and lifting the ring land. The production spec from Merc is 0098-.0161", that is pretty tight. I could see a motor built on the tight side of the spec butting a ring when wicked up. I'll make sure to measure after one of mine blows up lol.
11-10-2015, 10:02 PM
#38
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This thread actually makes me wonder if some of the stock 496 piston failures when hopped up aren't just random piston failures or detonation, but rings butting and lifting the ring land. The production spec from Merc is 0098-.0161", that is pretty tight. I could see a motor built on the tight side of the spec butting a ring when wicked up. I'll make sure to measure after one of mine blows up lol.
Think closed cooling is just for 'corrosion protection' on these motors ? I say no. Tight clearances with hot pistons + cold cylinder bores = what ? Yup. OOps !
11-10-2015, 11:44 PM
#39
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Rage,
I propose you get your engine back together and come down here and spend a week or so before you put it back in the boat. We'll beat the snot out of it on the dyno, where it hopefully won't blow up if something's out of kilter. In our spare time you can help me building the Cobra, or maybe I'll have it running by then. Even better! Between your afr meter and mine we can look at 4 cylinders at a time. You're retired, so time is not an excuse.
Bob
I propose you get your engine back together and come down here and spend a week or so before you put it back in the boat. We'll beat the snot out of it on the dyno, where it hopefully won't blow up if something's out of kilter. In our spare time you can help me building the Cobra, or maybe I'll have it running by then. Even better! Between your afr meter and mine we can look at 4 cylinders at a time. You're retired, so time is not an excuse.
Bob
11-11-2015, 05:19 AM
#40
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Rage did you take all the hard edges off the top of the piston before install? I usually run a tootsie roll over all the hard edges on the piston and smooth out a little. It doesn't look like this was done. Hard edges that see combustion can and will turn into hot spots acting like a glow plug and can cause issues. Doesn't look like the valve reliefs were smoothed out at all. On a small bore motor you really need to get that top ring away from the top of the piston. Where the piston broke is the weakest part of the piston. A lot more material in that area on larger bore stuff.
