Pictures of the carnage (JE piston)
Apr 19, 2004 | 11:16 PM
#61
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Pictures of the carnage (JE piston)
just my .02, but two thing that show that it was detonation,
1. in the picture look at your ring, a piston ring is a simple spring, when a spring is over heated it looses tension. Over heating means detonation, compare that ring to a new one.
2. where the hole is melted through. during detonation, the almost instantaneous igniton of air/fuel mixture causes such a rapid pressure wave, it creates a shock wave that bounces of the inside of the chamber. This is what makes the pinging sound, these shock waves are strong enough to sweep away the unburned bountry layer of air/fuel. On a flat top piston the burn through would be on the outside next to the cylinder way, on a high compression cyclinder, it would happen near the ridge of the cutout for the valves. The shock waves bounces off and pushes the fuel away and the metal is exposed.
I would say that you need at least 106 for 11:1's and 110 if higher.
Good luck, I feel your pain.
1. in the picture look at your ring, a piston ring is a simple spring, when a spring is over heated it looses tension. Over heating means detonation, compare that ring to a new one.
2. where the hole is melted through. during detonation, the almost instantaneous igniton of air/fuel mixture causes such a rapid pressure wave, it creates a shock wave that bounces of the inside of the chamber. This is what makes the pinging sound, these shock waves are strong enough to sweep away the unburned bountry layer of air/fuel. On a flat top piston the burn through would be on the outside next to the cylinder way, on a high compression cyclinder, it would happen near the ridge of the cutout for the valves. The shock waves bounces off and pushes the fuel away and the metal is exposed.
I would say that you need at least 106 for 11:1's and 110 if higher.
Good luck, I feel your pain.
Apr 19, 2004 | 11:36 PM
#62
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Pictures of the carnage (JE piston)
I'm going to try some of the race gas before long and see how it run's. But so far I haven't had a problem with 93 on my Ross 11:1. We'll see how it turns out...but I'm sure it will run better on the 100+ octane.
Apr 19, 2004 | 11:46 PM
#64
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I made a mistake in my last post, I just ran to the shop and checked we do use the 11:1 JE. They usually work out to high 10 - low 11:1
We then "0" deck the cylinder and cut 0.025"-0.030 off the heads to give us 12.6:1
Sorry for the confusion, we build a couple of hundred engines a year and it's hard to remember everything.
Ray
We then "0" deck the cylinder and cut 0.025"-0.030 off the heads to give us 12.6:1
Sorry for the confusion, we build a couple of hundred engines a year and it's hard to remember everything.
Ray
Apr 19, 2004 | 11:47 PM
#65
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Dynoray the reason alot of people bought the 11:1 is they were originally touted as a PUMP gas piston.
What does Yamaha say to run in the 12:1 YFZs? Pump gas I bet but I don't know.
What does Yamaha say to run in the 12:1 YFZs? Pump gas I bet but I don't know.
Apr 19, 2004 | 11:48 PM
#66
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Originally posted by: Dynoray
Hey wshrdskin, you don't need 105 or 110 octain fuel with 11:1 we run our Raps up to 12.6:1 on 91 octain. (I'd have to check my dyno computer for sure but we make high 55-60 rwhp on pump gas.
The cracking definitly was the initial problem and based on the fact that there is no stress riser at the crack location one would have to lean towards a high peak cylinder pressure or high heat over a long period as the likely cause.
There is no sign of detonation (based or what can be seen in the pictures, det will pull up the ring lands long before showing any other signs) so a timing issue is a posibility.(causing high cylinder pressure)
We only run JE 12:1 pistons in our 686's (why bother with an 11:1 when you can run 12:1+ on pump gas)
We did have a similar problem with one of our custom Wiseco pistons in our restrictor plate stock car engines. If we didn't have a large enough radius at the root of the valve relief they would crack along there. (but that was a stress riser)
If you are running the stk header with 686 cc's and do alot of high rpm running you could be running into a serious heat retention problem.
with an exhaust that will scavenge properly, (check with other guys that have had this problem and see how many are running the stk header) even with N2O, we don't see heat tracking on the bottom of the piston as is seen in your pictures.
PS throw that MBRP slipon away. If your looking for a tuff decent looking exhaust great, if your looking for HP look elseware.
I was testing another MBRP a couple of weeks ago on a pump gas V force I'm working on, we topped out at 65 HP with MBRP duals. HMF sent us a system to test and BAM 9.8HP and it started responding to more porting. Last run was 84.7HP. We should breakl 90 and probably get into the 4 sec range in 300ft (on pump gas ) ran mid 5's with MBRP.
Hope this info helps,
Ray
Hey wshrdskin, you don't need 105 or 110 octain fuel with 11:1 we run our Raps up to 12.6:1 on 91 octain. (I'd have to check my dyno computer for sure but we make high 55-60 rwhp on pump gas.
The cracking definitly was the initial problem and based on the fact that there is no stress riser at the crack location one would have to lean towards a high peak cylinder pressure or high heat over a long period as the likely cause.
There is no sign of detonation (based or what can be seen in the pictures, det will pull up the ring lands long before showing any other signs) so a timing issue is a posibility.(causing high cylinder pressure)
We only run JE 12:1 pistons in our 686's (why bother with an 11:1 when you can run 12:1+ on pump gas)
We did have a similar problem with one of our custom Wiseco pistons in our restrictor plate stock car engines. If we didn't have a large enough radius at the root of the valve relief they would crack along there. (but that was a stress riser)
If you are running the stk header with 686 cc's and do alot of high rpm running you could be running into a serious heat retention problem.
with an exhaust that will scavenge properly, (check with other guys that have had this problem and see how many are running the stk header) even with N2O, we don't see heat tracking on the bottom of the piston as is seen in your pictures.
PS throw that MBRP slipon away. If your looking for a tuff decent looking exhaust great, if your looking for HP look elseware.
I was testing another MBRP a couple of weeks ago on a pump gas V force I'm working on, we topped out at 65 HP with MBRP duals. HMF sent us a system to test and BAM 9.8HP and it started responding to more porting. Last run was 84.7HP. We should breakl 90 and probably get into the 4 sec range in 300ft (on pump gas ) ran mid 5's with MBRP.
Hope this info helps,
Ray
Apr 19, 2004 | 11:54 PM
#67
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Pictures of the carnage (JE piston)
Originally posted by: Dynoray
even with N2O, we don't see heat tracking on the bottom of the piston as is seen in your pictures.
even with N2O, we don't see heat tracking on the bottom of the piston as is seen in your pictures.
Apr 19, 2004 | 11:58 PM
#68
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Yep, your right sixsix the 12:1 YFZ runs pump gas.
If your engine is setup properly you safely run 12.6:1 on 91 octain.
Ray
PS wshrdskin,
I do see that you have the Alba now and also noticed that you are running a stg 1 Hotcam. This will increase your lower RPM dynamic cylinder pressure as well. We have never used that small of a cam on a Rappy, but that could be an issue?? (although it shouldn't be with 11:1)
ie: we run 12.6:1 on our 44rwhp 400EX's with the big Web cam(I'm not at the shop so I can't tell you the grind number) but can only run12.2:1 with a stg2 Hotcam and 11.8:1 with smaller Web cam.
Ray
If your engine is setup properly you safely run 12.6:1 on 91 octain.
Ray
PS wshrdskin,
I do see that you have the Alba now and also noticed that you are running a stg 1 Hotcam. This will increase your lower RPM dynamic cylinder pressure as well. We have never used that small of a cam on a Rappy, but that could be an issue?? (although it shouldn't be with 11:1)
ie: we run 12.6:1 on our 44rwhp 400EX's with the big Web cam(I'm not at the shop so I can't tell you the grind number) but can only run12.2:1 with a stg2 Hotcam and 11.8:1 with smaller Web cam.
Ray
Apr 20, 2004 | 12:07 AM
#70
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Pictures of the carnage (JE piston)
Originally posted by: Dynoray
Heat tracking is the dark stain on the underside of the piston .
Ray
Heat tracking is the dark stain on the underside of the piston .
Ray