#7 Always Broken
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
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#7 Always Broken Hey,
We have been researching the well known "#7 broken piston" under boost issue. (factory LSx pistons) 10 psi seems to be the magic number were failure occurs.
We have eliminated the following;
Injectors, Intake manifold, Fuel Rails and Exhaust manifolds.
It would appear to us that it is a cooling issue. We note that there is always a lack of a rear coolant crossover in most cases.
An open discussion on the subject would be great.
Cheers
We have been researching the well known "#7 broken piston" under boost issue. (factory LSx pistons) 10 psi seems to be the magic number were failure occurs.
We have eliminated the following;
Injectors, Intake manifold, Fuel Rails and Exhaust manifolds.
It would appear to us that it is a cooling issue. We note that there is always a lack of a rear coolant crossover in most cases.
An open discussion on the subject would be great.
Cheers
I'm not sure if the #7 is a myth or not but I ran a rear crossover vent line and T'ed it into my front vent pipe where I then ran it to my lower radiator hose. this should help with cooling. some say to just make sure all the air is out by keeping them loose on start up and then tighten them down
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
Likes: 0
He is saying that the lack of coolant flow to the rear of the heads creates detonation causing the notorious #7 issue. And aiding in the coolant flow to the rear of the heads helps prevent this problem
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
Likes: 0
So adding that extra crossover at the rear of the motor and returning it to the top of the rad is the correct path?
Cheers
When I did my Vic Jr intake swap I switched to a LS1 cross over tube that ties all 4 together. I also have noticed that in some of big FI builds I have see and stuff that Kurt Urban has done he ties all 4 together.
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I just got a new cross over and put it on the back side of my heads. Couldn't get the LS1 style so I connected the two cross overs together and then to the radiator. No clue yet if it really will make a difference. It really doesn't seem like much room for coolant flow, but I can see it preventing air from getting trapped back there.
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I was always under the impression it is due to the factory-style intake.
The #7 and #8 runners draw suction from against the back wall of the intake. When you are jetting air in at a high velocity you are going to setup a slightly higher static pressure against the back wall. A higher static pressure at the #7/#8 suction point means higher air density which means leaner #7 and #8 cylinders.
Measure the boost at the back and front of the intake - I think you'll be surprised.
The #7 and #8 runners draw suction from against the back wall of the intake. When you are jetting air in at a high velocity you are going to setup a slightly higher static pressure against the back wall. A higher static pressure at the #7/#8 suction point means higher air density which means leaner #7 and #8 cylinders.
Measure the boost at the back and front of the intake - I think you'll be surprised.
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
Likes: 0
I was always under the impression it is due to the factory-style intake.
The #7 and #8 runners draw suction from against the back wall of the intake. When you are jetting air in at a high velocity you are going to setup a slightly higher static pressure against the back wall. A higher static pressure at the #7/#8 suction point means higher air density which means leaner #7 and #8 cylinders.
Measure the boost at the back and front of the intake - I think you'll be surprised.
The #7 and #8 runners draw suction from against the back wall of the intake. When you are jetting air in at a high velocity you are going to setup a slightly higher static pressure against the back wall. A higher static pressure at the #7/#8 suction point means higher air density which means leaner #7 and #8 cylinders.
Measure the boost at the back and front of the intake - I think you'll be surprised.
However, we run LS4 Turbos. The motor is fed air from the rear not the front. And it still breaks #7. It breaks them with the stock LS4 intake and it breaks them with the LS6 intake.
So since our LS4's are fed at the back of the motor as opposed to the front, I have to rule out intakes altogether. If this where the case, #1 would be breaking instead of #7.
Cheers
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
Likes: 0
With intake, injectors, rails and all kinds of exhaust manifolds tried, Its gotta be a cooling system issue.
Cheers
Yup, cooling issue for sure. People have run the LS6 intake backwards on the bigger setups with the intercooler in the car and pipe through the firewall and it still hurts 7 and 8 first.
It seems that when a motor goes it is going to hurt one of the back cylinders first but if you get a good tune up on one of these engines you will find it bending the rods before hurting pistons and that happens at quite a bit of HP.
Anyone know of a writeup on how to rig the cooling system to get liquid to #7 better? Been searching but all I'm coming up with is a TON of people who are having #7 issues but no real fixes.
Thread Starter
Staging Lane
Joined: Jul 2010
Posts: 94
Likes: 0
Ok,
Some great information coming forward. Thanks to those who have contributed.
Let summarize what the consensus is;
1)There is cooling issue at the rear of the motor.
-An extra crossover draining to the rad helps remove trapped steam keeping the head cooler.
2) At the end of the day Detonation is the culprit triggered by heat.
-Timing should be pulled off peak horsepower for a protective measure.
-Additional fuel should be added to cool the combustion process.
So this leaves some questions.
Other than adding the extra coolant crossover, is there anything that can be done to improve cooling around the rear of the motor?
If peak HP is made with 18 degs of advance, how much timing should be pulled to ensure a safety margin?
In keeping with the concept of maintaining a safety margin, where do you feel AFR should be set?
If you have any other information to add please feel free to chime in.
The suggestions put forth will be instituted in my LS4 build. The LS4 is already known as a 10 psi piston breaker. So we shall see on a more analitical basis if these measures are effective.
Cheers
Some great information coming forward. Thanks to those who have contributed.
Let summarize what the consensus is;
1)There is cooling issue at the rear of the motor.
-An extra crossover draining to the rad helps remove trapped steam keeping the head cooler.
2) At the end of the day Detonation is the culprit triggered by heat.
-Timing should be pulled off peak horsepower for a protective measure.
-Additional fuel should be added to cool the combustion process.
So this leaves some questions.
Other than adding the extra coolant crossover, is there anything that can be done to improve cooling around the rear of the motor?
If peak HP is made with 18 degs of advance, how much timing should be pulled to ensure a safety margin?
In keeping with the concept of maintaining a safety margin, where do you feel AFR should be set?
If you have any other information to add please feel free to chime in.
The suggestions put forth will be instituted in my LS4 build. The LS4 is already known as a 10 psi piston breaker. So we shall see on a more analitical basis if these measures are effective.
Cheers
Last edited by LS4_DeltaV; Sep 15, 2010 at 10:56 AM. Reason: I never said I could spell
