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runnin' the valves

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I've been really curious about that, how far can you go safely? As in before the piston decides what IT wants the lash to be :lol:

Good way to gain HP/TQ and quicker spool... But my question is how about longevity and MPG's?

One of my trucks is a 93 12valve that had 124K on the odometer when I bought it. That being my first diesel I took it to Cummins for a tuneup. Cummins set the intake to .008 and the exhaust to .017 the truck now has about 190K and is running fine. I tried one time to set the valves back to .010 and .020 but was disappointed at the power and MPG loss (several MPG) so I set them back to .008 and .017. I haven't changed my 98.5 valve clearances from .010 and .020 but I'm tempted to try it. Jim

Mines going to 8 intake 15 exhaust when I get back. I ever since I got that new turbo shaft and put it back together that thing spools like crazy. I did set the valves to 8/18 right before that but don't think it made too much of a difference. I only noticed it because when I am in kansas city making turns and have to punch it to beat the red light and am usually lugging it, it didn't smoke at all. I looked back and said what is wrong with this thing. It usually smokes at least a little. So of course I had a few more red light runs and I started watching the boost and that thing was building boost very fast so it wasn't able to smoke due to low boost. I am not sure what did it but it might have been a combination of the 2 but I will set that exhaust down to 15 (since intake is already at 8) when I get home, then I think I can match the HY35 spoolup.

wow 15 thats pretty tight!!!

straight from the FSM

INTAKE 0.152mm(0.006in.)MIN. 0.381mm(0.015in.)MAX. EXHAUST 0.381mm(0.015in.)MIN. 0.762mm(0.030in.)MAX. note: If measured valve lash falls within these specifications, no adjustment/reset is necessary. Engine operation within these ranges has no adverse affect on performance, emissions, fuel economy or level of engine noise.

so still within specs
    • Like 4

    I am somewhat leary about setting exhaust valves on the tight side. Certainly though within specification I wouldn't think anything bad would happen.I know I need to run through my valves. Especially when cold my engine has a characteristic tick that has seemed to get louder over the last few months.I still haven't decided if I want to drop the coin for a barring tool or attempt to use the alternator. I have heard methods of turning engines over with the alternator in the past and it always seems the belt slips when I try it. :banghead:

    I am somewhat leary about setting exhaust valves on the tight side. Certainly though within specification I wouldn't think anything bad would happen. I know I need to run through my valves. Especially when cold my engine has a characteristic tick that has seemed to get louder over the last few months. I still haven't decided if I want to drop the coin for a barring tool or attempt to use the alternator. I have heard methods of turning engines over with the alternator in the past and it always seems the belt slips when I try it. :banghead:

    The alternator won't grip the belt the way the engine turns. It grips if you turn the engine over backwards. It grips it fine and everything you just have to work in reverse.
    • Like 1

    The alternator won't grip the belt the way the engine turns. It grips if you turn the engine over backwards. It grips it fine and everything you just have to work in reverse.

    I guess you learn something new everyday. I am sure it will come in handy in the future. :smart:

    I guess you learn something new everyday. I am sure it will come in handy in the future. :smart:

    Yeah I told Mike he was full of crap over and over until I tried it the other way :lmao: Not sure why it only grips one way *insert shrug smiley that Mike seems to have deleted*

    I don't have cam specs to work from but I know a bit about how this works in relation to other engines, so this is just theoretical, food for thought.If the lash is tightened up, the duration of "valve open" is longer. This will result in longer overlap, which drops cylinder pressure in most cases. The question arises then, if the exhaust valve opens faster, it should push the turbo sooner, and get faster spool up. If thats true, does it then counteract the loss of cylinder pressure from longer overlap? My guess (a WAG so to speak) is that it may. As to MPG, because its a diesel, stuffing raw air in there still results in more power as it still expands when heated. So any way to get more air in without having to add fuel, should result in a power gain. How much? no idea. I have not tried on my Cummins, but years ago I did this mod on my 225 /6, and it ran MUCH better. At least it seemed to. and it was far quieter. All of this is worth exactly what I got paid to post it,:2cents: and your mileage may vary.:)

    I don't have cam specs to work from but I know a bit about how this works in relation to other engines, so this is just theoretical, food for thought. If the lash is tightened up, the duration of "valve open" is longer. This will result in longer overlap, which drops cylinder pressure in most cases. The question arises then, if the exhaust valve opens faster, it should push the turbo sooner, and get faster spool up. If thats true, does it then counteract the loss of cylinder pressure from longer overlap? My guess (a WAG so to speak) is that it may. As to MPG, because its a diesel, stuffing raw air in there still results in more power as it still expands when heated. So any way to get more air in without having to add fuel, should result in a power gain. How much? no idea. I have not tried on my Cummins, but years ago I did this mod on my 225 /6, and it ran MUCH better. At least it seemed to. and it was far quieter. All of this is worth exactly what I got paid to post it,:2cents: and your mileage may vary.:)

    Well lets draw this out so everyone can picture this. We have 4 strokes, intake/compression/power/exhaust. The power stroke happens and then the exhaust stroke and tightening the valves would cause the exhaust valve to be open sooner right after the power stroke, this could cut into the power stroke pressure at the very end but I doubt it would hurt power much/at all. As the exhaust stroke comes to an end the intake valve opens, if the valve is tighter it would open sooner, since the exhaust valve is tight it will close later, this means there is more time for the intake pressure to shove out the exhaust gasses. Then the intake stroke pulls in more air because the intake valve is open longer. This helps high RPM operation out too. I think it would be very beneficial and if it is what made my turbo light a lot sooner but just going down 2 numbers from the original 10/20 (to 8/18) then it is definitely worth it.

    Well lets draw this out so everyone can picture this. We have 4 strokes, intake/compression/power/exhaust. The power stroke happens and then the exhaust stroke and tightening the valves would cause the exhaust valve to be open sooner right after the power stroke, this could cut into the power stroke pressure at the very end but I doubt it would hurt power much/at all. As the exhaust stroke comes to an end the intake valve opens, if the valve is tighter it would open sooner, since the exhaust valve is tight it will close later, this means there is more time for the intake pressure to shove out the exhaust gasses. Then the intake stroke pulls in more air because the intake valve is open longer. This helps high RPM operation out too. I think it would be very beneficial and if it is what made my turbo light a lot sooner but just going down 2 numbers from the original 10/20 (to 8/18) then it is definitely worth it.

    I can easily understand where this would lower noise levels from valve train, but I question how much extra power a couple of thousandths would produce. That said, only a dyno could say for sure. Since the only drawback is clearances and expansion of the valves due to heat, it would certainly pay to set up at the factory minimum, beyond that its a game of what gets to the point of problems first.

    I can easily understand where this would lower noise levels from valve train, but I question how much extra power a couple of thousandths would produce. That said, only a dyno could say for sure. Since the only drawback is clearances and expansion of the valves due to heat, it would certainly pay to set up at the factory minimum, beyond that its a game of what gets to the point of problems first.

    not arguing here, but you are till in cummins official specs between INTAKE 0.006in and EXHAUST 0.015in

    One of my trucks is a 93 12valve that had 124K on the odometer when I bought it. That being my first diesel I took it to Cummins for a tuneup. Cummins set the intake to .008 and the exhaust to .017 the truck now has about 190K and is running fine. I tried one time to set the valves back to .010 and .020 but was disappointed at the power and MPG loss (several MPG) so I set them back to .008 and .017.

    i would say there is little to no danger if your truck is STOCK. if your head or block have been decked, you have a custom cam, etc. then at the INs .010 EXs .020 they are not true anyway
    • Owner

    Comes down to the simple fact that all metal expands... So at full operating temp how much expansion of metal do you have? Now that things tighten up and running higher RPM's how much valve float or interference would you have? There is a reason for why Cummins has these valve set this way... (Yes. I know the shrug smiley is AWOL!) :banghead:

    Well I had a few minutes today and so I did it. I .08 and E .015 HOT. I did notice right away it is quieter. I started it and free reved it did not drive it. I also have 5k springs so i dought i will see valve float. Time will tell.

    Comes down to the simple fact that all metal expands... So at full operating temp how much expansion of metal do you have? Now that things tighten up and running higher RPM's how much valve float or interference would you have? There is a reason for why Cummins has these valve set this way... (Yes. I know the shrug smiley is AWOL!) :banghead:

    This. Its one thing to cheat the specs on a slant 6... on a Cummins, might oughta take a hint from the engineers unless ya got free cash to spend on a couple of hp. Just my :2cents:
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