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| Some thoughts after a recent valve lash adjustment https://slantsix.org/forum/viewtopic.php?t=47144 |
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| Author: | krytellan [ Tue Nov 29, 2011 12:05 pm ] |
| Post subject: | Some thoughts after a recent valve lash adjustment |
About a year ago, when I got my Volare, I did a valve lash adjustment as I assumed it had never been done. At the time I did it, the valves were pretty clacky. I did the adjustment to the .010/.020 specs and it quieted down a good bit. Recently, after noticing a real loss of power and economy out of the car, I decided to revisit the adjustment. I did the adjustment while the car was at operating temps and running, same as last time, with the idle at 750 RPM. In doing the process this time, maybe being a bit more precise than the first time, I noticed I had to back off the valves sometimes pretty significantly. In fine tuning for 6 total times through the motor, I noticed 2 things: 1) the engine got pretty clacky again, and 2) my RPMs rose to about 875 (I had a tach hooked up from start to finish) without any other adjustments being made. This leads me to 2 questions: 1) On a 1-10 scale of quiet to loud, how loud do you all find your valves to be at top tuning? 2) Is it reasonable to assume that the increase in RPM represented a significant increase in power/efficiency after adjusting the valves? I have not been able to take the car out yet as I am in process of replacing speedo stuff. |
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| Author: | Sam Powell [ Tue Nov 29, 2011 12:29 pm ] |
| Post subject: | |
Looser valve adjustment means more overlap for exhaust and intake events, and thus greater build up of compression pressure. This will equate to better low end power, and more torque. Do a compression test and see if the compression is normal, and even. I think decent is in the 125 -150 range. Sam |
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| Author: | Nongan [ Tue Nov 29, 2011 12:43 pm ] |
| Post subject: | |
Mine are about a 3 on the scale, you can't hear them unless you listen for them or it's a cold start. But mine are apparently abnormally quiet according to some of the Oregon gurus, Ceej and THOR both have made comments on them before. There were virtually silent prior to rebuilding the top end of the motor and now they are there if you listen for it. |
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| Author: | WagonsRcool [ Tue Nov 29, 2011 12:53 pm ] |
| Post subject: | |
1) I'd say 4-5, like a BIG sewing machine (clicky, not clacky) 2)If setting lash changed the idle speed, then it was too tight. (& that will affect engine operation across the board) Having valves that are a little loose is better than a little tight. I wonder if the engine was truely up to full operating temp when you first set clearance- it takes a LONG time to heat up all the engine parts at idle. As I understand, intake valve closing event has the most influence on when engine starts making power/torque. Early closing builds cylinder pressure sooner (lower rpm) while later closing (ie.- loose adjustment) tends to build pressure at higher rpm. |
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| Author: | SlantSixDan [ Tue Nov 29, 2011 1:17 pm ] |
| Post subject: | |
Other way round, Sam. Tighter valve adjustment = more overlap. A looser adjustment in effect makes the cam lobes smaller. Effective duration, overlap, and lift are all reduced because the start and end of each lobe is "used up" taking up the extra lash, and only once there's no more lash can the valve be opened. With a loose adjustment, opening events occur late, and closing events occur early. With a tight adjustment, opening events occur early and closing events occur late. Less overlap + earlier intake valve closing = increased low-end torque at the expense of high RPM breathing. Properly adjusted valves with a stock or near-stock cam will be not at all loud. Slight "tick", yes, but no prominent "clack". |
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| Author: | Aggressive Ted [ Tue Nov 29, 2011 1:29 pm ] |
| Post subject: | |
As always Dan is right on! My stock 79 engine sounds like the following: At 750 rpm idle mine sound like a slight tick. At 600 rpm in gear under load at a light with lights on, heater fan on, wipers, they are hardly noticeable. |
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| Author: | wjajr [ Tue Nov 29, 2011 2:21 pm ] |
| Post subject: | |
Krytellan: Quote: In doing the process this time, maybe being a bit more precise than the first time, I noticed I had to back off the valves sometimes pretty significantly.
Just my 2 cents: I wonder if the change in position of intake & exhaust from front three cylinders and back three had anything to do with this. In other words EI, EI, EI, IE, IE, IE. If by chance you forgot this, the adjustment first time around could have been opposite where Intake got an exhaust adjustment. This would have made a difference in idle quality, and a reason you had to make a few big adjustments second time around. This has happened to me more than once when in a pre coffee fog… |
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