Rocker ratio and PV

[1972ho]

If I go from a 1.7 rocker ratio to a 1.6 will that increase the PV clearance.Im running pretty tight on my rebuild of the V-6,I’m at .095 exhaust with .022 lash and only .035 with .020 lash on the intake.Something that was happen while checking pv with zero lash the exhaust was .075 and the intake was .050 this was with 4 degrees advance.Now should the intake decrease with lash or should it increase,every time I have added the lash the exh would increase the pv but the intake would decrease.Shold that happen like that.

[Baprace]

I think you have plenty of clearance. The intake seems to have good clearance if you don't advance the cam much more. More lash on intake should be more clearance, the events are happening later.

[DaveMcLain]

It will increase the piston to valve clearance by lowering the rocker ratio but not all that much. Why? Because you're multiplying the lift of the cam by the ratio of the rocker and the lift of the cam isn't all that much where it is getting pretty close. This means that the change is also much less than it would be at or near max lift.

[MadBill]

For example, if the valve lift is 0.250" at its closest approach to the piston, a change from a 1.7 to 1.6 ratio will increase the P-V by 0.250 - [ (1.6/1.7) x 0.250] = ~ 0.015".

[andyf]

I learned the hard way that adding rocker arm ratio will reduce piston to valve clearance! I put a set of Jesel 1.85 rocker arms on a BB Mopar once and the intake would chase the piston down the hole. That engine had a 7.100 long connecting rod so the piston didn't move to quickly leaving TDC. The high ratio rocker arm moved the valve faster........

[MadBill]

Do you recall what the crank angle was at closest approach?

[Geoff2]

You need to check not only PTV at TDC, but a few degrees before & after TDC. Check the exh before TDC, the intake after TDC.

[andyf]

Do you recall what the crank angle was at closest approach?

Not without digging thru some old notes. I plotted out the PV clearance every few degrees of crank angle and I remember that it was an interesting curve. The valve was accelerating faster than the piston after TDC. The valve probably got closest at 10 to 15 degrees ATDC and then the piston started to pull away.

It can all be modeled without too much work since it is just a lever arm type of mechanism. The rocker arm ratio changes both the travel of the valve as well as the acceleration of the valve while the rod ratio determines the acceleration away from TDC. You can get into trouble like I did when combining lots of rocker arm ratio with a long rod engine.

[1972ho]

I’m using a longer rod on this build it is 6.320 long in the v-6 with a 9.188 deck height and a 3.515 stroke.Do you think that my be why I’m seeing less ptv clearance on the intake with lash or is it something else.So do you think I can get away with the intake side only being at .035 or so this is with new valve springs,new tight timing chain 7/16 pushrods that are .125 wall or greater new set of lifters and turning the engine up to about 78-8100 rpm.

[hondo383]

I have run intakes that close, it was fine but be 100% positive the radial clearance is adequate. Sometimes the pocket is not parallel to the valve and you are way in there lol and you may wanna check all the pistons...The way I understand it when the engine is running at speed your cam will retard a degree or 2 with a chain maybe more? You may try checking at 2 degrees retarded from where you are now and see how that works for a real life view?no clue on the lash thing

[piston guy]

1972ho,
Something else is happening IF you increase lash and LOSE PtoV. It is impossible for that to happen as both Baprace and Dave Mclain mentioned.

[geraldtson]

If I go from a 1.7 rocker ratio to a 1.6 will that increase the PV clearance.Im running pretty tight on my rebuild of the V-6,I’m at .095 exhaust with .022 lash and only .035 with .020 lash on the intake.Something that was happen while checking pv with zero lash the exhaust was .075 and the intake was .050 this was with 4 degrees advance.Now should the intake decrease with lash or should it increase,every time I have added the lash the exh would increase the pv but the intake would decrease.Shold that happen like that.

Make certain your setting valve lash in proper sequence (E O I C ) and with 1.7 rocker ratio check close your pushrods are not binding or rubbing on pushrod guide holes in head or pushrod guide plates. Just sounds like they may be rubbing. And no i wouldn't run an engine with that close (.035) vtp clearance.

[1972ho]

I’m running the jesel rocker system so there is nothing rubbing or binding and I have checked the ptv 3 times with zero lash and with .020 lash and keeps increasing and decreasing ptv.I think I’ll put a different rocker on and check it maybe it’s something going on with the rocker.

[digger]

on intake side last time i check about 8* ATDC was worst

[twl]

I’m using a longer rod on this build it is 6.320 long in the v-6 with a 9.188 deck height and a 3.515 stroke.Do you think that my be why I’m seeing less ptv clearance on the intake with lash or is it something else.So do you think I can get away with the intake side only being at .035 or so this is with new valve springs,new tight timing chain 7/16 pushrods that are .125 wall or greater new set of lifters and turning the engine up to about 78-8100 rpm.

Longer rod means the piston dwells longer around TDC, so it could have an effect.