73, 340 compression question

[crash340]

 

[mrbill426]

So there are no maximum "yes it will fit" aftermarket cams for say a '68 340?  I understand the (should check it) aspect, just wondered about tried and true.

Thanks

Generally I think you need at least .1 inches clearance piston to valve clearance. That's what I was told when changing cams on my 440. But taking some measurements initially is the right thing to do.

st

[cudabob496]

So there are no maximum "yes it will fit" aftermarket cams for say a '68 340?  I understand the (should check it) aspect, just wondered about tried and true.

Thanks


 st

I'm sure there are guys on this site that know what worked for them. But I would also call the cam manufacturer, tell him what you got, and he should probably
know if it will fit.  that way, if you buy it, and there's problems, at least you have some recourse.

[Chryco Psycho]

the taller pistons have valve reliefs so .500 lift is fine , you actually often have more clearance than you would with lower comp pistons because of the reliefs

[dpcd67]

I used a Comp 268H cam and there are no valve to piston relief issues; the pistons have big notches in them for valve clearance.  Sealed power pistons.

[cudabob496]

I assume at assembly, there is a way to verify no contact, before actually starting engine?

[mrbill426]

Some folks put a thin layer of (I think oiled) modeling clay on top of the piston and rotate the engine through both cycles with the head on, them pull the head and note the valve impressions in the clay.

I assume at assembly, there is a way to verify no contact, before actually starting engine?

[Strawdawg]

Some folks put a thin layer of (I think oiled) modeling clay on top of the piston and rotate the engine through both cycles with the head on, them pull the head and note the valve impressions in the clay.

As you said--You put the clay over the valve reliefs and then smear a coat of oil on it to prevent from sticking to the valve(s) when you rotate the the engine as you describe.  You put the gasket on the block and then set the head on...just lightly snug the head bolts-not enuf to compress the gasket.  Most manufacturers provide the uncompressed and compressed gasket thicknesses so you can add the difference to amount of the cut in the clay...that way you don't ruin a head gasket.

Normally clearance is only a concern when using high lift cams as Neil pointed out.  Most pop up pistons are cut with deep reliefs from the factory

[cudabob496]

If keep heads on, then need to verify there are valve reliefs in the pistons.
Maybe a boroscope through a spark plug hole.

[Strawdawg]

So there are no maximum "yes it will fit" aftermarket cams for say a '68 340?  I understand the (should check it) aspect, just wondered about tried and true.

Thanks

If you are starting with a low compression engine and not rebuilding it for higher compression, then you will want to keep the cam matched to the existing compression which means a relatively mild cam and no worries about valve interference.  Given that the factory pistons are down in the block, by the time you got enuf lift to create potential problems, the car would probably not run anyway

If you have any doubts about the current compression ratio, simply do a compression test on a few cylinders.  If it is above 200 psi, you might have a problem.  If it is more like 140, you will not as long as you stick with the cam companies recommendation for your advertised compression ratio.

[72cudamaan]

If keep heads on, then need to verify there are valve reliefs in the pistons.
Maybe a boroscope through a spark plug hole.

Good thought, but you still need a way to verify how deep the reliefs are ie: valve to piston clearance. Usually the minimum recommended
is .060" on the intake and .100 on the exhaust. If heads were on then you would have to get each piston on TDC, remove
the valve springs, and put a dial indicator on the stem tip. Lower the valve until it touches. Measure the total travel
of the valve and subtract your minimum clearance from the total. Personally, I would pull the heads and measure with the clay.

[Chryco Psycho]

Valve clearance is a dynamic changing all the time , the piston is chasing the exhaust valve upward , total lift means nothing , it is a variable between lift & duration , the longer the duration the later the exhaust valve closes as the piston is rising towards it or earlier the intake will open as the piston is moving away so even if you roll the engine over with the current cam knowing the lift you may have plenty of clearance but a similar lift cam with longer duration may be tight for clearance , the Only way  to know for sure is install the cam & turn it over using putty , , the closest point may be roughly 6* before TDC between the piston & exhaust valve or 6-10* after TDC as the intake is opening as it chases the piston down the bore . If you change the installed centerline of cam by degreeing it in advanced or retarded  the clearance may go away also . The overlap in the cam is created by closing the exhaust valve late & opening the intake early , the exhaust & intake valves are actually open over TDC allowing the exiting exhaust to create a vacuum & pulling in the intake charge , it is actually more effective at filling the cylinder than the piston moving down the bore with a longer duration cam @ higher rpm 

[mrbill426]

What I plan to install are a set of +.020" Speed Pro forged pistons which do have valve reliefs; I think they are #2316F-20.  The heads are stock '72 with the smaller exhaust valves though.  This is a April '72 engine that has a steel crank.  I have a LD340 intake that I was thinking of using but I'm told it might not accept the factory A/C compressor which I AM retaining.  Stock exhaust will be retained too.

  

If you are starting with a low compression engine and not rebuilding it for higher compression, then you will want to keep the cam matched to the existing compression which means a relatively mild cam and no worries about valve interference.  Given that the factory pistons are down in the block, by the time you got enuf lift to create potential problems, the car would probably not run anyway

If you have any doubts about the current compression ratio, simply do a compression test on a few cylinders.  If it is above 200 psi, you might have a problem.  If it is more like 140, you will not as long as you stick with the cam companies recommendation for your advertised compression ratio.

[Tonker1]

[mrbill426]

Me too, but maybe no one knows.