340 Static Compression Numbers and Test Procedure

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or the cam is off a tooth
or
do the easy stuff first
check your gauge
peek in the cylinder
pull a valve cover and give us the lift
we can estimate the cam from that
then put a degree wheel on it and tell us the timing
you can even rotate it past TDC on overlap and tell us the difference in height of the intake and exhaust valves say -10 TDC +10- degrees
we can tell if it is advanced or retarded roughly

aj where did you post that comparison of 3 crs?? not the usual? great post
 
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Wyrmrider said:
or the cam is off a tooth
or
do the easy stuff first
check your gauge
peek in the cylinder
pull a valve cover and give us the lift
we can estimate the cam from that
then put a degree wheel on it and tell us the timing
you can even rotate it past TDC on overlap and tell us the difference in height of the intake and exhaust valves say -10 TDC +10- degrees
we can tell if it is advanced or retarded roughly

aj where did you post that comparison of 3 crs?? not the usual? great post
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Right. The OP can take the valve cover off and look at the valves on number one cylinder and you can tell in the cam is advanced, retarded or straight up.

That's an easy place to start.
 
340inabbody said:
HF, US General 0-300 PSI Gauge, I suppose it could be off?
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Yes! See if you can rig the compression gauge to a compressed air system with a decent gauge (NOT a cheap HF compressor with their tiny junky gauges) and compare. (A large shop compressor will more likley have a decently accurate gauge.)

As for backing into the CR... for a stock-ish cam, (SCR-1)*20 gets pretty close to cranking compression. But you're not in that boat. As noted.. 200 is quite high for NA, and pump fuel. Explains the detonation as noted.

There is a less invasive way to do this in-car. Pull the intake and measure the cam duration at .050" and .006" lifter lift, plus valve lifts, and check cam timing, so you will have that cam info. With the intake closing angle measured (lifter at .006" from the intake valve being fully seated on closing), and an accurate cranking compression number and your altitude, you can back into the Static CR with the Wallace Dynamic Compression Calculator or with several other such tools. (The Wallace one accounts for you altitude so is useful in that way.)

And you would get a pretty decent cam duration numbers and timing, including that .006" lift intake closing angle, for a solid cam if you measure carefully at the top of the pushrod, so you could avoid taking the intake off if you just wanted a decent idea of things. But for best accuracy do it right at the lifter. And if it's hydraulic, then it pretty has to be done at the lifter body; pushrod movement when not running will be inaccurate with a hydraulic lifter.
 
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I had a 34O with 195 cyl pressure compression was 11.4 ran well on 93 octane , would ping with 89 octane .it had alu heads if that makes a difference . Not sure the cam it had but was nothing much as it had good vacuum for the power brakes .
Good luck with your 340 those are good healthy numbers for cyl pressure
 
I took some shots inside the cylinder. I did not see any numbers but the shape of the piston may be some of you can comment on.
Also when I had it in the shop we theorized that the timing was indeed off one tooth. I don't know how to read these markings but they reset it after this picture was taken and buttoned it up. I actually felt it ran better (like in the picture off by a tooth) before they moved the timing over one tooth (not pictured).

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340inabbody said:
I took some shots inside the cylinder. I did not see any numbers but the shape of the piston may be some of you can comment on.
Also when I had it in the shop we theorized that the timing was indeed off one tooth. I don't know how to read these markings but they reset it after this picture was taken and buttoned it up. I actually felt it ran better (like in the picture off by a tooth) before they moved the timing over one tooth (not pictured).

View attachment 1715455157

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That's a 12.5:1 TRW.
 
340inabbody said:
I took some shots inside the cylinder. I did not see any numbers but the shape of the piston may be some of you can comment on.
Also when I had it in the shop we theorized that the timing was indeed off one tooth. I don't know how to read these markings but they reset it after this picture was taken and buttoned it up. I actually felt it ran better (like in the picture off by a tooth) before they moved the timing over one tooth (not pictured).

View attachment 1715455157

View attachment 1715455158

View attachment 1715455168
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Ignore the dots. You MUST degree a cam to know where it is.

Like I posted above, pull the drivers side valve cover, roll the engine to TDC number 1 cylinder on crossover (overlap) and LOOK at the rocker arms.

If the intake rocker is further open, the cam is advanced. If the exhaust rocker is further open, the cam is retarded. If both rockers are open the same amount, the cam is in straight up.

How far advanced or retarded the cam is can only be known by degreeing the cam.

That piston is a claimed 12.5:1 but you have to be about 64 cc's on the combustion chamber and the piston is out of the bore at least .020 or more, depending on the head gasket thickness.

I'm betting unless the heads have been milled to hell and the block decked a ton, you're probably at a true 10.5-10.8:1 range and that thing should be able to run full timing on 91 pump premium with no issues.

I doubt you'll be able to run a vacuum advance with that, unless you run it off of manifold vacuum and curve the distributor for that.

The real issue is even with manifold vacuum the mechanism works way too slow to pull timing back out and you get tip in rattle.
 
yellow rose said:
Ignore the dots. You MUST degree a cam to know where it is.

Like I posted above, pull the drivers side valve cover, roll the engine to TDC number 1 cylinder on crossover (overlap) and LOOK at the rocker arms.

If the intake rocker is further open, the cam is advanced. If the exhaust rocker is further open, the cam is retarded. If both rockers are open the same amount, the cam is in straight up.

How far advanced or retarded the cam is can only be known by degreeing the cam.

That piston is a claimed 12.5:1 but you have to be about 64 cc's on the combustion chamber and the piston is out of the bore at least .020 or more, depending on the head gasket thickness.

I'm betting unless the heads have been milled to hell and the block decked a ton, you're probably at a true 10.5-10.8:1 range and that thing should be able to run full timing on 91 pump premium with no issues.

I doubt you'll be able to run a vacuum advance with that, unless you run it off of manifold vacuum and curve the distributor for that.

The real issue is even with manifold vacuum the mechanism works way too slow to pull timing back out and you get tip in rattle.
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Great information YR! Thank you! I will try what you suggest WRT looking at the rockers as soon as I can.
 
I think a vac advance could be made to work.
Maybe even with this distributor.
Vacuum/Mechanical Advance Dilemma SB340

The more we learn the less surprised I am that the actual driving experience is as described.
I think with this distributor, best would be to see if it will like a little more initial. Say 20* @ 1000 rpm, then back down the speed to 800. It might want a little more fuel from the idle mixture screws. Put it in gear and see if it rpm drops less than before.

Check the timing at 800 rpm or whatever lower rpm it should idle at with more timing. If its 20* at 1000, it very well may be less advanced at lower rpm.

PS. No vacuum advance. Just forget about it for now.
 
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Mattax said:
I think a vac advance could be made to work.
Maybe even with this distributor.
Vacuum/Mechanical Advance Dilemma SB340
The more we learn the less surprised I am that the actual driving experience is as described.
I think with this distributor, best would be to see if it will like a little more initial. Say 20* @ 1000 rpm, then back down the speed to 800. It might want a little more fuel from the idle mixture screws. Put it in gear and see if it rpm drops less than before. Check the timing at 800 rpm if will run there.
PS. No vacuum advance. Just forget about it for now.
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Yeah I am running a new MSD billet into a new 6AL box. I want to try and keep the mechanical advance for now and make it all work.
 
340inabbody said:
Yeah I am running a new MSD billet into a new 6AL box. I want to try and keep the mechanical advance for now and make it all work.
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Going from that old thread, base timing of 17* was set at 1100 rpm.
Unlikely that's enough advance with your engine. By 1000 rpm, it should be up around 20*
If it was 17 or 18 at 600 rpm, that would be OK starting point assuming it starts advancing before 1000 rpm.
Just MHO
 
Mattax said:
Going from that old thread, base timing of 17* was set at 1100 rpm.
Unlikely that's enough advance with your engine. By 1000 rpm, it should be up around 20*
If it was 17 or 18 at 600 rpm, that would be OK starting point assuming it starts advancing before 1000 rpm.
Just MHO
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Hot off the press I just measured the actual curve in the car. May need to increase the timing at around 2700 that dip does not look great. It also maybe the data is not that great right there.

IMG_4640.jpg
 
Whistler looks like something dreamed up by tech inspection
run another compression check after moving the cam
let us know what you find on the tdc valve lift check
UDHarold was an expert at that check- he could get more out of it than I ever could
degree the cam with degree wheel before pushing advance
cheers
 
340inabbody said:
Hot off the press I just measured the actual curve in the car. May need to increase the timing at around 2700 that dip does not look great. It also maybe the data is not that great right there.

View attachment 1715455261
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Are the both springs the same?
Factory distributors can work with two different springs and make a curve, but a lot of the aftermarket ones can not work well that way.

I'd first work on reducing the rpm drop going into gear.
Is it better than when you had it at 17* ?

Then I'd go with less initial tension on the weights. That might mean a pair of lighter springs - which also mean quicker advance. With no vacuum advance, that should not be a problem. If there is ping (detonation), note the rpm and whether its part or full throttle. I'll be surprised if its at part throttle. May have to limit the total with a bushing or by heating and bending. Crackedback sells bushings in sizes MSD does not.
Heating and bending MSD/Delco advance info here. (do not join tapatalk, just click past any pop up)
MSD-Delco Distributor Weight Modification For Less Advance
 
Very roughly sketched on your graph are the timing curves you'd see for the DC/MP distributors if they were set up for approx 33* BTC at 4000 rpm.
Blue is the mechanical advance race unit. Black is the vacuum advance equiped unit. We can see your current timing might be in the ballpark if you were running vac advance. So that was reasonable when you were trying to set that up.
Notice the difference with the mechanical only distributor's advance. It can be very quick in. I wouldn't go quite that fast wiith the MSD, but it should be quicker.

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As far as initial (base) timing with the DC distributors goes, they are too little for what you're doing.
The vac advance unit was for a milder engine, so 12* was tolerable.
On the race unit they didn't care so much about idle when these were made, but did want easy restarts. The penalty for easy restarts was the timing hunted up and down with rpm at race engine idle speeds. On an automatic especially, this will drive most people nuts except for racing. Been there done that.
But look where timing is at 800 to 1000 rpm.
 
Mattax said:
Are the both springs the same?
Factory distributors can work with two different springs and make a curve, but a lot of the aftermarket ones can not work well that way.

I'd first work on reducing the rpm drop going into gear.
Is it better than when you had it at 17* ?

Then I'd go with less initial tension on the weights. That might mean a pair of lighter springs - which also mean quicker advance. With no vacuum advance, that should not be a problem. If there is ping (detonation), note the rpm and whether its part or full throttle. I'll be surprised if its at part throttle. May have to limit the total with a bushing or by heating and bending. Crackedback sells bushings in sizes MSD does not.
Heating and bending MSD/Delco advance info here. (do not join tapatalk, just click past any pop up)
MSD-Delco Distributor Weight Modification For Less Advance
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From my documents I am running both heavy silver springs to give me the shallowest curve and ease into the advance. I am also running a 14 degree advance limiter.
 
Well I'm with YR. If your going to be running it as is, I'd go with lighter springs and maybe limit more (12*).
Not sure where you're at with this.
Maybe the priority is checking when the valves open etc.

Just went off on this tangent because the timings relationship ties into compression and valve events.
 
Mattax said:
Well I'm with YR. If your going to be running it as is, I'd go with lighter springs and maybe limit more (12*).
Not sure where you're at with this.
Maybe the priority is checking when the valves open etc.

Just went off on this tangent because the timings relationship ties into compression and valve events.
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I thought that if I was getting some ping under load say at 2500 that a slow advance curve would help. If I speed it up I will have higher timing at 2500 which would add to the pinging or detonation.
If YR and you think I need more timing at lower RPM’s then why not just take that theory out and lock it out at 32?
 
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