I was out in the guys room today,changing oil. Since I have never checked the compression on the motor that was built with your help , I desided to do it.

Well here are the results.
390 .030 over
-20 cc pistions kb's
crane cam 941
stock heads cced @72
.040 head gasket
.000 deck height

167to 172 lbs throttle closed

192 to 198 throttle open

then I started it ran to 3000
and recheked 1 cylinder wile running 218

just for fun

mike

 

What kind of vacuum are you getting with that setup. Thats the same setup I have except I have silovlite 1131 pistons...not sure of the difference though, not a piston expert. Oh and I don't remember my deck height either. I'm only getting 14-15" of vaccum unless I crank my initial timing up to about 16. Then I can get 16" of vacuum.

 

vacume is 13 to 14 @ 750rpm

17 inital advance

stock dizy is curved @ 20 total starting @1500 and in mall the way @ 3500

adjustable vacume advance 10 total @ 18 inches of vacume

must burn 92 or ping ping

 

Ok good, I was wondering if I had something wrong to only be gettin that little of vacuum. I'm still workin on curving mine. With 17 initial yours doesn't buck at hot startup? Mine bucks at only 15-16 initial. Seems like I can't win for loosing on this motor for some reason. I don't mind learning but I hate going in circles.

 

This is where Bear would step in and remind me of my evil ways, but I'm gonna say it anyway...

73F100*8Mile, you might need to go to an inverted vacuum advance like I did. My setup was such that I was lucky to get 11 inches of vacuum at 900RPMs. And that was with a huge initial already.

Like you, it would not want to start when hot - too much advance. I had to hook the vacuum advance to manifold vacuum, adjust it just right, and got 18 degrees of advance at idle to get the vacuum up to 15 inches or so.

The good part is, with the vacuum advance hooked up this way, you have NO advance when starting because it's not making any vacuum yet. So it would fire right up very easily.

And, when leaning on the gas, the timing would RETARD so that it wouldn't ping so easily.

Of course, the cam, it's timing, and other factors should be "fixed" instead of using a vacuum advance band-aid like I just described. But sometimes it's more fun to turn things around

 

Well see thats the thing, I don't knnow much about cam timing when it comes to advancing or retarding it. I don't know what the results are so I'm not gonna spend all that time going in there to do something I know nothing about. What are my options with cam timing? Is that where I should be looking? Ignition timing isn't doing hardly anything for me and like I said, I'm just going in circles with it. I will probably do the reverse vac advance like you did as a bandaid for now though.

 

218 is 14.8:1 at sea level!!!!!!! What are you using for fuel?.......

 

He did that with the engine running - quite a different story than using the starter for a "normal" compression check

 

Hey how do you figure compression anyhow? I think I checked mine and it was around 190psi. Whats that good for ratio wise?

 

Depends on the cam timing and duration too

 

Krewat, can you take a stab at my post above about cam timing. I'm not experienced at all with changing my cam timing. What does advancing/retarding do for your motor? Is that something I should seriously consider since I'm having troubles with pinging?

 

What dizy are you useing? Did you check your mechanical advance ? From my little time playing arround with your set up, I think @ 2000 you want it to start and @ 4500 end. but think this is with a truck with a auto and 3.01 gears. The heaver the car the slower the curve. A stock dizy with 2 light stock springs will get you very close. May need to bent the tabbs out to slow stuff down a bit.
The vacume advance should have a good stout spring in it. G et a few of the shelf and push the arm in you should feel the differnce. Just my 2 cents.

 

Volumetric air pressure at sea level is 14.7 Lbs per sq inch.......A naturally asperated engine pumping 147 psi would then be 10 to 1. Dividing the compression gauge reading by 14.7 will get you in the ballpark. However, as mentioned, cam timing, and especially overlap will drop the actual results. The fact that the 218 PSI was obtained with the engine running, (not sure how) doesn't change what the air/fuel charge is seeing in the cylinder.....IMO his compression gauge may need a little calibration......You can also do the math. Convert cylinder volume to CCs, and divide that number by the combustion chamber CCs......Yes, there are varibles, valve reliefs, domes, etc etc, but you can figure it to a knats *** using the numbers instead of gauges....aMP

 

This is how. Install a screw in compression guage, start engine, rev to rpm wanted,, shut off engine, read guage,,, just for fun

 

what is this "inverted vacuum advance"?