Cranking PSI 72 351C 2 bbl

[Johnny57]

Can't seem to find it on google. TKS!

 

[Cleveland Crush]

Lots of factors to consider on cranking psi, ie; temp, cam specs, battery charge, etc. Main thing all cylinders reading within about 10% of each other using the same number of cranks (I use 5 for each reading) but you asked for "The Number" so here goes: my 11:1 solid roller 427c with all plugs removed at about 60 degrees=195 psi +/- 5 psi. My mild 351c stocker shows 160 psi low to 170 high. I'd be OK with anything from about 140 on the low end all the way up to 200 high end as long as all eight readings are within 10% of each other.

 

[Don C]

To add to the variables affecting cranking pressures, actual barometric pressure and altitude. Less air pressure equals lower cylinder pressure. The spread sheet in this post will let you play with the variables:
https://7173mustangs.com/threads/compression-altitude-calculations.11267/

 

[Johnny57]

Anyone have the factory spec psi cranking compression for a 1972 351C 2bbl?

 

[boilermaster]

8.6 to 1 cr. should yield 142 psi cranking compression on a warm engine with the throttle wide open.
this would be for a factory camshaft.
if it's a well sealed engine you are looking for, look for all cylinders to be within 10 psi. of each other.
also, one should have all spark plugs removed when doing a compression test.

 

[71ProjectJunk]

Somewhere in the 130-150 range would be fine for a stock 351C low compression engine. Just check it and post your findings

 

[Johnny57]

I just found (actually thought to look for a calculator) for the conversion 8.6:1 is 126 at sea level. Which I'm getting 120 psi at 5500ft alt. So I'm good.
https://calculator.academy/compression-ratio-to-psi-calculator/#f1p1|f2p0

 

[machphil]

I have three sets of manuals from Ford for '71-'73. The compression numbers should be within 75% of the highest number.

 

[boilermaster]

Machphil is absolutely correct, I must have looked for half an hour for my 1972 vol2. engine manual.
johnny57, you are correct about having to account for altitude but I believe you need to add one atmosphere to your equasion.

 

[Johnny57]

Machphil is absolutely correct, I must have looked for half an hour for my 1972 vol2. engine manual.
johnny57, you are correct about having to account for altitude but I believe you need to add one atmosphere to your equasion.

I was just looking for the factory "base" PSI. I built performance 2 stroke motorcycle engines as a profession for the last 22 years, so I know the parameters. But it seems some guys don't realize the base psi is very important. As many post "as long as they are with in 10% differential" or what ever you are good. That's not really valid as a single statement, as you have to know the base psi also. You may be within the 10% difference, but if you are down 40 psi from your base psi, your motor is sick. Which actually negates the 10% differential saying you are in a acceptable range.

 

[Cleveland Crush]

I was just looking for the factory "base" PSI. I built performance 2 stroke motorcycle engines as a profession for the last 22 years, so I know the parameters. But it seems some guys don't realize the base psi is very important. As many post "as long as they are with in 10% differential" or what ever you are good. That's not really valid as a single statement, as you have to know the base psi also. You may be within the 10% difference, but if you are down 40 psi from your base psi, your motor is sick. Which actually negates the 10% differential saying you are in a acceptable range.

 

[Cleveland Crush]

Restated slightly different: If your Engine is within 130 and 190 cranking psi (no one has a Ford assembly line produced baseline engine mentioned in 50 year old shop manuals) AND a variance of less then 10% you're good. The old corporate shop manuals stated 25% variance to avoid warranty repair and getting the vehicle out of the warranty period. Anything even approaching 25% variance is terrible.

 

[boilermaster]

Very well stated Cleveland Crush, nowdays , I usually only use a compression tester as a yes or no tool.
If I want to know the health of any particular engine, I break out the leakdown tester, it doesen't care what the cranking compression should be.

 

[Cleveland Crush]

That's for sure.
As a side note, if a "Classic" non racing street engine runs good, doesn't smoke or use oil, I'm happy and won't go looking for trouble with a compression or leakdown test!!

 

[Johnny57]

Then we will agree to disagree. As you posted, compression ratio has no effect on power. 130psi converts to 8.9:1 and 190psi converts to 13:1. Which engine will run circles around the other?

And wanting to know a baseline psi tells me the motor is not worn out, in a quick and dirty way. Even if my motor showed 110 psi, I would know there is a problem with wear somewhere, and the motor wasn't healthy, no mater what the variance was.

 

[c9zx]

"130psi converts to 8.9:1 and 190psi converts to 13:1." I don't know where you got this information but, there are too many variables that affect cranking PSI for it to be accurate. Cam timing events, altitude, cranking speed, WOT or not, are all plugs removed, and is the battery kept fully charged. "As you posted, compression ratio has no effect on power." Perhaps you just worded that poorly. Chuck

 

[Cleveland Crush]

No disagreement, totally agree 110 psi is well out of range of the 130 min. mentioned above. Also agree, more is better IF within octane tolerance and and design specifications.

 

[Johnny57]

"130psi converts to 8.9:1 and 190psi converts to 13:1." I don't know where you got this information but, there are too many variables that affect cranking PSI for it to be accurate. Cam timing events, altitude, cranking speed, WOT or not, are all plugs removed, and is the battery kept fully charged. "As you posted, compression ratio has no effect on power." Perhaps you just worded that poorly. Chuck"

If you keep everything the same spec and only change the comp ratio, this is the outcome. If you look at the rest of the posts, I was looking for the stock 72 351C 2bbl cranking psi, that was all. And this is the calculator. 8.6:1 is 126psi at sea level, I got 120 at 5500 ft, which is in line with the factory spec. So I know the motor is in good condition compression wise.

https://calculator.academy/compression-ratio-to-psi-calculator/#f1p1|f2p0

 

[Hemikiller]

This is a snip from the 66 Ford service manual. It illustrates that the compression ratio does not have a direct correlation to the cranking pressure readings. The sixes have a higher pressure range than the 289s with a higher CR, yet have a nearly identical range as the 10.5:1 390 engines.





Your 126psi at 5500 corrected to sea level at 70°F is 154psi. That's a healthy reading no matter what.


The linked calculator is simply multiplying the atmospheric pressure by the theoretical CR. It doesn't take into account the intake closing point, which will produce the Dynamic Compression ratio. A short duration cam at 9:1 will produce significantly higher cranking pressure readings than the same engine with a cam having 20° more duration. George Pence compiled a chart on the various 351C engines, which demonstrates the differences. The 72 engines shared the same compression ratio, but the 2V would produce a higher cranking pressure than the 4V due to the shorter duration 2V cam.

 

[Johnny57]

This is a snip from the 66 Ford service manual. It illustrates that the compression ratio does not have a direct correlation to the cranking pressure readings. The sixes have a higher pressure range than the 289s with a higher CR, yet have a nearly identical range as the 10.5:1 390 engines.

View attachment 61261



Your 126psi at 5500 corrected to sea level at 70°F is 154psi. That's a healthy reading no matter what.


The linked calculator is simply multiplying the atmospheric pressure by the theoretical CR. It doesn't take into account the intake closing point, which will produce the Dynamic Compression ratio. A short duration cam at 9:1 will produce significantly higher cranking pressure readings than the same engine with a cam having 20° more duration. George Pence compiled a chart on the various 351C engines, which demonstrates the differences. The 72 engines shared the same compression ratio, but the 2V would produce a higher cranking pressure than the 4V due to the shorter duration 2V cam.

View attachment 61262

Thanks for your post.

Then I guess I'm back to square one. I need to know the factory psi for the H code 351C 2bbl. I guess it is nowhere to be found. I don't see how anyone can figure out if their ring/valve seal is any good without a total stock rebuild to test. That 40psi "range" is ludicrous, and literally useless for a comparative. It can't be that impossible to accurately calculate. And my results were 120 psi at 5500ft, the 126 psi was at sea level.