Stock compression???

[Arvizu9618]

Stupid question but what compression readings should i see on a used (but still running, not abused) stock 350?

 

thanks

 

Art

[bobsmc]

i would think if your readings are even and above 100 psi you should be good.

rule of thumb is 10% between highest and lowest reading.

[TrunkMatt]

100psi would be too low.

 

From what I recall, It should be minimum 160 psi on a bone-stock 350 with factory cam in good condition.

 

[72-CLASSIC_RIDE]

If motor not tired, 140 to 160 compression at cylinders.

Even 100 is doable but getting tired.

 

Doug

[footballubet]

Make sure you are set up for the compression test correctly. All spark plugs removed, ignition system disabled and throttle plates wide open. With the gauge installed crank the engine until you hear 5 "puffs" then take your reading. Repeat for all cylinders. Then compare your readings. As Bob said the highest and lowest should be within 10% of each other.

[TrunkMatt]

For fun as I was changing plugs I put my Monte / 383 to a compression test today (only ~5k miles on this build)

 

Dry compression:

 

1. 220

2. 230

3. 220

4. 220

5. 230

6. 220

7. 220

8. 215

 

I am still safely within pump-gas range. I think #8 was off / bad test as I did a 1/2 crank short... will keep my eye on it though.

[MCfan]

That's interesting, Matt. Your PSI readings suggest a much higher CR than you could ever support with pump gas (unless you pumped it at an airport or race track ) .

 

What is the advertised CR of your motor or what was the target CR when it was built? Also, what brand of compression tester did you use? Thanks.

[TrunkMatt]

That's interesting, Matt. Your PSI readings suggest a much higher CR than you could ever support with pump gas (unless you pumped it at an airport or race track ) .

 

What is the advertised CR of your motor or what was the target CR when it was built? Also, what brand of compression tester did you use? Thanks.

 

Ahh, the good ole pump gas conundrum... cranking compression isn't the end-all of octane tolerance... cam duration / profile play a lot into it. Also Aluminum heads help a bunch :-) However, I am on the edge of not being able to use 87 octane...

 

For my build, 10.5:1 compression ratio with a cam profile of 488/.510 - 234/244 at around 220psi was the target. The 230 reading could be that I am still a bit wet in the cylinders as I am running really thin Royal-Purple oil right now... and just getting a bit of blow-by until I raise the viscosity and make the switch to synthetic (engine is just passing final break-in).

 

As far as compression testing - we used an electronic testing computer as part of my Dyno tuning I am doing at PSI motorsports: Link

 

Here is a video of the dyno-test: Link to Testing Video

 

The big pull starts at 0:40... you can hear a flat spot at the beginning that I am trying to have tuned out. Almost got it worked out. Damn Holley Carbs...

 

Anyhow, even with the flat I am breaking 350 at the wheels. SAE Net HP should be 425+-ish at the crank with matching torque. Power drop-off is at 6250 RPM.

 

My target for the build was a nice street-able 383 with 400/400 Torque/HP at the crank.