350x compression reading

[whiteman350x]

What should the compression read on a 350x with 10.25 Wiseco piston at .040" overbore? I just got one of my 350x's back after being on loan for about 15 years. It was freshly built when it left. They were saying hard to start when parked it about 8 years ago. Trying to do some testing before I tear into it. Working on 7 350x's right now so don't want to tear down one that is ok. I can find stock compression readings, but not for the 10.25 comp.

[Jim mac]

can't help on a actual reading, but the different cams will effect the compression test too. jim

[ATC King]

If the static compression ratio wasn't verified during the build, that 10.25 number doesn't mean much. Can't really just go by that. Besides, that's just static compression. You'd need to know other important factors to calculate dynamic compression.

Another thing that hasn't been verified is the camshaft timing. A worn timing chain will retard the timing. That is very seldom discussed when people are trying to diagnose issues. If at any time the cylinder head was resurfaced, compression and cam timing aren't stock anymore. A thousandth here, a degree there, adds up.


Without knowing the actual CR, a better indicator of engine condition would be a leak-down test.


Don't forget about checking the ignition. At the very least, and inexpensive spark energy tester is going to be a big help. https://shop.advanceautoparts.com/p/performance-tool-hi-energy-ignition-tester-w84600/9020659-p?c3ch=PLA&c3nid=9020659-P&adtype=&product_channel=Online&store_code=&msclkid=7dab5642c41a1881c90b3554cf369a3c&utm_source=bing&utm_medium=cpc&utm_campaign=b_shopping_all_hand%2Fspecialty%2Ftoo ls_gen_gen_gen_evergreen_eng_aap_us_all&utm_term=4580428014406805&utm_content=Bing_All_Hand%20%26%20Specialty%20Tool s_Specialty%20Tools&gclid=7dab5642c41a1881c90b3554cf369a3c&gclsrc=3p.ds

[MrConcdid]

The short answer found in the manual is 178psi +/- 14psi

[whiteman350x]

That is stock compression. How much for the 10.25 wiseco piston?

[ATC200X4716]

If stock is 8.5-1 and you have 10.25-1 you have 1.75 points higher compression, so all other things being equal it would be 1.75 x 14.6 psi more so about 25psi more. Now that would be in an ideal world on a hot engine, throttle wide open while kicking; all of the textbook compression test stuff. As pressure goes up, you have more losses so you won't necessarily get all of it. On a fresh engine with stock cam and that piston you might see 190-200 psi. Bigger cam, decking, stretched chain will all affect it. Honestly if you have better than about 160psi it should run well. Check the valve lash too. I think my 12-1 motor was about 220psi when it was fresh.