Mercuiser 357 mag low compression on #6 (120 psi)

[JustJason]

So I have all the books.

Service manual 17 is for 1993 through 1997 5.0 and 5.7 engines, which is not what OP has.

Service manual 18 is for 1993-1997 V6 engines, which is also what OP doesn't have.

To add to that, you wrote "to get to 200 psi cranking pressure, you need to have well over 14:1 compression", which is untrue. If it was true, how in the world is Suzuki and Honda getting OVER 200 PSI of cranking compression pressure on engines that have 9.4 to 9.5 to 1 compression ratios?

It's no magic here. It's valve timing and volumetric efficiency.

BTW OP's engine is a 5.8 not a 5.7.

 

[Scott Danforth]

BTW OP's engine is a 5.8 not a 5.7.

OPs engine is a Mercruiser SBC 350 reman with anywhere from a 0.020 to 0.060 overbore sold as a "357" so it could be a 352 cube, 355 cube or 357 cube. Unless you look at the reman tag, neither you or I know the displacement.. you are arguing for the sake of arguing

 

[dubs283]

FWIW my compression tester guage reads 35ish psi static (0 psi). I routinely test many sbc v8's that read 180-190 psi on engines that are sound, no miss/dead cylinder exhaust sound, wot 4500ish rpm/spec

If I do the math: 180 - 35 = 145

I've dropped, bumped and used my guage on a weekly basis for 20 years. That, I believe, is a sound reason for the 35 psi offset

 

[alldodge]

Compression ratio is compression ratio
Math does not lie, incorrect calculations can

If atmospheric pressure is used, turbo changed, supercharged the formula is correct. The DF300AP list 123 to 213 psi which leads me to think this covers more then just the 300, as with the older Verado V6 OB they were supercharged. Supercharging will increase the compression ratio

 

[JustJason]

Compression ratio is compression ratio
Math does not lie, incorrect calculations can

If atmospheric pressure is used, turbo changed, supercharged the formula is correct. The DF300AP list 123 to 213 psi which leads me to think this covers more then just the 300, as with the older Verado V6 OB they were supercharged. Supercharging will increase the compression ratio

You literally have no idea what you are talking about.

 

[JustJason]

"OPs engine is a Mercruiser SBC 350 reman with anywhere from a 0.020 to 0.060 overbore sold as a "357" so it could be a 352 cube, 355 cube or 357 cube. Unless you look at the reman tag, neither you or I know the displacement.. you are arguing for the sake of arguing"

You also have no idea what you are talking about. Back when it was sold (It's no longer in production).) The engine was literally sold as a 5.8 liter. https://www.albernipowermarine.com/...vo-4v-remanufacturing-plus-series-engine-only

 

[tpenfield]

Just catching up on this thread . . .

The OP has run off, not been logged in or posted in the past 10 days.

The compression test spec on my Mercruiser 7.4L w/ about an 8:1 compression ratio was 150 psi. (Manual #16) Once in a while you might see 160.

I know the 357 MAG is a bored 350 w/ 9.4 compression, so you might see about 175-180. I would think readings in the 200 psi range is more of a gauge issue.

But . . . The OP was wondering if running the engine for a few hours with one low cylinder was OK. Probably so . . .

 

[alldodge]

Ron White

I'm out

 

[Scott Danforth]

Going to close this thread. @PNBOOTH if you need to re-open, PM the mods