compression

[elwood]

so how much compression is to much ?
engine is,
49 to 53 block.
merc crank.
isky max1 cam, and springs.
lots of relieving, port work, and port matching.
matter of fact, its the block in my avatar.
edelbrock heads.
eddie myers intake, 2 97s.

i think i have the wrong pistons in it.
crazy compression, 13.5 to 1
think is crispy as hell and runs like a small block ford.

[Mike B]

If it doesn't ping/knock...ya done good!

Wow though, I didn't know you could get that tight (13.5 :1) with a FH .

Sounds like a real fun engine!

[Bassman/NZ]

How did you calculate the comp. ratio? The reason I ask is there's some gurus on here who reckon the maximum that can be wrung from a flattie is a lot less than that.

[scooder]

13.5:1 !! Honestly? Need to know how you calculated that number. Did you CC the heads? 50cc would have you in the 10:1 area, and 50cc is very small/flat combustion chamber.
Martin.

[scooder]

As to the question how much is to much. If it don't ping or knock, I'd say it's good to go. On the street more compression is probably the best thing you can do to these engines.
Martin.

[JWL]

Sponsored Links (Register now to hide all advertisements)

Must have converted to 2 cycle utilizing port openings to provide inlet and outlet. There is certainly no way to have 13+CR if the valves are opening into the head and transferring.

[scooder]

Did a little math, you'd need a 31cc combustion chamber to get 13.5:1 not sure that's achievable. For comparison, the offy 400 has about 58cc.
Martin.

[Mart]

The relieving lowers the compression ratio. Have you took that into account?

In reality, it doesn't matter what the number is, like said above, if it is running good and not pinging it's all good.

Mart.

[Mike51Merc]

Elwood,
Please tell us how you calculated compression.

[Ol' Ron]

The cylinder head volumn must have a Fudge factor added to it for the gasket and valve eye brows. I use 15cc's as a guestamate after many checks the old fashion way by bringing the piston to TDC and filling the chamber with fluid. This is for a non relived block. Thus, an Edelbrocl 64 cc head would have a 79cc combustion chamber. I like to limit CR to 8.5 max to allow the use of regular gas.

[elwood]

well guys, this definitely would not be the first time Ive been wrong about something,
ok, the heads are eddelbrock, they are 65 ccs,
some how the center head bolts backed off some and around 4000 miles I blew the passenger side head gasket, the head was warped, it took .022 to clean it up.
when I went to put it back together, with a .070 thick felpro gasket, the piston hit the head.
so I did a little work on the head where the piston hit, took about .040 out of them so the piston dome would clear, used a old gasket, torqued one head down, and had .030 piston to head clearance,
put everything together, with no plugs in it, I put my compression gauge in each cylinder, all where within a 1/2 to each other.
spun the engine for 5 revolutions, and this is what my gauge showed.
am I doing the compression check wrong ?
bet im reading the gauge wrong ...

[Charlie Stephens]

Your picture is great. The gauge reads 135 pounds per square inch (psi), not 13.5 compression ratio. I think you are about right but will leave that to the flathead experts.

Charlie Stephens

[Ross F-1]

Using atmospheric pressure of about 14.7 psia for Dallas, that would indicate a static CR of 9.2

[elwood]

thanks Ross, that sounds a lot more reasonable.
I just talked with my friend and he told me I needed to divide by barometric pressure.

[scooder]

With 65cc heads (is that before the. 022" came off?) and 255ci you'd be about 8 and a bit :1. With the 0.022 cut, maybe 8.5:1.
You can sort of get compression ratio with a gauge used like you did, but you gota factor in valve timming and some math than has be done here.
Proper way is measured, via cc ing the heads/engine.
John's book covers the pressure gauge to compression ratio stuff.
Martin

[elwood]

hey scooder, thanks.
yes, the heads where 65cc before the removal of .022
so im not positive what they are, im just glad there not what i thought they where at first.
the engine runs real good, and its driven every day, so im happy with it.

[scooder]

Your happy with it, that's the important thing. It's surprising what a little extra compression does ain't it. I remember the first time I fitted high comp heads to a flatmotor, stock heads off and Navarro 9:1 heads on, I gave it a boot up the road, the difference was incredible! It actually made my hands shake.
Martin.

[BILL WZOREK]

Originally Posted by scooder

Your happy with it, that's the important thing. It's surprising what a little extra compression does ain't it. I remember the first time I fitted high comp heads to a flatmotor, stock heads off and Navarro 9:1 heads on, I gave it a boot up the road, the difference was incredible! It actually made my hands shake.
Martin.

Martin:

I am afraid OLD AGE will do that also , LOL

[scooder]

Ok Bill, I'll take your word for that, I be only 40:-D
Martin.