higher compression ratio?

[Mr. T. Minnesota]

Forum Members,
Curious if it is possible to increase the compression ratio when rebuilding an 8N engine to higher than 6:1? Any thoughts are appreciated!

Mr. T. Minnesota
 

[HCooke]

Sherman made a 'high' compression head - it was made out of aluminum. 8Ns have a compression ratio of 6.5:1. Many times these heads get trimed (decked) to correct a warp. Caution there is not much room before the piston begin to contact the head.
 

[Dean]

Only slightly without the NLA pop-up pistons.

The late 8N engine CR is nearly maximized.

Dean
 

[Dell (WA)]

Mr T..........take yer sleeves OUT and run bigger pistons. By definition, you have increased yer compression ration. (ratio of swept volume to combustion chamber volume)

As the flathead V-8 guys have found out, It is really difficult to increase compression. Since ALL cylinder heads are interchangeable, Buy a NEW 8N cylinder head and gitt 6.5:1 compression.

Howsomevers: you want more power? Gitta 454 Chevy smog pump and NAA carb and SUPERCHARGED yer engine. You doubt? Google "Dirt cheap supercharger by Steven Richmond" ........Dell
 

[kylemorley]

6.5 to 1 is pretty much the limit because above that everything is too crowded to let the valves and heads breath effectively - you can't run a piston right up to the valves like an OHV can do, when they are off to the side.

Harley Davidson put huge amounts of effort into developing the flathead long after it was obsolete, well into the '60s, due to a loophole in the US racing rules that let HD run 45 c.i. sidevalves against 30 c.i. OHVs. While stock sidevalve Harleys ran compression ratios similar to the 8ns, the fastest racers usually had even LOWER compression ratios, 5.5 or so, since there was no way to port them without lowering the ratio, and good breathing was found to be more important than compression ratio.

It's likely some 8ns are now nibbling at the upper limits of compression without benefit of trick heads due to the heads and/or blocks having been resurfaced.
 

[souNdguy]

shave an 8n head a lil.. then check valve clearance with putty and no head gasket.. then relieve as needed and bolt on.. that's about it.. as dean and the others said. other goodies not so available.

and the merc pistons in the parent bore.. well.. it works.. bat can cause issues later and requires overbore to correct.

also don't give you much more hP.

if you have an 8n and want more hp.. get a naa or 6xx ( or 8xx and have near 2x hp )
 

[Mr. T. Minnesota]

Forum Members,
Thanks for the info. Reason for the question is the 8N I purchased this summer had been newly overhauled. The seller said he had added special pistons, shaved the head and decked the block. He said the compression "may be an 8:1 ratio" ?? Even as a novice to mechanics the math just did not seem to work out. There is only so much space to work with.
With that being said, the 8N seems to be a lot more powerful than my not so worn out 2N. Had to change the 33 to a 243 carb to make it run right which may also add to the power.
Again, any other thoughts are appreciated. In retirement my goal is to learn something every day.
Mr. T. Minnesota
 

[Bob Harvey]

Or go the Baumann way and install a turbine/V-8/Valkirie !!
 

[souNdguy]

yep.. jet engine HP
 

[Dean]

It's not 8:1.

Probably closer to 6.5:1.

Your 2N most likely has flat-topped pistons in it while the 8N almost certainly has late 8N domed pistons, which are good for a noticeable increase in torque and power vis-a-vis the flat-topped pistons.

Dean
 

[HCooke]

8:1 seems like an exaggeration, but my '51 8N does seem stronger than any of my 5 9Ns.
 

[HCooke]

8:1 seems like an exaggeration, but my '51 8N does seem stronger than any of my 5 9Ns.
 

[Dean]

It will be if everything is as manufactured. Late 8Ns do have higher CR and more HP than 9/2 Ns.

Dean
 

[Hobo,NC]

[Ken(Ark)]