Lets talk ratio`s VS cams..

[bikerinthenight]

Seeing how the shovelhead area is a bit slow these days..LOL
I figure I`l throw out a question I`m a bit curious about..

anyway..lets suppose for example, the perfect cam is in your bike..runs perfect, it`s the recomended cam for your motor (or kit if a ya had one etc)..anyway, your motor originaly had 8.2:1 pistons/compression..but now your gonna throw a 7.5:1 set in it..

how much effect, would that have on your cam?

would that throw things off a noticeable amount, or??

Also, lets suppose...knowing that compression is raised when either cylinders are decked, heads are milled etc...lets now suppose
one were to knock off about .012...how much would that bring up the 7.5:1`s? (ratio wise)..i know that aproximently every .001in, is ROUGHLY equal to 1lb compression...so roughly what ratio would one end up when running 7.5:1`s, with about .012in tighter squish??

thanks for any and all thoughts..

biker

[Heretic]

I would not run compression that low. Ignition timing is precise enough now that there is really no reason. My last shovel had 9.5 /1 compression weisco forged pistons. The heads were ported, and I had an Andrews B grind. I ran the crane HI-4 single fire ignition. I timed it and rode it for the break in. Then I used a laser thermometer and established the difference between front and rear cylinder operating temp. I retarded the rear cylinder until they operated at the same temp. This allowed me to take the initial timing up a little more.

I built that engine in 1996. My dad still rides that bike. he came to me recently complaining it was smoking a little. I told him that it was probably time for rings and valve guide seals....Running that many years, and being pushed on enough to run with the evos and twinkies....I think maybe it was an OK build...LOL

[viking-bob]

Hey bikerinthenight

Here is a link to Andrews cam were you can determent the best cam vs. compression and model year
http://www.andrewsproducts.com/PDF_f...009-retail.pdf
Measure your comp. With a gauge ore let your local Harley doctor do it for you.

Is your shovel a 1200ccm. ore a 1340cc? And what year is your ride?

If you brake down the numbers the result is af follows…
Lets say you take off 0,1 inch you will app.
1200ccm. increase the compression from 7,5 to 8,3
1340ccm. increase the compression from 7,5 to 8,2
and still with 0,1 inch off.
1200ccm. increase the compression from 8,5 to 9,5
1340ccm. increase the compression from 8,5 to 9,4

// Viking bob aka. Lars

[junior]

uless you are planning on adding a blower to that bike there is no reason to go that low on compression. as heretic noted most bikes today run well in the 9.0-9.5:1 range. however, a few thoughts....

static compression (ratio) and cranking compression (pressure) are the two factors to think about. you can choose a piston that provides an average 9.25:1 staic compression ratio, but can then yield abnormal cranking cylider pressure if the intake valve timing is too aggressive. make sure that you choose the proper cam for the application you are after.

secondly, shovels are bascially a hemi (OK- the plug isn't dead center, but the overall layout fits). this means that in most cases unless you have a dual plugged head you will by nature end up with varying degrees of incomplete combustion due to the flame path being interrupted by the increased height of the piston dome. even though heretic didn't mention it, i bet that his dad's shovel is dual plugged. this increases burn efficiencies and often allows for easier cranking/kicking and less chance of pinging (to a point).

just my $0.02....

j