Max Compression

[FreeBeeTony]

FWIW.......haven't even pulled the engine yet.
​Wanna get this worked out before I even go to the machine shop and he starts suggesting things.
​I would like to know what I'm looking for before that.

​So far considering a Comp Cam XM270HR, deck the block to 0.015 and find a 0.025 head gasket (not sure of the numbers, want a 0.040 quench), and find matching pistons.

 

[Scott Danforth]

Sounds good, have any thoughts on setup pistons, deck height, gasket?

most machine shops will cut the deck to true it up to the crank center. they will also line-hone the crank bearing bores. this will put the deck between .005 and .010

many pistons are available with a .010 combustion height increase to forego zero-decking. with that combination, .041 head gasket.

the KB +12cc D-dish or the ICON (economy KB) +12cc D-dish

FWIW.......haven't even pulled the engine yet.
​Wanna get this worked out before I even go to the machine shop and he starts suggesting things.
​I would like to know what I'm looking for before that.

​So far considering a Comp Cam XM270HR, deck the block to 0.015 and find a 0.025 head gasket (not sure of the numbers, want a 0.040 quench), and find matching pistons.

Tony, talk to your machine shop before buying parts. infact, have your block sonic tested and hot-tanked prior. I know too many people that bought parts....found out block was junk... sent parts back and bought different parts.

if it was me, I would get a scat/kb stroker rotating kit from someone like competition products. call them, they will package a kit (most of what they do is NOT on the webpage). and they are less money than Summit http://www.competitionproducts.com/Engine-Kits-and-Rotating-Assemblies/departments/198/

also, keep an eye out in Craigslist for someone's project motor. co-worker just picked up a 509BBC that is fresh from a machine shop from a guy going thru a divorce. he paid $800 for $3500 worth of motor because the guy needed rent money for the apartment he needed to get.

what ever you do, get the rotating assembly balanced

 

[Lou C]

Tony who is going to do the build up for you, SK or someone else? Great discussion, I think I finally understand the Quench Effect after reading a whole bunch on it. Makes you wonder why GM did not incorporate this into the marine engine design as a regular thing instead of using the regular old full dished pistons that nearly all of these engines have...

 

[FreeBeeTony]

Will definitely talk to the machine shop before ordering parts.
Plan on doing the build myself over the winter........should be fun.

 

[Scott Danforth]

Tony who is going to do the build up for you, SK or someone else? Great discussion, I think I finally understand the Quench Effect after reading a whole bunch on it. Makes you wonder why GM did not incorporate this into the marine engine design as a regular thing instead of using the regular old full dished pistons that nearly all of these engines have...

many of the pistons are the flat-tops with the 4 valve reliefs in them. the 4-reliefs are so that the piston can be installed by an idiot on the assembly line who is hung-over from the weekend. round dish same reason.

D-dish requires thought to assemble. GM is not interested in making the best motor for one application. boat motors, genset motors, trash pump motors, fork-lift motors, etc. pull parts from the same parts bin as truck motors to keep cost down. if you want purpose built power, then build it.

 

[Lou C]

Its always about the money!

 

[FuelishBehavior]

Here are some interesting numbers for you just as an FYI based on an XM270HR cam with an IVC (intake valve closing) at 65 degrees ABDC (above bottom dead center):

No. of cylinders: 8
Piston bore: 4.030"
Stroke: 3.48"
Rod length: 5.7"
Deck height: 9.015" (assuming 0.010" deck job)
Compressed gasket thickness: 0.025"
Head combustion chamber voume: 64cc
Piston dish volume: 15 cc
Piston compression height: 1.56"

This will yield a very friendly 9.33:1 STATIC compression ratio.

More importantly, the DYNAMIC compression ratio, calculated using a 65 degree above bottom dead center intake closing (this is when compression actually begins) and I'm assuming you're on the coast at 0' sea level elevation, you will yield a 7.46:1 dynamic compression ratio. You can run on any pump gas all day long with this.

If you're at say elevation 2500' above sea level, your dynamic compression ratio actually decreases to 6.96:1.