TheSamba.com :: View topic - 74mm crank and 94mm P&C

Tue Jun 07, 2011 12:22 pm

I am currently assembling a 74 x 92 A motor using a CB 4140 crank, CB 5.4 I-beam rods and thick wall AA pistons. No case clearancing needed on an aluminum case.

Measuring deck height I came out with a deck height of between + .045 and + .050 on all cylinders. I just ordered some .110 spacers to give me a - .060 minimum final deck.

--louis

Tue Jun 07, 2011 3:34 pm

Nice to have the option of an affordable VW journal, Porsche length rod.

Is the B piston and 5.5 combo only good on the 76-78mm cranks?

Sun Jun 12, 2011 10:01 am

I had to clearance a litle more than useual on a As41 case with 74mm. Some .40 shimms would make the piston almost sit flush to the top of the barrel. It jus goes to show that evey case is different.
Had to shave the top of the case, barrel holes etc...

Sun Jun 12, 2011 11:28 am

Has anybody tried the smaller 2" Buick (Chevy) rod journal on a 76mm crank with H beam rods? Just wondering if that would be an easier fit in the case and decrease the need for barrel shims.

Sun Jun 12, 2011 1:11 pm

I swear i had a 2" journal 78 crank and eagle h-beams, and it dropped in to a AE case with no clearance required.

So yeah, if it was a 76 stroke it would have fit too.

Sun Jun 12, 2011 2:30 pm

Yeah, I was thinking it might fit. Might be a good answer to fiddling with barrel spacers or lots of shims too....at least according to my crusty mathematics.

Sun Jun 12, 2011 7:09 pm

newbie___ following this thread

AirCooled said__
74 with Porsche length rods...no clearancing... NOT sure how to figure that rod ratio and what is a good number...

Would the 74 with Porsche length rods___give you a good rod number?

I am trying to decide 94x69 1915 or 74x94

Sun Jun 12, 2011 7:51 pm

btrapr wrote: newbie___ following this thread

AirCooled said__
74 with Porsche length rods...no clearancing... NOT sure how to figure that rod ratio and what is a good number...

Would the 74 with Porsche length rods___give you a good rod number?

I am trying to decide 94x69 1915 or 74x94

Stock 1600 rod ratio (69mm stroke, 137mm rod) is 1.99.
74mm stroke x137mm =1.85. The lower rod ratio is part of the reason a slightly longer stroke crankshaft produces so much more torque.
74mm stroke x 136mm ( porsche rods are 1mm shorter?) =1.84
74mm stroke X 135 (or are they 2mm shorter? I can never remember..)-1.82

The ease of installation with a Porsche length rod comes from the compact design of the big end of the aftermarket rods; it doesn't have any thing to do with the rod length.

Go with the longer stroke crank; it will change the motor completely (and I mean that in a good way! :lol: )

Sun Jun 12, 2011 8:21 pm

82mm crank with a 5.325" rod is a fine rod ratio, don't worry about rod ratio. Choose rod length to make it easy to assemble.

Sat Jun 18, 2011 7:26 pm

Thanks___ALB and John

So much to learn static/dymanic comp., stroke, deck heigth, etc...I want to try and stay std. width/or close as possible on the build.

still trying to decide which stroke ro run...82 is the magic answer for most, but I want to stay 78 or less.

what about
76 with a 3.532" rod
or
78 with a 3.532" rod *would this require B's? not sure of pin heigth on A/B's?
I guess I need to figure out my deck heigth first- then go for deck heigth as i would like to keep shims to a minimum.

good thread def answers some Q?'s

Sat Jun 18, 2011 7:29 pm

Way too short of a rod....much shorter than even that which is used for a stocl stroke 69mm crank.....try stock length rods of 5.4" or CB's unitech rods which are balanced and clearanced fro a stroker crank...they are 5.4"

Sat Jun 18, 2011 7:39 pm

fivelugshortaxle wrote: Way too short of a rod....much shorter than even that which is used for a stocl stroke 69mm crank.....try stock length rods of 5.4" or CB's unitech rods which are balanced and clearanced fro a stroker crank...they are 5.4" He meant 5.325 length rods, not 3.532. Plenty long enough at between 1.74-1.78 rod ratio depending on which crank they are about the perfect rod ratio.

RC

EDIT-corrected my OWN dyslexic 5.352 to 5.325....Sheesh, I can't even get a break from myself! :roll:

Sat Jun 18, 2011 7:45 pm

some may be missing , wat RC and or john meant

those rod ratios given (the shorter rod config) actually lets the engine breathe harder/stronger = a good thing

but let me close the can... as the worms may escape hahaha

Sat Jun 18, 2011 7:47 pm

Lol!...I know what he meant......the first Mars surveyor craft hit Mars like a missile.....somebody had transposed numbers in one of the calculations.......Lol!

Sat Jun 18, 2011 7:52 pm

fivelugshortaxle wrote: Lol!...I know what he meant......the first Mars surveyor craft hit Mars like a missile.....somebody had transposed numbers in one of the calculations.......Lol! How about the mixed Metric and US figures that went through the Hubble lens calculation and left that all botched up.....don'tcha think they would have CHECKED the lens for correct operation BEFORE they launched the whole project into the unknown?

Sat Jun 18, 2011 8:11 pm

fivelugshortaxle wrote: Lol!...I know what he meant......the first Mars surveyor craft hit Mars like a missile.....somebody had transposed numbers in one of the calculations.......Lol!

LOL and i think i was directing wat i said, to go into the same realm as wat john posted, reg'd choose rod length to suit an easier build
rod ratio = not so big of a factor compare to excessive shimmage

not your post :)

yours was actually indirectly showing the mis punched number by the other poster, now it is corrected

glad we wernt flying a mars lander HAHAHAHAAH