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Allis Chalmers Discussion Forum
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advice/formulas for figuring stroke/compression

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CAL (KS)

11-14-2005 10:20:55




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I have a wd 45 im rebuilding and the crank will have to be ground on the rod journals. Im looking for advice or some math to figure how much i could possibly stroke the crank without any other modifications and using stock wd45 pistons adn rods. -(wd45 have the lowest compression of any of the crater pistons right?)

pistons and sleeves are in good conditioin, had been replaced not too long before block cracked and i acquired tractor. not gonna be doing any pulling or anything but im just tossing some ideas around and want to get as much power as possible without getting rediculous $$$.

i know i can put in d-17 or 170 pistons buti want some advice on this route.

i want to chore usable tractor that will run on pump gas.. so preferably less than 10:1 compression. Wondering how i could math this out without actually building it.

thanx in advance

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CAL (KS)

11-15-2005 05:06:35




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 Re: advice/formulas for figuring stroke/compressio in reply to CAL (KS), 11-14-2005 10:20:55  
HMM>.. I made some assumptions that the journals coulde be built up (welded) and ground offset to add stroke. I have seen places that do crankshaft work that have done welding.. maby the stroking is not done that way... i didnt know.

but anyhow using the formula (x+stroke)/x as denny stated thank you btw... here is where i was going with this post

using dennys formula with stock wd45 pistons distance .8257 if the stroke is increased to say 4.75 .25 increase in stroke yields .125 or 1/8 reduction in clearence
then .8257-.125=.7007 and (.7007+4.75)/.7007 yields 7.7789:1 compression

a 5in stroke yields 9.6:1 compression

so a 4.75in stroke could run on pump gas and 5in stroke could not. the maximum would be somewhere in between but im not gonna figure it now. Right?

does this formula work like this or is there some other factors that need to be accounted for since the stroke/volume have changed?

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Tramway Guy

11-15-2005 09:38:08




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 Re: advice/formulas for figuring stroke/compressio in reply to CAL (KS), 11-15-2005 05:06:35  
The formula does not account for a dished piston, which was standard on the stock WD45 engine. There is a certain amount of volume in the top of the piston.



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steve(ill)

11-14-2005 18:29:08




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 Re: advice/formulas for figuring stroke/compressio in reply to CAL (KS), 11-14-2005 10:20:55  
Denny has the right plan. the old WD45 has a big gap between the top of the pistona nd the top of the block. the volumn on top of the piston at top dead center is several square inches. if you use a piston that is 1/4 inch taller from the wrist pin, to the top of the piston, then you have decreased the combustion chamber volumn, but not changed the total stroke or displacement. this raises the compression suggnificantly. looking at some sort of crater piston that is higher on the side wall, and dished inward in the center is a good way to raise compression and confine the combusion area.--- if you take the head off, measure the distance to the top if the block with the piston all the way up and all the way down. the ratio is the combusion ratio. now refigure it if the piston was a little taller.

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Texas Denny

11-14-2005 14:09:09




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 Re: advice/formulas for figuring stroke/compressio in reply to CAL (KS), 11-14-2005 10:20:55  
I believe Power Crater referred only to WD-45 engines. I would guess D-17 might fit and are 7.5:1. The 45 had the highest compression to date when released.



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Texas Denny

11-14-2005 10:59:15




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 Re: advice/formulas for figuring stroke/compressio in reply to CAL (KS), 11-14-2005 10:20:55  
Cal, It is pretty easy to calculate a close estimate of compression on a WD-45. Since the head is flat (no combustion chamber), you can calculate compression with a caliper measurement. Without even an engine to look at, the distance from the top of the block to the top of the piston on a WD-45 is 0.8257". If you reduce this by getting a taller piston, the ratio will increase. If you reduce it by 0.2" (0.6257"), the new ratio is 8.19:1. The formula is Compression Ratio = (x+4.5")/x where x is the distance from the top of the piston to the top of the cylinder.

Reducing by 0.25" to 0.5757 will yield 8.81. You could probably still burn regular gas with 8.0:1 but may need premium if you go higher. The cylinder is not a best choice shape for combustion chambers so you may get pinging with compression too high. Honda Motor Company has new regular gas engines with compression of 9.6:1. In the old days, premium was needed for anything over 8.5:1 or 9.0:1.

With out installing the pistons you can estimate the change on the bench. With a caliper, measure the distance from the top of the wrist pin to the top of the piston on the existing 45 piston and the same measurement on any that you may try to use. The difference in these two measurements will be the amount to reduce the 0.8257 in the calculation. So you can estimate it without even buying the piston.

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mace56

11-14-2005 17:28:15




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 Re: advice/formulas for figuring stroke/compressio in reply to Texas Denny, 11-14-2005 10:59:15  
I'm not sayin its a good idea but you could grind a crank offset from original pin diameter and increase stroke. Of course you would loose pin diameter and cranks strength pretty quick. Not many engines could allow for that.



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Tramway Guy

11-14-2005 12:01:09




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 Re: advice/formulas for figuring stroke/compressio in reply to Texas Denny, 11-14-2005 10:59:15  
Denny, don't forget that the question involved grinding the crank to increase the stroke. Your formula assumed only a taller piston. If you increase the stroke, the formula would be (x+(4.5+y))/x where (x)=the final distance from the top and (y)=the amount of stroke added. However, the WD-45 has a dished piston, and I think the formula would only be true for a flat piston. Probably have to measure the chamber in the piston first to figure that in.

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Kent of SW MO

11-15-2005 08:19:47




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 Re: advice/formulas for figuring stroke/compressio in reply to Tramway Guy, 11-14-2005 12:01:09  
Normal Hot-Rod thing. You weljd the conrod area of the crank big and then you can change the radius of the throw and increase the stroke. Did this on a Chrysler 392 Hemi to increase stroke .625 inches. On a 45 you can install WD pistons to increase compression but this WILL NOT increase the engines displacement.

Kent



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Texas Denny

11-14-2005 14:04:01




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 Re: advice/formulas for figuring stroke/compressio in reply to Tramway Guy, 11-14-2005 12:01:09  
I remember that comment. You can not grind a crank to increase or decrease stroke. You grind a crank to put undersize bearings in the rods and journals. I didn't answer because it is not a possibility.



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