Help with some Stroker mathematics

[timos]

Hello,

I’m building up my stroker and I need someone to check if I had make no mistakes with the stroker mathematics. Some hints to adjust some parameters are welcome.

Application:
My 91' YJ offroad vehicle. I do not spot to high RPM and high horsepower. Instead I want the best low-end torque I can archive.

Parts (so far):

• 4.0l Engine (stock) with 4.2l crankshaft (heavy-weight) and the 4.2l rods
• Sealed Power cast aluminum 525P (.030)
• COMP cam 68-232-4
• Head Gasket Stock
• Stock head (7120 Cast) with Comp springs 926-12 and harald sharp roller-rocker (S40196)

So I get this values:

0.030 over bore:	3,905
4,2l Rod length:	5,875
4,2l Crankshaft stroke:	3,895
Piston pin height:	1.580 (?)
Piston Dish-Volume:	17,5cc (?)
Head Gasket Stock:	0,051
Gasket bore of:	4,000
Combustion Chamber:	58cc (?)
Deck Clearance:	0.0215 (?)
Cam Intake :	250
Cam Exhaust:	256
Cam Lobe Separation:	113
Cam Retard:	3 (?)

Static/Dynamic Compression Ratio Calculator result:


Conclusion:
Are the values right and is that calculation OK?

Problem:
Comp springs (926-12) with the comp retainers (744-12) do not fit to the valves. I’m working on!

[SilverXJ]

Your piston to bore clearance is not 1.580". It is more like .002", depending on the piston alloy. Doesn't make that much difference on the calc though. I would recommend the 68-231-4 instead. It has a little more upper end power. I would run it straight up, as the factory has 4* advance already ground into it (68-232-4 has 3*). You also list the dish being 17.5cc, but in the calc you have 13cc.

[timos]

Hello,

Your piston to bore clearance is not 1.580". It is more like .002", depending on the piston alloy. Doesn't make that much difference on the calc though.

Oh, it seams „piston to bore clearance“ is not the same as „Piston pin height“! OK, I clear this field.

I would recommend the 68-231-4 instead. It has a little more upper end power.

I have already bought the cam, and spend all my money on the machine shop, so I have to life with the 68-232-4 cam.

I would run it straight up, as the factory has 4* advance already ground into it (68-232-4 has 3*).

I don’t understand this advance/retard thing (can’t find it in the cams data sheet). Anyway it is as it is!

You also list the dish being 17.5cc, but in the calc you have 13cc.

Yes you are right, I put in the wrong piston dish volume (default value). I read the 525P piston have a 17.5cc dish volume --> I correct it.

But what is with the .0215 deck clearance? I think this value is wrong for my setup!

Deck height = Rod length + stroke/2 + piston pin height + deck clearance
For a normal 4.0 HO I have:
9.453" = 6.125" + 1.705" + 1.601" + 0.022"

Assuming, the deck height is identical for the stroker:
deck clearance = Deck height – Rod length - stroke/2 - piston pin height
0.051" = 9.453" – 5.875" - 1.947" - 1.580"

Where is my mistake/misunderstanding?

[SilverXJ]

Assuming all the numbers are correct (AFAIK they are) .0515" is your deck clearance. Using a .051" gasket your quench is 0.1025". That is assuming you haven't decked the block. The best way to find out is to mock the block up with #1 and #6 piston, rods, crank and bearings installed and then measure it with a dial indicator at the two pistons.

[timos]

Hello,

Thanks, it will take some time to measure it, because the crankshaft is at the machine shop for conditioning.

Assuming the 0.051” deck clearance is right, than I get SCR=8.96, DCR=7.72 and a Quench height of 0.102. Right?

Will it even run without Problems with these values?
What is the disadvantage of that high Quench?

What could I do if the measure confirms the 0.051” deck clearance?
Is it OK to deck the block by 0.040” (1mm) and get SCR=9.69, DCR=8.33 and a Quench height of 0.061?

[gradon]

You can deck the block .040"(or even .050" if you'd like). The scr and dcr numbers w/ the deck look fine by me. You can also pick up a .043" HG if you want to lower the quench more. Do take into account that the pushrods should be shorter as well and I'm not sure about the clearance of the dowels(if there are any on your year).

[timos]

Hello,
Oh thanks, I totally forgotten the pushrods. So decking the block by 0.040” results in 0.040” shorter pushrods, right?

How do Hydros come in play? What is the maximum compensation?
Is it possible to measure if the pushrods are right (eg. valve pre-load)?