Maximum Milling of Block (Decking)

I am doing some calcs for a stroker... I'm using the ever popular "AndreasStrokerCalc.xls" from MadXJ.

I am wondering what is the maximum material that can be removed from the 242? I was thinking of having mine decked, to bring the numbers back around to 9.69:1 with a quench of 0.0635... to do so, I'd have to deck the block .025 ... Is that a bit excessive? What are the issues I'm running into by removing this much material? (there are always issues)...

Also, How do you factor in cam specs to compute the practical compression ratio... With more duration, you can lower the CR, right? How are you gurus managing the math? I hate to see everyone ask you guys to calculate thier specific (and sometimes exactly the same as someone else's) numbers. I am a bit of a numbers geek on this, since I am poor and cannot afford to screw up. I CAN trust everyone else that has a running motor, but I'd ALSO like to see the numbers and understand thier relationships directly. I LOVE that spreadsheet... it is very illustrative.

Also, does anyone know the piston dish volume and compression height for the 2229's? (KB Silvolite) or the H802's? (sealed power/FM) or at least a source for the specs? I have had NO luck finding these.

I've got the 677's (sealed power/federal mogul) at 17.5cc's and 1.585 respectively.

I HAVE done a search, but found no specific tolerances for decking a block, or for the piston specs.

Thanks Gurus

There really isn't a spec for milling the block that I know of. We always assembled everything, measured down to the piston, figured gasket compressed thickness and whizzed it off. Sometimes we run the piston out of the hole a bit for compression too. Yes, you can figure it all, but with it assembled, it is going to be 100% correct. Also, stroke variations from hole to hole need to be measured before milling.

I have no idea if it is too excessive, but I did notice that the default or starting decking block was 0.028. So I would think it is not bad, but the piston height also started out at 1.581 and yours is at 1.585 so your specs would be .001 closer to the head. Just math, no actual experience, but someone has to be using the same pistons in their stroker. Chime in.

5.0 Junkie:

I applogize for what will probably turn out to be a stupid question but:

"Sometimes we run the piston out of the hole a bit for compression too" ?

Color me stupid, but I have no clue what this is refering to... can you fill me in?

I had to remove .022 on the MJ - there is lots of material in the 4.0 block so milling .025 shoild not even be noticed.

I think what 5.0 junkie was saying is that with some engines to get higher comp. the piston is set to just barly stick out of the block at TDC. Which means that the piston either goes up into the gasket or even into the bottom of the head. But when stroking the 4.0 the problem is keeping the comp down. I do not think you will have problems decking the block so little on this motor, anything important is much lower than .025 on the 4.0. Also if you are having a shop do this, which i hope. The shop should have the specs on this or be able to get them, just call up a shop and ask them.

Did you take a starting deck height from the original piston/crank/rod combination?? If not you need to mock up the reciprocating assembly and take your measurements before you can decide how much to mill off the block. I have found that Jeep is very liberal in the tolerance range...I have seen several that were nearly .100 higher than spec.

As for the cam...a cam with a lot of duration and/or overlap will give you a lower cranking pressure because the swept volume is effectively reduced at low engine speeds...I would recommend a "medium" cam..and if you have detonation, you can advance the cam 2-4 degrees. That will cut the cylinder pressure back a bit.

I used the Zollner/Sealed-Pro coated pistons...they have held up well despite frequent visits to the 6,500 rpm zone.

MaineJeepah said:

Also, How do you factor in cam specs to compute the practical compression ratio... With more duration, you can lower the CR, right? How are you gurus managing the math? I hate to see everyone ask you guys to calculate thier specific (and sometimes exactly the same as someone else's) numbers. I am a bit of a numbers geek on this, since I am poor and cannot afford to screw up. I CAN trust everyone else that has a running motor, but I'd ALSO like to see the numbers and understand thier relationships directly. I LOVE that spreadsheet... it is very illustrative.

Click to expand...

Just to make sure, I believe you're talking about figuring the Dynamic Compression Ratio, rather than the Static Compression Ratio, no? The difference is this:

The Static Compression Ratio (abbreviated SCR or just CR) is figured using the total stroke included in cylinder volume, making no allowance for the remainder of the intake vavle open time. This is what is usually referred to when motorheads talk about "Compression Ratio."

The Dynamic Compression Ratio (DCR) is the compression ratio figuring cylinder volume using the stroke remaining after the intake valve has closed. The DCR is always lower than SCR/CR, and is usually a more reliable predictor of power and fuel octane requirements.

DCR is a number bandied about by serious motorheads and engineers, and I think you are the first to really bring it up here in a while (with the possible exception of myself - but I think I've saved most of my comments on the subject for a couple engineering fora...)

I bashed this around pretty solidly a few months ago, since I wanted to know what I was talking about before I wrote about it. If you go to one of my Yahoo! groups (groups.yahoo.com/group/JeepPower) and sign up, you can get the files I included on the CD that ships with my book (and there's information on the book there as well.) If memory serves, you'll want one of the compression calculators, which will be titled CR95 (for Excel 95/97) or CR2000 (for Excel2000/XP.) There is also a MSWord file called AutoMathRef95/2000 (same deal on the numbers, for Word) that explains how to do it all if you happen to be a masochist or a maths junkie (or both, like myself :wierd: ) It's actually a lot easier to do than it looks at first gloss, but you really want to pay attention, and don't round anything off until you're done.

In case you are interested, there are also a couple .zip files which contain camshaft and cylinder head data for AMC I6 engines (199/232/242/258, but I need more head data) that will drop right in to Desktop Dyno 2000.

Give me a bit before you try to download the files - I'm upping them now (about 1940PDT.) Probably be about a half-hour or so before they're all up - I want to be done with this before Stargate comes on...

I have checked the execution of these spreadsheets, and they are (as well as I can determine) accurate.

5-90

MaineJeepah said:

Also, How do you factor in cam specs to compute the practical compression ratio... With more duration, you can lower the CR, right? How are you gurus managing the math?

Click to expand...

If you use a longer duration cam you can go for a higher static CR, not lower. On a 4.0 or stroker, my rule of thumb is basically this:

Intake duration @ 0.050: 192.....198......204.....210.....216
Maximum static CR: .......8.75.....9.1......9.5.....10.0....10.5

I would recommend a "medium" cam..and if you have detonation, you can advance the cam 2-4 degrees. That will cut the cylinder pressure back a bit.

Click to expand...

It's the other way round. If you retard the cam timing 2-4 degrees and close the intake valves later, the cylinder pressure and dynamic CR is reduced and this can cure a pinging problem.

You'll find piston and other specs here:

http://www.jeep4.0performance.4mg.com/tech_specs.html

Thanks for the correction Doc. I was having a brain fart and didn't catch that. :speepin: Guess the ole Dawg wasn't in his best form.....