Jeep Strokers

I'm trying to learn about the in's and out's of strokers and it is a bit overwhelming. I have a worn out 1988 Cherokee with a 300,000 mile Renix. My goals are a strong, reliable engine that can be driven daily on 87 pump gas. I want to use the Renix head if at all possible to keep things simple and plug and play. I may look into a bored throttle body.

I have played with a SCR/DCR calculator and have come up with the following ratios. SCR 9.71 and SCR 7.13

4.2L 12 wt crank
4.2L short rods
H802CP-030 pistons (15.5 cc dish)
.043 head gasket
Deck clearnace .0215
OEM Renix cam
Quench Height .0645

According to the numbers I achieved, I should be on the right track. Can anyone verify this? Anything you would change?

Get your quench down to .035 so the quench can do it's job properly. .065 will have you knocking or running premium gas.

johnj92131 wrote:.065 will have you knocking or running premium gas.

That quench calculation is incorrect anyway but I agree. With H802CP pistons, 4.2 crank, and 4.2 rods, the deck clearance will be more like 0.038" with a quench of 0.081".
The 9.7:1 SCR, stock cam, and 0.081" quench combination is a recipe for detonation. Substituting the CompCams 68-231-4 cam would help some by reducing the DCR but even then, the engine would probably still need a diet of premium fuel.
If PacificEd wants to run a stroker on 87 octane fuel, he'll have to rethink the combination (perhaps the modified poor man's stroker?).

Thanks for the responses. I'm trying to figure this out. This stuff is new to me but it's like a giant puzzle and kind of fun. The toughest part for me is understanding what ranges I'm looking to be in for the SCR, DCR, and quench utilizing the parts and goals I have.

After playing with the calculator with a friend this afternoon, we found that the only two things on the calculator that can change the quench height is decking the block from the .0125 and changing the head gasket thickness. Are we correct in this? Also the lowest quench we were able to get was .0443 which boils down to basically the thickness of the head gasket with the deck milled to .00.

I have included an attachment showing where I got my initial numbers. Where did I go wrong to not get the value of .081?
I read the modified poor man's stroker build and it uses a 4.0L rod and custom pistons. I see where the original poor man's build used a 4.2L rod and it drove up the quench causing it to be more prone to detonation issues. I'm trying to use the 4.2L rods along with a corresponding piston to keep the overall cost down.

What combination of parts would get me closer to my goal? After talking things over with my friend, he pointed out that since this rig isn't a DD, and isn't really run over 3000/year, dealing with having to run 89 or 91 octane gas really wouldn't be that big of an issue.

Thanks for guiding me.

Ed

PacificEd wrote:I have included an attachment showing where I got my initial numbers. Where did I go wrong to not get the value of .081?

You plugged in the stock 4.0 deck clearance of 0.0215" when you should have plugged in 0.038".

After talking things over with my friend, he pointed out that since this rig isn't a DD, and isn't really run over 3000/year, dealing with having to run 89 or 91 octane gas really wouldn't be that big of an issue.

Then just copy my original poor man's stroker. I wouldn't get too hung up on the quench alone 'cause there are other factors that can predispose the engine to detonation anyway.[/quote]

Where did the value of .038 come from? If the stock deck height is .0215, milling the block would lower this number downward? If you deck a head .020, wouldn't that lower the deck height to .0015?

Or is the .038 number come from the fact that I'm working with a Renix block?

I did find that by using a 4.2L rod combined with a piston that can be dished to 25cc's, I can get my SCR to 9.18, SCR to 7.88 and a quench of .0445. This is a variation of the original poor man's stroker formula.

PacificEd wrote:Where did the value of .038 come from? If the stock deck height is .0215, milling the block would lower this number downward?

Here's your answer:

With H802CP pistons, 4.2 crank, and 4.2 rods, the deck clearance will be more like 0.038" with a quench of 0.081".

The stock deck height of 0.0215" is with the stock 4.0L crank, stock 4.0L rods, and stock 4.0L pistons in a stock 4.0L block.
The only stock part you'd still be using is the block.
Here's a more detailed explanation:

Deck clearance = Block deck height - rod length - stroke/2 - piston compression height

Plug in the stock 4.0L engine component numbers and you get:

Deck clearance = 9.453" - 6.125" - 1.706" - 1.601" = 0.021"

Plug in your proposed stroker engine components and you get:

Deck clearance = 9.453" - 5.875" - 1.948" - 1.592" = 0.038"

Does that make sense to you now?

Yes! I was overlooking several important numbers!

Sorry I'm such a slow learner.

So if the new deck clearance is .038 and some say to deck the block .020, does this now give you a new deck clearance of .018?

I also ran the numbers going to a IC944 piston and long rod combo.

I get a deck clearance number of .029. That number seems better than the .038 to lower the overall quench.

9.453 - (6.123 + 1.353 + 1.948) = .029

PacificEd wrote:So if the new deck clearance is .038 and some say to deck the block .020, does this now give you a new deck clearance of .018?

Yes.

I also ran the numbers going to a IC944 piston and long rod combo.

I get a deck clearance number of .029. That number seems better than the .038 to lower the overall quench.

Indeed it IS but for roughly the same cost as the IC944 pistons, you could order Bulltear custom pistons so you'd end up with the modified poor man's stroker shown below.

Jeep 4.2L 3.895" stroke crank
Jeep 4.0L 6.125" rods
Custom forged +0.020" bore pistons, compression height 1.380", dish volume 26cc
9.25:1 CR
Stock 4.0 camshaft
Stock Renix cylinder head
Mopar/Victor 0.043" head gasket
0.043" quench height
Ford 24lb/hr injectors for '87-'95 engines