compression ratio for 87 octane with MPI

BurntOrange

Hook 'em Horns!
City
Oklahoma City
State
OK
#1
For those of you running Mopar fuel injection setups (OBDI or OBDII, not Renix) on your 4.0 & 258s, if you are able to successfully run on 87 octane what is the compression ratio of your inline?

I'm carefully planning my 4.6l stroker and am trying to calculate my compression & quench values to allow running on 87 octane without pinging. I've heard somewhere around 9.1 or 9.2:1 is the max.

I follow the Yahoo! 4.0/258 strokers group but want to gather data from some other sources too.

I just scored a NOS '96-'98 4.0 bare cylinder head from eBay so I should be able to keep my milling/decking/blocking to a bare minimum to help keep compression ratio low and quench in the right range. Just trying to decide which pistons to use. I could end up anywhere from 8.7 to 9.5 depending on my piston & head gasket selection.
 

CBRogers

Perpetual Parts Collector
LIFETIME
CJ-8.com Member
City
Yorktown
State
Va
#2
Honestly the guys on the stroker board are the best source of info.

From what I can tell the silvolites are the way to go. You need to get the dish milled out some more and that will help bring CR down. To get quench fairly tight you will need to mill the block in the 0.010" range and then use the thinner performance head gasket. There is a guy on the newsgroup that will mill the pistons for you for a reasonable charge. Also if you polish the top of the piston and the combustion chamber to get rid of any sharp edges that will act as focal points for heat then you can go a little higher on the CR.

I am still collecting parts, so I cannot give real world experience. But if you are looking for the most low end grunt but the ability to use cheap to bad gas I would hold CR to 8.8 and go with the Isky cam. That will limit upper end HP by reducing redline. But it will still pull to 5000 RPM. In a CJ that should be fine. And it will give you more low end grunt than the Crane Cam.

Carl
 

BurntOrange

Hook 'em Horns!
City
Oklahoma City
State
OK
#3
Yup Carl, I've pretty much picked up on all the points you made from watching the strokers group too. Some really good technical info over there!

Reason I am curious about compression ratio with 87 octane is I'm wondering if I can get away with using the Sealed Power H802CP pistons with the graphite skirts to help prevent piston slap. These pistons with my new bare head and only .010 off the block would put me at 9.1 with the right head gasket according to my spreadsheet calculations. Then I could avoid using the custom milled silvolites but still run 87 octane. I'll post this question up to the strokers group later tonight too.

Are you collecting parts for a stroker or a regular 4.0 MPI build? Which model Isky cam are you looking at? I was thinking about a comp cams or a custom ground (not reallly much more $$). I want low end torque right off idle with a cruising sweet spot from about 1800-2200rpm.
 

2K1TJ

Still Scramblin'
City
Rockmart
State
GA
#4
I'm not sure if anybody has tried this but you might be able to do some bowl work on the head. Have it shaped to optimize flow and also increase combuston chamber volume, therefore lowering compression ratio. I'm not familiar with the 4.0 head so I'm not sure if there is enough extra material there to do this. This might be cost-prohibitive too. The custom pistons might be cheaper. Like Carl said, ask at the Stroker board. I'm sure somebody over there has tried it or at least researched it.
 

CJ-8_Jim

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
NW Chicago Suburb
State
IL
#5
Can't answer your question, but this tip that was passed along to me was valuable:

Get the variable-pressure fuel regulator that HESCO sells. It's pre-set a few pounds higher as I recall (and you obviously can subsequently change it since it is a "variable" fuel pressure regulator).

For the first 10K on my ATK reman 258, I didn't get ping using 87 octane and MPI. Then I started getting it and even getting with premium (91?).

I speculate it may have something to do with the first 10K being on non-ethenol gas in IN... now I'm running it in IL where ALL gas is 10% ethenyol.
Plus the trips here in IL are a lot shorter jaunts around town.
 

BurntOrange

Hook 'em Horns!
City
Oklahoma City
State
OK
#6
Yes, good point on the Hesco adjustable fuel pressure regulator. $189 - ouch! I may end up running a '96+ OBDII setup with a 49psi in-tank pump system instead of the OBDI setup with the regulator on the fuel rail.
 

CBRogers

Perpetual Parts Collector
LIFETIME
CJ-8.com Member
City
Yorktown
State
Va
#7
On a stroker you do not need to run the FPR because you usually go to the 24lb/hr Mustang injectors. This already increases your fuel delivery much more than the increades pressure. Now this is too much for a 4.0 or 258 to handle but hte 4.6 stroker at WOT actually needs that much more fuel.

One of the big draw backs to the MOPAR MPI setup is the intake on the system. That short tube and then leaving the open filter directly above the exhaust leads to a really high intake temp. This in turn will lead to more pre-ignition. It is much better to route the intake to the far side and use a factory TJ box, or some sort of aftermatrket like the K&N FIPK or the AEM that has the aluminum shield and gets air directed straigt off of the grillle behind the light. Or you could use a snorkle.

Also a heat blanket betweeen the header and the intake is good. Look at the stuff that Dino did to lesen teh heat in his engine. Another good thing would be to install vents in the hood to let out excess heat but that is a body mod that you may not want to do.

Check out the Quench info on Accurate's sight. It is more abotu race engines but it was an interesting read. And with a good polish job on the head that shoudl help a lot in reducing points of high heat that would act as a glow plug for pre-ignition.

Carl
 
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