Jeep Strokers

I got my 4.7 stroker installed in my YJ a few weeks ago, and I have been chasing preignition problems... I have it better than it was but I still get it under a load on the highway. Its 3.915 bore and stroke (.020 offset ground crank), .040 H802CP pistons, 7120 ported head, 1.94/1.5 chevy valves, .000 deck, polished chambers and pistons, 2000 intake, 62mm TB, Ford 24# injectors, Hesco FPR set to 46psi, Plugs are RC9YC4, 180 thermostat, dual electric fans, Spal PWM controller, stock computer, dynamic compression is 204 psi on the low and 214 on the highest cylinder.

It runs good but even with premium gas it is running a tad lean at idle and I sometimes get some preignition when I let off the skinny pedal. I also sometimes get ping under load when on the highway going up hill.

I am considering getting an adjustable map sensor. I would also like to retard the timing just a few degrees but I am not sure how I can accomplish it, do I invest in a hesco crank sensor relocation kit and adjust it or can I move the distributor?

It suck because this same engine with a carb on my CJ ran great....but I did have to retard the timing some. Any advice would be appreciated. Tim

Your cranking compression is very high for pump gas. You may be out of pump gas range. Moving the distributor on a computer controlled Jeep won't change the timing at all as it is controlled by the PCM. Make sure your distributor is indexed correctly. http://www.angelfire.com/my/fan/dist_index.html. Also, you mention running lean, what are your air:fuel ratios like?

Your engine's displacement is 4634cc so it's still a 4.6, not a 4.7.
I've calculated your engine's static compression ratio at 10.5:1 so it's no wonder the cranking compression is sky high, and I'm not surprised it pings even on premium fuel.
I suggest you retard the cam timing by 4 degrees with a 3-way Cloyes dual roller timing set. It might lower the cylinder pressure just enough to get rid of the pinging.

It also has a crane 753901 cam, when I did the calculator. It came to 9.6 with a Felpro head gasket. I replaced the computer, got it from a 95 cherokee, and it runs much better, the old one kept killing the engine. Now I need to get the correct computer for the wrangler so it doesnt pull any wierd codes. Tim

Does it ping at cold idle or once warm? If only warm, try unplugging the coolant temp sensor in the t-stat and see if it still pings at warm idle. You could also use a split sec ftc1 to retard timing a bit and adjust the map.

That cranking compression is so high it makes me wonder if he's got too little pre-load on the lifters or something... even a performance stroker shouldn't have much over 160-180psi should it?

IIRC The 901 Crane is not an RV cam, it's a a warmed over factory profile if I remember correctly (I could be misremembering but I do have one lying around somewhere and I recall it being a street grind) so if its a street oriented cam it should have some overlap that should bleed cylinder pressure...

Over 200 PSI sounds like something wasn't done right either with the machining/piston dish or the valve train specs.

As everyone has already pointed out you need control over your timing and richer fuel mixture for sure. If you have E85 near by run it until you can get something programmable and have it dyno tuned. I would not run the factory ECU with that much compression going on with even 93 octane. With the lean condition and factory timing you're running a real risk of hammering a piston land or valve into submission.

gradon wrote:Does it ping at cold idle or once warm? If only warm, try unplugging the coolant temp sensor in the t-stat and see if it still pings at warm idle. You could also use a split sec ftc1 to retard timing a bit and adjust the map.

What is an FTC1?
I am using a different computer and it hardly pings at all now, I did a 200 mile trip on the pony express trail south of the Bonneville Salt Flats, I kept my speed and rpm's down and did a little playing and I didnt notice any ping. On the highway coming home the faster I went the hotter it got. I think the winch is blocking the air to the new 3 core radiator. If I slowed down it cooled down porportionately. At idle it would stay at about 180 with the electric fans coming on and off to maintain. Now I have a cracked exhaust to deal with.

The suggestions are good.

I dont have the money now to convert from the original OBD1 system and I have not found anyone that can reprogram a JTEC computer to retard the timing a couple of degrees.

I am going to try and pull the valve cover and see if I remove the rocker shims if it would lower the compression by loading up the valve train.

Thanks
. Tim

There was some talk about being able to retard the timing a bit by adjusting the crank positioning sensor. you might only get 2 or 3* out of it, but that might be all you need?

You have to oval the mounting holes on the CPS and while tightening the bolts pull the sensor to counter clock wise, or toward the driver's side to retard the timing a little.

I have a wrangler and the CPS is different than the XJ, it only has 1 bolt and it goes into a really tight hole in the bellhousing part of the auto tranny. I think that I will have to use the hesco relocation kit to make it adjustable. Tim

gradon wrote:Bonneville Salt Flats, I kept my speed and rpm's down and did a little playing and I didnt notice any ping. On the highway coming home the faster I went the hotter it got. I think the winch is blocking the air to the new 3 core radiator. If I slowed down it cooled down porportionately. At idle it would stay at about 180 with the electric fans coming on and off to maintain.
Thanks
. Tim

overheat at speed is airflow problem, idle overheat is radiator size issue... basic rule of thumb but can change based on other factors.

you need more air getting through radiator at speed-winch prob issue . shrouding your fans will help if not already.

I didn't write the above--maybe Plechtan did.

When my 4.0L was over heating on the highway it was due to coolant flowing too fast through the radiator. I has a highflow t-stat housing, high flow t-stat, high flow water pump and used both the stock radiator and the CSF 3 core. Replaced the water pump and the over heating stopped... it was a flow kooler with the plate on the back.

Well I finally decided to fix my jeeps compression problem, I did an INFRAME rebuild of the bottom end. I got a set of H802CP-40's machined to 20cc by Oleshot, installed them with new bearings and rings. Pulled the head and 4 cylinders had exhaust valves leaking oil, after 13,000 miles, so I got a valve job. The seats and valves were good, just cleaned the head and put new seals on. Got everything back together and now my cranking compression was 145-150. I didn't have any ping but I still had some premium left in the tank. When I get it down some more I am going to put in some 85 octane and see if I am getting any ping. So far so good! Tim