Super Dragpack

I have a totally stock 1998 4.0 Auto with 90k miles - except it runs on propane/lpg as well as petrol. It is running fine, with no issues other than poor economy, but I have a few plans on the drawing board. I think we could do better than 12mpg UK (that is just 10mpg US) around town whilst also creating a fair bit more power.

As you may know, propane/lpg has a higher octane and therefore prefers/benefits/copes with a higher compression ratio (CR). The stock 4.0 has a lowly 8.8:1 CR, so not ideal for high octane fuel. However, with an octane of anywhere up to 115, LPG is suggested to run safely with up to 12.5:1 CR. Some have gone as far as 15:1 - but lets not risk it!

As I only use petrol to fire the engine (hardly needed actually) and run on LPG all the time, I am thinking about rebuilding the engine, renewing any worn parts and increasing the compression ratio. The next step would be to adjust the timing for more initial advance but no more total advance.

My questions are therefore:

1/ What is the best way of increasing my CR on these engines? Head skim, block skim, custom pistons???

2/ Can the timing be adjusted either manually or electronically to benefit from the extra octane?

3/ Would I benefit from a cam swap? If so, which one? Would a simple cam advance/retard do the trick?

Thanks in advance,

Mick

weebur

Have you done a compression/leak down check? If your numbers are on the low side you would probably benefit from an engine rebuild/shaving the head a bit/three angle valve job/port matching the intake/head with the gasket/cold air intake/experimenting with different heat range plugs/bored throttle body/reducing exhaust backpressure.

If you were forced to run on gas (petrol) I don't think these engines would tolerate 12.5 compression, you'd probably have to run racing fuel to prevent blowing it up.

The initial timing is what it is, no adjustments on your part.

There are aftermarket cams available. You'll need to decide how much lift/overlap you want based on if you want more low end torque or high end horsepower. The makers will be able to guide you.

Interesting post, get back with what you did and the results.

salad

What you want is called a "stroker". Try searching some threads in the Modified Tech section, there are a quite a few. On of my favorite ways of upping the displacement and compression ratio is the crank shaft from a Jeep 4.2L engine.

Super Dragpack

A stroker kit will increase stroke, and therefore cubic capacity, but will not always alter the CR unless specced that way.

I build a lot of engines, normally Ford V8's etc, but am new to the Jeep 4.0. Although it appears to be a simple lump, I thought I would ask to see if there is a known route (or known risks) of raising the CR on these engines without going the normal route of custom pistons.

To aid in working out my options for this engine, does anyone know the following please:

1/ How much can I safely skim from the head?
2/ Does anyone know the stock CC's of the head and what effect various skims will have on this figure?
3/ How much can I safely skim from the block deck?
4/ What is the stock piston height?
5/ Are various length pushrods readily available?
6/ What is the stock headgasket thickness?
7/ Are there thinner headgaskets available (do they work)?

Thanks for your help guys.

Mick

Rockville

If you have 10 MPG, there is a problem.
Diagnose it first and then decide what to do.

Super Dragpack

I've checked everything through but sadly LPG/Propane does a fair bit less MPG than Petrol, so with the journeys I do, this is likely as good as it will get with the current engine state. I know it is slightly down on compression (can't tell you how much as my guage is known to be inaccurate), but it starts and run fines still.

There is a lot of traffic where I live, loads of long traffic lights and always long queues, this kills my MPG a lot. I get a nice 17mpg if I do longer trips and nicely over 20 if on a run.

Domaas

low MPG is normal to LPG converions, they are not as thermally efficient as gas"petrol" over seas.

you could build a stroker to a zero deck clearance, then use some doomed pistons to bring up the compression ratio, but would have to work with a cam mfg for valve clearance verus the piston. Im sure it could be done.

also, are you using a vacuum operated converter? od a electric feed bak style tied to a 02 sensor?

many of the "dumb" vac units run fat all the time(impco model k,J..ect)

but we used some electric models based off the mazda fe series engies that ran a "feed back circuit" to the mixer control. It was signaled off a 02, they seemed to get way better MPG.

once you get the C.R. up, no need to bump over with "petrol"

Super Dragpack

The system I have is a single (simple) point system. Allow me to explain.

The LPG/Propane comes from the tanks (via several solonoid safety valves) to a vaporiser. This reduces the pressure right down. The vaporiser is connected to the coolant lines to aid it in turning the lpg to a gas (and stop it freezing up). The lpg leaves the vaporiser (in gas form) and runs through a stepper motor valve which controls the amount of gas permitted through. The gas then enters a mixer which is just a venturi type mixer affair bolted to the top of the throttle body. The gas is drawn through via manifold vacuum.

The stepper motor is controlled by an ECU which monitors TPS and O2 sensor readings to permit the correct volume of lpg in. It works very well, and it is very simple.

The system includes emulators to make the Jeep think it is still running on petrol. Basically, all the fuel injectors have a pigtail connector on them which turns them off (stops petrol being injected) but lets the Jeep ECU think they are firing fine. The car therefore runs fine on LPG whilst thinking it is on Petrol. I have no warning lights and the only issues is the console MPG readout if WAY inaccurate obviously.

Things I have noticed about my Jeep:

1/ I have a lot less power (on petrol or lpg) when the engine is stone cold.
2/ I have a very slight ticking when the engine is laboured - pulling up a hill, accelerating hard, towing etc. Some days it is loud, other days it is almost un-noticable. Lifters?
3/ I need a new thermostat (rarely goes over 1/4 way)
4/ I loose noticable power with the AC on (affects MPG too)
5/ I have a tranny fluid leak (from front seal).

I am hoping that an engine rebuild, and CR increase, will fix all these issues whilst it is apart/removed.

Cheers for you help guys.

Mick