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Fast and Easy way to raise the compression


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As some of you know, for the last 3 months or so my 88 MJ renix has been running cleanly on LPG, the oil is cleaner than ever (still has a clear reddish color), based on rough calculations i estimate a fuel economy around 20-21 MPG which is great considering that on gas it averaged 18-19, i`m running it as lean as possible with a 10%-15% power loss penalty but still drives great.

 

today i completed on the basic install on the 95 ZJ HO, using an  impco 300 reducer and a custom mixer, it starts and runs flawlessly, even better than the MJ :-( now that both will run on LPG i was wondering how to gte some of the power back by raising the compression, lets say from 8.8:1 to 10:1, of course i want to start with basic mods to do so...

 

any ideas are welcome

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I had a 72 Chevy P/U running on propane. It had over 300,000 mils on it when I sold it. For a clean burning engine it can't be beat. To help with the higher octane raising the c/r will help There is no way t raise the c/r. except by milling thr head or installing new pistons.

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changing pistons involves dissasembling nearly the entire engine, maybe as part as a rebuild job...

 

but milling the head? that sounds like an easy, more straight forward way to gain some compression...

 

how about using a thinner gasket?

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When you mill the head, things like pushrod length, bracket location, and lots of stupid other little details change. There really is no easy "Bolt in and go" way to raise compression.

 

Pistons are by far the least involved.

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i though you only shaved the head a little and put it back...

 

not building a stroker here...

 

Now that you've mentioned "stroker," the big hurdle when trying to do a budget stroker is that minor differences between the 4.0L and 4.2L engines add up, and you get an engine with too high compression for street gasoline -- unless you're VERY careful about things like quench height, timing, and probably enlarging the combustion chambers a bit. So a budget stroker might be a natural for LPG.

 

But I don't think that qualifies as either fast or easy.

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yeah i saw the article about strokers, they even mention a special lpg setup with 11:1 compression (pure propane), without all the trouble that comes with the stroking process (better fuel pump, larger inyectors, ECU tweaks) so that option is not discarded in the future...

 

new pistons...

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Suggest you go to the jeepstrokers web site and use their compression calculator to figure out how much you need to cut the head to get to 11: 1 compression.

 

Stock 4.0 compression is 8.7 according to the calculator.  The head gasket is .051 thick.  Reduce the head gasket thickness to zero and you get 9.78 : 1 compression

 

Then set the head gasket to -.049 and you get 11.14 to 1 compression.

 

So, cut the head by .100, then replace all the pushrods with new shorter pushrods.  That keeps your valve train the same.

 

Will the stock head stand a .100 cut?  I have no idea.  Do you know if your existing head has been cut before?

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I am running 9.7 static compression 8.5 dynamic on 1996 stock block, stock rods and pistons.

Added 1.7 yellaterra roller rockers, 99 up intake, 96 zj down pipe, no mechanical fan, ported head meaning bowl cleanup and shaping valve guides, matched intake to head, took off the 90 degree edge on the bottom of the throttle body.

 

I can run through all the numbers but You end up with a piston to head clearance of 0.030" it's on the tight side but it works

 

You'll love the higher compression as it unlock more power every where in the rpm range. Yellaterra 1.7 works great on a 1996 camshaft because you can run stock valve springs but you will have to pull the head and clearance the pushrod holes. The 1.7s on a stock 4.0 96-98 camshaft had unexpected gains from idle to red line. These are the best two modifications to do to a 4.0

 

My 1991 is a blast to drive 5 speed with this engine and 2628 lb weight

 

Search jeepstrokers.com and mini stroker. I can say first hand I can be done with stock rods and pistons!

 

With propane you do the 232 crankshaft and take 0.020" or more off the head just pickup new shorter pushrods to get into 10 to 1

 

I hope this helps

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Suggest you go to the jeepstrokers web site and use their compression calculator to figure out how much you need to cut the head to get to 11: 1 compression.

 

Stock 4.0 compression is 8.7 according to the calculator.  The head gasket is .051 thick.  Reduce the head gasket thickness to zero and you get 9.78 : 1 compression

 

so the head gasket alone can raise it to an hypotethical 9.78:1? interesting, anything above 9 is fine, after all, the crappy LPG i get in my country is like a 50/50 mix of propane and butane and that lowers the octane number a bit...

 

i found this

 

 

If you shave you head you will raise you compression ratio but Quench will be unchanged......

 

good point, LPG mixes well with air, quench is not as critical as when running on gasoline

 

shaving some 0.000 of an inch off the head plus a thinner gasket got to make some change in static CR... but with stock rods it will raise the time of valve opening/closing so less dinamic CR... so i have to shorten those too?... then the camshaft profile, too many variables...

 

oh and i forgot to mention, i prefer low end torque over max HPs

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oh and i forgot to mention, i prefer low end torque over max HPs

 

 

Then how about just an RV cam? Similar lift to stock (maybe even a bit more), but shorter duration and less overlap will generate more dynamic compression.

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I had considered milling a head to raise the compression and running a 0.043" compressed head gasket which gives a 0.065" quench

 

Every 0.010" you take off the head you reduce the volume in the head by 0.5cc

 

I was looking at taking 0.110" off the head which reduces the head volume by 5.5cc. Using shorter push rods from a 2.5 engine and the thinner 0.043" compressed head gasket. The static compression works out to 9.5 to 1 and a 0.065" quench

The 232 crankshaft gave me slightly more compression 9.7 to 1 and a tighter 0.030" quench

 

On propane, You could mill a head 0.110", shorter push rods, 0.043" compressed head gasket. While the head is off install a used 96 cam with new lifters. Run it then install 1.7 rockers when money is available. It's worth it! The 96 cam and 1.7s will give you more bottom end power mid and upper. I may install a roller timing chain and advance this cam as it pulls hard straight to red line.

 

Now propane, a 232 crankshaft, 0.051" compressed head gasket and 0.110" would give you 10.5 to 1 and a 0.030" quench

 

Beyond this things get more expensive

Other options I was looking at:

1.milling the head 0.110" and decking the block 0.022" getting a 0.043" quench and 10 to 1 static compression

2.buying eagle 6.150 rods and using stock pistons (if no ridge in cylinder ) would get a 0.038" quench with 0.043" head gasket,stock block and head =9.2to1 static if then milling 0.110" off the head it would get 10.1to one

 

For me the 232 crankshaft swap was quick and cheap way to get 9.7 to 1 static and a 0.030" quench

I wanted to get the dynamic compression up at 8.5 to 1 with a tighter quench.

 

Tell us more about this propane setup you have.

 

I hope this information helps. I wish it was as easy as just running with no head gasket

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NO.  Not what I was trying to say...  I was trying to say that milling the head by .051 will get your to a 9.78 compression.  Also, milling the head by a total of .100 will get you to 11.14 compression.  Both with the standard head gasket in place.  There is a thinner .043 head gasket that would add just a touch (like .2) more compression.

 

 

 

Suggest you go to the jeepstrokers web site and use their compression calculator to figure out how much you need to cut the head to get to 11: 1 compression.

 

Stock 4.0 compression is 8.7 according to the calculator.  The head gasket is .051 thick.  Reduce the head gasket thickness to zero and you get 9.78 : 1 compression

 

so the head gasket alone can raise it to an hypotethical 9.78:1? interesting, anything above 9 is fine, after all, the crappy LPG i get in my country is like a 50/50 mix of propane and butane and that lowers the octane number a bit...

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Tell us more about this propane setup you have.

 

the MJ has a piece of crap made by an unknown mexican company called Lamsa, on demand type system, made for carburators so i had to make a small adaptor so it fits a renix TB

 

the ZJ has a more respectable Impco 300 but the mixer i got with it was also for carburated engines so i made a custom mixer that plugs to the HO TB

 

 

For me the 232 crankshaft swap was quick and cheap way to get 9.7 to 1 static and a 0.030" quench

I wanted to get the dynamic compression up at 8.5 to 1 with a tighter quench.

 

a 232 crankshaft with stock 4.0 rods and 4.0 pìstons? that`s affordable

 

 

NO.  Not what I was trying to say...  I was trying to say that milling the head by .051 will get your to a 9.78 compression.  Also, milling the head by a total of .100 will get you to 11.14 compression.  Both with the standard head gasket in place.  There is a thinner .043 head gasket that would add just a touch (like .2) more compression.

 

of course i got the idea, use the thinnest head gasket AND shaving off some 0.000 of inch, with that in mind what about the pushrods? if i leave them stock wouldn`t the valves touch the pistons? or may even have the side effect of more valve duration?

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way beyond my budget, i could buy a complete engine with the cost of a custom camshaft in here

 

RV cams aren't "custom" -- there are several companies who offer them off-the-shelf. Swapping a camshaft should be a lot less work and a lot less expense than doing things like decking the head.

 

http://www.cranecams.com/product/cart.php?m=product_detail&p=23907

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Search jeepstrokers.com and mini stroker. I can say first hand I can be done with stock rods and pistons!

 

With propane you do the 232 crankshaft and take 0.020" or more off the head just pickup new shorter pushrods to get into 10 to 1

 

I hope this helps

 

232 crank?

 

The AMC 232 is a 3.8L engine. Did you mean 258 (4.2L)?

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way beyond my budget, i could buy a complete engine with the cost of a custom camshaft in here

 

RV cams aren't "custom" -- there are several companies who offer them off-the-shelf. Swapping a camshaft should be a lot less work and a lot less expense than doing things like decking the head.

 

http://www.cranecams.com/product/cart.php?m=product_detail&p=23907

 

 

This was the standard thing to do with propane conversions in the 80s.  In fact it was a great way to score a better cam for a SBC/SBF for nearly free, look for a truck in the boneyard with a propane conversion, 95% of the time they would have a name brand 'RV' cam in them just ripe for the picking.

 

I never swapped the cam on my 4L.  I just ran the timing on the ragged edge.  Same with the current truck I have on propane with a SBC in it.  IMHO shaving the head is the more work way to do it, and if you do that and wind up doing a cam swap you will not be able to run as much timing advance as is ideal.  The cam swap would also likely make more power than shaving the head.  The flame front speed is a little low on propane, timing helps more than on a comparable gasoline engine.

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my only concern when it comes to swapping a used cam is how much wear it may have, in here people trash the engines by not changing the oil or things like that, so cars end up in the junkyard mostly because the engine is deader than dead...also the chances of finding a good RV cam on a lpg truck in here are slim, people just thow in the lpg system, advance the timing and call it a day (kinda like me)

 

about the 238 crank,  there was a 3.8 inline 6 made, but it was discontinued back in late 70`s or so i read...

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my only concern when it comes to swapping a used cam is how much wear it may have, in here people trash the engines by not changing the oil or things like that, so cars end up in the junkyard mostly because the engine is deader than dead...also the chances of finding a good RV cam on a lpg truck in here are slim, people just thow in the lpg system, advance the timing and call it a day (kinda like me)

 

about the 238 crank,  there was a 3.8 inline 6 made, but it was discontinued back in late 70`s or so i read...

 

I was going to say that the 232 (3.8L) doesn't have a long enough stroke to help a 4.0L, but I think I could be mistaken. The 4.0 has a stroke of 3.41 inches. The stroke for the 3.8L engine is 3.5 inches, and the stroke for the 4.2L is 3.895 inches. When you do the math, the 3.8L crank does increase the displacement a tiny bit -- but not enough to matter. Mathematically, the 4.0L is 3.96 liters. With a 3.8L crank that increases to 4.07L. Not much of an increase, but the increased stroke would possibly help increase the torque by a little bit.

 

Since you're in Mexico, how about a crank from one of the big VAM engines?

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You won't (likely) find any aftermarket cam for a 4L in the junkyard.  I was talking GM stuff, if anything they seemed to be the most common converted vehicles out there.  And a cam could be had for them relatively cheap, even at that time, so it was common to slap one in as part of the LPG conversion.  Same with the Ford stuff, but I don't care much for them.

 

I'd just run more timing and leave it alone otherwise, personally.  The cost vs return is poor on doing anything beyond that.  If you wanted to make the engine perform I would order an aftermarket cam, the Yellaterra rockers, a HEI and weight/spring kit if you didn't go that route, an Offenhauser intake manifold, a generic Holley square bore mechanic secondary throttle plate, an eBay/Amazon header, and an Impco 425 (and a Model E regulator to go with it).

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Since you're in Mexico, how about a crank from one of the big VAM engines?

 

they weren`t popular here (weird he?) you don`t see many around not even in junkyards, maybe most were exported, the old FSJ i wanted to buy and a working green one are the only ones i have seen, not even the old full size cherokees the few ones you see are 1 step away to be scrapped, and 90% of the time are V8s

 

now that you mention headers, are the renix era exhausts optimal or is there some extra power to be had there? because the exhaust pipes in my MJ are rather rusty and they gonna fall apart some time soon and i was planning that when the time comes, put a larger pipe, shorter one that exists just behind the drivers side door, no point in going all the way back of the truck, or maybe a vertical exahust, serves no purpose but looks cool...

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