Highest compression ratio?

Looks like I'm going to do a rebuild next year and I'm wanting to get the CR up to 12.5:1 I know that's kinda high but if I can get my piston to head clearence down to. 035 - .045 that should help with detonation and heat build up. I was also thinking of fabricating a CAI that runs freon through it. I plan to make it out of a rear unit made for a conversion van or something along tose lines. That is if I do it before the TB. The coils for it might be too restrictive and too cold for sensors is my only concern with it. I figure if I can get the intake charge down to around 70°f then I could run 89 octane gas easy. I'm looking for at least 370ft. lbs of torque out of the engine. I will probably fab up my own intake manifold and have it machined. I'm thinking this because cooling the air that much before the throttle body could mess with some sensors? Don't really know about that but it sounds good in my head. I know I have a lot more reaearch to do but I have the rough numbers in my head and on paper. And before anyone says turbo or supercharger I'm an all motor guy. I like my engine to sound like quater sticks are lighting of at high idle speeds. So if anyone has any good input I sure would love it. I'm sure there is a lot of stuff I haven't thought of yet, but research will get me there and I alway start with .

Why bump the compression? Seems like it would be a lot easier to get the power out of a stroker or supercharger.

O well it will be a stroker also. I wouldn't waste the money with out getting the most cubes out of it. I just prefer the feel of high compression rather that a supercharger. Also I have no $$ or experience with superchargers. I can build the engine stroked and high compression for about $1000 more than just a good rebuild would be. I don't know if this is the best price for the sprintex? But $4000 could build me a very nice small block chevy and a TH350. http://boostecus.com/index.php?option=com_content&view=article&id=40&Itemid=26

High compression isn't going to add hardly any horsepower. High compression is useful on a race motor where you choose a cam that makes power in the 7000rpm range. With a torquey cam like you'd want in a Jeep you'd have so much cylinder pressure at low rpms that you'd destroy head gaskets and be guaranteed to detonate.

It's just a bad idea. Shoot for 10-10.5:1 and pick a good cam, you'll be much happier.

Disclaimer: i've had several 12:1 motors.

Yea I know that a long duration cam is gonna destroy my compression at low rmp I was thinking of something around a 260-270 with about 4-6 degrees of advance would let me keep that compression up at lower rpms. I had in my mind that I could really get my thermal efficiency to jump up and get the quench good to help with the detonation.
So what is a realistic number in torque that I could shoot for going all motor. The last enigine i built was a 327 and I really only put the heads on and put the motor in the car. It has to run 114 octane fuel it has 14.5 domed pistons and about a. 26 quench. The piston come so cloae to the head I think that they bararely smack the bottom of the head around 7500 rpm. That's way more extreme than what I'm looking for. The idea is too make enuff power to destroy the axles so the wife will have no choice than to let me upgrade to d44's front and rear then maybe ill step up to a d300 transfercase. Sucks all things have to be cleared through her.
Atill wanna know what CRpeople are running.

If you have a 12.5:1 4.0L you will a) never be able to run pump gas, or b) have a cam which such a large duration that it will be a complete dog (the 4.0L doesn't lend it self to be reved high either, where the cam would make most of the power.

Yea I know that it doesn't want to rev I just want gobs of torque.

That won't happen with your 12.5:1 compression ratio.

High compression + big cam = want to turn 8000 rpm's and is a baby kitten on the bottom end...and the valve train is not going to want to turn like that.

There's absolutely no reason to run high compression inline 6 in something you want to drive with pump gas. If you want torque, throw a diesel in it..

Personally..if you want go fast/big power goodies go with a LS motor. They're cheap, and easy to make big power.

Going diesel was what I thought about doing at first but I do not know jack about diesel motors.
So what is a good setup to make uber torque with the 4.0l. Like I said I want about 350 ft. lbs at the rear wheels in 2wd. That should be doable with a naturally aspirated stroker I would think. Just need to know what is needed to get it done

350ft-lbs is very doable, just not with a 4.0L Jeep i6.
You want an LS swap.

Bore and stroke to 5.0L and you can probably pull off 350lb-ft. At that point a 5.3 swap would be cheaper and only require a cam to get that.

I'm running a 10:1SCR, 8.57DCR 4.6 stroker with a 256/256 mopar stage 3 cam(30ab) and ~.062" quench. My quench isn't horrible, but I have to use 93 octane to keep it from pinging(if I ever have to take the head off, I will use a .010" thinner HG to make quench better). What you could do is build a 4.7l stroker using KB's IC945 10.8cc dish(small) pistons in +.060" size and the 4.0l rods. Measure your deck height and have your machinist remove the ~.030" off the top of the block in order to get the 945s flush with the block(zero-decked) then use a .040-.043" mls head gasket. You'll be at 11:1 with 58cc combustion chambers(you'll have to polish them and measure). Pick your cam wisely to have a DCR that will be able to work with pump gas(Dave Vizard recommends <8.5:1 with an iron-headed build; you could spend $2K on Hesco's aluminum head which will be more friendly to higher comps). You could also go crazy and plan an e85 build using custom non-dish pistons and have a 12.6:1 SCR with them zero-decked(edit: if you custom order pistons, order them with pin heights that don't require you to have the block decked in the first place--what I'll do with the 6.150" Eagles: 1.35" pin height forged pistons). Go over to Jeepstrokers.com and play with the calculator. My stroker build cost me ~$3K and I would and will do it again. Don't be caught up with the end result #s, just have fun doing it and driving a higher powered Jeep.

I think I'd give up on the freon cooled intake also. The added weight and drag of the compressor and added coils/piping would more thean kill any gains from the cooler charge... If your that concerned about cooler intake temps get a cowl hood or a real cold air intake.

When I rebuilt/modified my 88XJ 4L, at 385,000 mi, I used Venolia forged flat-head pistons and a static compression ration of 11.25 and a quench height of about 0.050". My first cam (Crane) produced a dynamic compression ratio of 8.8. The Crane cam was defective and died at about 10,000 miles and Crane replaced it. However instead of using the Crane cam, I decided to have Comp Cams regrind my stock cam for better low end performance. The Comp Cam regrind produced a dynamic compression ratio of about 9.1. The engine has been running well for about 20,000mi and any occasional knock is taken care of with my knock sensor. My tach indicates this Comp regrind is a much better choice for the power range I normally drive in with my automatic trans.

Best regards,

CJR

CJR said:

When I rebuilt/modified my 88XJ 4L, at 385,000 mi, I used Venolia forged flat-head pistons and a static compression ration of 11.25 and a quench height of about 0.050".

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How? Even with a .060" overbore I am only getting 11:1.

Talyn,

To answer your question; How? My block was bored 0.030" over and after some calculations the block was milled down so that the top of the Venolia pistons were close to the top of the block. The head was ported and Manley SS valves installed with Manley bronze valve guides. The head combustion chambers were CC'd, some calculations (w/gasket thicknesses available) made to determine how much needed to be milled off the head to get the right dynamic compression ratio and quench height. After the head was milled, it was CC'd again to check each combustion chamber volume. The selected gasket (to get proper quench height) was temporarily installed and the valve-to-piston clearance checked for the Crane cam. Final check was made with my Performance Trends Engine Analyzer software to determine what my static and dynamic compression (cam grind dependent) ratios were. When I replaced that Crane cam with a Comp cam regrind, the lift was less so valve-to-piston clearance was greater and not a problem.

Likewise, all the rods were polished and shot-peened and the rotating assembly was balanced. The ARP rod bolts were tightened using bolt stretch. The top piston rings were ductile iron.

That's how I did my 88XJ 4.0 L engine. I'm very happy with how it turned out! It was after I changed out my dash panel to instrument gauges I realized that the power band of the Crane cam was not in the right RPM range. So when the Crane cam died, I decided to go with a Comp Cam regrind and get the cam's power band in the right RPM range.

Best regards,

CJR

CJR sounda like you have done what I am looing to do. Do you have the specs on the regrind that was done on your stock cam?

CJR said:

Final check was made with my Performance Trends Engine Analyzer software to determine what my static and dynamic compression (cam grind dependent) ratios were.
CJR

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Before you spend $$$$ and not know where you are going with the build, buy Performance Trends EA3.4 (or the older EA3.2) to model and research the relative output gained from the various combination of parts you are thinking about.

If you are not familiar with SCR, DCR, overlap, BSFC, valve sq-in degrees, fuel #/hr, and the difference between mild and wild cam ramp intensity, the engine analyzer will help you teach yourself.

You could also purchase Internal-Combustion-Engine-Theory-Practice (volumes 1 & 2) for the same money and spend a year digesting the knowledge (with less favorable results).

Either method of modeling (computer modeling or old school calculations) will save you more than the cost of the education in parts and in knowing the parameters to have double checked during the machine work.

1985XJLaredo,

The drill with Comp Cams goes like this. You talk to Comp Cams about what you want, i.e. low end or high end performance. Then you send your cam to them and they measure it and determine what regrind can be done.

As mentioned, I used flat-top forged Venolia pistons, made to my specs, with free-floated lightweight off-set pins, Trulocked in place, and the top ring located higher on the piston. With the thinness quality gasket, calcs/fitup are done to determine how much to "deck" the block to get the desired quench height; i.e. must be less than 0.060 inch. The idea is that when the head is bolted over the gasket you'll get the quench height desired.

Next the head is ported and new valves installed and each combustion chamber CC'd for volume. With Performance Trends software and the cam regrind specs, the software is run to determine what static CR is required to get a dynamic CR of about 8.8 to 9 (i.e. for regular gas). Then calcs are done to determine, with the head gasket used, how much the head must be milled (reduce combustion volume) to get the static CR determined by Performance Trends.

My Comp Cam regrind specs are as follows:

Lobe separation: 111.5 deg.
Intake duration @ 0.050" : 201 deg.
Exhaust duration @ 0.050": 208 deg.
Intake valve lift: 0.416"
Exhaust valve lift: 0.433"

I used a Cloyes roller timing chain set and degreed my cam with a 2.0 deg. advance. With a high altitude CPS this cam starts pulling strongly at about 800 RPM. Eventually I'll bore out my Throttle Body for more response. Likewise, one reason I reground my OEM cam was that the Crane cam lobes were significantly narrower than the OEM cam. In my view-that's not good. AMC knew what they were doing when they used wider cam lobes and large diameter lifters.

Hope this helps.

Best regards,

CJR