1987-2006 Jeep 4.0L Inline Six - Inside The Inline

Part 1: Stroking Jeep's 4.0L Six

Jim AllenPhotographer, WriterCourtesy Mopar PerformanceWriter

If any engine can give a grizzled Jeeper the warm fuzzies, it's the '87-'06 Jeep 4.0L inline six. Designed by AMC and refined by Chrysler, its basic architecture was based on the '64-and-up AMC 199-, 232-, and 258ci OHV sixes. The fuel-injected 4.0L ruled supreme in Jeeps until forceably dethroned in 2006. We would like to say, "The King is dead, long live the King," but, thus far, nothing has emerged in diehard Jeepers' eyes worthy of wearing the crown. Before the late King's body cools too much, let's walk through tweaking this legendary engine from the inside out.

The old saying goes, "There's no replacement for displacement." Darn few single mods you can make to the 4.0L will give you more than a handful of ponies or pound-feet. One of them is to increase the displacement via an increase in stroke. Several companies sell high-quality stroked Four-O's as complete engines or kits, most notably Hesco, Golen Engine Service, and Custom Design Performance. Engine gurus have commented that, while stroking an engine is not to be taken lightly, it ain't brain surgery on the 4.0L. We wondered if the Average Joe, capable of an engine overhaul on his own, could do the job.To find out, we enlisted the aid of the University of Northwestern Ohio (UNO) and its High Performance Motorsports program to provide the facility and manpower for the job, as well as companies known for providing great products and services to enhance the Jeep 4.0L. In this article, we'll walk you through the job of stroking the engine; we'll cover other performance mods and finish up with dyno tests next month.

Like most modern engines, the 4.0L (bore and stroke: 3.88 x 3.44 inches) is oversquare, meaning the bore dimension is larger than the stroke. The 258ci (4.2L) Jeep six (3.75 x 3.89-inch) used before the 4.0L was undersquare, meaning the bore dimension is smaller than the stroke. Generally speaking, the oversquare engine is decent at low rpm, most efficient in the middle rpm range and has good high-rpm performance. The undersquare, long-stroke engine produces the best low-end torque, has a good midrange, but is so-so on the upper end.

Increasing displacement can also come via a larger bore, but a 0.060-inch maximum overbore on the 4.0L results in an increase of only 8 cubic inches. If you install the long-stroke 258 crank into the big-bore 4.0L block, it gains 32 cubic inches and becomes almost square, with the bore and stroke the same dimension (3.875 x 3.895 inches). The usual overbore, 0.030 inch, makes it slightly oversquare at 3.905 x 3.895 inches, and it gains 34 cubes. An 0.060-inch overbore gives a 41-inch increase.

There are two roads to building the 4.0L stroker. The budget method uses a 258 crank, 258 rods (5.875-inch long), and pistons with the stock 4.0L piston pin height (around 1.60 inches.). This is called the "short-rod" engine. The more expensive "long-rod" method uses the 258 crank, the longer 4.0L rods (6.125-inch) and a shorter piston pin height dimension (slightly variable but around 1.38 inches).

While the differences in a long- and short-rod motor can be huge in a high-revving', mega-power V-8 engine, the performance differences between the long- and short-rod Jeep engines is small. That's partly the inherent design limitations of an inline-six with a long stroke, and partly the way the engines will be used. The long-rod Jeep strokers offer a somewhat broader rpm range and, in theory, the short-rod setup is subject to more wear. Frankly, we doubt the wear differences will amount to much for most of you. The rod ratio (the ratio of rod length and stroke length) of the short-rod engine is identical to the stock 258, and they were known for a long life. Because we wanted to make this an "every man's" budget stroker, offering mainly a big boost in torque in the ranges where most 'wheelers work, we stuck with the short-rod option and off-the shelf parts.

We learned the hard way that not all Four-O's are created equal. Our engine was destined for a 2000 Wrangler. The low-mileage original engine was being saved for another purpose, so a used engine from the early '90s was procured. We learned the hard way that there are significant external differences between the '00-and-later engines ('99-and-later for the WJ) and the earlier ones. Eye exams for all were ordered. It was as obvious as a garish Aloha shirt at a funeral. Water pumps, accessory mounting, engine mounts, induction systems, and more-all changed. Installing an older engine into a newer rig, or vice versa, involves adaptation-doable but adding to the financial burden. The best advice is to start with your original engine or a similar year range.

The biggest problems faced by 4.0L strokers is pinging (aka spark-knock or detonation) and it can be deadly to the engine. Stroker conversions end up with a higher compression ratio (CR) than stock, sometimes higher than can be supported on pump fuel. It pays to take heed of your particular combination of camshaft, pistons, head gasket, and deck height.

The static compression ratio (SCR) is simply the ratio of cylinder volume versus combustion chamber volume. Our 4.0L stroker has a total cylinder volume of 855.12 cc (52.16ci) and at top dead center (TDC, with the piston at the top), that air-fuel mixture has been squeezed into 90.59cc (5.5ci). Ergo: 855.14 90.59 = 9.44:1 compression ratio.

The dynamic compression ratio (DCR) is a more useful way to think about this. It factors the cam profile-more precisely, the intake valve closing point-with the SCR to get an operating, or "effective," compression ratio. On virtually all engines, the intake valve stays open after the point where the piston starts up on the compression stroke. Until the intake valve actually closes, compression is not occurring and the true compression stroke does not start until it does. Because the cylinder volume is reduced, so is the dynamic compression ratio. The later valve closing takes advantage of air velocity at higher engine speeds to help fill the cylinder. At low speeds, yes, that reduces the effective compression ratio, but at higher speeds the increased volume of air more than makes up for it.

The big question now is, "Why does this matter?" The bottom line is that the DCR is a major factor in whether you can run on regular, midgrade, premium, or race fuel, and not ping. Also, if the DCR is too low, you will lose bottom-end torque. The generally acknowledged safe "sweet spot" is an 8.0:1 DCR for an EFI engine, and it offers good low end and a decent high end, usually on 87-octane regular. Going below 7:1 usually costs too much low-end power. With fuel injection and a good quench dimension (read on) you can sometimes reap the benefits of going higher, perhaps up to an 8.7:1 DCR on regular or midgrade. Dynamic CR above 8.7:1 almost always requires 90- or 92-octane premium. Above 9.0:1 DCR, it's almost always ping city for pump fuel. For reference, the stock 4.0L has an 8.7:1 static ratio and a calculated 8.45:1 dynamic ratio. You can download a DCR calculator from the Internet, but the Keith Black Silvolite Piston site has a good one to play with (see sources).

The formula for getting the right combination of static compression ratio and cam profile starts with selecting the cam. First, pick the maximum rpm range you need. If it's over 5,000 rpm, decide how much low-end torque you are willing to sacrifice. Seldom do 'wheelers need more than 5,000 rpm, and here's a clue: The short-rod stroker is happier below that. The cam manufacturer will usually list a maximum SCR at which the cam works best. You can double-check them by using a DCR calculator. The next step is for you to determine what parts will give you the right SCR to match. If you can't find the right mix of parts for the job, the easiest route is usually a longer-duration cam.

The elements that make the SCR on a stroker 4.0L are the cylinder-head combustion chamber, deck height, piston dish volume, and head gasket compressed height (see page 76). All of these are changeable, with varying degrees of difficulty, to build an SCR compatible with the cam profile and desired DCR.

You can grind out material from each combustion chamber to increase the head volume but you must be able to measure that volume via the procedure called "cc-ing" and make each combustion chamber the same volume. Ditto for the piston dish, which varies by aftermarket manufacturer, but which can be ground out slightly to increase its volume. See the sidebar below for some common piston dish specifications.

Head gasket thickness seems like one easy way to adjust compression ratio, but compressed gasket thickness can be difficult to determine exactly. In the high-performance gasket realm, the compressed thickness is listed and the manufacturers stand by those numbers. The standard gaskets are less controllable. The range runs from about 0.040 to 0.055 inch on the commonly available 4.0L gaskets. A rule of thumb for the 4.0L stroker is that with a 4-inch gasket bore, an 0.010-inch change in head gasket thickness equals a change of about 0.180:1 in CR.

You can get a rough idea of what your compressed dimension will be by measuring the thickness of the gasket body (not the flame ring around the bore or sealing strips imprinted on the body) and deduct about 0.005 inch. This applies to the gaskets advertised as "permanent torque" or "no-retorque." Cheaper gaskets that require retorquing can compress two to three times that much and are less predictable. Bottom line, you cannot say with certainty that X gasket has Y thickness unless it's a performance gasket that has a listed specification.

Quench is the "Great Equalizer" when it comes to a high compression ratio. The quench area is the area between the highest flat spot on the piston and the lowest part of the roof of the combustion chamber. Less quench distance induces more turbulence in the combustion, which promotes better fuel mixing and fewer hot spots-all of which reduce pinging. Many experts think that 0.040 to 0.045 inch of quench is ideal, but without making other changes, decking the block for zero deck height (removing about 0.045 inch) will increase the static compression ratio to about 9.7:1 with the standard combustion chamber, an 0.052-inch head gasket and largest available 4.0L piston dish volume (18.83 cc). If you put in a 218-degree cam (intake at 0.050 inch, 36 ABDC), the dynamic ratio is about 8.56:1, and most experts consulted agree that this engine would run on midgrade, maybe even regular, with timing and EFI adjusted to optimal power settings.

Calculated, not observed. The volume dimensions are averages, not absolutes. The only way to know for sure is to test your parts.

*Note: This piston dish is commonly listed as 17.5cc. The number shown is our measured amount, including ring land volume (about 1.3cc)
** The Golen Engine. Piston dish estimated from advertised compression ratio.
SP= Sealed Power, KB =Keith Black Silvolite, OE= Original Equipment
Bold text indicates the setup used for the UNO engine.

Bold text indicates the parts used in the UNO buildup.

To Get Deck Height (DH)
deck height = block height* - rod length - piston pin height - 1/2 stroke

To Get Compression Ratio (CR)
CR = total cylinder volume combustion chamber volume

To Get Combustion Chamber Volume (CCV)
CCV = cyl head volume + head gasket volume + deck clearance volume + piston dish volume

To Get Cylinder Volume (CV), Deck Clearance Volume or Head Gasket Volume
CV = 0.7853942 x bore2 x stroke
Note: For head gasket, used compressed distance.

To Get Quench Height (QH)
QH = deck height + head gasket compressed distance
*Block height = Distance from crankshaft centerline to block deck On a 4.0L= 9.450-9.456-inch

Head Piston Part No./GasketDeckCam Part No./Static CRDyn. CR
CCDish CCCC/Comp Ht. (in)Ht. (in)Intake Closed ABDC
58OE/13.510/0.0520.031OE-'96-up/14 9.66:19.29:1
58OE/13.510/0.0520.031Crane 750501/21 9.66:19.01:1
58OE/13.510/0.0520.031Crane 753905/26 9.66:18.91:1
58SP-H802CP/18.8*8.85/0.0430.025Crane 753905/26 9.44:18.71:1
58KB-2228/11.58.85/0.0430.025Crane 753905/2610.10:19.39:1
58SP-677P/17.58.85/0.0430.025Crane 753905/26 9.{{{57}}}:18.83:1
58SPH802CP/18.8*8.85/0.0430.000Crane 753941/35 9.92:18.77:1
58SPH802CP/18.8*8.85/0.0430.000Comp 68-2394/36 9.92:18.72:1
{{{62}}}Custom/25**8.44/0.0400.000Comp 68-2394/36 9.02:17.95:1
PartCasting
Number
YearNote
Block53005535'87-{{{90}}}Non-HO
Block53008405'91-95HO, early
Block53020569'96-99HO, late, best for pre-2000 Jeeps
Block53010449'99-06HO, metric fittings
Crank3199738'71Drilled for Borg-Warner automatic, bad pick
Crank3214723'72-{{{80}}}12 counterweights, long nose, strong & heavy
Crank3235477'81-864 counterweights, long nose
Crank3727'87-904 counterweights, short nose, best pick
Con Rod154'64-665.875 length, 232ci, weak rod!
Con Rod207'64-796.125 length, 199 & 232ci, weak rod!
Con Rod707'71-815.875 length, best pick short rod
Con Rod352'82-905.875 length, ok
Con Rod53020126'87-066.125, 4.0L rod, best pick long rod
Cyl Head2686'87-90Non-HO, Renix engine
Cyl Head7120'91-95HO, good pick, 55.22-58-22cc
Cyl Head0630'96-99HO, good pick, 55.22-58-22cc
Cyl Head0331'99-06HO, so-so pick, 55.22-58-22cc
Piston-'87-064.0L, 1.60 pin height, 13.5cc dish
Piston2229c-Keith Black, 1.58 pin ht, 11.5cc dish
Piston677CP-Sealed Power, 1.592 pin ht, 17.5cc dish
PistonH802CP-Sealed Power, 1.585 pin ht, 17.5cc dish
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