Frequently Asked Questions

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Q. What is a stroker engine?
A. A stroker engine utilizes a different crankshaft to achieve more displacement with the same engine block. A crankshaft that has a longer stroke increases the engine's displacement while the engine itself remains unaltered from the outside.

Q. What do I need to build a Jeep I6 stroker?
A. The Jeep 4.0L block ('87-'06) and AMC/Jeep 258 crankshaft ('72-'90) form the basis of a Jeep I6 stroker.

Q. Which block should I use?
A. A 4.0L block from any year will do. Although blocks from '96 onwards have thicker main webs and a main bearing cap girdle to increase bottom end rigidity, the reduction in noise/vibration/harshness (NVH) is small and the AMC/Jeep I6 engine still remains gruff and unrefined. If you want refinement, buy a Lexus!

Q. Which cylinder head should I use?
A. The '91-'95 (7120) and the '96-'99 (0630) high port castings are preferable. The '00+ (0331) castings have smaller exhaust ports and are prone to crack, while the '87-'90 (2686) Renix castings have low intake ports that don't flow very well. If you can afford to splash out the coin, the Hesco aluminium head offers a near 30lb weight reduction and a raised valve cover rail to allow the use of roller rockers.

Q. Which stroker crankshaft is better?
A. The answer depends on what type of use (or abuse) that the stroker engine will be subjected to. For offroading and endurance racing, the heavier 12-counterweight crankshaft ('72-'80) offers better damping against vibrations and a greater "flywheel" effect to reduce the risk of stalling in low rpm offroad use. For normal street use or for drag racing, the 20lb (9kg) lighter 4-counterweight crankshaft ('81-'90) allows the engine to rev up faster, has less windage horsepower loss, and offers slightly better fuel consumption.

Q. Do I need custom pistons?
A. Yes and no. The added stroke of the engine would push the piston out of the bore by .221" if you used stock 4.0 pistons and rods. If you run stock 4.0 pistons and AMC/Jeep 258 rods, you can get away without custom pistons. If you use stock 4.0 rods, you will need custom forged pistons with a compression height of ~1.380". Alternatively, you could use Keith Black's recently introduced forged stroker IC944 and IC945 pistons with the 4.0 rods. If you use Eagle forged 6.150" length rods, you will need custom forged pistons with a compression height of ~1.355".

Q. Which connecting rods are better and why?
A. The longer 4.0 rod reduces the rod to cylinder wall angle theoretically resulting in slower cylinder wall wear. The longer rod increases piston dwell time at top dead center (TDC) providing free compression, maximum power from less ignition timing advance, and a reduced risk of detonation. The longer piston dwell time at bottom dead center (BDC) also allows more time for cylinder filling at the end of the intake stroke. The Eagle forged rods are the best of all and will stand up to the abuse of forced induction at high (>8.0psi) levels of boost without failure.

Q. Will there be any problem if I use shorter 258 rods?
A. No. There are many budget stroker engines running these rods and still living happily with high mileages. The difference in rod length:stroke ratio between 4.0 and 258 rods is small (1.57 v 1.51). If you use the 258 rods, you don't need to buy expensive custom forged pistons.

Q. Where can I buy custom forged pistons?
A. The Hesco stroker kit already includes Diamond stroker pistons and Keith Black's forged stroker pistons are less expensive. Otherwise, you will need to order custom pistons from companies such as Ross Pistons, Wiseco, Childs & Albert, and JE pistons. Although these are expensive (typically $600+ per set of six), you can specify the compression height, bore size, and dish volume that you require so it's like having a suit made to measure instead of buying one off the shelf. They are stronger than cast pistons and will therefore tolerate the higher cylinder pressures produced by forced induction (turbocharging, supercharging, nitrous oxide injection) without breakage.

Q. Is cold start piston slap a problem?
A. Forged pistons expand at a faster rate than cast or hypereutectic aluminium units and therefore require a greater piston to bore clearance. The engine may sound almost diesel-like until it warms up because the pistons rock in the bores at TDC and BDC, but oil consumption isn't increased and engine longevity isn't compromised.

Q. Will the block require any machine work?
A. In most cases only a cylinder overbore and hone is required. If the block has previously been subjected to overheating, the main bearing bore alignment will need to be checked and align boring carried out as necessary. Blocks should be magnafluxed for cracks before any machine shop work is carried out. The block does not need to be clearanced for the stroker crankshaft and counterweights unless the rod journals have been offset ground to increase the stroke even further. Engines from '96+ fitted with main bearing cap girdles will need to have the girdle raised using steel washers to clear the rod journals.

Q. What is the maximum safe cylinder overbore?
A. Oversize Jeep 4.0 pistons are available up to +0.060". Cheap and readily available Small Block Chevy (SBC) type 4.00" bore pistons have been used on some strokers but since this requires a +0.125" cylinder overbore, the cylinder walls of the block must be sonic tested to ensure there is adequate thickness. Minimum cylinder wall thickness should be 0.180" on the thrust side.

Q. Should I balance the motor?
A. In most cases weight matching of the pistons and rods/rod bolts is all that's required. The inline six is naturally balanced against first and second order harmonics and the crankshaft/counterweights usually don't require spin balancing, but having this done will improve engine smoothness at high rpm and lessen harmful vibrations. The Jeep I6 motor is internally balanced so the addition of the flywheel and harmonic balancer isn't necessary for the spin balancing procedure.

Q. How much displacement can I gain by stroking the Jeep 4.0L engine?
A. If you use the standard AMC/Jeep 258 crankshaft, a displacement ranging from 4.5L (standard bore) to 4.8L (0.125" overbore) is possible. A 4.9L displacement requires a 0.125" overbore plus an offset-ground 258 crankshaft with a 3.98" stroke, while a 5.0L displacement requires the same overbore plus an offset-ground 258 crankshaft with a 4.06" stroke.

Q. How much HP/TQ will my stroker produce?
A. This will depend on the final displacement, compression ratio, camshaft, and cylinder head combination used in the stroker engine build-up. Ancillary performance modifications such as a free-flow cold air intake, bored throttle body, header, performance exhaust, larger injectors, and chip upgrade will increase engine efficiency and maximize performance. This page gives you an idea of what's possible.

Q. What is quench and is it important?
A. This article gives a very good explanation. The Jeep 4.0L engine is blessed with a cylinder head that has kidney-shaped combustion chambers and dished pistons that promote swirl and reduce the tendency for the engine to detonate. The piston/head combination also has two quench areas where the pistons come into very close contact with the head at TDC, thus pushing hot gases back into the combustion chamber and promoting more swirl. A tighter distance between the flat part of the piston crown and the head (quench height) promotes even more swirl, allowing you to run the engine on lower octane fuel without experiencing detonation.

Q. Do I need to change the chip or reprogram my engine computer?
A. In the vast majority of cases (especially the pre-'96 OBD 1) the stock computer will work fine. Custom mapping of the fuel and ignition curves will maximize performance, fuel economy, and reduce emissions. The Apexi SAFC 2 air/fuel controller piggybacks to the stock computer and can be used to optimize the fuel curves under both light and heavy loads. OBD 2 engine computers (especially from '00+) are less able to adapt to performance modifications, but the aftermarket has come to the rescue with the Unichip programmeable computer that plugs straight in between the stock computer and wiring harness.

Q. Do I need oversize injectors?
A. In most cases yes. Ford Racing 24lb/hr injectors are the most popular choice for strokers and they work very well. The fuel curve may need to be fine-tuned using a MAP sensor voltage adjuster or an adjustable fuel pressure regulator. The air/fuel ratios may not be optimized at all rpm points and under all load conditions though, and this is where the Apexi SAFC 2 excels.

Q. Will my stroker pass the emissions test?
A. There's no reason why not. A properly built and tuned stroker should run even cleaner than the 4.0 that it was based upon.

Q. Will my stroker match the 4.0L engine's legendary reliability and longevity?
A. Again, there's no reason why not. The key to success is accurate machine shop work, selection of good quality parts, matching the parts to the application, and careful assembly. Some strokers have needed to be rebuilt because of poor machine shop work and mistakes made during assembly.

Q. Will my stroker consume more fuel than a 4.0L engine?
A. Since you're adding displacement (~15%) to the engine, there's no such thing as a free lunch. You need more fuel to feed a bigger fire. That said, a stroker can match a 4.0L engine's fuel consumption during highway cruising but you can expect your mpg to drop 2-3% in mixed driving conditions. That's only a small price to pay considering the extra performance on hand. The temptation to use the stroker's extra performance may be hard to resist and a heavier right foot will definitely hit fuel economy!

Q. What is the maximum safe redline rpm on my stroker?
A. The stock 5250 rpm ('91-'95) or 5300rpm ('96-'06) rev-limiter will suffice. The torque curve of most strokers will be tailing off above 4500rpm so there's little use in revving them beyond 5000rpm. Most strokers are torquers, not revvers.

Q. Do strokers run hot?
A. Not necessarily. Mine runs as cool as a cucumber. The combination of a good 2-core radiator (either factory HD unit or Modine), high-flow water pump (Hesco, Flowkooler), high-flow thermostat (Robertshaw is best; 180* for hot climate, 195* for temperate climate, 205* for Arctic conditions), and single or dual electric fans pulling a total of 3000+cfm will keep just about any stroker cool. Forget about hood vents and ported thermostat housings; they simply don't work. If you have the '87-'90 closed cooling system, convert it to the '91+ open system.