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4.0 engine torque curves The torque curve of the engine in stock form was already broad and flat. The addition of some simple bolt-on performance modifications increased torque throughout the rev range, particularly at higher revs. Maximum horsepower increased from 193hp @ 4850rpm to 235hp @ 5200rpm, and maximum torque from 239lbft @ 3500rpm to 269lbft @ 4000rpm. The torque curve is broader, with at least 242lbft (1.0lbft/ci) available from 1600rpm to 5100rpm. This translates into faster acceleration at any speed in any gear making passing manoeuvres safer. Gas mileage remained the same despite the higher HP/TQ outputs thus reflecting the engine's greater efficiency. |
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| Budget stroker build-ups Robert Bryce built the world's first budget Jeep I-6 stroker engine, reusing as many stock components as possible in his assembly to keep cost to a minimum. The key to success was to keep it simple. Click on the image to the right to find out more. Not to be outdone, I also jumped onto the stroker bandwagon and built the world's first "square" Jeep I-6 stroker engine with a bore and stroke of 3.895". It's a slight variant of the 4.6L low-buck stroker option 1 shown below. I transformed a junkyard 4.0L short block plus a pile of parts into a rip-snorting torque monster with a displacement of 4563cc (4.6L), hence I entitled my stroker story "From Junker to Stroker". |
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Accurate Power stroker kit Limeyjeeper built the 4.0L engine from his '99 WJ into a 4.6L stroker engine using this kit. It produced 210.5hp @ 4800rpm and 262.4lbft @ 3800rpm on the chassis dyno. Assuming a 20% drivetrain loss (RWD auto.), that's 263hp and 328lbft at the flywheel. Here's the link to Limeyjeeper's stroker story. Specs are the same as the medium-buck stroker shown below except for 0.030" overbore, Ford Motorsport 24lb/hr injectors, stock intake, and 62mm TB. Accurate Power ceased trading in April 2005. |
| Hesco stroker kit Available for all Jeeps in .030 or .060 oversize. Click on the image to the right. A Hesco 4.7 stroker dyno'ed 267hp @ 4700rpm and 332lbft @ 3400rpm at the flywheel. The specs of that engine are in this Yahoo Stroker Group message. Another Hesco built 4.6 stroker did even better on the engine dyno, producing 269hp @ 4800rpm and 339lbft @ 3200rpm. |
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Other stroker kits Other stroker kits worth mentioning are supplied by Speedomotive short rod/ long rod and by RPM Machine You can also get a complete 4.6L stroker longblock engine from Golen Engine Service or Custom Design Performance The Golen 4.6L stroker has been reviewed by JP magazine, producing 268hp @ 4900rpm and 324lbft @ 3900rpm on the engine dyno. Accurate Power and Jakes Racing Engines have ceased trading, while Clifford Performance continue to trade but no longer supply stroker kits. |
| Injector sizing Stroker engines require oversize injectors to provide adequate fuel and prevent pinging due to lean air/fuel mixtures. Click on the image to the right. To calculate optimum injector size, use the following formula: Injector flow rate (lb/hr) = [Expected HP x 0.5 (BSFC)] / [no. of cylinders x 0.8 (injector duty cycle)] The actual injector flow rate from each injector depends on the fuel pressure (FP) in the injector rail. Most injectors have rated flows at 43.5psi fuel pressure. The actual flow rate from each injector is: Actual injector flow rate = Rated injector flow rate at 43.5psi x square root of (FP/43.5) |
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Adjustable fuel pressure regulator Fine tune your engine's fuel curve with a Hesco adjustable fuel pressure regulator (FPR). Click on the image to the left. The fuel pressure can be adjusted to provide the optimum air/fuel ratio. Part no. for the '87-'90 FPR is HES8790FR Part no. for the '91-'95 FPR is HES9195FR There is no adjustable FPR available for '96 and later engines. |
| Camshaft Stroker engines need a longer duration than stock camshaft to maximise horsepower and torque. Click on the image to the right to go to the CompCams catalog. The aim is to produce maximum torque over as wide an rpm range as possible. Stroker engines built with a compression ratio (CR) of about 9.5:1 will derive maximum benefit from these camshafts: Crane #753905 204/216 degree cam CompCams #68-231-4 206/214 degree cam A stroker built to a CR of 8.8:1 that'll run with 87 octane fuel will benefit from a shorter duration dual-pattern cam for more low rev torque: Crane #750501 192/204 degree cam CompCams #68-115-4 192/200 degree cam |
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Long rods vs. short rods If the Jeep 4.2L 5.875" rods and stock Jeep 4.0L pistons are used, the pistons will be 0.008" further down the bore at TDC than those in the stock Jeep 4.0L engine. The deck clearance is thus increased to 0.030" and the quench height is increased to 0.081". The resultant CR is 9.7:1 with stock bore pistons and 9.9:1 with 0.060" overbore pistons. If the block is decked 0.010" to return the quench height to stock, the combustion volume is reduced by 2.0ml and the CR is increased to 9.9:1 and 10.1:1 respectively. Premium fuel will be required to prevent detonation. If the stock Jeep 4.0L 6.125" rods are used, expensive custom forged pistons with a shorter pin height than stock (1.380") are required to achieve zero deck clearance. Using longer rods does enable you to choose pistons with a larger dish volume than stock. These reduce the CR and allow the use of low octane fuel without detonation. You can also use a shorter duration camshaft for increased low-rev torque, and select the head gasket thickness required to achieve the desired 0.040"-0.060" quench height. If you use the shorter Jeep 4.2L rods with cheaper cast aluminium or hypereutectic pistons, the quench height can be reduced by milling the block deck and installing a thinner head gasket. The CR can be reduced by increasing the combustion chamber volume and the piston dish volume. *For each 0.010" milled from the block deck (or taken from head gasket thickness) to reduce quench height, the combustion volume decreases by ~2.0ml and the CR increases by ~0.2*. |
| Stroker engine math Here are some formulae, and how they apply to the stock Jeep 4.0L engine: 1. Deck height = Rod length + stroke/2 + piston pin height + deck clearance 9.453" = 6.125" + 1.705" + 1.601" + 0.022" 2. Combustion volume = Combustion chamber volume + head gasket volume + deck clearance volume + piston dish volume 84.6cc = 56.7cc + 10.5cc + 4.3cc + 13.1cc 3. Compression ratio (CR) = Cylinder volume/combustion volume + 1.0 CR = 660/84.6 + 1.0 = 7.80 + 1.0 = 8.80 4. Quench height = Deck clearance + compressed head gasket thickness 0.073" = 0.022" + 0.051" (Picture courtesy of Muddypaws) |
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Stroker combinations I've outlined eleven stroker combinations below based on budget and desired horsepower levels. To get maximum benefit from the stroker engine you need a free-flow intake with Powerstack or K&N cone filter, 62mm bored HO throttle body, header, high-flow cat, performance muffler and 2.5" exhaust piping. |
| 4.5L Low-buck simple stroker Jeep 4.2L 3.895" stroke crank Jeep 4.2L 5.875" rods Jeep 4.0L 3.875" standard bore pistons 9.7:1 CR CompCams #68-231-4 206/214 degree camshaft Ported HO 1.91"/1.50" cylinder head Stock 0.051" head gasket 0.081" quench height Ford 24lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Ford 24lb/hr injectors with stock 49psi FPR for '96 and later engines 266hp @ 4950rpm, 322lbft @ 3500rpm 4.6L Low-buck stroker Option 1: Same as above except: Speed Pro H825CP +0.030" bore pistons 9.6:1 CR Mopar/Victor 0.043" head gasket 0.081" quench height* 265hp @ 4900rpm, 325lbft @ 3500rpm Option 2: Same as "rockcrawler" except: Piston dish volume 22cc 9.5:1 CR Crane #753905 204/216 degree camshaft Ford 24lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Ford 24lb/hr injectors with stock 49psi FPR for '96 and later engines 262hp @ 4800rpm, 325lbft @ 3500rpm |
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4.7L medium-buck stroker Jeep 4.2L 3.895" stroke crank Jeep 4.0L 6.125" rods Custom +0.060" bore pistons with 1.38" pin height & 25cc dish 9.5:1 CR Crane #753905 204/216 degree camshaft Ported big valve 2.02"/1.60" cylinder head Mopar/Victor 0.043" head gasket 0.043" quench height Flometrics F&B 68mm billet TB Accel 26lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Accel 26lb/hr injectors with stock 49psi FPR for '96 and later engines. 269hp @ 4800rpm, 334lbft @ 3400rpm 4.8L medium-buck stroker Option 1: Same as above except: Custom 4.0" bore pistons with 1.38" pin height & 23cc dish 10.0:1 CR CompCams #68-235-4 210/218 degree camshaft 284hp @ 4900rpm, 345lbft @ 3600rpm Option 2: Same as Option 1 except: Jeep 4.2L 5.875" rods bushed for 0.912" floating pin Small Block Ford UEM-KB364 hypereutectic pistons Mill block deck 0.017" 0.051" quench height |
| 4.9L high-buck stroker 3.98" offset-ground stroker crank Jeep 4.2L 5.875" rods Small Block Chevy UEM-KB142 hypereutectic pistons 10.5:1 CR Crane #753941 216/228 degree camshaft Ported big valve 2.02"/1.60" cylinder head Mill block deck 0.017" Mopar/Victor 0.043" head gasket 0.053" quench height Flometrics F&B 68mm billet TB 3.0" exhaust system Ford 30lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Ford 30lb/hr injectors with stock 49psi FPR for '96 and later engines. Custom PCM programming 300hp @ 5000rpm, 354lbft @ 3700rpm 5.0L Very high-buck stroker 4.06" offset-ground stroker crank Jeep 4.2L 5.875" rods Small Block Chevy UEM-KB142 hypereutectic pistons 11.1:1 CR Crane #751111 238/248 degree solid lifter cam Ported big valve 2.02"/1.60" cylinder head OEM 0.051" head gasket 0.038" quench height Flometrics F&B 68mm billet TB 3.0" exhaust system Ford 30lb/hr injectors with adjustable FPR or MAP adjuster Custom PCM programming 328hp @ 5250rpm, 375lbft @ 3800rpm |
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