after I posed the question of is it possible, or plausible to get your engine bored out for more horsepower somone suggested getting my engine stroked. Here's what I got when looking it up and boy does it look like something that I want to do!!!
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 @ 5250rpm, 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.
When 4.0 litres and 280lbft aren't enough...
There's no replacement for displacement so if you want even more torque, you need a bigger engine.
The 4.0L engine has a 3.411" stroke, 3.875" bore, 6 cylinders, and a displacement of 241.5ci (3956cc).
You can increase the displacement of your 4.0L engine by installing a stroker kit.
At the heart of the kit is the 3.895" stroke crankshaft that was used in the AMC/Jeep 258ci (4.2L) I6 engine.
On an engine with the stock 3.875" bore, the stroker crank increases displacement to 276ci (4517cc or 4.5L).
Add 30 thou or 60 thou overbore pistons and displacement is increased to 280ci (4587cc or 4.6L) and 284ci (4657cc or 4.7L) respectively.
If you're feeling really brave, you can overbore your engine to 4.00" and use custom forged pistons. The block must be sonic-tested to ensure that there's adequate cylinder wall thickness before overboring this far. Displacement with the 3.895" stroker crank is increased to 294ci (4812cc or 4.8L). The next step up the ladder includes an offset-ground 3.98" stroker crank that increases displacement to 300.1ci (4917cc or 4.9L). The ultimate stroker includes an offset-ground 4.06" stroker crank producing a displacement of 306.1ci (5016cc or 5.0L).
Here's some information on how you can build a stroker engine and the kits available.
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".
My stroker engine's producing a calculated 258hp @ 4850rpm and 320lbft @ 3500rpm. At least 278lbft (1.0lbft/ci) is available from 1500 to 4850rpm. The difference in performance over the old modified 4.0 is very noticeable, particularly from 1500-4000rpm where I need it most. Performance-wise, the stroker engine has hit the bullseye!
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.
Other stroker kits
Other stroker kits worth mentioning are supplied by Speedomotive 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)
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
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"
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 @ 5250rpm, 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.
When 4.0 litres and 280lbft aren't enough...
There's no replacement for displacement so if you want even more torque, you need a bigger engine.
The 4.0L engine has a 3.411" stroke, 3.875" bore, 6 cylinders, and a displacement of 241.5ci (3956cc).
You can increase the displacement of your 4.0L engine by installing a stroker kit.
At the heart of the kit is the 3.895" stroke crankshaft that was used in the AMC/Jeep 258ci (4.2L) I6 engine.
On an engine with the stock 3.875" bore, the stroker crank increases displacement to 276ci (4517cc or 4.5L).
Add 30 thou or 60 thou overbore pistons and displacement is increased to 280ci (4587cc or 4.6L) and 284ci (4657cc or 4.7L) respectively.
If you're feeling really brave, you can overbore your engine to 4.00" and use custom forged pistons. The block must be sonic-tested to ensure that there's adequate cylinder wall thickness before overboring this far. Displacement with the 3.895" stroker crank is increased to 294ci (4812cc or 4.8L). The next step up the ladder includes an offset-ground 3.98" stroker crank that increases displacement to 300.1ci (4917cc or 4.9L). The ultimate stroker includes an offset-ground 4.06" stroker crank producing a displacement of 306.1ci (5016cc or 5.0L).
Here's some information on how you can build a stroker engine and the kits available.
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".
My stroker engine's producing a calculated 258hp @ 4850rpm and 320lbft @ 3500rpm. At least 278lbft (1.0lbft/ci) is available from 1500 to 4850rpm. The difference in performance over the old modified 4.0 is very noticeable, particularly from 1500-4000rpm where I need it most. Performance-wise, the stroker engine has hit the bullseye!
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.
Other stroker kits
Other stroker kits worth mentioning are supplied by Speedomotive 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)
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
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"