Jeep
Stroker Motor
|
|
Stroking
the 4 litre - engine torque curves
The
torque curve of the 4.0 Jeep engine in stock form
was already broad and flat. The addition of some simple
bolt-on performance modifications increased torque
throughout the rev range and most noticably, at higher
revs.
This
translates into faster acceleration at any speed,
making passing safer. Gas mileage remained
the same despite the higher HP/TQ outputs thus reflecting
the engine's greater efficiency. Just remember
- if you use that power, you will notice a difference.
Doesn't matter how you work it - it takes more fuel/air
to fill more space.
|
|
When 4.0 litres and 280lbft aren't enough... |
After
stroking the motor, maximum horsepower increased
from 193hp @ 4800rpm to 240hp @ 5250rpm, and maximum
torque from 240lbft @ 3300rpm to 280lbft @ 3500rpm.
The torque curve is broader, with at least 242lbft
(1.0lbft/ci) available from 1600rpm to 5200rpm. |
|
|
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 thousandths or 60 thousandths overbored 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, though. 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 265hp @ 4900rpm
and 325lbft @ 3500rpm. At least 278lbft (1.0lbft/ci)
is available from 1500-5000rpm, and over 300lbft is
on tap from 2000-4500rpm. The difference in performance
over the old modified 4.0 is very noticeable, particularly
from 1500-4000rpm where I need it most. Performance-wise,
this 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.
|
|
|
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 340lbft
@ 2500rpm.
|
|
|
|
|
|
Stroker
engines require oversize injectors to provide
adequate fuel flow rates and prevent pinging
due to lean air/fuel mixtures.
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).
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.
|
|
|
Stroker engines need a longer duration camshaft
(than stock) to maximise horsepower and torque.
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"
|
|
|
|
|
|
Stroker
combinations
I've
outlined ten 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 "Poor man's" simple stroker
~ Jeep 4.2L 3.895" stroke crank
~ Jeep 4.2L 5.875" rods
~ Speed Pro 677P standard bore pistons
~ 9.05:1 CR
~ Stock 4.0 HO camshaft
~ Stock HO cylinder head
~ Stock 0.051" head gasket
~ 0.097" quench height
~ 2.25" exhaust
~ Ford 24lb/hr injectors with stock 39psi FPR
for '87-'95 engines, stock injectors with stock
49psi FPR for '96 and later engines
226hp @ 4600rpm, 304lbft @ 2000rpm
4.6L Low-buck, low CR "rockcrawler"
~ Jeep 4.2L 3.895" stroke crank
~ Jeep 4.2L 5.875" rods
~ Keith-Black Silvolite UEM-2229 +0.030" bore
pistons
~ Increase piston dish volume to 30cc
8.8:1 CR
~ Crane #750501 192/204 degree camshaft
~ Ported HO 1.91"/1.50" cylinder head
~ Mill block deck 0.035"
~ Mopar Performance 0.043" head gasket
~ 0.058" quench height
~ 2.25" exhaust
~ Ford 24lb/hr injectors with stock 39psi FPR
for '87-'95 engines, stock injectors with stock
49psi FPR for '96 and later engines
~ 231hp @ 4400rpm, 327lbft @ 2000rpm
|
|
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
~ 260hp @ 4850rpm, 321lbft @ 3500rpm
4.6L Low-buck stroker
Option 1: Same as above except:
~ Speed Pro H825CP +0.030" bore pistons
~ 9.6:1 CR
~ Mopar Performance 0.043" head gasket
~ 0.082" quench height*
~ 260hp @ 4800rpm, 324lbft @ 3500rpm
Option 2: Same as "rockcrawler" except:
~ Piston dish volume 22cc
~ 9.5:1 CR
~ Crane #753905 204/216 degree camshaft
~ 2.5" exhaust
~ 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
~ 256hp @ 4750rpm, 324lbft @ 3500rpm
|
|
|
|
|
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
~ 9.6:1 CR
~ Crane #753905 204/216 degree camshaft
~ Ported big valve 2.02"/1.60" cylinder head
~ Mopar Performance 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.
~ 270hp @ 4800rpm, 335lbft @ 3400rpm
4.8L medium-buck stroker
Option 1: Same as above except:
~ Custom 4.0" bore pistons with 1.38" pin height
& 24cc 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 0.015"
~ 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 0.015"
~ Mopar Performance 0.043" head gasket
~ 0.055" 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
~ 299hp @ 5000rpm, 353lbft @ 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
|
|
|
|
|
