Ultimate Diesel Builder's Guide - - Contents -

4BT Ba­sics and Per­for­mance Op­tions

T he pop­u­lar­ity of diesel ve­hi­cles in the U.S. has jumped in a pos­i­tive di­rec­tion in re­cent years. More man­u­fac­tur­ers are start­ing to of­fer all-new diesel pow­er­plants in half-ton trucks, cars and mid­size Suvs—the in­tro­duc­tion of a 3.0L diesel in the Ram 1500 trucks, a diesel in the sec­ond-gen­er­a­tion Chevro­let Cruze pas­sen­ger car, the Nis­san Ti­tan XD with a Cum­mins V8, and we hear about more half-ton diesel-pow­ered trucks com­ing down the pipe­line. With the big car builders start­ing to of­fer diesel up­grades from the stan­dard gas pow­er­plants, there must be le­git­i­mate rea­son­ing.

Maybe it’s bet­ter high­way fuel efficiency and the gi­ant in­crease in torque of­fered by a diesel en­gine. In re­al­ity, it’s prob­a­bly an at­tempt at meet­ing ever-in­creas­ing CAFÉ (cor­po­rate av­er­age fuel econ­omy) num­bers. Ei­ther way, these smaller V8, V6, and I4 en­gines will be stick­ing around. But back in the day, it was Cum­mins that de­vel­oped their own small diesel en­gine mar­ket with the 3.9L 4BT.


The 4Bt—which stands for four-cylin­der, B-se­ries, Tur­bocharged—was used mostly in mid­size box trucks, agri­cul­tural equip­ment and small in­dus­trial ve­hi­cles, and is ba­si­cally a smaller ver­sion of the pop­u­lar 5.9L 12V Cum­mins found in the 1989-98 Dodge trucks. The 4BT shares vir­tu­ally all its parts with its big brother the 6BT; i.e., pis­tons, con­nect­ing rods, in­jec­tors and valve train de­sign.

Ob­vi­ously, the ma­jor in­ter­nal dif­fer­ence comes from the lack of two cylin­ders, which means two less jour­nals on the crankshaft, a shorter camshaft, two less pis­tons in the in­jec­tion pump, etc. But even these pieces are the same as the 6BT, just in an in­line four-cylin­der ver­sion. Be­cause of these sim­i­lar­i­ties, the 4BT Cum­mins has an ex­tremely strong af­ter­mar­ket, since many 5.9L 12V per­for­mance parts can be used in the smaller 3.9L en­gine. This, paired with the some­what com­pact size of the 4BT, makes it a great can­di­date as a trans­plant en­gine for just about any ve­hi­cle. We’ve seen 4BTS in ev­ery­thing from Jeeps to old school rat rods. The sim­plic­ity of the me­chan­i­cal in­jec­tion sys­tem also helps make it an easy swap, as there’s lit­tle to no wiring or elec­tron­ics re­quired.

Since the en­gine was orig­i­nally de­vel­oped for in­dus­trial ap­pli­ca­tions, big horse­power wasn’t a ma­jor con­cern and the 239-cid mo­tor most com­monly came from the fac­tory at around 105 hp at 2,300 rpm and 265 lbs-ft of torque at 1,600 rpm. But these num­bers can vary up to as much as 150 hp de­pend­ing on the ap­pli­ca­tion it was in­stalled in.

The 4BT en­gine weighs in at around 750-780 lbs, which is heav­ier than most of the gas en­gines it would be re­plac­ing in a conversion project, but much lighter than the six-cylin­der 6BT ver­sion. The in­line-four also of­fers a fairly com­pact size at just 30.6 inches long and 37.7 inches tall, help­ing with space con­straints. The en­gines came with a few dif­fer­ent in­jec­tion pump sys­tems, but the P7100 is, by far, the most pop­u­lar, since it’s ca­pa­ble of pro­duc­ing larger amounts of fuel and more eas­ily mod­i­fied by the af­ter­mar­ket to help in­crease power out­puts.


As pre­vi­ously men­tioned, the 4BT can eas­ily be mod­i­fied to pro­duce higher than stock power lev­els when out­fit­ted with the P7100 (P-pump). Fuel­ing mods can be taken to the same ex­tremes as with the 5.9L 12V Cum­mins; 12mm and 13mm pumps with laser-cut de­liv­ery valves, hirev gover­nor springs, full-travel rack plugs along with mod­i­fied in­jec­tors can all be used in the 4BT plat­form. Ob­vi­ously, to go along with ma­jor fuel up­grades, com­mon cylin­der head and tur­bocharger up­grades would be re­quired. But the af­ter­mar­ket al­ready has most of that cov­ered as well with head studs, bet­ter valve springs, larger valves, and even per­for­mance ex­haust man­i­folds and camshafts that are be­ing built for the four-cylin­der diesel. While these up­grades can take the 4BT to all-new lev­els, one thing that needs to be men­tioned is that be­cause of its lack of cylin­ders and nearly iden­ti­cal op­er­at­ing rpm range, each pis­ton is go­ing to un­dergo a com­bus­tion cy­cle much sooner than that of a 6BT Cum­mins, so things like camshaft pro­files and pump tim­ing be­come crit­i­cal for max­i­mum efficiency at higher horse­power lev­els.


Like any en­gine plat­form, it’s a known fact that more cu­bic inches gen­er­ally means more power and there’s only so much a 239-cid mo­tor will be ca­pa­ble of, at least safely and ef­fi­ciently. With this in mind, ACD En­gines of Salt Lake City, Utah, has de­vel­oped what they re­fer to as a “Stro­ker Kit” that will take the 4BT to all-new per­for­mance heights.

Also known lo­cally as “All Cum­mins En­gines,” ACD has been a full-line Cum­mins dealer for more than 20 years, spe­cial­iz­ing in midrange and in­dus­trial en­gines. They of­fer vir­tu­ally ev­ery­thing Cum­mins—new or re­man­u­fac­tured en­gines, new gen­uine Cum­mins parts, used en­gines and even sal­vage parts. Since they deal in nothing but Cum­mins, any­one look­ing for that hard-to-find part can most likely call ACD and find they’ll have it in stock or at least know where to get one. Through their years of ex­pe­ri­ence, they’ve be­come ex­tremely well versed on the 4BT plat­form and have the conversion process down to a fine sci­ence, in­sert­ing the small Cum­mins en­gine into Jeeps, small SUVS, pick­ups and even a mid-’50s am­bu­lance.

In the search for more power, owner Robby Ped­er­son be­gan de­vel­op­ment of a Stro­ker Kit

that would in­crease the length of stroke and add some ad­di­tional cu­bic inches to the in­line-four diesel. In the gas world, Stro­ker en­gines are nothing new, as the GM small-block 383 Stro­ker en­gines have been around for nearly 35 years. The 383 is built by us­ing a stan­dard GM 350-cid en­gine block with a mod­i­fied 400-cid crankshaft, which changes how far the pis­ton trav­els.

New Rods and Pis­tons

Not look­ing to do so much crankshaft work, as the fac­tory 4BT crankshaft is a ro­bust piece, Ped­er­son opted for an all-new pis­ton and con­nect­ing rod de­sign to gain that ad­di­tional stroke he was after. A fac­tory 4BT en­gine runs a 4.02-inch bore with a 4.72-inch stroke; this is how we come to a 239-cu­bic-inch mo­tor. The new parts from ACD will al­low the over­all pis­ton bore to ex­pand to a 4.402-inch and a much longer 5.430-inch stroke is used, which will ef­fec­tively take en­gine output to 333 cu­bic inches, or 5.46-liters, al­most that of the 5.9L 12V Cum­mins.

Know­ing that the clien­tele for such a kit would be after ex­treme power lev­els, only the best ma­te­ri­als were used to be sure the ACD Stro­ker Kit would stand up to high-boost and high-cylin­der pres­sures. Rather than use stan­dard-cast pis­tons, like the stock units, ACD went with a much stronger forged pis­ton de­sign that of­fers a dif­fer­ent bowl de­sign to im­prove the air/fuel swirl ef­fect, help­ing cre­ate a more ef­fi­cient burn in the com­bus­tion cham­ber. The pis­ton de­sign also has a much shorter over­all height, along with the wrist pin lo­ca­tion be­ing moved closer to the deck; this is where most of the ad­di­tional stroke comes from.

To match the new pis­ton, con­nect­ing rods had to be de­vel­oped, which were made from bil­let 4340, like those be­ing used in all the high-horse­power diesel en­gines. The new H-beam rod de­sign is not only stronger than a fac­tory con­nect­ing rod, but it also has a much smaller wrist pin jour­nal, which will only work with the ACD pis­ton. While the pis­tons and rods are the true heart to the Stro­ker Kit, ACD has also de­vel­oped a host of 4BT parts to go along with them to en­sure the true peak per­for­mance can be met. Spe­cific camshaft pro­files have been de­signed, along with cus­tom cylin­der head work, and even 4Bt-spe­cific ad­justable in­jec­tion pump tim­ing gears. The short time be­tween in­jec­tion events on the four-cylin­der en­gine re­quire a lit­tle dif­fer­ent en­gi­neer­ing of these parts than that of a six-cylin­der Cum­mins, and ACD thinks they’ve de­vel­oped the per­fect pieces to help turn your run-of-the-mill 105-hp 4BT into a tire-shred­ding 800-hp mon­ster.

ACD will soon be in­stalling this com­pletely built 4BT en­gine into their shop ve­hi­cle, a H1 Hum­mer Army body sit­ting on the chas­sis from a Dodge Ram 2500. The Stro­ker 4BT will help push fully built axles,

1 The 3.9L 4BT Cum­mins diesel en­gine has be­come an ex­tremely pop­u­lar plat­form for conversion projects over the past cou­ple years and for good rea­son. Shar­ing vir­tu­ally ev­ery in­ter­nal and ex­ter­nal piece with its big brother, the 5.9L 12V Cum­mins found...

2 The 4BT en­gine has been used in com­mer­cial util­ity trucks and in­dus­trial con­struc­tion equip­ment since the late 1980s and be­cause of its ex­tremely ba­sic func­tion­al­ity works well as a trans­plant en­gine when space con­straints re­strict the use of the...

4&5 ACD En­gines of Salt Lake City, Utah, is a strong Cum­mins en­gine and parts dealer with years of ex­pe­ri­ence in both the 6BT and 4BT plat­forms. Through their busi­ness prac­tices, they’ve be­come well known for their 4BT conversion projects and have just...

6 A close-up of the two con­nect­ing rods pis­ton ends show where some of the ad­di­tional pis­ton stroke comes from. No­tice how much smaller of a di­am­e­ter is used to en­case the pis­ton’s wrist pin? This new con­nect­ing rod de­sign was de­vel­oped to be run...

3 The 4BT en­gine uses a sim­ple two-valve per cylin­der sys­tem, ob­vi­ously one for in­take and one for ex­haust. While this de­sign is some­what “out­dated” with the in­tro­duc­tion of the four-valve-per-cylin­der de­sign found in the 1998.5+ 6BT en­gines, it’s...

20 Since the cylin­der head in­take and ex­haust sides were ported and pol­ished to max­i­mize air­flow through the head, it’s only nat­u­ral that an ex­haust man­i­fold up­grade be used as well. While there are dif­fer­ent Oem-style man­i­folds for the 4BT that can be...

19 Turn­ing to their back­ground work­ing on the in­dus­trial side of the Cum­mins en­gine plat­forms, ACD will be us­ing these OEM Cum­mins valve cov­ers on this high-horse­power build, which of­fer dual port ven­ti­la­tion. While the gap­less pis­ton rings should cut...

17&18 Again, turn­ing to their knowl­edge of the 12V Cum­mins af­ter­mar­ket, the fuel in­jec­tors for the 4BT en­gine were also re­worked to max­i­mize the amount of fuel that could be ef­fi­ciently in­jected and burned in the cylin­ders. The dual-feed in­jec­tors and...

15 Be­cause this en­gine will be run­ning higher boost pres­sures, the cylin­der head will be ma­chined to use a cus­tom fire-ringed head gas­ket kit and ARP head studs. Due to the swap from the fac­tory head bolts to the stronger ARPS, like any 6BT Cum­mins...

14 In high-revving and high-horse­power en­gine ap­pli­ca­tions, the fac­tory press-in freeze plug de­sign is a com­mon fail­ure point. To pre­vent that, ACD En­gines has the cylin­der head wa­ter ports tapped to ac­cept threaded plugs.

16 The Bosch P7100 in­jec­tion pump has come a long way from what it was orig­i­nally de­signed and built to do on the 1994-98 Ram trucks. Orig­i­nally de­vel­oped to sup­port the mid 200-hp range, the 4BT plat­form with just four cylin­ders moves even less fuel,...

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