Chevrolet Conversion Overview
There are a lot of older model GM trucks on the road that are prime for a conversion. Our conversion kits supply the difficult to manufacture hard parts, all connected to a technical know-how support staff that’s a phone call away. We can even offer suggestions for sourcing the remaining items. We leave the frame and suspension intact, the engine mounts are bolt-in and only require some minor modifications to the mounting pad on the frame. Just browse through the many forums and you’ll find many creative ways to accomplish the GM to Cummins conversions. We don’t pretend to offer the only solution but believe with our parts and support we can get you the conversion you’re after.
1978-1988 Straight Axle, Non IFS trucks
The 4x4 trucks have a taller transmission tunnel making it a more likely candidate than the 2WD but the conversion has been done to both. 2WD truck owners have had to do body lifts or tunnel modifications depending on which transmission they wish to use. Regardless of 2 or 4 wheel drive you’ll need to drop the cross-member under the oil pan about 2” to clear the taller Cummins engine. It is possible to fit the engine into the truck without modifying the firewall slightly but removing the #6 valve cover will not be possible. Just below the windshield wiper motor you will need to massage the firewall into the cab about ½” – ¾” so it’s not anything major but necessary for valve cover removal after the engine has been fitted into your truck. Another thing to be aware of if you choose to run the mechanical fan is bending the pinch weld, where the firewall and the tunnel meet, back to make room for the transmission bell housing. You may have to modify the pinch weld all the way across the firewall on the passenger side depending on what diameter exhaust you wish to use.
1988-1999 Independent Front Suspension (IFS)
Fitment of the Cummins engine is tight but results in a great conversion because you get to keep your IFS. The space from the firewall to the core support makes it extremely difficult to use the mechanical cooling fan. Anything is possible just be prepared to use your creativity and doing a significant amount of front clip modifications and fabrication. We used side by side electric fans and even then space was tight. You will have to do some cutting to the engine mount perches to make room for the wider oil pan rail. A 3” body lift is a must with the mounts we offer and even then clearance is tight. To keep the engine and transmission angle correct you’ll need to space the transmission up 1 inch.
We understand body lifts aren’t for everyone. For those of you that have a suspension lifts that use brackets to drop the front differential, it may be possible to use the Non-IFS mounts, however we haven’t done this ourselves so there could be other issues to work through.
All GM transmission adapter plates use a 2003 – 2007 Ford 6.0L Power Stroke starter. We offer two styles of transmission controllers for the electronic transmissions- Please refer to the Automatic Transmission Controller article for more information.
“Turbo” TH 350 / TH400
Typically speaking you probably don’t want to use this transmission behind a Cummins because the input shaft and internal gears were not designed with low RPM torque in mind. Using either of these transmissions is usually not recommended but if your truck has 3.55 or higher gears you may be happy with keeping this transmission, even though a lock up torque converter and an overdrive is very beneficial when mated to a Cummins engine. One of the benefits of using this transmission is that it is completely non-electronic so an aftermarket transmission controller is not needed. The trade-off is that you will probably need to rework the governor, and/or valve body. You will also have to either use an adjustable vacuum modulator delete plug or a throttle vacuum valve which controls the vacuum modulator. These parts control transmission line pressure and shift points, which will need to be different because the Cummins engine has a much lower RPM limit. A reputable transmission shop can assist you with setting up these transmissions to work best based on your conversion specifications.
Basically the same information applies to this transmission as the TH350 &TH400 but the 700R4 has a beneficial overdrive. You’ll need to make a kick-down linkage for the transmission to engage and disengage overdrive. Generally speaking you may not want to use this transmission behind a Cummins because the input shaft and internal gears were not designed with low RPM diesel torque in mind.
This transmission is found in the half ton applications such as the Yukon, Suburban, Avalanche, etc. This is a fine transmission but you probably don’t want to do any performance modifications to the engine if you want your transmissions to last any amount of time. The biggest benefits to using this transmission are that it has an electronically controlled overdrive unit and torque converter lock-up. Some folks may think that the electronics are a drawback but the efficiency of the electronic controlled transmissions is noticeable and maximizes power transfer and economy. If you are towing often, or heavy pay loads this may not be the ideal transmission for the application. You may want to consider the 4L80E or possibly the Allison transmission for heavy hauling and towing. Of course there are exceptions to every rule! It may very well be easier to have your transmission "built" and avoid fabrication work to swap in a different transmission.
Please note: Some 4L60E Transmissions use a 300mm torque converter. If your 4L60E transmission has a bolt hole at the top of the "tent" on the bell housing you have a transmission that will require the 300mm flex plate option.
Found in three quarter and one ton trucks; it’s stronger than the 4L60E and still has all of the benefits of electronic overdrive and torque converter lockup. Those things coupled with today’s advanced transmission control modules bring more functionality to the transmission than the factory control modules. An example of this is the option to have a Calibration A & B in which A could be “economy” mode and B as a “tow / haul” mode. The 4L80E is a good upgrade option for the folks that have 4L60E transmissions.
We do not currently make adapter kits for Chevy manual transmission applications. Due to the small clutch that would be required we recommend upgrading to a Dodge or Ford 5 speed transmission. If you already have a Chevy 5- speed it can be converted to bolt to a factory Dodge adapter plate. This allows for the use of a 13” clutch which is much better suited for the Cummins torque. If you have a 5 speed manual transmission in your truck, by swapping the bell housing and input shaft, it’s possible to convert the "GM" into a “Dodge” transmission. Not all Chevy 5 speed transmissions can be switched over because of a different bolt pattern on the case of the transmission.
Our adapter plates are built out of aircraft quality aluminum and they replace the existing adapter on the Cummins Engine creating a factory quality connection between the engine and the GM transmission. All of the adapter plates we offer for GM transmissions use a 2003 – 2007 Ford 6.0L Power Stroke starter and require our billet flex plate.
ALTERNATOR REGULATOR KIT
When using the 12 Valve Dodge alternator, an external voltage regulator is needed to replace the Dodge’s PCM regulating function. In our experience these work better than an internal regulator that is available for these alternators. The kit also includes a wire pigtail connector. 99-02 Cummins swaps may use the Dodge PCM to regulate the alternator. Most GM alternators have a single wire connection to the battery and can be desirable but we have concluded that no custom pulleys or belt lengths is much better for routine maintenance reasons.
We sell exhaust manifolds that work much better than the stock Dodge Cummins manifolds in the conversion for the 12 valve and 98.5-02 24 valve engines. These manifolds place the turbo in a much better place for a/c box clearance, and they also make exhaust and turbo oil drain connections easier as well. For the 24 valve engines, other exhaust pieces are necessary when using our exhaust manifolds. Using these manifolds requires re-orientation, “clocking,” of the turbo housings for proper oil draining. Either fixing the waste gate shut or using a different waste gate actuator we can provide is also necessary because once the turbo is re-oriented the actuator will hit the engine block. Fixing the waste gate shut does not usually have any adverse effects on the engine, but we do recommend using a boost gauge to monitor and limit boost pressures to less than 40 psi. We also offer a flexible oil drain tube for the turbo that makes the drain tube a snap, and gaskets for the new manifold for a reasonable extra cost. The manifolds do not come with turbo mounting studs, but you may use the studs in your original manifold. It will take some heat to get them out so if you would rather not fight them we have new ones.
The tach kit consists of three parts; the Mounting Bracket, Sensor, and Tone ring. These parts are necessary to make your factory tachometer work.
Providing this signal to your computer also is necessary for the air conditioning, transmission temperature, and other gauges to function properly. All gasoline and diesel trucks require the tach ring that bolts onto the front of the Cummins crankshaft balancer for the sensor to send the correct signal pulse which makes the tachometer work.
It is not necessary to buy a diesel instrument cluster for your truck.
This manifold bolts to the back of a ’94 and newer Dodge a/c pump, making the a/c plumbing less of a challenge. All you will need to do is have some new hoses crimped between our manifold and your Chevy pieces at the drier and evaporator. If you would like, you may send your lines and have us crimp the new hoses to the new manifold for a reasonable price.
The 94 and newer pump works fine in these trucks, but you may have to move the power steering lines on the engine cross member and cut a hole in the cross member to allow access to the manifold mounting bolt.
SOME OTHER PARTS TO SOURCE ON YOUR OWN
If you have ever done a custom conversion before you are no stranger to the fact there are lots of small details that need to be worked through. Because the way we have done the conversion may differ completely from your build this can be a challenge to provide a detail specific list of each and every part. We’ll do our best to summarize.
Intercooler - We used water-to- air cooler as the space in front of the grill was limited (typically 1988+). This would also include the tubes, clamps, silicone boots, water pump, heat exchanger, and reservoir.
Electric fans - We typically don’t recommend using electric fans but they are a necessity for this build. You may prefer one brand over another. Just remember to measure depth before purchasing.
Exhaust work - A stock Dodge downpipe can provide for a good start. Your local exhaust shop is a fine resource too.
Throttle Cable - Dodge stock throttle cable. We used a 454 gas pedal from an earlier GM truck to pull the cable.
Make of Truck: Chevy/GMC
Year of Chevy/GMC Truck: 1978-1988, 1988-2000
Chevy TH350, TH400, 700R4 AT
Chevy 4L60E, 4L80E AT
Chevy 2001-2005 Allison 1000 5-Spd AT
Chevy 2006+ Allison 1000 6-Spd AT
Chevy NV4500 5-Spd MT
Chevy ZF-6 MT
Adaptable Cummins Engines:
1989-1993 5.9L 12-Valve (Gen-1)
1994-1998 5.9L 12-Valve (Gen-2)