Isuzu Impulse 3.90 Diff Gears

[IT Manta]

I am going to start looking for an Isuzu Impulse rear end that has a 3.90 ring and pinion in it. What year(s) exactly am I looking for? Also do I need the whole diff, just the carrier with ring and pinion, or just the ring and pinion. It will most likely go into a non 3.67 rear end housing...

[yellaopelgt]

I was looking into this a while back, and this thread might be some use to you.
HTH
Joe
http://www.opelgt.com/forums/engine-...ibilities.html

[IT Manta]

I was looking into this a while back, and this thread might be some use to you.
HTH
Joe
http://www.opelgt.com/forums/engine-...ibilities.html

I looked through that thread and a few others but some of the imformation was contradictory or unclear.

Basically I need verified the years/models I should look for. I found information saying impulse up to 1985 but also 1981-1989.

And whether the entire carrier works in all manta rear end housings. I found some information that said I would need a 3.67 carrier.

[RallyBob]

I looked through that thread and a few others but some of the imformation was contradictory or unclear.

Basically I need verified the years/models I should look for. I found information saying impulse up to 1985 but also 1981-1989.

And whether the entire carrier works in all manta rear end housings. I found some information that said I would need a 3.67 carrier.

It's been so long, I can't remember exactly what years are inclusive. But in a nutshell:

*rear wheel drive Impulse only. I *think* they switched to front-drive in 1986 or 1987, but I honestly don't recall. I'm sure you could google or Wiki for a correct year.
*non-turbo only. The turbo cars used an entirely different rear axle.
*the 3.90's bolt right onto a 3.67 Manta carrier. If you have a 3.44 carrier, then you will need a spacer shim to make the pinion engage correctly, OR you can use the Isuzu carrier and bearings (all bearings interchange).
*Use an Opel pinion nut, the OD is smaller and easier to fit a socket onto. You must use the Isuzu crush sleeve however, it is different than the Opel one and they don't interchange.

That's pretty much it. Ernie Bello told me about this about 15-16 years ago, I've done many such swaps over the years and they work just fine.

EDIT: Research shows they were available in the US from 1983-1989. HTH.

Bob

[GTJIM]

Yep - the Imulse diff to use - or get gear parts from - was used right up to 1989 in RWD Non-Turbo Impulses and has a torque tube that looks very like Opel diffs (mainly because they were made in the same GM factory and supplied to both Isuzu and Opel - plus early years of Vegas, Astres and Chevettes - but with different lengths of driveshafts/torque tubes and disc brakes in Isuzu's case).

Turbo Impulses with auto trans used the same type of diff till about mid-1987 when it was found the 180ps from the turbo motor shredded them (thus the limit on hp that they can sustain - about 150/160 hp).

The opel torque tube bolts on to the Impulse housing and the Opel driveshaft slides on to the Isuzu barrel spline on the front of the pinion - so as long as the wider Isuzu diff fits your chasis and disc brakes are allowed in your racing class - the whole rest of the Impulse diff can be used in a Manta with a little spring cup and location bar alteration ... If you can find an early Vega or Astre or Chevette LSD unit (Aurburn made one!) that will fit too! Pic of a NOS one that was on EBay a couple of years ago attached below.

BTW: The Impulse front suspension is also very similar to the Manta/Ascona/1900 front suspension - and as an aside ... the Impulse front spindle can be fitted to the GT/Kadett front suspension with a 10mm spacer above the Opel top ball joint; with the Opel lower ball joint bolting up with the same taper as the Impulse one .... nice, light 246mm vented discs and Isuzu calipers on GTs and Kadetts!

[RallyBob]

The opel torque tube bolts on to the Impulse housing and the Opel driveshaft slides on to the Isuzu barrel spline on the front of the pinion - so as long as the wider Isuzu diff fits your chasis and disc brakes are allowed in your racing class - the whole rest of the Impulse diff can be used in a Manta with a little spring cp and location bar alteration ... If you can find an early Vega or Astre or Chevette LSD unit (Aurburn made one!) that will fit too! Pic of a NOS one that was on EBay a couple of years ago attached below.

Sadly, Jim, the class he races in only allows 'bolt-on' mods, and the wider track and rear disc brakes are strictly verboten...

[IT Manta]

Just on a whim, you don't have anything "lying" around do you Bob?

[bello]

Hello! Here is the scoop: Impulses diffs ONLY without turbo- 83 thru 85 0r 86 only! If it has a torque tube it will easily fit into the Manta/Ascona. With a few caveats: You will have to reuse the shock brackets off the Manta diff, the Impulse diff uses lower shock mounts like a GT or Kadett B. I think that the stabilizer bar is backwards: it hinges (pivots) on the body on the Impulse, the Manta bar hinges on the differential axle sleeves. It is wider than the Manta diff, but I am not sure exactly how much wider. The brakes are awesome- maybe too good: I ended up having to install a Pressure limiting valve from an '86 Porsche 944 Turbo to "dial" the rear brakes in (in my 78 2-liter Kadett C GTE- OE brakes identical frt & rr to a 75 Manta) Small detail- no one makes a racing pad for the Isuzu Impulse calipers- it is a pain in the ass & pricey to have them custom made... If you go the gearset way, you need the carrier from a 3.67 Opel diff to hang the gears on or just use the Impulse carrier- that is the easy way, since all the bearings and their locations are the same. The pinion bearings look different but will interchange! If you have a limited slip for an Opel- same story- must be for a 3.67 or higher gear set. Or you have to use the spacer between the carrier & the ring gear that Bob mentioned. I do not know how thick that is. I stumbled into this years ago (1990) when we changed a noisy Impulse diff at the shop & ended up with a free core. Took it apart & found a bad front seal & bad pinion bearings. Replaced said items & had myself the "hot setup" for my Kadett C- I transferred the limited slip into the Isuzu diff and I am still racing it! Please ask if you have any questions! I may even have an extra set of 3.90 gears. Also a set of 4.22 & 4.30's. Ernie Bello

[RallyBob]

Just on a whim, you don't have anything "lying" around do you Bob?

I have lots of stuff, but nothing for sale....

[IT Manta]

You are killing me Bob!

[IT Manta]

Well I have had some offers on the needed parts to install a 3.90 but I have also located a complete assembly out of an 84. The yard is unable to tell me the rear end ratio. Doeas anyone know if there were different ratios for manual and auto transmissions?

[GTJIM]

They were all listed with 3.90 ratios - the only different ones I have found are the ones with Twin Cam motors (3.587:1 - from the Japanese home market only). Both manual and auto SOHC and Turbo were listed with 3.9 in the US market - so you are on a pretty safe bet!

[namba209 (R.I.P.)]

Peter, if the rear-end in out of the car, you could do the old tried and true method of guestimating, fairly close to what the ratio is. Mark the driveshaft and roll the right rear wheel one complete revolution, and count how many times the driveshaft rotates. With a 3.9:1, you should have real close to 4 turns of the driveshaft for one complete wheel revolution. HTH.

[IT Manta]

Thank you with the help everyone. I scored a complete diff out of the Sacramento area. I will just put the third member in a spare Opel housing. It was only $325 to my delivered!

[yellaopelgt]

At first glance, It looks just like an Opel diff(cept the spring buckets) Good score!!!
Joe

[Rally Manta]

Isuzu Amigo Frequently Asked Questions, FAQs