Isuzu Impulse (non-turbo) rear disc brake swap

Isuzu Rear Discs

Cliff

Have just finished this project for my GT (and for a Colorado Buddy too)

Charles Goin has written an excellent article about this as well.

There are two types of Isuzu Inpulse rear discs:

1) the 266mm diameter ventilated rotors that really need bigger ventilated discs on the front as well to be a balanced system.
They donot fit inside standard (or mag) 13" wheels so larger 14" or bigger are needed.

2) the 239mm (9.5") solid discs that are the same diameter as the standard GT front rotors and will fit inside 13" wheels.

I used the 2nd type making up an adaptor plate to mount the caliper which also has to have a recess machined to fit over the wheel bearing. The bearing MUST be clamped in by the plate as this is all that stops any end play and prevents the bearing from spinning in the axle housing.

The Isuzu caliper brackets can be made to fit too by redrilling the axle mounting holes - then an adaptor does not need to be made but it is not as "nice" since welding and redrilling is required.
(Just a toolmaker talking, Charles! - my appologies)

It also needs a hybrid handbrake cable to be made and a bit of ingenuity to get the brake lines to conect up - though the Isuzu brake line nuts are the same thread - on mine anyway.

This all works very well and give a good upgrade for the old drums.

An adjustable proportioning valve to balance front/rear braking and some modifications inside the master cylinder will also be required to complete the job.

When sourcing the discs just check that the Isuzu diff has a LSD or not - some do and they fit in the GT 1970 on diff along with the 3.90:1 gears. Also the front bakes are worth a look too. Nice wee 16mm thich ventilated rotors and the hub fits onto the GT front stub axle. Haven't investigated the caliper mounting probem yet but they dDO NOT just bolt on. I am trying to see if the whole Isuzu spindle will bolt in to the GT front suspension .....

The Isuzu rear disc approach has been discussed in great length in the past elsewhere. A concern over the safety of the conversion due to the larger volume of fluid needed to operate the brakes was brought up. I've never done the conversion myself so I can't comment but I would proceed with caution...

What hasn't been discussed are the affects on vehicle dynamics. For a given vehicle there is an 'ideal' proportion of braking front to rear. If you stray much outside this ideal proportion you are looking for trouble. If your brakes are biased too much to the front your stopping distance will be increased and your front brakes may lock during heavy braking, both of which are bad. If your brakes are biased too much towards the rear you will very likely lock them up during heavy braking. Remember that when braking you get a load transfer to the front of the vehicle. This makes it MUCH easier for the rear brakes to lock up. If your rear brakes lock up and you're not going perfectly straight you'll soon be looking the other direction. This is when really bad things can happen.

If your looking for improved braking performance, adding rear discs along with upgrading the front brakes will get you what you want and keep the correct proportion. If you just do the rear you will need to add a brake proportioning valve to keep the car drivable and you'll be back where you started in terms of stopping distances, though you will likely be able to do more repeated stops before the brakes start to fade. This is really only an advantage if you're racing. If all your looking for is the coolness factor or the fact that you don't have to adjust the discs, then I guess this is OK.

For my money I'd buy a good set of pads and shoes and save the time and money for something else...

-Travis

First - Listen to Travis!

A balanced system is esential - your life and others depend upon it.

In 90% of street driving the standard system that has been brought up to top notch condition will be the least expensive and do the job - as it has been for 30 years.

However if you significantly up rate your motor power and indulge in fast open road or spirited winding road driving then any improvement in braking is a safety plus - but it MUST work correctly.

I am in an unusual situation - long way from GT spares and in the position of having a good supply of Impulses with the bigger brakes. Also a history of hot rodding and car fixing. I like trying better setups and seeing what can be fitted to what as improvements.

I have had both front suspensions on the shop floor together and done some preliminary sizing up and it looks good enough to continue investigation. My GT front suspension went back under the GT at the body shop as it is finally being undercoated and being moved about for the finish coats. This means that the project is on hold till it comes home - after a years worth of serious rust removal.

A complete braking system from an Impulse is something to base a balance upgrade on and there are many other parts that can be very useful as they were built on the OPel "B" platform.
Take a good look at it and see how similar the diff is and how good those discs look. They can be used but it depends on how much you can do - SAFELY!! - for yourself.

Ok, I guess the ten dollar question is, Has anyone done the rear discs successfully on a GT? Please correct me if I am wrong. Wouldn't you need a single cylinder master to proportion front to back? It appears that the GT master has proportioned for the two front discs, leaving the rear drums paired together on the larger of the two master cylinder pistons. If this is in fact the case wouldn't it be biased to the rear anyway? Is the small block at the rear axle a metered type or just a manifold? I guess the best answer is the displacement of the rear calipers vs the stock cylinders to determine which way the bias will go. I know this is alot of questions but the idea looks too inviting to pass up. Besides the motor and tranny have been beefed up, now I'm looking at 30 year old brakes. I'm a bit worried now that I have plenty of go, the stopping needs attention.

GT Rear Discs

The physical fitting of the Non-Turbo Impulse discs to the Opel GT diff is remarkably straight forward.

The discs themselves fit on to the GT axle without modifications but the Isuzu caliper needs either the Isuzu mounting plate modified as in Charles Goin's article or a new plate made to position it correctly.

I have fitted a set but do not have the car on the road yet to see how it all works. The information I have suggests that the standard GT master cylinder will not work unmodified and I am leaning towards using an Impulse master cylinder but have not connected it to the GT booster yet. I have not gone down the track of master cylinder modification yet. Another pending job.

Also, to retain balance, the front discs may need to be upgraded to preserve the front/rear bias. The front discs do most of the braking and if the rears are too efficient they will just lock up.

The Impulse does have a proportioning valve too and is heavier with more of its weight on the front wheels.

I am working on the engine at the moment and am still cogitating about the rear discs so am an interested party too - at this stage.

A set that I sent over there has just turned up and the new owner has contacted me so between us we should have something more concreate.