Thread: Boost compensator, how does it work ??

There are no downsides to a working and well tuned boost compensator on these engines. Here's what you need to do.

Step 1. Remove the hose and blow down it. If it leaks, then you need a new diaphragm inside. They cost about $100.

The diagrams John has put up show the function well, but what they don't show is the adjustable screw on the rocker. To adjust this you will need a long 10mm tube spanner and a long flat blade screwdriver. The tube spanner is to crack loose (and retighten) the locknut, the screwdriver is to adjust the screw on the rocker.

Step 2. Remove the boost compensator from the pump, (a few bolts, easy, it will dribble some oil but no flood). Mount it in a vise, unscrew the cap over the rocker and familiarise yourself with the locknut and adjuster screw. I had to mod a tube spanner to fit inside (skimmed the outside on a lathe). Do this with it in the vise, you'll never get it on the vehicle first time.

Step 3. Unscrewing the adjuster screw increases fuel. 2 turns is a lot. Adjust it 1 turn in the direction you think it needs to go (unscrew gives fuel sooner, with less boost).

Step 4. Put it back on the vehicle, test drive and keep incrementing it half a turn until you get no visible smoke but still rapid spoolup.

Mine has a roughly 3psi range of operation, I used to run it starting to move at 4psi and fully open by 7psi but I had to back it off a little with a bigger and more laggy turbo.

It is also possible to replace the spring under the diaphragm with a weaker or firmer one for further tuning. But I didn't find this necessary. Setting the start point worked well for me.

Thanks Dougal,
So they are adjustable. Do you know what the factory setting might be ?

Originally Posted by Dougal

.....a few bolts, easy.....

It sounds like a fairly uncomplicated devise. That's what I like about these engines, it's not too scary to tackle things you're unfamiliar with as they usually turn out to be relatively straight forward.

Cheers, Murray

Sorry no idea of the factory setting. Mine had been tampered with untold number of times and the 4BD2T manual doesn't list any pressures.

The original diaphragm in mine had worn out many years ago. Since then the pump had been sent for a rebuild (previous owner) and reset while the compensator wasn't working.
I suspect they simply applied 15psi hosing through the compensator (moving it maybe a fraction) and set fuel at that point.

When I put the new diaphragm in, it was suddently getting a lot more fuel than it ever had before, it was very wet first drive with it working, out of my drive, halfway around the corner and I put my foot down. It went sideways. I have turned the fuel down three times since and still hit 750C with 21psi max boost.

If you need to deactivate the boost compensator, simply unbolt it from the back of the pump and take out the push-rod which enters the pump body. Reinstall the compensator to cover the holes.

Originally Posted by Dougal

It went sideways. .

Still awaiting video footage

Originally Posted by rovercare

Still awaiting video footage

Keep waiting then.

I just found some will not be able to see some of the pics from the manual that I posted above, so here they are again for all to see.

IP complete


Aneroid - reduces fuel to compensate for lower atmospheric pressure as altitude increases


Boost compensator - reduces fuel when boost pressure is low

Originally Posted by rijidij

So, as SteveG suggested to me in the first place, the compensator is just to prevent smoking under load at lowish RPM. If it's reducing the fuel supply, it's certainly not there to enhance performance.

Correct, looking at the boost comp and IP calibration settings in the manual, assuming I have interpreted them correctly, the boost compensator only has effect (reduce fuel) when boost pressure is quite low (I can't remember pressure as it was given in mm mercury) and for rpm less than 1500 (750 rpm of IP).

The pump governors for 4BD1T's fitted with boost compensators will most likely have a different torque cam to those with turbo and no compensator (like I have), and certainly different torque cam to those with no turbo. If so removing the compensator, or a faulty compensator could cause a lot of smoke.

The compensator is fitted for pollution requirements. If the fuel is turned up on a turbo'ed 4BD1/T so it makes smoke at low speed and boost, then fitting a boost compensator could be beneficial.

Edit:
The following diagram taken from a 4BD1T manual (1986 - 88) for the IP calibration shows control rack travel vs pump rpm (1/2 engine speed). The dashed line shows rack travel when boost pressure is < 20 - 40 mm Hg (0.39 - 0.77 psi) - it is not shown beyond about 710 rpm (1420 engine). The stock boost compensator reaches full travel at boost pressure > 110 mm Hg (mercury) (2.13 psi).

Point 'A' is 11.2mm rack travel for 70.2 - 72.2 cc/1000 stokes at full load/torque and 900 rpm (1800 engine) with boost pressure > 110 mm Hg (mercury) (2.13 psi).

At point 'B' the rack is pulled back 0.25 mm from its position at 'A' (i.e. 10.95mm) for 49.7 cc/1000 st at 550 rpm (1100 engine) with boost pressure > 110 mm Hg (mercury) (2.13 psi).

At point 'C' rpm is same as at 'B' but the boost compensator has pulled the rack back a further 0.75mm (i.e. 10.2mm) for 29.7 cc/1000 st when the boost pressure is < 20 - 40 mm Hg (0.39 - 0.77 psi).

Originally Posted by steveG

As you suggested earlier, I'd try cranking up the fueling a bit more and see how it behaves. When I was playing around with mine I found that it built boost MUCH faster with a bit more fuel.

...

Steve

Correct, turbos convert heat energy in the exhaust gasses to mechanical energy to drive the compressor. More fuel gives more heat.

Edit: I should have added the conclusion - incase it isn't obvious to all - that the extra energy in the exhaust gasses causes the compressor to spin faster, thus increasing the air flow.