Thread: Boost compensator, how does it work ??

You could try disconnecting the red hose at the turbo end and connecting it to a constant 15-20psi supply of compressed air to test it.

If working properly it should limit smoke/soot, but not really affect power.

If you blank off the hose to the compensator you will get non boost fuelling through the whole rev range.
There is just a diaphram attached to a rod inside which pushes the rack to more fuel (Simplified version, someone might be able to explain better), so the more boost the more fuel. You can get holes in the diaphram so might be worth checking yours, if you're getting extra fuel under boost I doubt this would be your problem.
It's not adjustable but as Ben mentioned if you connect a separate source of constant pressure it would give you boost fuelling all the time.
That's the way I understand it, more than happy to listen to other peoples thoughts on this.

Eric

Thanks,
So, by disconnecting the hose to the compensator and say, turning up the fuel, would that simply mean the compensator is disabled and the engine just works like it has a N/A injector pump, or is there more to it than that.
I should point out, it does not blow smoke. As far as I can see in my mirrors it never blows black smoke like it's over fueled.
Once the boost starts coming on, it comes on fairly quick and strong, it's just that it's taking a while to start boosting.
I'll try disconnecting the hose just for interest and if I can work out a way to keep the compensator pressurised it will be interesting to see the difference.
I have a GT2556v VNT on my 130 Tdi. I'd love to try it on the Isuzu. I haven't got time to muck around with that at the moment, but maybe a VNT would work well with the compensator.

Cheers, Murray

Originally Posted by THE 109

If you blank off the hose to the compensator you will get non boost fuelling through the whole rev range.
There is just a diaphram attached to a rod inside which pushes the rack to more fuel (Simplified version, someone might be able to explain better), so the more boost the more fuel. You can get holes in the diaphram so might be worth checking yours, if you're getting extra fuel under boost I doubt this would be your problem.
It's not adjustable but as Ben mentioned if you connect a separate source of constant pressure it would give you boost fuelling all the time.
That's the way I understand it, more than happy to listen to other peoples thoughts on this.

Eric

Originally Posted by rijidij

Thanks,
So, by disconnecting the hose to the compensator and say, turning up the fuel, would that simply mean the compensator is disabled and the engine just works like it has a N/A injector pump, or is there more to it than that.
I should point out, it does not blow smoke. As far as I can see in my mirrors it never blows black smoke like it's over fueled.
Once the boost starts coming on, it comes on fairly quick and strong, it's just that it's taking a while to start boosting.
I'll try disconnecting the hose just for interest and if I can work out a way to keep the compensator pressurised it will be interesting to see the difference.
I have a GT2556v VNT on my 130 Tdi. I'd love to try it on the Isuzu. I haven't got time to muck around with that at the moment, but maybe a VNT would work well with the compensator.

Cheers, Murray

No, your assumptions about boost compensators are somewhat back to front.

Boost compensators are intended to reduce the fuel rate when the boost pressure is low.

The particular boost compensator fitted to the IP used on some 4BD1T's has a diaphram in the canistor with a spring on the front side - so that boost pressure opposes the spring.

The mechanism actuated by the boost pressure, or the spring acting on either side of the diaphram allows the stop cam to travel all the way to the full load stop screw (which we adjust for more power/torque) when boost pressure is high. When boost pressure is low the compensator reduces that travel.

Further to my previous post, here are copies of pages from manual about the aneroid (altitude compensator) and boost compensator.







Edit: here is another pic from manual showing reduction in rack travel by boost compensator at low boost pressure.

Originally Posted by Bush65

Boost compensators are intended to reduce the fuel rate when the boost pressure is low.

The particular boost compensator fitted to the IP used on some 4BD1T's has a diaphram in the canistor with a spring on the front side - so that boost pressure opposes the spring.

The mechanism actuated by the boost pressure, or the spring acting on either side of the diaphram allows the stop cam to travel all the way to the full load stop screw (which we adjust for more power/torque) when boost pressure is high. When boost pressure is low the compensator reduces that travel.

So, in theory, Ben's suggestion of pressurising the compensator to full boost pressure (or removing the spring ?) should supply the maximum set fuel right through the rev range ??

You might be able to fit the spring on the other side of the diaphram and not use boost pressure at all, I guess it would behave something like a N/A pump then???

When I get time I'll have a look inside and see what can be done.

Murray