Boost compensator, how does it work ??

**Dougal** · 8th November 2011, 06:21 AM

Originally Posted by bushhack

is it possible to source a compensator separate as my 4bdt doesn't have a boost comp and blows smoke at low rpm. new or second hand ?

Sort of.

I swapped the boost compensator from my current pump onto my spare pump before I found out my spare pump had a different shaft size and wouldn't fit the drive gear.
The mounting holes are a straight bolt up. There is a depression on the back of the case which needs drilled through for the needle. But there is also a small rectangular flag on the rack lever inside which the boost compensator needle pushes on. This little flag isn't there on my spare pump.

I swapped across the whole lever. But then swapped everything back again when the drive gear wouldn't fit. It would be very easy to weld a flag on internally.

**rijidij** · 15th January 2012, 02:17 PM

Well, I finally got around to playing with the boost compensator to see what mods could be done. Here's what I did...........

Remove the compensator from the I/P

Remove the end cap to reveal the diaphragm. Under the diaphragm is a spring which is compressed with boost pressure.

There doesn't appear to be much allowance for adjustment (maybe a bit, but not enough for me

)
So, I undid the locknut, withdrew the diaphragm and completely removed the return spring.
I placed a spring on the opposite side of the diaphragm so it would be fully depressed all the time. I used a different spring that fit in the housing better.

I put it all back together and took it for a drive.

The result........excellent, no lag any more. As you can see from the video it starts to boost pretty much straight off the mark. I forgot to take 'before' video, but there was very noticeable lag in the lower rev range before the turbo really kicked in.
Probably where I notice the best improvement is when I slow down, but don't change down gears, then accelerate again from fairly low RPM, not to the point of 'lugging', just lower RPM.

I have to back off in 4th gear as I get a bit of clutch slip now and then, even though I have the turbo pressure plate.

Cheers, Murray

[ame=http://www.youtube.com/watch?v=YkEDJOM3cKM]4BD1t Boost Gauge.mpg - YouTube[/ame]

**Dougal** · 15th January 2012, 03:30 PM

That's one way to do it. Easier is just removing that plunger you can see in the first photo.

How is the smoke now?

**slug_burner** · 15th January 2012, 04:26 PM

Originally Posted by Dougal

That's one way to do it. Easier is just removing that plunger you can see in the first photo.

How is the smoke now?

Was that a cabin full of smoke oe just a hazie lens?

**rijidij** · 15th January 2012, 05:26 PM

Originally Posted by Dougal

That's one way to do it. Easier is just removing that plunger you can see in the first photo.

How is the smoke now?

Originally Posted by slug_burner

Was that a cabin full of smoke oe just a hazie lens?

Just a hazie lens on my phone, no noticeable smoke, so I can probably turn the fuel up a bit more, but I might wait until I fit a bigger intercooler which I have in the shed.
I'm happy with the improvement so far anyway.

Cheers, Murray

**Blknight.aus** · 15th January 2012, 08:46 PM

Originally Posted by rijidij

Well, I finally got around to playing with the boost compensator to see what mods could be done. Here's what I did...........

Remove the compensator from the I/P

Remove the end cap to reveal the diaphragm. Under the diaphragm is a spring which is compressed with boost pressure.

There doesn't appear to be much allowance for adjustment (maybe a bit, but not enough for me

)
So, I undid the locknut, withdrew the diaphragm and completely removed the return spring.
I placed a spring on the opposite side of the diaphragm so it would be fully depressed all the time. I used a different spring that fit in the housing better.

I put it all back together and took it for a drive.

The result........excellent, no lag any more. As you can see from the video it starts to boost pretty much straight off the mark. I forgot to take 'before' video, but there was very noticeable lag in the lower rev range before the turbo really kicked in.
Probably where I notice the best improvement is when I slow down, but don't change down gears, then accelerate again from fairly low RPM, not to the point of 'lugging', just lower RPM.

I have to back off in 4th gear as I get a bit of clutch slip now and then, even though I have the turbo pressure plate.

Cheers, Murray

4BD1t Boost Gauge.mpg - YouTube

sounds like you may have had a problem with the compensator or it was poorly setup, or worse youve got the industrial static application style torque cam in place....

OOC do you get a decent extra kick in torque/power at about the 1800RPM mark?

**rijidij** · 15th January 2012, 09:50 PM

Originally Posted by Blknight.aus

OOC do you get a decent extra kick in torque/power at about the 1800RPM mark?

Before I did this mod I got a decent extra kick in torque/power at about the 1800RPM mark, but now it boosts nicely right off idle.
The engine is a factory turbo engine from a NPR59 truck which I bought and wrecked......County V8 to 4BD1turbo conversion

**Blknight.aus** · 15th January 2012, 10:32 PM

reason Im asking is because in all of the places the donk turns up I've been having some fun trying to get one to tune nicely.

Turns out that the engine in question was setup as a static hydraulic plant engine set and forget at one engine speed.

power delivery was basically

0-1800RPM enough to over come the drag of the machine alone
1800-2000 RPM enough torque to reverse the rotation of the sun
2000-governor a fairly rapid drop off in power

Whatever it was attached to apparently had a disconnect so that if the RPM's dropped down below 1900 RPM then the load was throttled off. if it dropped below 1800 RPM the load was disconnected till the engine got back up to to 1900 rpm when the load was throttled back on.

Great engine for towing with, crap for general driving.

**Dougal** · 12th February 2012, 07:58 AM

I've long considered these boost compensators needed another adjustment point. So I've fitted one.

Stock you have just one adjustment on the rocker arm, which sets how far out the plunger moves for a certain boost and consequently how far the fuel rack can move to increase fuelling.
But, there is no lower stop. Trying to adjust the compensator to stop smoke when just on boost limits the fuel you have off-boost. Making driving awkward when you get caught with no boost.

So I popped my wastegate open (zero boost) and did some tests.

First run, the EGT's would hit 550-600C maximum and it's very sluggish. Confirming what I've thought for a long time, it needs more fuel before boost.
So I pulled the compensator off and measured where the stop sat. It was roughly 24.3mm from the end of the plunger to the back of the fuel pump. Reducing this distance would reduce the fuel available off-boost, increasing this distance increases the fuel available off-boost.

The main modification is simple, I turned up a sleeve that goes over that little spring to provide a lower travel limit. Thus stopping the boost compensator from interfering with my off-boost fuelling.

I also added some 5mm washers for good measure for the first test. The total stack was 27.9mm measured from the head of the pin to the end of the compressed spring, sleeve and washers.

This was smoke propelled, seriously bad. I removed the washers and trimmed down the sleeve over the next four test runs. In the end I had a stack length of 25mm (0.7mm more than my original adjustment) which gives me sustained EGT's of around 750C and minimal smoke under acceleration and no boost. This is about 50% more power off-boost than I had before.

The final dimensions for that sleeve are 10mm OD, 7mm ID and 12.3mm long. Even 0.5mm makes a big difference.

The drivability improvement is great, there is no longer a hole in the fuelling off-boost, it starts pulling well and then the turbo takes over. This also means I can better tune out the smoke when boost comes on.

My fuelling now has three adjustment points.
Fuelling with no boost. (tuning of the spacer length).
Fuelling during the transition when boost arrives.
Fuelling maximum (the fuel screw on the pump).

I highly recommend this mod to anyone with the boost compensator on their pump.

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