Gas Woes

[miles]

My vehicle is a Series I Disco 3.9/4.0L fitted with a Series II Thor inlet manifold. Plus it runs on duel fuels.

Now the outside diameter of the Bosch throttle body is 82mm. (Air cleaner side). It is much bigger than the gas mixer that was fitted on the old seriesI manifold.

I have had trouble saucing a larger mixer locally. I was given a mixer that was apparently off a Ford. The outside diameter is 82mm but the section that fits over the throttle body is only 74mm. So I made up a stainless steel sleeve to fit it onto the throttle body.

Last week I had my Disco dyno tuned and the results showed that the vehicle was 20 to 25 hp down on previous tunes.

My dyno tuning man and friend David, told me that the most likely cause was the gas mixer I fitted. The gas mixer has an internal hole of only 46mm and this was placing a high restriction on the incoming air, through the throttle boby. When I switch over to gas, the performance is like disconnecting two cylinders. Dave recommended that I find a larger gas mixer and if I cannot get a larger mixer, start saving my money for a sequentiel injection system.

Now I know a lot of you guys have fitted the Thor manifold to the 3.9 V8, but do you use lpg as well and have you come across this problem?
What we need is a gas mixer with a 82.5 mm section to go over the throttle body and a throat around 55 to 60 mm. Is this possible?

Cheers

[bee utey]

My vehicle is a Series I Disco 3.9/4.0L fitted with a Series II Thor inlet manifold. Plus it runs on duel fuels.

Now the outside diameter of the Bosch throttle body is 82mm. (Air cleaner side). It is much bigger than the gas mixer that was fitted on the old seriesI manifold.

I have had trouble saucing a larger mixer locally. I was given a mixer that was apparently off a Ford. The outside diameter is 82mm but the section that fits over the throttle body is only 74mm. So I made up a stainless steel sleeve to fit it onto the throttle body.

Last week I had my Disco dyno tuned and the results showed that the vehicle was 20 to 25 hp down on previous tunes.

My dyno tuning man and friend David, told me that the most likely cause was the gas mixer I fitted. The gas mixer has an internal hole of only 46mm and this was placing a high restriction on the incoming air, through the throttle boby. When I switch over to gas, the performance is like disconnecting two cylinders. Dave recommended that I find a larger gas mixer and if I cannot get a larger mixer, start saving my money for a sequentiel injection system.

Now I know a lot of you guys have fitted the Thor manifold to the 3.9 V8, but do you use lpg as well and have you come across this problem?
What we need is a gas mixer with a 82.5 mm section to go over the throttle body and a throat around 55 to 60 mm. Is this possible?

Cheers

OK, first question, does your mixer restrict power on petrol? If not then it is not the problem. A 46mm bore mixer is what I use on the D1 and D2 engines and they go just fine on gas. Anyway, as I have pointed out to another forumite, putting the mixer on the throttle body means your IAC valve is drawing air from the clean side of the mixer. Where did you draw the air from for the IAC valve? A picture of your mixer installed would help diagnosis. If you have the IAC connection from behind the mixer then you can machine out an 80mm mixer to fit. It would take a lathe operator about 2 minutes on a good day.
Lastly I have seen people fit mixers on backwards when doing the stainless tube trick. That makes them run awful bad.

[miles]

Thanks Bee Utey .
Attached is some pictures of the Gas Mixer that I was talking about in my previous thread. As you can see the red silicon gasket goo, that was the end which went over the throttle body.

When I said my Disco was down some 20 hp, that was running on petrol.
Prevous tuning on the dyno gave readings of 135ph and the reading from the last run on the dyno indicated 115hp.

I was wrong when I said the throat diameter of the mixer was 46mm. I measured it again, this time using v/calipers and the correct diameter of the throat is 42mm.

Also the internal section of the mixer can be tapped out. Could this be in back to front? And there is a hole with a threaded plug in it which goes right through into the centre (air stream). Does this have any purpose?

Cheers

[miles]

I am trying to add more pictures tomy last thread

cheers

[bee utey]

Your mixer is the right way around.

The brass gas inlet stack-of-fittings is too restrictive, you will never feed the V8 through that. Drill out a plain straight inlet fitting to the max possible ID and use a rubber 90 degree elbow hose. 19mm heater hose is ideal if you can't source the correct 18mm lpg vapour hose-with-elbow. Use thin wall tubing for joiners if needed.

You can probably increase (in a lathe) the bore size of the mixer to 46mm. Even 45mm will work better (Area =square of diameter etc.). Check how thick the alloy sleeve is around the slot. It is important to have the diameter slightly bigger just past the gas outlet, as you see in your first picture. The gas forms 1/15th or so of the air volume past this point. There must be a space for the gas flow otherwise you cause a pressure wave, further reducing the apparent diameter.

The plugged port appears to be into the gas/air region, you could feed the IAC from here. Remember the idle air control valve should be sucking gas/air mixture, so needs to be connected to the intake downstream of the mixer. I have seen one drilled into the side of the throttle body just before the butterfly. You could also connect it into your sleeve adaptor as the port looks too small.

[miles]

Thanks mate. I appreciate your help.

I'll do what you recommended. Give it all a go.

cheers