Amount of air vs gas consumption for CNG?

[toxicdevil]

Hello
This is my first post here. I am from Pakistan and CNG vehicles are very common here.

My car is Civic 98 VTEC 1.6l automatic. The fuel average I am getting on petrol is 10.5 km per litre and on CNG it is 12.5 km per kg.

Cost of petrol here is (after conversion to USD) around $1.03 per litre and CNG is $0.65 per kg.

I drive around 50 km daily 5 days per week. So fuel charges per month are:
Petrol: $98 == 9600PKR
CNG:52$ == 5000PKR

I save around 47% in fuel costs.

Now the CNG system is almost like a carb system.
Let me explain.

The regulator converts high pressure to low pressure gas. And it goes through a pipe to a venturi/mixer attached to the opening of throttle body.

Gas is adjusted through a high speed screw and an idle adjustment screw.


Now here is the deal.
We have to place some blockage near air filter to reduce the amount of air entering the throttle body. In some cars there is no such blockage.

Initially I had around 75% of the air pipe blocked.
A fellow had the same car and he removed the blockage completely and tuned it and the car was running fine. I tried the same and but could not tune it, the car idled smoothly but I could not make the mixture rich enough for higher RPMs.
So I modified the blockage to around 40% and I tuned the cng kit accordingly.

The fellow who removed the blockage completely did not talk about any decrease in the mileage but rather a very minimal increase!


Now my question is that How can increasing the gas and air flow NOT cause the mileage to decrease?(I haven't tested mine yet. For some reasons. And cant until February.)

And how to determine the ideal amount of air that should be blocked?


Thanks.

[hamsterpower]

I see two possibile answers,

One, He is wrong. it did not increase.

Two, it is more accurate to adjust the flow of CNG. So he is getting a better(leaner) mixture. AND has to use less throttle for the same work. Netting better overall fuel economy.

[arcosine]

You may need to advance ignition timing for lean mixtures.

[foozil747]

Does he have a larger regulator? Or is his mixer a different diameter than yours is? If his mixer is a better design or his regulator has a higher flow rate that can still allow sufficient mixing it might be why he was able run it with out any other blockage.