Hi guys, long time no AZ. I've even come back with a new name! Had a pretty tumultuous six months but that's a story for another day. Still got my old Sierra, still loving it, and still running straight LPG. It's still running great too - but in the quest to get this car perfect for my needs i'm considering another revision to my intake system.
I went ahead and installed a dedicated LPG throttle body some months back. Nothing too hardcore, just a 40mm TB from an old Hyundai... it seemed about right after doing the maths. On the plus side it's improved my 1.6's performance - no more elastic accelerator response, a massive bloom to low RPM torque, sitting on a steady highway speed it's more efficient than ever, and I can white knuckle it to 140kph if i'm feeling brave. It's greatly simplified the intake and neatened up the engine bay too.
On the negative side - jeez it can smash the fuel when I put the foot down. Even though the TB is very modest in size, if I decide to drive my Sierra like a rental i'm using about 20L/100Ks. This reduces my range from 500km+ per tank to 300... yikes! When I had the Aisin on I could drive like stink or like a grandma and i'd always be using 12-13L/100K - but after 10 refills with the Hyundai TB setup i'm yet to better 14L/100K. And from point A to point B i'm not convinced it's moving any faster. I'm also finding with all that low end torque gain it's very touchy when feathering the accelerator. It's very easy to get this car bunny hopping in first gear, though i've somewhat mastered it now. In short, as much 'percieved' performance as this install has introduced it hasn't really granted me any of my desired gains.
My main 'like' with the Hyundai TB install is that the intake system is now bulletproof - the only moving part is the TB butterfly, and near zero chance of a vacuum leak. That's a big plus for me. But ultimately i'm all about getting the most miles out of a tank of gas. As such i'm considering reverting back to my Aisin carb. But if I do there are two major issues i'd want to correct first:
(1) idle RPM had a tendency to drop really low when hot. After a long drive and getting the engine right up to temperature the RPM would drop from a happy 1000 RPM to a near stalling 400 RPM... I don't know why this was... perhaps a vacuum leak that only presented itself when hot? Something to do with the heat-driven choke?
(2) the Aisin had a very elastic relationship with the accelerator pedal. Feather the pedal with this setup and the RPMs would flounder rather than rise - you needed to give it a quick little pump of the foot to get the RPMs headed in the right direction. With my current TB setup if you drop the accelerator a millimetre the response from the engine will be a linear rise in RPM - much better.
So what do you think. Should I revert back to a sorted Aisin, or should I just stick it out with my current setup?
I remembered this carby sat a little too high under the hood with the LPG mixer sitting on top. Last time I paid a shop to do the machining... this time I used a hacksaw to shave 9mm from the profile. Unconventional, but it worked out perfectly.
backyard machining 101