Ruff posted a picture of the mixer / plate that he uses for his LP set up - I am wondering if any one has any thoughts as to how well this type of set up would work.

The one pictured is a unit made in Australia but it looks to be easy enought to make one on a lathe and then just mill some ports in it for the gas...

I am using the OEM throttle body on my 4.0l I6 jeep motor so that I can retain the TPS for the auto tranny - Because the throttle body has a throttle plate and an idle passage way all I should need to do is supply a source of propane to the plate - the throttle plate itself with control the metering correct... By keeping the AIS motor I should be able to electrically raise and lower the idle speed as well

I have my system planned out like this so far - tank/vacuum lockoff valve/model J converter/ and then a mixer plate right above the throttle body...

Any thoughts how this should work or problems I am glossing over

Matt

Here is the mixer mentioned above:
http://www.pirate4x4.com/forum/attachment.php?s=&postid=1399733

The Impco Model J is only rated at 100hp max,,,,,,,I'd suggest a Model E rated at 325hp max,,,,,,even if your motor may not make 325 HP, why limit it by the regulator's flow.

That's great if you like to make your own parts but when a rebuilt 425 mixer is only 80 bucks plus 10 or so for the adapter,,,,,,,,,,,,,I'd just buy one that I know works.

I had seen the model J rated by cubic inches?! and the 242cu" motor seemed to fit within the range - kinda besides the point but maybe either one will work fine - if not I will swap to the larger ;) (I already have a J)

As for the 425 space is an issue as all this is going in a WWII flatfender - I was palnning to build an air box w/ filter off the backside of the glove box and the just run an air tube to the throttle body...

Plus with the 425 I would have to make an adapter to mount just the mixer portion to my throttle body and then figure out a way to fit a tube to the head of the 425 to the airbox...

I guess my real question is: does the mixer do anythin other than meter the air and gas flow via a throttle plate and associated idle passage ways?

Thanks for the heads up on the reg. - I will look into that...

Matt

Originally posted by PYRO
The Impco Model J is only rated at 100hp max,,,,,,,I'd suggest a Model E rated at 325hp max,,,,,,even if your motor may not make 325 HP, why limit it by the regulator's flow.

That's great if you like to make your own parts but when a rebuilt 425 mixer is only 80 bucks plus 10 or so for the adapter,,,,,,,,,,,,,I'd just buy one that I know works. EXACTLY, i wouldnt mess around with the flow of lpg when teh parts arent that expensive anyways, jiMMy

A conventional mixer has a diaphragm in it that controls the fuel flow. This is what keeps the engine from being flooded with LP at low RPM or when it is stalled with the fuel lock open.

I personally do not like the style of mixer you have pictured, it is a hack job IMO, all it is is a calibrated orifice supplying always on LP to the engine above the throttle plate. With out some kind of mechanical or vacuum control you are getting the same fuel flow at WOT and at idle. This leads to a rich idle and a lean WOT. At this point the only thing you are controlling with the throttle pedal is engine RPM. You have to remember LP is under pressure when it is delivered to the carb, it is not like gas where it is sucked out by vacuum. The setup pictured is just going to flow unregulated low pressure propane gas to the motor.

A "proper" mixer is using a vacuum signal to meter the proper amount of fuel in relation to the vacuum of the engine so it compensates for need.

Where it my setup on a 4.0 I would use a model E and a 425. The converters are rated in CFM but that translates into HP, you need xx cfm to feed xx HP. For a 4.0 I would use nothing smaller then a model E and a 425 mixer, or possible 2 model J converters and a pair of smaller carbs but that will create more mounting issues then you already have. I would make an adapter to go from square bore holey to you intake and use a holley throttle plate with the mixer. Or retro fit an older 4bbl 258 intake to your motor.

This is from Impco:

http://www.wps.com/LPG/LPG-carb-theory.html

Ok I see the pressure draw issue and was concerned about that from the start - how much distance could be placed between the 425 and my throttle body - I was toying with the idea of cutting a hole in the airbox and just plugging the impco mixer directly into the side of the air box (removing the throttle plate in the 425) and then a 9 or 10" tube (maybe less) to the OEM throttle body... This would solve the idle vs WOT demand issues but I dont know about the placement - once the lp is vaporized I would think there is enough heat from the tube running over the valve cover to keep it in gaseous form :confused:

Any thoughts about version 2.0 of this plan...

Matt

According to impco my engine could use either a 200, 225, 300A, or 425 mixer - based on a 250 cfm requirement

IMPCO Max.
Model CFM
50 91
50-500 108
100 170
125 202
175 210
200 276
225 329
300A-1, -20 348
300A-50, -70 432
425 460

Engine speed, RPM 400 - 2400 RPM
CID 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
50 5 7 10 12 15 17 20 22 25 27 30
100 10 14 18 23 28 33 37 42 46 51 56
150 14 21 28 35 42 49 56 63 70 77 84
200 19 28 37 46 56 65 74 84 93 102 110
250 23 35 47 58 70 78 93 105 116 128 139
300 28 42 56 70 84 98 112 126 140 154 168
350 32 49 65 81 98 114 130 146 162 178 195
400 37 56 74 93 111 130 148 167 185 204 223
450 42 63 83 109 129 149 169 189 209 230 251

Engine speed, RPM 2600 - 4000 RPM
CID 2600 2800 3000 3200 3400 3600 3800 4000
50 32 34 37 39 42 44 47 49
100 60 65 70 75 79 84 88 93
150 91 98 104 112 118 125 132 139
200 121 130 139 148 158 167 177 185
250 151 163 174 186 198 209 220 232
300 182 196 208 224 236 250 264 278
350 212 228 244 260 276 293 309 325
400 242 261 280 298 317 335 254 372
450 272 293 314 335 357 378 399 419

Any of those mixers should work and can be placed well upstream of the actual throttle plate. All a LP carb uses to meter fuel is vacuum and there *should* be enough of that to work it properly even a good ways upstream.

Impco also makes several carbs that are made to be inline away from the throttle plate, they are common in dual fuel setups. They meter fuel just as well and work good on their own over a throttle body. The rpoblems with dual fuel setups that gives LP a bad rep lie in the ignition, not the fuel metering.

I would seriously doubt a 250 CFM mixer is enough for a 4.0, go big or go home.

The farther away the mixer is from the throttle plate, the slower the thottle responds will be.

Another concern with a large volume area between the throttle plate and mixer is the propane being left in this area when the engine is turned off. This propane can slowly "leak down" past the throttle plates, past open intake valves, past rings and into the bottom crankcase. I've seen this happen on engines with no shut-off valves, the results were very explosive, blew the crap out of the motor.

I run a 300A mixer 50 & 70 series on a 350 chev dual fuel set up. Easy to tune, and the powerloss is hardly noticeble from gas to propane.
I think a 425 maybe a little too much for a 4.0l and you may have problems setting up the air/fuel mixture which is very important with propane. From what i've been told and seen the 425 works well with V8s but i'd be a little hesitant to use it on a 6, unless you're running a hopped up race motor that needs lots of fuel.

Originally posted by PYRO
Another concern with a large volume area between the throttle plate and mixer is the propane being left in this area when the engine is turned off. This propane can slowly "leak down" past the throttle plates, past open intake valves, past rings and into the bottom crankcase. I've seen this happen on engines with no shut-off valves, the results were very explosive, blew the crap out of the motor.

When you say "shut off valve" do you mean a lockoff or something else...

A lockoff with limit the amount of lp that can be made into a gas by the converter - I was actually thinking of using an electric soleniod as a secondary lockoff right at the converter instead of a vacuum activated one - letting the engine cycle out instead of just killing the coil...

Basically I am hearing to put the mixer as close as possible to the throttle plate and maybe use 300cfm as my working flow over the 232cfm figure afforded by the chart - and use a 225 and up sized mixer... Thanks for the info so far guys!

Matt

Edit: just a thought - the CCV's in the valve cove would draw off any of this stray lp gas if I keep it connected to the intake like it normally is set up in the OEM app. - correct??? ;)