Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

ktbundy (Automotive) (OP)
20 Nov 02 14:24
I'm about to convert my car over to propane using standard Impco parts. I have a 360 Mopar engine that will be running 11.5 to 1 compression. The heads have stainless valves, bronze guides and hardened seats. Everything internally is pretty much new, as is the machining. I have chosen a Weiand 2x4 tunnel ram, and will be using 2 425 mixers, 2 vaporizors(model E), and 2 fuel lock offs. Are there any additional parts that would optimize this set-up? And I was thinking of fabricating my own mixer adapters and removing the current top plenum of the intake(to help a bit with hood scoop issues. Is this going to cause any problems if there is no front to back balance tube between the intake runners, like the top plenum provides? Thanks in advance.
Helpful Member!(2)  franzh (Automotive)
20 Nov 02 15:11
With the Weiand tunnel ram, each intake runner comes up to a plenum which should help even out the mixtures.  This setup should provide up to 700 bhp of air and fuel, assuming the engine can handle it (it would be VERY radical and hard to drive).

One area you did not cover is the ignition, it should be top-line.  The OEM ignition is barely adequate as it is.  Try initial of around 8 degrees, and all in by about 3000, somewhere around 32 degrees.

Since propane vapor displaces about 5 to 8% of the incoming air, anything you can do to improve airflow is a benefit, but remember that larger ports and runners will hurt low speed perforamance, so a trade-off is in store.

Personally, unless this is an all out performance application, you may not like the way it runs at low to moderate speeds, but once the throttle is opened. . . .

ktbundy (Automotive) (OP)
20 Nov 02 16:52
Thanks for your reply Franz. Ironically enough, it was your information on the web that led me here. After reading your information on engine build-ups, I felt I was on the right track. I forgot to mention that I'll be running a 284/484 camshaft. Not too radical, but enough to wake the motor up. I am looking at an MSD ignition also. Is there any kind of engine management system associated with propane? I have heard terms like closed loop, but don't really understand what it means. My ultimate goal is to turbocharge/intercool this car. I have another 8 to 1 shortblock ready for assembly, but really want to get to understand propane and how it works before cranking up the boost.
franzh (Automotive)
21 Nov 02 9:15
Glad I was able to help;

The same company that owns MSD also owns Dualcurve, and provides closed loop fuel control systems for LPG vehicle.  With a little adaptation, one could be installed on this engine.

The closed loop devices take engine RPM, Oxygen sensor output, and either manifold vacuum or throttle position to determine engine demands, and adjust the fuel mixture accordingly.

There are basically two types of closed loop controls for a mechanical LPG-CNG carburetor system:

By controlling the pressure at the regulator/vaporizer, thus changing the delivery pressure/volume to the carb.
By controlling the volume of the gas output to the mixer/carburetor.  This device uses a vacuum operated restrictor in the vapor line.  There are some electric models that work very well too.

Both have advantages over the other in certain applications, but both have drawbacks too.  The pressure type can be a little slow to respond to rapid throttle openings and may lead to intake backfires, where the the volume type allows fast response but may be a bit difficult to initially tune.  The same electronics can be used on each.  The pressure type is the one most used here in the US, where the rest of the world uses the volume type.  Sounds like a good article for my webpage.

Using dual vaporizers should not be too difficult to plumb.

ktbundy (Automotive) (OP)
21 Nov 02 22:22
I guess the next obvious question would be, does this type of application need a closed loop system? Or will this function properly without it? Most of the older cars and trucks that I have seen at the wreckers just run the fuel lock/vaporizer/mixer set-up. Also I looked at that cam website from Australia you mentioned, and most of their cams seem to be a split pattern design. My cam is not the greatest for engine vacumm(single pattern 241  at .050)and again I wonder if this affects how the propane system functions. Obviously at half or full throttle it makes plenty of vacumm, but would it affect the fuel at idle? Sorry for so many questions, but I want to be fully prepared when this set-up is first fired.
franzh (Automotive)
22 Nov 02 8:47
The vacuum that is used is called "air valve vacuum" and is measured in inches of water column.  Any application with an Impco mixer will generate a stable vacuum here.

As for if your engine could benefit from closed loop, it all depends on what the final application will be.  If its straight competition, then no, it would be of no benefit.  If its street use, your entire engine setup seems a bit radical for comfortable use, but it would help some, but personally, I wouldn't recommend it.  You will find throttle lag unacceptable, and a bit more aggravated with closed loop.

Closed loop works best with a single plane manifold where the mixer is close coupled, meaning less plenum volume to control.

The 425 offers 3 gas valves for mixture tuning, and you may have to experiment to obtain the best ones.  You should also run a balance line between the two.  When it is running, I would try to locate a gas analyzer, or install an O2 sensor in the exhaust for tuning.  If you run open exhaust, tuning can be difficult due to air pulses.

Helpful Member!  kimbo1 (Mechanical)
27 Nov 02 19:30
you seem to know your stuff.  i have done a lot of tuning on aftermarket computers and find almost universally engines seem to like 15 to 20 degrees at idle.  My own runs 24 degrees and 30 total.  I was wondering why why has this not caught on with the traditional tuners?  I can run the engines at a much cleaner idle with this sort of advance, especially big cammed cars which seem to just love it, and the engine that responded most impressively to this treatment was a 351 windsor with a single plane manifold.  It had 38 degrees advance at light load at 2000 rpm (but still only 32 total)( both arrived at by tuning for maximum torque).  I know the 20 degress advance might be a bit radical if you are still using mech advance distributor without the benefits of a totally programmable 3D map.
Was just wondering as I still come across people that underestimate the benefits of ignition and ignition control.  
I agree with you that especially on LPG a top quality ignition system is crucial.   
ktbundy (Automotive) (OP)
30 Nov 02 22:15
I have run into a bit of a stumbling block with regards to my intake height and the mixers. Doing a quick measurement, I will not only need to cut a hole in the hood, but the mixers will need just over 8" of clearance above the hood. I don't know if I can put a scoop on this car that will be tall enough, and still be able to see around it. I may have to either do something with the top plenum(like get rid of it)to adjust for clearance, or go to a lower dual carb intake. The drawback to this is having to use a dual plane intake, which to me would not be optimal. The car in question is an AMC Spirit, so under hood area is at a premium. I would like to have a single plane 4bbl intake to be able to measure volume. It must be quite low on these intakes as the runners are not very long. I really would like to just do away with the top plenum on the tunnel ram.
franzh (Automotive)
2 Dec 02 8:30
As for the timing issue, 99%+ of the engines with LPG are not performance oriented, and smooth idle and low emissions are mandatory.  Although I personally like more aggressive timing profile for crisp throttle response and a bit more power, emissions take precedent.

I did a study at a major research lab in the early 90's with timing and we found a huge difference in emission production with not so much an improvement in power (LPG only, not so true with NG).  I also did a timing map on an engine dyno and came up with some numbers, but all this is out the window when working with non-stock compression ratios and more aggressive cam profiles, as it would with any fueled engine.

Increasing base timing in the regions you describe increases the cylinder pressure at BTDC, which may indicate some deficiencies in other areas of the engine, and also increases NOx production.  Ideally, you want maximum combustion pressure just after TDC, and before the 90 degree mark.

No argument that a good effective 3D ignition map is necessary, along with a corresponding fuel map.  This is only possible with a good engine management system.  Someone with a VERY good mechanical system can get close, but it's not adaptable.

franzh (Automotive)
2 Dec 02 8:37
Can't really comment on the hood clearance issue, but I strongly recommend against removing the plenum, unless you plan on using 8 individual carbs.  Going to a lower profile dual plane intake is far superior to removing the plenum.  There are plenty of autos running VERY well on a "X" type single plane intake with a single carb, gasoline or propane.

I think you are placing too much emphasis on runner length and plenum volume, which only provides minimal gains anyway, and then only in certain rpm ranges.

In the "old days" I had a small street engine (292 cid) with an early small single plane manifold and 700 Holley that outperformed the replacement engine (350, large single plane with 850 Holley) by a big margin.  Just because its larger, doesnt mean its better (grin).  There are plenty of posts on Eng-Tips dealing with pipe sizes and harmonics.

ktbundy (Automotive) (OP)
10 Dec 02 1:12
I think I may have come up with a solution to my hood clearance problems, but thought I would run it by here first. I have mocked up a custom plenum that positions the mixers off to the side. I have one mixer/vaporizor combo hanging off the drivers side, and the other on the passenger. I removed the original plenum and have replaced it with aluminum tube(square design much like the plenum). I have also mocked up a balance tube between the two. This gives much better hood clearance compared to the vertical set-up. My question now is, can I mount the throttle plates on top of the tunnel ram base, and bolt my new plenums on top of the throttle plates? This should work in theory, as the dual mixer/single throttle plate adapter creates a large plenum area before the throttle plate. With my intake there would be no bottle necking at the throttle plate though, as there are still two of them. Am I out in left field with this idea, or does it sound feasable? All I am concerned about is whether mounting the mixers this far away(about 5")from the throttle plates will cause a problem.
franzh (Automotive)
10 Dec 02 9:30
This ought to work ok.  My guess is that this engine will see primarily high rpm use, so any transition from idle to mid range is not particularly important.
You should insure that the plenum volume is the same for each side.
The plenum volume may help with additional homogization, something the tunnel ram with individual port runners will not have.

comptech (Computer)
9 Jan 03 12:12
i have recently converted my 86 chev 3/4 ton 4x4 350 engine over to propane and would like some information on tuning the ignition and mixer. i had build the engine with specs that i thought would be good for propane. block is bored 30over decked and all the other goodies camshaft is a comp cams extreme 4x4 designed for low-mid performance intake is edelbrock performer heads are world products 67cc chamber high compression heads with 202 160 valves. approx 10.5 -1 compression ration. im also running comp cams 1.6ratio roller rockers for increased lift headers and good dual exhaust. on gas this engine seemed very strong but now on propane it seems to be lacking. any suggestions or recommendations on tuning. thanks
franzh (Automotive)
13 Jan 03 16:44
A lot depends on what type of carburetion you are using.  This engine seems like its pretty warm.  I would recommend installing an oxygen sensor in the header collector to test for fuel mixtures.  I suspect you will find the problem there, assuming you have enough air-flow.

TaZMaNiaK (Automotive)
23 Dec 04 1:03
Hi, I stumbled across this thread through Google, and even though it's going on 2 years since the last post, I'll give it a bump and see what happens..

I am about to start building a Mopar 360, which will be put in an '89 Ramcharger 4x4. I will not be doing any sort of competitions for speed that I can forsee, except maybe drag with other trucks here and there, due to the fact that this era of Dodge trucks were designed with the aerodynamics of a cinderblock. Anyway, what I will need is low RPM power for occasional offroading, but this will mainly be my daily driver, and I'm looking to save some cash on gas in the longrun (since propane is still a lot cheaper than gas here), and do my part for the environment.  At this point, I am leaning heavily toward going with propane, but I am concerned with a few things.

First and foremost is the lower energy content of propane.  I will most likely be using an Edelbrock dual-plane 4bbl intake, because dual-planes are better for the low powerband.  Tunnel-rams and single plane intakes were mentioned above as better for propane power, but that was for high-rpm/horsepower applications - their low rpm power is almost non-existant.. How well will a dual-plane work?

The stock 360 uses I believe 8.5:1 compression. From what I've gathered from the net, propane works better with higher (10:1 or better) compression due to the higher octane rating.  What kind of gains can I expect going with a 10:1 over stock using propane?

Im not going to be using a radical cam, - just an RV cam made for power.. (don't have the specs here in front of me, but it's very close to stock)

I'm interested in a Closed Loop system, but I can't seem to find any real info on them.. MSD's website only shows their new Diesel Digital Propane systems, which wouldn't work with my application..  Can you point me in the right direction??

Thant's all I have for now.. Any advise would be appreciated. Thanks..

franzh (Automotive)
24 Dec 04 10:57
A regular dual plane manifold will work fine.  The 10:1 compression takes advantage of propanes higher octane and should provide a pretty solid foundation.  The MSD site also provides information on a closed loop engine controller, (www. but there are some others available around the world.  Each have advantages, but this is not the forum for manufacturer recommendations.

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

turkeyokato (Automotive)
29 Dec 04 21:19
Hello Franzh,
I have a turbocharged 265 CI Australian "Hemi 6cyl", running 10psi, straight LPG, fully mappable ECU with 6 coils off a Nissan RB30 ignitor, 9.2:1CR, 270deg 520/520 valve lift cam 114 centre, 1.64 2.02 valves in flowed head, individual runner manifold inlet and exhaust, intercooler,2000RPM stall auto, 3.23 diff, 1650KG vehicle weight.
What would be an ideal ignition curve?
Hope you can help.

franzh (Automotive)
29 Dec 04 21:32
That would be like recommending a favorite cut of steak as the only acceptable cut there is.
There are too many factors to consider here, more than what you plugged in, even though, I hesitate recommending a spark curve across the board.
Your ECU should have the capability of including a knock sensor which I recommend with what you have set up.  Propane will indeed detonate under the right (wrong) conditions, and with your engine setup, a slight almost or barely audible detonation may result in blown pistons or head gaskets.
Your ECU may have the capability of mapping via snapshot your knock sensor output based on inlet pressure, current timing, air temp, coolant temp, etc.  This is what the OEM's use.  A rolling road (chassis dyno) is invaluable here, otherwise, a good road trip is needed along with a good helper.  Either way, this engine combination sounds like a real runner.

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

turkeyokato (Automotive)
29 Dec 04 22:29
Thanks Franzh,
It was a fairly generic question I know but any base lines are welcome information as in New Zealand this kind of LPG application may as well be from another planet, and very few people can get their head around it (though the price of petrol may soon change that).
I currently run around 20BTDC at idle and 17BTDC at 10psi boost. Dont know if its right or wrong as I am on my own with this at home, but its a start.
It goes very well with massive torque and completley driveable. As you suggest, a decent day on a DECENT dyno will be invaluable. Again, its getting my knock sensor baselined correctly so the ECU can read whats actually happening.
Injection with LPG is next on the to do list. I replied to another thread about 120psi injectors and mentioned that I have replies from half a dozen companies that can supply kits to sequentially inject LPG up to 500HP. I can email the 3 pages of replies to anyone who is interested.
Thanks for your time.

patprimmer (Publican)
30 Dec 04 1:59

Please send me a copy to

If you send it, I promise never to cheer if ever we beat the all blacks again.

PS, we did build you that nice hemi 6



eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

turkeyokato (Automotive)
30 Dec 04 2:21
Hi Pat, Yes that engine is mine. Here are some links to some photos.;start=1&stop=20

My engine is the black one with stainless inlet runners.
Go to "whats new" then "Photographs" on the Hemi perf site for shots of the block.  
I will send the file when I next get a minute.
Did you win again? I have been away along time and not been up to speed.
Pine tree meads said it best about the current state of our teams with "there are too many mirrors in the changing rooms"
franzh (Automotive)
30 Dec 04 9:38
Of general interest, when retarding the ignition to prevent detonation, your exhaust temps climb thusly increasing boost, which increases inlet pressure until the wastegate pops.  Its a wild circle.
You may have luck playing with air to fuel ratios when at hard boost.  I have leaned the fuel mixture from L=1 to L=1.4 at 20 psig boost, reducing exhaust temps to 900 Deg F.
By the way, VERY VERY NICE engine!  Liquid injection has not caught on here in the states, primarily due to the injection pressures.  125 psig is about 1/4 the amount seen in a typical liquid injector (propane saturation pressure + engine absorbed heat + added heat from road and radiation)  I have seem pressures in excess of 400 psig!  Add in one Texas day of 90 Degrees, plus road absorbed heat, and your tank temperature can reach 125 degrees F.  That equates to 250 psig tank pressure.  Liquid injection works by circulating propane through the injectors to flush the partially vaporized propane and returns it back to the tank.  Usually, dual fuel pumps are used, one for maintaining a circulating pressure, the other for purging.  We then add about 50 psig above the saturation pressure to keep the propane in liquid phase (you may have seen that name in marketing material).
Its not as simple as you may think.  Supplying fuel for 500 bhp is no problem, but keeping it in liquid state is.  One glug of vapor at an injector and its 270 times leaner than it was.  Toss in that the fuel mapping program must operate at 270 times more precise/shorter pulse width profiles, and may injectors cannot operate that fast with an injector pressure that high.

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

turkeyokato (Automotive)
30 Dec 04 15:21
Thanks for the kind comments. Excellent points you make and fantastic claims they are making to me. I will pursue their claims and get to the bottom of it.
Maybe they have cracked it.
Im gonna find out and let you know.
Thanks very much for your time.
Have a great new year everyone.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close