AirGap intake mods for Propane fuel?

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AirGap intake mods for Propane fuel?

Post by bigblockmopar » Sat Apr 28, 2018 6:01 pm

Wondering if there would be any benefit modifying an AirGap intake manifold, like trimming the divider wall with use of propane/LPG fuel to create a larger/open plenum. RPM range of interest is low/mid range on a 360ci Mopar smallblock engine.

The general consensus is that propane is also happy with a single plane intake.
Using an IMPCO 425 carb/mixer and Holley-style 4-bbl throttle plate. The IMPCO carb is basicly an 'open' tube-style mixer.
Mixture coming from the carb is 'divided' through the 2 or 4 throttle blades and then enters the dual plane intake.

I personally doubt there will be any benefit if the intake divider is partly open again below the throttle plate, but maybe someone with more experience in this field could perhaps shed some light on this.

Underside of IMPCO 425 carb
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Re: AirGap intake mods for Propane fuel?

Post by MadBill » Sun Apr 29, 2018 5:00 pm

Assuming the mixer has a fairly limited flow capacity, e.g. 500 CFM (be nice to test one), it should be subject to the same effects as a liquid carburetor, namely that the greater flow pulsations in each bore resulting from a divided manifold will limit the average mass flow at higher RPMs. That said, for low to mid range the potential high RPM flow increase would be irrelevant so removing the divider would probably not be effective.
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Re: AirGap intake mods for Propane fuel?

Post by Scotthatch » Sun Apr 29, 2018 5:11 pm

The one I dealt with years ago did not even come close to 500 cfm I think it was like 380 so I hope that one is bigger ... I'm with the single plane group ... a small runner single plane is better then a duel plane....

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Re: AirGap intake mods for Propane fuel?

Post by bigblockmopar » Sun Apr 29, 2018 6:41 pm

The mixer is rated at 450-475 cfm I recall. On average they can only support up to 300hp, hence their popularity on 'smaller' street engines. They are pretty much 'done' at 5000 rpm.
If one needs more power, you have to add more carbs (and vaporizers).

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Re: AirGap intake mods for Propane fuel?

Post by cjperformance » Sun Apr 29, 2018 6:48 pm

The cut divider is kind of pointless with the impco 425 mixer as it is such a restriction, it flows enough fuel/air mix to achieve around 320hp N/A and that is about the limit. I have used them on 6cyl, 8cyl all of varying sizes and the end hp result ceiling is always close to the same.
Unless you have a small ci mild dual plane engine that is maxxed out for power below 300hp the divider will gain nothing. It will however alter partial throttle characteristics so try open spacers before you cut your intake.
You need the right plenum volume as the vapour fuel takes up a lot of 'space' in the air fuel mix compares to a mainly liquud fuel. So dual plane with a big plenum (pipemax plenum sizing works well) and also single plane on mild engine works well due to the air displaced by fuel the plenum, runners and ports need to flow more volume of gas (air and gas*vapour*fuel) for a given rpm/hp than using a liquid fuel so the typical boggy low end of a single plane on a small/mild engine is not seen.
If you want to make good hp N/A with the 425 mixer then you simply need to use 2 of them.
They work on a constant depression value so it is almost impossible to over carb an engine with them.
20171222_160932-800x600.jpg
308 ci engine running 2x 425. Previously with a single 425 and milder 22*@.050" cam and sinlge plane it was good but not great. Now 230@.050 very 'minor' head work its has come alive.
I used to modify Offy 360* Equaflow intakes to take 2x 4barrel LPG carbs. Those intakes suck wirh a single 4 barrel on regular fuel and suck as a 2x4 bl on regular fuel but 2x4bl vapour fuel they are like a whole different intake.
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Re: AirGap intake mods for Propane fuel?

Post by bigblockmopar » Sun Apr 29, 2018 7:41 pm

Thanks for the info.
I daily drive on LPG with an IMPCO 425 powered 8.6:1cr stockcammed 318ci Mopar SB engine, and will swap the engine one of these days for a freshly built 11.3:1cr 360ci with a XE256H cam w/solid lifters, lightly worked '302' heads, 1.6 rockers and the AirGap intake.

While putting it all together I started thinking about the divider wall, which btw was cast lousy and shifted so much that one of the throttle blades hit it when opening the throttle.

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Re: AirGap intake mods for Propane fuel?

Post by cjperformance » Sun Apr 29, 2018 8:15 pm

11.3 with that cam! It will be ok unloaded but hot weather once the coolant gets even up to 175+f and with any sustained load it will ping. You will have the timing backed down to 15/18* loaded to control it and it will just be a fuel guzzler. I have learned that the hard way and fixed up CR combos for DIY customers who would not listen or some who have bought engines from other builders.
They spend a fortune on dyno time, insane cooling system mods etc etc and still, 34*C+ day and same story, so they richen it up thinking its lean and they burn stuff, headers glow red hot, melted carpet even in the rear floor above the mufflers! Coolant blows out, burned ex valves and seats. Its not fun.

Lpg/vapour fuel has no ability to absorb heat like a liquid fuel so chamber temps go thru the roof with high CR vapour fuel application. And the richer you run it the hotter the chamber gets, also leaner equals cooler with lpg or propane. Forget its octane rating, you simply dont have a fuel related temp control other than leaning it out to reduce chamber temp at which point you lose power too. You need to use CR to control chamber temp at the correct fuel mixture. Its quite different to playing with a high CR liquid fuel engine.
Personally, wait for the canning haha. Without knowing more about the combo or use i would start my thinking around 9.7 - 10:1 and may raise or lower from there. I have done a lot of tow LPG engines and Performance LPG including Boosted and n2o over the last 25/28 years so i have some experience there.

Air gap intake, cold air induction, coolant below 160f and at 11.3 you will be ok unloaded in the cool weather.
All that aside. This ci with a small cam i would just clearance your divider so you can move the throttle properly but leave it divided for now and if you have room try a 1/2" or bigger open spacer once you have it running. See what suits your driving best.its still going to restrict top end hp potential but will daily drive great.
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Re: AirGap intake mods for Propane fuel?

Post by bigblockmopar » Mon Apr 30, 2018 5:40 am

I realise the cam is building pressure quickly and everything needs to stay cool for it all to keep working. If I'm to change the cam I will use a roller next time.
Got a MegaSquirt ECU controlling ignition so I can have it pull timing whenever and wherever needed to in the rpm, temp and load-range.
The current engine has max. advance at 32° where it made most power. It idles with 24° advance @ 550rpm.
Will add propane port injection (vapour) later on to the 360 engine.

I'm aware not to richen up the mixture to combat heat. The climate is fairly mild here. We luckily only have a small number of 30°C days here, with 32°C the max usually.

A 160F stat is in the engine already and cold air intake is present in the car. Also has coated headers, thermo-viscous fan and a good shroud.
Eventhough the viscous-fan is freewheeling currently, the existing 318 engine has a 180F stat but hardly gets over 150-160F with normal use. There's also a low front spoiler on the car to help draw out (hot) air from the engine bay.

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Re: AirGap intake mods for Propane fuel?

Post by joe 90 » Mon Apr 30, 2018 7:34 am

When using a carburettor and petrol , you've got all sorts of limitations.......like the intake manifold.
Once you switch to gas as a fuel, gas (air) and gas mix really well, not like gas and liquid.

The best solution is to use propane with EFI. Those impco type mixers are last century's technology, they're a big restriction themselves.

Modifying the manifold while still using a propane mixer is probably a waste of effort, you won't notice anything.

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Re: AirGap intake mods for Propane fuel?

Post by cjperformance » Tue May 01, 2018 9:13 pm

I wouldnt waste money on vapour injection personally. The best solution is Liquid LPG/Propane injection.
Then you get the benefit of huge charge cooling ability so you can easily run a CR compatable to octane.
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Re: AirGap intake mods for Propane fuel?

Post by bigblockmopar » Wed May 02, 2018 3:58 am

I've installed a 4-hole 1" spacer once on the Performer intake which had a noticable effect on throttle response and mid-range, similar to what I've found on an gasoline engine. Hood clearance will probably prevent adding spacers on the taller AirGap intake.


LPI would have been perfect indeed, but that would also mean having to replace pretty much the entire fuel system in the car. That's too involved at the moment.
It would also add the fuel's temperature as an important tuning factor into the mix.

Along with the single stage vaporizer(s) needed for vapour injection, I plan on keeping the current vaporizer as a fall back system while I tune the MegaSquirt for vapour injection.
I'm aware that newer (fuel injected) cars have their original gasoline fuelsystem still operational for the cold starts, and switch to LPG when there's some heat in the coolant. This is to prevent still frozen LPG droplets to enter the engine at too high demands when cold.

The gasoline system in the car has been gone for awhile so I think I can have the engine cold start on the current 'old' dualstage vaporizer and have MegaSquirt automaticly switch over to vapour injection when the coolant has reached a certain temperature.

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Re: AirGap intake mods for Propane fuel?

Post by Leftcoaster » Wed May 02, 2018 4:36 am

LPG specifications can vary dramatically, especially between countries

Mods which work well in Oz for instance, may be troublesome with your local supply

Decades ago Western Canadian provinces pushed lpg retrofits with rebates etc - - then, realising they had decreased their tax base raised lpg prices - - sales volumes dropped, lpg providers lowered product standards, and many who had committed to lpg suffered

Caveat emptor :?

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Re: AirGap intake mods for Propane fuel?

Post by bigblockmopar » Sun May 06, 2018 5:28 pm

Had my first long drive today with the engine, and just so happened it was a warm day here (25°C / 77°F), so this was also a good test for pinging but I didn't notice any. But I didn't really let the engine work hard yet. Some short 3/4 throttle sprints at most.
As a precaution I took some 4° out of the max. timing, topping at 28° advance @ 2800rpm (compared to 32° with the previous low cr 318 engine).
Maybe that AirGap intake with its blocked heatriser works nicely after all at keeping the charge cool :wink:

Still dealing with some idle issues however where the vacuum needle is fluctuating rapidly between 19 and 21". As I converted the hydraulic lifters to solids (on a hydraulic CompCams XE256 cam) so I suspected a too tight lash eventhough I reset the lash twice on the engine stand at .003". Rechecked the valves today while it's quite a bit warmer. Found some a tiny bit too tight but even after relashing them to .004-5" the issue still remained. Maybe I should up the lash to .006"?
When the engine is warmed up to 160-180°F the vacuum fluctuation at idle widens to 18-22". The needle smoothes out at higher revs.
Steel heads and block. Lash should grow with engine temp imo. Plan to check lash with a hot engine to make sure and then do a compression test.

Heads were rebuilt with bronze guides, seats refreshed, new exhaust valves. Backcut intake valves, 1.6:1 rockers.
Before installating the heads, I 'leak' tested them by spraying some brakecleaner in all the ports and found it took some 20 seconds or more before just a very thin wet line appeared on the chamber side at the valve heads.

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Re: AirGap intake mods for Propane fuel?

Post by Circlotron » Sun May 06, 2018 8:00 pm

cjperformance wrote:
Tue May 01, 2018 9:13 pm
I wouldnt waste money on vapour injection personally. The best solution is Liquid LPG/Propane injection.
Then you get the benefit of huge charge cooling ability so you can easily run a CR compatable to octane.
Got my first LPG fuelled car going on April 26th 1978, just gone 40 years ago. :shock: Was an LC with a 192. Anyway, back then you could run 12:1 CR with a standard cam and it would never ping no matter what I did. Must have been pure propane back in those days. 3 years later had a Pontiac with a worn out stock 283 and on hot days it would ping. LPG appeared to be going downhill from that time on. I read that they started putting propylene in it among other things. Floor sweepings that would previously have been flared off at the refinery. Nowadays I think of LPG as the meat pie of fuels; you never know what’s in it from one day to the next. The expensive stuff you get for bbqs might be only propane so it doesn’t stratify into separate components when sitting still for a long time, but the stuff you buy at the bowser is anybody’s guess.

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Re: AirGap intake mods for Propane fuel?

Post by cjperformance » Sun May 06, 2018 10:00 pm

Yes the more you can do to keep intake temp down the better. The Airgap type intakes definetly help. Insulate it from the engine as much as possible as once its heat soaked and up to engine temp its no longer such an advantage. You are lucky to be able to call 25C a warm day !
Steel/iron heads and block, get it as hot as you can then check clearances. With iron heads and block it is quite possible to have the valve length grow more than the block/heads will !!

Circlotron- yes now we get all sorts of lovely stuff in our lpg, i think last i heard Propane, Butane, Propanol, Butanol, various light ends, and ethyl mercaptant for odour. Cant remember levels of each.
Craig.

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