Heavy towing

[C3H8]

I should have added one more statement about purging even though it seems ludicrous. Do it outside in a well vented area without any ignition sources. Just CMA.

[Dan350]

Thanks, that is good info. Now that i know how the recertification is done, i am better equipped to convince the propane mafia here to do a once over on my tank. As for purging, I worked at Costco for many years as a tire installer and part of my job was to purge and fill bbq tanks (hundreds daily) and we purged tanks exactly as you described with methanol and propane vapor. Since what you described very closely matches my experience, it should be easy for me to purge the autogas tank after it has been recertified.
Once my system in completely installed can i check for leaks using only vapor from the 20 pound tank i will be purging with? That way if i screw up the install i won't have to deal with liquid propane if and when i need to disassemble some components?

[C3H8]

That's a good idea. Just let the vapour pressure build up to a fairly high pressure and then spray the valves with a solution of soap and water or windshield washer fluid with dish soap added to it. WW fluid won't freeze during leak tests once liquid fills the lines. Another alternative is to pressurize the tank with compressed air, leak test it and then purge it.

[Dan350]

Hi, just thinking out loud here. How do you guys feel about bolting a 425 mixer/holley base plate to a magnacharger eaton style supercharger designed for a carbureted draw through setup for small block chevys? I think that this is the only roots type supercharger that can run with a dry fuel. With 355 cubic inches and 9:1 compression ratio and 5-6 pounds of boost starting near 2000 rpms, at what engine speed would the single 425 become too restrictive? I think big block torque from a small block. Any thoughts?

[storm]

Hi Dan
To work this out you need to work out the max revs Naturally aspirated and then divide that by this equation Pressure ratio = (boost pressure + atmospheric pressure) ÷ atmospheric pressure.

So a naturally aspirated 350 running at 85% VE should be able to rev to approximately 5500 rpm.
Now work through the pressure ration equation (6+14.7) / 14.7 = 1.408 (this indicates you are forcing about 40% more air into the engine).
So divide 5500 by 1.408 and you have 3906 rpm.

I think, personally, 3500rpm is a safe limit because we don't know the engines real natural aspiration VE. If it is higher the carb would need to be larger to give the same max revs.

[Dan350]

Thanks Storm, that is exactly what i thought. Also, since a model E is rated for about 325 hp and if I carefully match the cam, compression ratio and boost to be optimal under 3500 rpm give or take, and my engine produces 325 hp at say, 3000-3200 rpm, it should produce enough torque to satisfy my needs. 325 hp at 3200 rpm equates to 533 lb/ft at that rpm, if i acheive 325 hp at 3000 rpm, i would be producing 569 lb/ft at that same rpm. This is about 200 lb/ft more than i currently get out of my tired 350. My cruising speed is 2600 rpm when i'm hauling so i wouldn't need to downshift when passing or going up hills. Is there any other mixer that i can combine with 2 model Es that flow more than 460 cfm? Maybe from across the pond or from oz? Something that would top out at a higher rpm with 355 inches of displacement and 5-6 pounds of boost?

[storm]

I've heard good reports with regards to GRA and supercharging but I've also heard people bag them out as well. GRA throttle bodies come in various sizes allegedly up to 510cfm but I'm not convinced they can pull that much cfm. In theory you could fit 2 and flow 1020 cfm. One day when I get my flowbench built I'm going to flow test a few. I personally like the GRA, I have one on my 350 Holden engine and a smaller one on my 3800 Ecotec Commodore wagon. I'd like to get s supercharged 3800 V6 and put one on it. Having said all that I have no personal experience with supercharging a GRA system.

[Dan350]

That is very interesting. That GRA mixer has about the same adjustability as a holley gasoline carb where the idle,the primary and high speed circuits are adjusted separately. Also, being a side draft it solves a lot of packaging issues. Especially if two mixers are used to increase capacity. One thing is not clear to me; can model E's be used with these? Or do they come with their own convertors? They are a little pricey though but at least the aussie dollar is at par with our loonie...one more question how reliable has your system been so far? Performance wise how do you compare it to the typical impco 425/model E setup?

[storm]

GRA uses a specially modified (different spring pressures) Aussie B2 (which is a copy of the OHG unit). You can use an Impco E but you need to change the spring.

I have found the GRA to be quite reliable, My 350 Holden engine is a little down on power but the GRA is more restrictive compared to the EFI throttle body due to the fuel feed pipe at the opening of the throttle body. It's difficult to compare the GRA in my case to a 425 because the manifold would need to be changed so automatically you would be installing a performance intake for a carb rather than an OEM (restrictive) intake for EFI. Having said that I think using comparably a sized GRA the 425 and GRA would be similar in outright performance but for cruise economy the GRA would be better.