2.5L Turbo -> 425 questions #1

[Melvin]

I've mentioned my project in passing but it's getting down to go time and I have a couple questions if anyone can help.

The project is an 89 Voyager SWB minivan with the factory 2.5L turbo and A413 3spd transmission. These vans are capable of mid 12 seconds with essentially stock internals at upwards of 30psi of boost. More here, here, and the guy that started it all. I'm not sure I'm going to push her that high but stock boost is a stepped 7&10lbs and and I think I'd like to double that eventually.

The van is a learning project, the eventual goal is to learn enough to turbo a 2.5L Dakota 4X4 and run it on propane. I would have jumped right into the truck but I didn't know squat about turbos and I though it would be best to learn how to run a factory turbo on propane before jumping into a turbo + LPG conversion. I'll be updating the your car section as I go once I've got enough pictures for a good first post.

Anyways to the questions. The plan so far is to use a impco 425 to feed fuel. The mixer I'm using was mounted on a 454 (so I was told) and has the pictured adapters mounted on the bottom. The plate pointed to by the blue arrow (a four barrell throttle plate) must and does obviously come off. It's the plate it attaches to that I'm wondering about. Is the plate pointed to by the green arrow a required part of a 425? I'd like to replace it with a piece of tubing 3-4" in diameter, either straight or a long 90 degree, welded to an appropriate flange.



The van will eventually have a medium size intercooler. I thought the best mounting position would have been directly in front of the throttle body; however franz indicated else where that the 425 will only handle 5psi of boost in blow through configuration. Is there anything to be concerned with in operating in draw through operation? As an alternative has anyone just completely enclosed an 425 the way that used to be done with carbs in blow through so that internal and external pressures are the same?

[sleepybu]

that's the adaptor(sp) to the throttle plate and is not needed for what your doing

[C3H8]

Your idea of enclosing the 425 completely should be valid. I belive Franz's concern with over 5 PSI of boost would be the pressure exerted on the idle diaphragm circuit. If you enclose the mixer it should provide equal pressures all around. You would need a pretty strong structure to contain 10 to 15 PSI though.

[franz]

If you are pressurizing a mixer for a 2.4 engine, you might try the 200 instead. I have run more of them successfully at higher pressures than the 425, and on your engine, the 200 would be more drivable at low speeds too. The 425 is designed for engines in the 5.7 range and larger. I dont think the 425 would work well enough at low speeds to be usable.
Franz

[michael]

Your idea of enclosing the 425 completely should be valid. I belive Franz's concern with over 5 PSI of boost would be the pressure exerted on the idle diaphragm circuit. If you enclose the mixer it should provide equal pressures all around. You would need a pretty strong structure to contain 10 to 15 PSI though.

can you boost reference that diaphram?

[pfer10]

can you boost reference that diaphram?

You could but I think you would have to rig up some kind of check valve / diverter so while under vacuum the diaphram would see atmosphere again?

[pfer10]

.... you might try the 200 instead. I have run more of them successfully at higher pressures than the 425, ...

Franz how high of a pressure have you put to the 200 before?

[franz]

The 200 diaphragm is already referenced to pressure, just at this point, its atmosphere pressure minus the water column pressure created by the air valve. As long as the air inlet and vaporizer vent are at the same pressure, it makes no difference what pressure, the limiting factor is where mechanican components begin to leak. Whereas the 425 is limited to a single 1/4" hold down screw for the bonnet, the 200 is not. Normally aspirated, a 200 can feed up to a 4.6 engine with few problems and more than adequately a 2.4 under high performance. Your limiting factor is the vaporizer, a single E is up to about 350 bhp.

I boosted it to 20 psig before, but I did have to change the lid to a HD iron unit from a 200D, the aluminum one leaked.

[Melvin]

I've never even seen a 200 before, they look nice and compact though. Anyone got one they'd like to trade for a 425?

Poor idle was a concern but I couldn't see anyway around using a 425 and still feeding 275hp. I was thinking of staging a pair of 225s but I couldn't figure out how the heck to manage that. The 425 seemed the simplest and just deal with poor idle like one would with a high lift, large lap cam. Not that it should make any difference but I'm running the 2.5 not 2.4.

You would need a pretty strong structure to contain 10 to 15 PSI though.

Guys run aluminized exhust pipe for intercooler piping up to 30 psi. The hard part is keeping the couplers attached. I was thinking a person could just get a piece of pipe large enough to stick a 425 in and cap it at each end. The only tricky part would be hooking up the gas input but I've got some ideas in that direction using a short nipple and a couple pipe flanges.

[pfer10]

I am in the same boat as you. I have a 425 also but only have 1.9 liters. I just got a 200T off ebay. Keep your eyes open. www.propaneguy.com has 225 for ~ $125 or so and they have a major rebuild. The 200 has the hat over the top as Fran mentions. Maybe he has one?

[franz]

No parts, sorry. PropaneGuy has pretty good parts selection and prices too. With the 200, AF ratios are real easy to change too. The top cover is integrated with the mixer body. The air inlet has a smooth inlet and keeping couplings intact at high boost is indeed a real problem.

Remember, that when an engine is boosted, the air density is increased which makes a small mixer sort of act like a big one when boosted, but a small one NA which helps with idle and starting. If the cam is big with lots of overlap, the idle quality will be lousy no matter what.

Franz

[pfer10]

Franz I understand how boosting a petro carb makes it act like a much bigger carb. The turbo / carb industry always said you you couldn't boost past 5 psi blow through.

I just had a question about propane carbs. Up to a point boost should work fine but once the gas valve reaches it's max travel how does more gas continue to flow? To me it seems the mixture would start to lean as you continue to make the air more dense. Maybe I don't understand the operation of propane carbs just yet.

Would propane carbs act the same in draw through as blow through.? I notice Impco say 217 hp max for a 200. Can I throw that number at the window when using a turbo? Does draw or blow make a difference in that number?

Thanks for your input.

PF

[Melvin]

I just had a question about propane carbs. Up to a point boost should work fine but once the gas valve reaches it's max travel how does more gas continue to flow? To me it seems the mixture would start to lean as you continue to make the air more dense.

I think I can answer this, someone please correct me if I'm wrong. Your evaporator is boost referenced. Therefor the gasous propane is entering the mixer at a higher pressure and a greater quanity is delivered.

[Melvin]

No parts, sorry. PropaneGuy has pretty good parts selection and prices too. With the 200, AF ratios are real easy to change too. The top cover is integrated with the mixer body. The air inlet has a smooth inlet and keeping couplings intact at high boost is indeed a real problem.

Remember, that when an engine is boosted, the air density is increased which makes a small mixer sort of act like a big one when boosted, but a small one NA which helps with idle and starting. If the cam is big with lots of overlap, the idle quality will be lousy no matter what.

Franz

I'm guessing this means you could get away with a 225 in my application but it would still suffer from the single screw holding things together problem.

[sleepybu]

use a studd