Jas,s 1200s

[dirtyleppa]

mate of mine did a blow through setup on his old mitsi lancer and it worked fairly well. he kept boost down to about 8 psi and made a small box on top of the carb like vaughan had suggested. the only problem he had was lack of fuel that caused it to die under full load. solution to that was to fit a secondary electric fuel pump hooked up to a throttle switch to pump extra fuel into the bowl at full throttle. worked perfectly.

but in saying that, draw through would be easier and if you kept the boost levels low or fitted a water injection setup, you can get away with no intercooler.

barts one runs no intercooler and no problems dealing with the boost created. and lewis' one is draw through to

[classicdat]

Next problem I see is that as the fuel level drops and the needle valve opens if you have a higher pressure in the float chamber than the fuel line you will blow air back down the line rather than fuel out into the bowl, lol. Will need a higher pressure pump with a pressure regulator that is boost sensitive. I think the easiest way to go would be put the carb in a box and pressurise the lot, kind of just increasing atmospheric pressure, sounds like it should work. You would need linkages that were air tight and would still need a fuel pressure reg. could work though, search the web.

[jas1200]

Sounds like this is going to be a big learing curve but i,m up for it........... There was a guy on 1200.com, he had a 350 holley with a top hat set up for blow through for sale....was going to buy it but he change his mind, it was all jetted and ready to go....it would have been a better start......Jas

[Bartman]

Was that Steveo Jase? I know he runs a holley on an A15 in a datto 1K.
For the work it's going to take to make the hitachi carb work, you'd be loads better off to change to a different carb. The fuel will be spewing out the accelerator pump boot because the bowl is vented into the venturi. Therefore when you pressurise the venturi, the bowl is pressurised too. And the boot on the accelerator pump plunger isn't that good of a seal. And when air leaks out the top of the bowl, you also run thye risk of having lower pressure in the bowl, than in the throat of the carb. This can cause boost to push back through the fuel delivery point in the air horn, and blow back into the bowl. Then no gas will be able to be drawn into the carb throat. (Motor no go!)
The other critical things that a blowthrough set up needs is the carb MUST have non brass floats. (Nitrophyl? sp?) Otherwise when the bowl is pressurised by boost, the float can collapse. The it'll sink like a stone, and fuel will flod your engine.
You're also going to need a Malpassi rising rate fuel pressure reg. A low pressure one. (1-15psi I think?)


You'll also need a fuel pump which can supply at least 3psi (give or take) more that your planned maxiumim boost level.
Yuu plumb up the RRFPR and set it up so that it's supplying around 3psi under no boost. Then, as it has a boost signal it'll raise the fuel pressure accordingly so that you alwaays have more fuel pressure than you do boost. Otherwise when you're on boost, the boost pressure will blow the fuel along the fuel line, and dback into your tank!
And Going by the above picture, it also looks like you're going to need a fuel return line, as the RRFPR's don't reduce pressure by dead-heading the fuel supply from the tank, so much as by diverting flow back to the tank.
I'd reccommend having a 3/8 inch pickup line fitted to your fuel tank, and then running a new 3/8 steel supply line to the FPR. The you might be able to get away with using the original 1/4 line as the retrun line.

The beauty of the holley is that boost hats are readily availabale for them. And they only have two jets to change. (I'm sure Steveo would tell you what jets he's using if you ask him nicely)
There is one other consideration too. Depending on how worn the throttle shaft on your chosen carb is, you may need to supply a clean boost supply (Unfueled) to the butterfly spindles. Otherwise fuel may leak out the ends of the throttle shafts when you're on boost. (I'd rather just lend you a book about it, rather than try explain that on here now.

I've heard of guys run the 32/36 weber in blowthrough set ups. But at least one of them (1200rallycar@1200dotcom) go the screamers with the bowl gaskets blowing out all the time.
People have successfuly blownthrough SU carbs too. Although to save yourself massive heartache, it'd be easiest (although probably not cheapest!) to track down a factory blowthrough SU off a mini metro.

Drawthrough is starting to sound pretty good aye!
Let me know if you wanna borrow a book about it.

Edit,
I dunno if this is a good price or not, but palmside sell them here http://www.palmside.co.nz/product_pcid_15786.html" onclick="window.open(this.href);return false;

[jas1200]

The guy that had the blow through Holley goes by the user name 'greendog' on 1200.com, he lives in Austraila.
Hey thanks for all the info......had an idea today and made some mods to a spare carb I have in parts. What I,ve done is made an external air feed by drilling and taping a small air feed into the top of the bowl and blocked off the feed from the venturi to the bowl..........this should stop all the fuel blowing out the accerater pump. I,ll fit it on Wednesday and see what diff that makes...

[Bartman]

Do you already have a rrfpr, and larger fuel pump?

[jas1200]

I,m using an electric fuel pump but have no idea what pressure it will run to. No I hav,nt got a rrfpr. What do you think about the idea of the external air to bowl feed??

[Bartman]

I think your accelerator pump boot seal will inflate like a balloon. And If your boost exceeds the fuel pressure, the boost will push the gas back into the tank. 6psi boost pressure will overwelm 5psi of fuel pressure. And once all the gas has been used, (Wont take that long when you're at WOT on Boost) the car will die. What fuel pump do you have? You must have more fuel pressure than boost. And you probably cant run that much pressure full time, as the needle and seat in the carb can probably only stand around 3psi. Hence why the rising rate regulator. I don't hold much hope for the hitachi unless the whole carb is boxed. The accelerator pump bootie was never designed to hold in boost.

[jas1200]

Wish greendog would just sell me his 350 holley and top hat, its been running on a turbo a14 and is all ready to go. I looked through the Holley site and did,nt find any top hats, did you say they were easy to come by.
I,m not sure what fuel bump I have, a friend of mine fitted it years ago, I works really well but a dont know what pressure it,ll run or what boost it could keep up with.....
So the pressure in the bowl will need to match the boost presure in the carb, if thats right then my mod I did today wont work.......

[Bartman]

The other problem you'll have is you're going to need a monster blow-off valve, and/or a pedal triggered swith to control your blower clutch. Otherwise when you lift off, the blower is going to keep on pumping air into your carb. 1.4litres of air per rev (Approx) going into your carb when the holes are closed is going to see huge pressure. It'll blow your bowl gasket, probably pop out your accelerator pump boot. And most likely pop off your intercooler piping too. So long as the intake of the blower is open, it'll keep pummping air. A WOT switch that shuts the blower off when you don't have the hammer down would be a a must. Blowthrough turbo carb is much easier than blowthrough supercharger. Infact, at less than wide open throttle, there's nothing to meter how much air is feeding into the charger. So you could have the throttle at say 50%, but given the pump will push the same amount of air, you could see stupidly high boost into your partially closed carb. It's a bit of a curly one. I think you'd have to make it wide open throttle, or nothing.