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| Re: Boost vs feul pressure |
Subject: Re: Boost vs feul pressure
by jmac on 2009/1/19 6:21:23
It's gotta track with boost. so it's always 2-3psi + boost pressure to be safe. Holleys in general can handle as much as 7-8psi static without trouble, so 7-8+boost pressure is the go. Other carbs - most of the sidedraughts and most downdraught webers will _not_ cope with that - 2.5-3.5psi should be about it - i.e. 2.5-3.5psi plus boost pressure. The reason for that is that above 3.5psi many of them aren't able to successfully close the needle and seat (it's basically due to the leverage of the float/link arm and how it acts on the needle and seat) - so it will over run the needle and seat and flood the carb as the fuel bowl overfills.
Another thing you'll find with most sidedraughts and downdraught webers (to a less extent) is that they'll tend to lean out under boost (even with adequate fuel pressure) - due to the way their fuel and air corrector/bleed/emulsion circuits work. you _can_ definitely work around this and get the fuelling right under boost, but they (at least sometimes) then are 'off' the optimal a/f curve when off boost.
There is a simple way around this - you run a restrictor ring at the mouth of the carb. It has to be just a few mm smaller than the mouth of the carb (still likely larger than the venturi/chokes for the webers) . for whatever reason, this allows the emulsion circuits etc work a lot better. It basically doesn't affect them much at all for normal driving, but once more rpm and boost is on board, and flow starts to really increase this restrictor ring will cause slightly less pressure through the mouth of the carb, which pushes more air pressure (relatively) to the fuel bowl, which then automatically enrichens (above and beyond the carb circuits ability from beforehand) under higher boost/flow.
Such a phenomenon isn't a big secret. If you lok at the mg metro turbo setup (basically a mini engined car that looked like a ford laser, which ended up with a turbo to account for hte fact it was in a larger car than a mini, and had a mini (i..e 1959 tech) engine powering it. The carb 'hat' or plenum has a sleeve inside it that works as a restrictor ring. Take it out, and it'll technically flow a little more, but fuelling under boost is way way way off, and generally not recoverable. Similarly, there was a turbo kit available (again for the mini) from Janspeed that used a dellorto (as far as I can remember) in a blowthrough setup. It too ran a restrictor - in the shape of two rings placed at the carb mouth.
Yet further on minis, a long time contributer to the mini-list mailing list had an mg metro turbo setup in a mini, then went to a dellorto,
found it on google
and tried extensively to get around the restrictor ring option, but nothing worked (and he tried all emulsion tubes and air correctors etc under the sun). There was a fair bit more detail on this on the min-list and the old blowthruturbol list at the time than is on the website but the main important stuff is there.
This _is_ a lesser issue on the smaller dgv style webers (pretty much all downdraughts save for the ida/idf stuff)
I'm adding all this because even _if_ you get the fuel pressure/supply into the carb right, you might still run into issues of it leaning out, or being right for boost but terrible off boost. If you manage to source some of the webers that had been used in factory blowthrough (and the carb hat if they had them) it'd be a better start.
With a holley 2 barrel, you can address the phenomenon almost solely by going to high speed air bleeds that are around 1/2 the area (area not diameter) of std for that particular carb
Only other thing to remember - on some oddball carbs - under boost in a blowthrough for fuel to flow you need to route full boost (from before th carb mouth) to the fuel bowls. This usually isn't a problem at all, because the fuel bowl vent is routed into the carb main body well inside the air cleaner base attachment point, so it automatically gets full pressure. then air travels through the carb, through hte venturi, and it sees a slight pressure drop (so let's say you have 14psi boost pre carb, by the time it goes through the venturi it might be about 12psi - so there's a 2psi difference between air above the fuel in the fuel bowl and the fuel introduction point in the carb throat, and fuel will flow. On those oddball carbs where the fuel bowl vent is to somewhere else, as soon as you get boost in the carb throat, there's none above the fuel in the fuel bowl, so no fuel will flow (in fact air will be pushed backward through the fuel circuit and into the bowl through the jets etc.) That's one of the 'sneaky' reasons the engine can die flat on its face when you get any boost pressure, even though fuel supply/pressure is perfectly adequate.
Lastly - on a supercharged blowthrough, when you close the the throttle or even at part throttle, the charger is trying to push heaps of air through there but the throttle won't let it - as a resuly you get stuff all pressure (or partial vacuum) in the inlet manifold but possibly 10-20psi or even more above the throttle plates. Apart from causing more heat, and possibly blowing a hose, it'll put massive air pressure above the fuel in the fuel bowl and push significant amounts of fuel through the idle and transition circuit and cause flooding/stall soon enough. In that scenario you need to have a bypass valve plumbed in there so that at part throttle there is somewhere to dump excess air from the supercharger when it's still sspining and the negine is at part throttle. On a blowthrough turbo, you can usually get away with it, as it only spikes a little when you lift off after a full boost run. The turbo soon enough spools down and the pressure pre throttle drops back to atmospheric, and it's 'all good' . you can get these bypass valves through places that stock stuff like 'pro-chargers' which are made for blowthrough carb, or you can diy one, or you can even rig up a turbo sourced blowoff valve.
I'm just trying to cover all possible bases as far as things that might still be playing a part and might make it 'appear' that something else is wrong or that some other change wasn't also warranted.
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