Superchargeing a Carby motor, 4k, blow thorugh setup

[Sherlock]

Ok

I'm trying to work out how to setup an sc12 on my 4k, Blow through setup, useing something like a weber carby, but the questions are not about fuel setup

i have several questions some might be abit confuseing so bear with me

1. Would it be possible to have a switch that will limit the boost level, much like the ecu does on a 4agze, but maybe setup with intake manifold vacuum connected to a relay connected to the clutch of the charger, just the question is there a relay that uses vacuum to tell when something should be switched on/off

2. if the supercharger is disengaged, then how will the carby get airflow, is there a gate or something that can be setup to open to allow air when the clutch is disengaged on the charger, i noticed on the outlet of the charger there is a smaller second outlet attached to the main one. if this could be open and closed when disengaged and engaged respectivly then this would releive pressure at high RPM where boost would be limited by the answer to question 1.

cheers
Alex.

[petergoudie]

Do a google search and look for the 10 year old jaguar supercharging set up.

The issue with a blow through set up is you need the carb at the same pressure as the boost otherwise you blow fuel everywhere. Old systems had the carb in a sealed box at the same pressure as the blower.

[petergoudie]

Do a google search and look for the 10 year old jaguar supercharging set up.

The issue with a blow through set up is you need the carb at the same pressure as the boost otherwise you blow fuel everywhere. Old systems had the carb in a sealed box at the same pressure as the blower.

[Sherlock]

by carb at the same pressure as boost are you talking about the fuel pressure the same as boost

because as far as i have looked into it
say standard fuel pressure is about 4 psi
i need 4psi plus whatever boost is at,
can be done with a riseing rate regulator, which i plan on useing

so at 5 psi, fuel pressure will be 9 psi, to keep it form being blown back down the line

[petergoudie]

I think you will find there is more to it as carbs also have all sorts of compensation ports and tubes etc to make them work properly throughout the rev range.

When blowers (turbos or superchargers) had a resurgence in the 70's but fuel injection wasn't so popular they experiencedd the same problem. I think you will find the Bentley Mulsane had it's SU carbs in a sealed box. I'm guessing the pressure was whatever the boost pressure was and yes I think you are correct, the fuel pump pressure should be compensated accordingly.

A good thing however is that the Mercedes racing cars of the 30's were set up as you plan and apparently they have a different sound which is quite distinct as there is no muffling given by the carb as it is behind the supercharger. What you hear is the growl of the supercharger lobes.

[oldcorollas]

rising rate increases at faster than 1:1.
maybe you mean boost referenced regulator?

as well as the fuel pressure being increased, the fuel bowl of the carby needs to be pressurised, since it is the pressure drop between the carby throats and the fuel bowl that squirts in fuel.
the higher line pressure is needed to get fuel into the pressurised fuel bowl. also need float that can withstand pressure..

back to your questions
if you mean limit boost as in choose what boost to run. not simply. i am not sure of the 4A setup, but i thought it turned off when throttle was less than 25% and rom was less than X000rpm (but kept it on for a couple of seconds after throttle lift, to help with gear changes)

you could have a pressure switch to determine when throttle is closed, to turn off the SC.. but by that time, the pressurised air hitting the closed ccarby butterfly migth blow the carby apart , even if you have BOV.
on 4A it is not issue because fuel is injected in the manifold...

having throttle before the SC helps in that you are controllign the total air in the system.. not just letting in full pressure air... otherwise you will have pumping losses all the time when you are not WOT, and need to vent the air by maybe a BOV or similar...

using a 4KE or 7KE manifold with injectors and a simple computer system would be so much better. and you can dial in timing retard.

not sure on the internal SC bypass, but even if the SC spins a but when not spun by motor, it is better to have bypass valve. using bypass valve also as a boost control valve might be a bit tricky..

a good thing to do is sit down and write out all the different situations that can occur..
ie, idle, mid and high rpm
closed throttle, mid throttle, WOT, and also closed throttle after WOT..

then work out what you want the air to do for each situation, and then work out how to control it that way...

[TERRA Operative]

Why don't you run the supercharger on all the time? At idle you won't produce boost, but it'll allow the air past.
There are fixed pulleys of various sizes you can get, or weld up the one on the charger.

The pipe you mention is for the ABV (air bypass valve) on the stock setup. basically it opens on idle to allow air to bypass the charger (when the charger is off). On overboost it also opens to bleed the excess pressure, but most people defeat this feature to enable greater boost.

On a stock pulley, you should see about 10-12psi max at redline (I get 10-11 on my AE101 GZE) with all the bypasses disabled ect. You can't overboost like a turbo as the boost is tied directly to RPM. I wouldn't bother with the boost control and just make sure the engine can handle the boost that will be produced with your pulley setup.


The SC on the stock setup is activated on WOT or at 8 inches vacuum, and is delayed off for the gear changes. There's no real reason not to run it all the time though, except maybe slightly increased wear.

[oldcorollas]

do the GZE have the TB before the SC?

with carby after SC, the SC will ALWAYS pump the max air it can, and pressurise the area between SC and carby, unless you provide some way to bleed this air off.

overpressurised carby = bang

[Sherlock]

hmm i see,
sounding like a suckthrough setup would be a much more trouble free way to go.

when the charger is disengaged, will air pass through it?
someone told me it did because they dorve around with ther clutch disengaged with there 4agze,?

but from what someone said above that could just be from air getting into the intake through the abv?

what are disadvantages of a suck through setup?
would it be more feasable to do this?

[Sherlock]

also what about a pressure releif, valve?

[oldcorollas]

pressure relief valve could work, but it presents operational issues.

ie, when you don't want boost, you turn off SC and suck air thru ABV or similar.
thats fine up to what would be WOT on NA (although tthe ABV is probably not big enough for that much air).
but then when you go from no SC spinning at say, 4000rpm, to SC spinning.. suddenly you have a lot more air to go somewhere... if the throttle is only halfway, then where does the rest of the air go?

you can have relief valve, but can you regulate the relief valve to allow the correct amount of air thru, and to close when you want it to?
certainly possible, but probably needs electronic control.

if you have suckthru, then can't have intercooling, and have issues with fuel pooling or whatever...

much much easier is to put EFI on it... EVEN if it is just using a simple TBI type "electronic carby" from mayeb camira or falcon or whatever..

that way, you don't have pressurisation problems, can tune easily etc, and you can get some fuel efficiency...

what you could do.. is to have a throttle body before the SC, to regulate the air going in (and use the ABV), and then have a theottlr body injection unit on the manifold, as blow thru, but with no butterfly.. just using it for fuel injection....

use with cheap ECU and you will have much much better setup than anything boosted and carby

[andrew_mx83]

jaycar sell a vaccuum operated switch as per ur first post. they are advertised as being to cut out ur air con compressor at high throttle loads & from memory is a pretty simple little gizmo.

also im pretty sure a weber will need to be in a pressurised box if ur running blow thru...i can think of quite a few standard cars that had them back in the day.

go for a suckthru twin weber, easier to setup than blowthru carby and much nicer than efi

[benjamin]

Instead of spending so much time and money trying to develop something that still may not work, I agree with OC and reckon you should get yourself a manifold from any of the later injected KE's and a cheap(ish) computer and run EFI.
I can understand having your heart set on something because it is interesting/different but in the end you may have something that is complicated and unreliable. Just out of interest what vehicle are you planning to put this combo in?

However a SC14 on a 4k is pretty awesome! Especially if you build it to rev aswell

[oldcorollas]

jaycar sell a vaccuum operated switch as per ur first post. they are advertised as being to cut out ur air con compressor at high throttle loads & from memory is a pretty simple little gizmo.

also im pretty sure a weber will need to be in a pressurised box if ur running blow thru...i can think of quite a few standard cars that had them back in the day.

go for a suckthru twin weber, easier to setup than blowthru carby and much nicer than efi

you'd need to wire up that switch backward but.. could be made to work..

or.. with a megasquirt (or many other EFI ECU's) you can tell it to only have the SC on above a certain throttle, rpm, and vacuum etc.. much more precise and flexible

weber is DMTT? or you can just pressure proof any carb with new float, seals on throttle shafts, sealing up the fuel bowl etc.

ahh suck thru... that would be fun to start on a cold day all the fuel will pool out in the SC

how would suckthru carb be better than EFI?
EFI would be 1 crank starting, auto warmup mixtures, no mucking around with a boost retarding dizzy, and run closed loop on cruise = less fuel....
i can7t see a downside, now that you can get an MS2 for cheaps...

if you can't find a 4KE/7KE manifold, the throttle body injection units i mentioned mean you can stick with stock manifold, and just plonk the TBI on top.. then you just need 15psi or so fuel pump, a couple of sensors (cheap holden ones), and a couple of wires to TPS and injector on the TBI... very very easy
tuning is also easy for NA, but you would want to dyno it to set timing etc as well.

[Sherlock]

apparently vl turbos had a pressure releif valve operated by spring, when the boost level gets above about 6psi, it would open because the spring could not keep it closed, and then when the pressure has dropped the spring can close again.