Supercharging and Ignition timing

[Fungus]

I'm struggling here...

Basically, I'm considering forced induction on my Polo. I've seen people succesfully use Renault 5 Turbo carbs (Solex 32 DIS) with either a Turbo or a Supercharger.

Now, I already have Aldon Amethyst (programmable ignition control) which works alongside the Distributor (with advance mechanism locked). I have the N/A version which has a vacuum input.

I understand that a Turbo would require variable timing with accordance to boost, as it would be possible to let off the throttle at high RPM and not be generating boost (so the timing could be advanced back to near N/A levels). BUT, would a Supercharged application run with a fixed advance curve? Seeing as the the boost level will be the same at partial throttle?

Thanks in advance for any help,

Fungus

[froggy]

Any forced induction build with a carb is like training for a marathon then tying your feet together at the start line . Carb really needs to be in front of the charger to work properly but that puts intercooling out of the window . Ready built micro squirt is under £300 new with edis4 and coil packs for a few quid and some fabrication for a proper control system .

[sunbuilder]

check out megasquirt forum

[Fungus]

I understand that EFI is the best way to go but I'm doing this as an engineering excersise and a bit of fun, I'd like to try and use a carb.

People have successfully ran Renault 5 Turbo carbs on turbo Polo's, they seem fairly easy to set up.

Thanks again

[v8]

Thing is, you need a throttle plate of some sort before the supercharger.

If you don't then it will be making boost all the time and the pressure will build up before the carb, lose power, lose mpg and make heat...

If you suck through the supercharger then you will need a backfire valve to prevent you inlet system blowing up. Intercooling would also be out of the question.

Also modern superchargers like the Eaton are not designed to have fuel go through them, from what I've read on the web the rear bearings can get the grease washed out of them and fail.

Be easier to turbo it if you really want to stick with a carb!

[pork]

Just go to efi, trust me, i started like you

I started with bike carbs, then turbo bike carbs, them supercharged/turbo bike carbs, different throttle placements, twin throttles ........then binned the carbs and got injection


They were very!!! Hungry for fuel (only a 1.3)
They ran very good at full chat, but low rpm dangerously lean
If your gonna go through the effort of fitting an ecu just for the ignition, you might as well do the whole conversion


.......but it was huge fun and a very good way to learn stuff

[pork]

Regarding the throttle placement, I ran mine post charger/turbo, and was instant power ALL the time, did have a decent vacuum bypass which sounded ridiculous driving round town, build up speed then off throttle and it deafens you with the whine and air

Plus you can get silly bov noises parked up



Or attach a tube and fire potatoes out of it

[ChasR]

Would some of it depend on just how much power he wants? I'm wondering if he could get away with a suck through design (maybe even using a trusty old SU, although getting the needles right may be tricky!), but it would still be quite a bit of time to muck around with it I guess. Or do you peeps reckon it is all of nothing regarding using EFI?

Just how much of a boost in power are you after?

[froggy]

You might get 100 beard points for sticking with a carb but once you move over to to the 20th century you won't bother with them again .

[ChasR]

I asked the question thinking about the popularity of the VMAX Scart kits for the Mini and the Moss Supercharger conversions for the Triumph TR6 and MGB.

[goldnrust]

Firstly, I agree with everyone else, a proper EFI solution is definitely the way forward.... but thats not what you asked, so lets go back to the original question.

If you really want to be simplistic you could just retard the whole lot, the most basic Mx5 turbo kits work on this principle. It says to adjust the crank angle sensor to retard the ignition by 6° across the board. I ran with this setup initially and was initially happy with it, but switching to EFI there was a notable step up in drivability and performance.

Seeing as you already have an aftermarket ignition module you have other options. As your ignition advance is calculated from RPM + vacuum currently, from what you've described, then it doesn't matter how the boost is made, or when the boost is made, the ignition module wont know any difference. Presuming this is one of those ignition systems that work with a centrifugal advance curve and a vacuum advance curve (rather than a proper 3d map) then you can retard the centrifugal advance curve by an appropriate amount to be safe for your maximum boos level. Then add the lost advance back in as extra vacuum advance. You will loose some performance and fuel economy at mid throttle positions (where you're making near 0 vacuum or light boost) but will make good safe power at full throttle and should retain stock drivability on light throttle.

If your ignition module can be mapped with RPM + throttle position then you have other options, but would be tricky to map safely and accurately, even with the more predictable boost provided by a supercharger over a turbo.

[froggy]

I'd be tempted to try an su if you have to stick with a carb after doing some tuning work on a fairly well specced mini with a wideband you can get a pretty good linear afr although it was n/a .
You would in all honesty end up with a low blow set up that worked well enough but the fairly small investment compared to the time needed to make a carb work makes efi a bit of a no brainer . If it's cost driven then save up and do it right

[gazz81]

How does the G40 do it?

[ChasR]

AFAIK the G40 was a MkII Polo with that being done by fuel injection.

[ftz313]

Hi-basically you take off 1 deg for every 1psi.
So if you normally run 30 deg advance & run at 10 psi then you need a retard module on your dizzy that'll retard 10 degrees on boost.
I would try www.h-h-ignitionsolutions.co.uk/

Oh good luck.

[Fungus]

Thanks to everyone for all the help on this.

The G40 uses digifant injection, but it's getting hard to find parts to use in a conversion!

[skylinedave]

Could you not use mk2 golf digifant parts ?

[ChasR]

Dec 13, 2015 11:22:21 GMT skylinedave said:

Could you not use mk2 golf digifant parts ?

If it is anything like the KE-Jetronic (probably more like EFI is Digifant) it will need to account for positive boost pressure in order to retard the ignition timing.

[sparkyt]

You can use the vacuum unit off the side of a saab turbo or volvo 240 turbo or 924 turbo
This will give your vac dizzy advance and retard under boost ...costs about 50 notes for a dizzy off eBay ...