twincharging boost control
Craigorypeck - 15/8/15 at 04:40 AM
I'm building a twincharged 2.3 lima using a t3/4 .63/.50 turbo blowing into a merc m62 with modified non mag clutch pulley
I supercharged a YB cosworth and had the throttle body before supercharger with a bypass butterfly from a mini cooper diverting the boost back to SC
intake on closed throttle and it worked perfectly, unlike the BOV i tried earlier in the build... Ha that was nuts!
This project will have the throttle body after all the boost makers, there will be a smallish intercooler between the turbo and charger and a large
fmic after the supercharger before throttle body. All this pipe work and air mass would have a terrible throttle response if TB was before everything
or even between the turbo and SC
So has anyone used a turbo wastegate to control boost? This setup will have a wastegate in the normal place doing its job on the turbo but there will
be another wastegate in place of a BOV/recirc valve after the SC (possibly venting to atmo for simplicity to start)
I have read quite a bit on an engineering forum where a wastegate has been modified with a very weak spring, and you find a gate with a large
diaphragm and a small valve. There needs to be a port above and below the diaphragm as most gates have, and a boost/vac ref from across the throttle
body. Hot side of TB piped above diaphragm and cold side below.
This can be tuned to close gradually according to throttle position and not snap shut like a BOV and have boost glide on nicely. It will also act like
a safety pop valve
Has anyone here heard of or tried this method of controlling a compound charged setup?
It sounds like a very simple idea? Am I wrong?
Cheers
[Edited on 15/8/15 by Craigorypeck]
Ugg10 - 15/8/15 at 07:39 AM
Just a thought, wouldn't it be better to run the turbo and supercharger in parallel and use a valve to divert the air depending on revs. I always
thought the idea of twincharging was to optimise the supercharger at low revs (using the merc style clutch pully to disengage it at higher revs) and
once the turbo is spooled up then divert the air flow through that. Iirc this is how the old lancia and the modern vag engines work, I also remeber a
cisworth being featured ib ppc a whike back. Would not running them in serial just cause a choke point with either the turbo choking the supercharger
at low speed and vice versa at high speed.
[Edited on 15/8/15 by Ugg10]
bi22le - 15/8/15 at 11:42 AM
I have posted on here and done quite a bit of research on using a wastegate to control boost on the ITB.
Have a search on here for my posts.
I wanted to use it on an M62. Pulley it up to max out at max revs. On a 1.6 with a wastegate this would create a steep boost curve nice and low down
on the rev range and then flatten to a desired level of boost. This is done by fitting the correct strength spring. A waste gate can also recirc as it
has a ported exhaust and vacuum feed.
Craigorypeck - 15/8/15 at 04:38 PM
quote:
Originally posted by Ugg10
Just a thought, wouldn't it be better to run the turbo and supercharger in parallel and use a valve to divert the air depending on revs. I always
thought the idea of twincharging was to optimise the supercharger at low revs (using the merc style clutch pully to disengage it at higher revs) and
once the turbo is spooled up then divert the air flow through that. Iirc this is how the old lancia and the modern vag engines work, I also remeber a
cisworth being featured ib ppc a whike back. Would not running them in serial just cause a choke point with either the turbo choking the supercharger
at low speed and vice versa at high speed.
[Edited on 15/8/15 by Ugg10]
I have looked at this setup where the supercharger blows through the turbo (VAG setup) and when the turbo spools up it can draw air past the
supercharger via a one way flapper valve.
The flapper valve can be something simple that will just fall open when the pressure between charger and turbo becomes negative (to let turbo breathe)
or it could be electronically activated via sensors etc.. BUT what you really have here is still a supercharged motor in lower revs and a turbo motor
at high revs.
Compound twincharging means the engine inlet side will have a higher pressure ratio vs back pressure from running a turbo alone, and that means one
thing, way more efficient cylinder filling, helps get all spent gasses out (I will be using an NA cam so will have a bit of overlap which will help a
lot), less prone to detonation and could result in more ignition timing.
As far as one choking the other thats not an issue, the SC will be able to draw air through the turbo to provide boost down low, then when the turbo
spools the SC will multiply the boost. A roots SC is just an air mover, it will gulp a certain amount of air per revolution and spit it towards the
inlet valves, so if there's 0psi behind SC or 10psi the SC can NEVER become a restriction, it just grabs what ever it sees and throws its faster!
To produce 25psi total boost the turbo will need to produce approx 12psi and the supercharger 7-8psi.
Running a turbo to produce 25psi alone on a 2.3l engine is not easy and would have serious lag.
See where I'm going with this??
[Edited on 15/8/15 by Craigorypeck]
Craigorypeck - 15/8/15 at 04:42 PM
quote:
Originally posted by bi22le
I have posted on here and done quite a bit of research on using a wastegate to control boost on the ITB.
Have a search on here for my posts.
I wanted to use it on an M62. Pulley it up to max out at max revs. On a 1.6 with a wastegate this would create a steep boost curve nice and low down
on the rev range and then flatten to a desired level of boost. This is done by fitting the correct strength spring. A waste gate can also recirc as it
has a ported exhaust and vacuum feed.
Hi Thanks, I will have a search here now!
richardm6994 - 15/8/15 at 08:11 PM
I think you may struggle keeping inlet temps down by compounding the boost pressures.
Why not fit a rotrex supercharger instead.....those things are bloody amazing!!!!
Craigorypeck - 15/8/15 at 09:36 PM
quote:
Originally posted by richardm6994
I think you may struggle keeping inlet temps down by compounding the boost pressures.
Why not fit a rotrex supercharger instead.....those things are bloody amazing!!!!
Inlet temps can be a serious issue with this setup especially when running high boost from the turbo, but with conservative boost from turbo and 2
intercoolers i think inlet temps should be acceptable.
editing to add-
A standard turbo setup will always have a lot of back pressure (more than the turbo can produce to inlet side) meaning exhaust gases will inevitably
remain or pour back into cylinder during overlap events which will increase internal temps during compression stroke by a few hundred degrees...
A twincharged setup can expel ALL these hot gases because inlet side will always have a higher pressure than the back pressure created by turbo...
soooo the extra heat generated by the twincharging before entering the cylinder is still less than the exhaust gases hanging around in a standard
turbo setup. . . dah dah!
A rotrex is a bit expensive, I have both SC and turbo for a fraction of a rotrex.
[Edited on 16/8/15 by Craigorypeck]