Technical Switched supercharger and compression ratios

[nedge2k]

Hola,

You guys seem to know more than the punto side about forced induction so shall ask this here:

I'm planning to supercharge my 1.2 16v, I want to use a Toyota SC12 'charger due to it's electromagnetic clutch (think air-con). Now, all other bits and bobs aside, I want to run the engine on a DTA S60 Pro as it has switchable maps and I can get it relatively cheaply via work. So, my question is this, with 'charger switched off, how well will the engine run in it's decompressed state?

The compression rate is likely to drop from 10.2:1 to 8.8:1, can the main map adjust the fueling and ignition enough to still make it drivable? I want the 'charger and second map to cut in on a switch or maybe just WOT - which is doable - so I can pootle about sipping fuel one minute and then cry as I watch the gauge drop the next

Thoughts?

 

[allanhelen]

Well a low compression engine isn't fun to drive when off boost

But have you fully thought it through? why are you making it switchable? I would look for a more OEM way of making it work if i were you.

Most original and after market chargers have either a vacuum operated bypass or electo magnetic clutch operated when needed based on load parameters from the ECU or from a vacuum switch as you don't want the charger engaged when lifting of the throttle for example.

If you do it this way then you only need 1 ECU map and you have a drivable car that unless you are pushing it will be running NA most of the time still keeping your fuel economy and having the charger load triggered you will have no down sides from lower comp or parasitic drag. (what boost pressure are you going for as you may be OK at standard CR)

 

[nedge2k]

well, prolly half a bar, nothing mad - for now at least. if it made 130bhp i'd be happy.

don't ask why i want it switchable. it's prolly half because it's different and half because i've wanted since seeing mad max as a kid

how much boost could i potentially get away with, with say a copper h/g, before the pistons go?

edit: i should add, i have seen davethetrike's thread on supercharging but his didnt run for any great length of time before the crank pulley gave up!

 

[allanhelen]

Probably more a case of how long before the crank key goes and you mash your valves into your pistons especially if you make it an on off charger. When you hit that switch you are going to be putting 20 to 30bhp's worth of drag on it and if you are starting the rotors from a dead stop at high engine speeds I don't know how long it or your drive belts would last.

The 1.2 16v's have seen 150+ with low comp and FI.

I do like superchargers though I have fitted an Eaton M45 to my MX-5

 

[nedge2k]

well, i am considering welding the pulley to the crank...

how feasible that is i dont know but as for belt wear, well, the mk1 mr2 that the 'charger comes from (US/JP only) seems to cope alright and from what i can tell - in normal driving - the charger is off more than it is on...

havent been able to find out exactly how much load the 'charger will put the engine under, i know the eaton m45 is around 25bhp, mebbe the sc12 is lower?

 

[allanhelen]

Might be an idea to make up a hardened key and beef the pulley up a bit or maybe even get one machined,

the number on chargers is usually an indication of the flow rate in CFM so the M45 displaces 45CFM the MP62 62CFM is the SC12 12CFM?

my charger has a Vacuum bypass, the charger is always spinning, but not compressing air until the actuator opens when the manifold pressure reaches a certain vac level and diverts air into the charger so is off boost a lot of the time but i know guys who run without a bypass and they have a mad throttle response as the intake is always pressurised.

The electro magnetic clutches are notoriously unreliable (well on mercs at least) the clutch slips when it is engaged to give a smooth transition onto boost and they end up just constantly slipping in the end so you never get full boost. but if it is small maybe it might be OK or you could always go for a Rotrex if you are feeling flush

 

[nedge2k]

Found this on another forum:

the sc12 , a 1200cc s/c that only produces like 920 cfm's per ful revolution , and maxes out at 16-17 lbs. due to rpms , and capacity.

...although from what I've read it can't handle that amount of boost, safe at 10psi, might be pushed to 13/14psi.

hmm, eaton is looking the more solid option. would be nice to be able to switch the 'charger on and off tho i was tempted by a small rotrex but, well, the prices are madness. g60 'charger is another alt..

oh and they key is apparently part of the crank itself so....

...unless i made a second keyway in the pulley and another in the crank but then the balance might go off...

 

[allanhelen]

Found this on another forum:
...although from what I've read it can't handle that amount of boost, safe at 10psi, might be pushed to 13/14psi.

hmm, eaton is looking the more solid option. would be nice to be able to switch the 'charger on and off tho i was tempted by a small rotrex but, well, the prices are madness. g60 'charger is another alt..

oh and they key is apparently part of the crank itself so....

...unless i made a second keyway in the pulley and another in the crank but then the balance might go off...

You do have to think a bit different with supercharger, it is not pressure that you have to worry about as the pressure is just a measure of the restrictions of your engine, it is the volume of air being flowed 10psi on another engine could be anything on yours.

As a charger is fixed displacement you change the amount of air flowed by spinning the charger faster. As you spin it faster the charger heats the air more and more so you reach a point where the extra flow is offset by the increased intake temperature so sizing is key (are you planning an intercooler?)

 

[nedge2k]

yeah intercooler is planned, nothing massive, might get a water/chargecooler combo. the mk1 mr2 was a 1.6l - so i thought it'd be ideal, shouldn't have to work it quite as hard - plus the all important switch! but, well, the more we talk about it, the more im leaning toward the m45

i still need to find out the sc12's drain though. if it's considerably less, it might still be the way to go...

edit: m45 is 750cc per rev vs sc12's 1200cc so it could be run slower but then what would the drain be!?

 

[Brooky]

i wouldnt bother welding anything on to a crank. may warp it and it'll change the hardening process.

wouldnt be too hard to make another keyway in the crank and pulley, you just need a well equiped workshop. My slotting maching still isnt wired in

 

[cc1]

The crank shaft pully end is something like 20mm dia so not much meat to play with.

 

[Brooky]

The crank shaft pully end is something like 20mm dia so not much meat to play with.

you could get atleast 1 other key in there, or dowl it

 

[nedge2k]

damn, yeah, didn't think about warping / messing up the hardening.

20mm aint much to play with but its either that or fabricate a fanifold and go turbo.

that being said, if the sc12 doesn't place as much load as the m45, might get away with std keyway.

what i should really do is find a scrap engine/crank or two, test how much torque one will take and keep other as spare/dev crank.

i can see the cost of this getting out of control quickly

cheers for tips guys