Getting Good Vacumn Signal from Quad Throttles to Air

[oldcorollas]

for reference, an alfa 2L motor (basically 18RG) with quads from a GSXR bike... had about 25kps vacuum at idle, and cruise was about 15-20kpa vac...

with this 25kpa range (and one point at about 50kpavac for decel) we tuned it pretty damn well.... it helps being able to space the load pints as close as you want.. we had them maybe 3kpa apart...

but anyway, even with low vac, it is possible to run very well... the reason ou get atm pressure at 20% throttle.. is not cos of poor signal, but cos the runners are seeing atmospheric pressure (usually) which means it is effectively at full throttle anyway..


but one last idea... if an engine generates NO vacuum, it cannot run. it must generate vacuum to pull air into the cylinders... it's just a matter of being able to measure it.

[takai]

Yeah, but with the lumpy cams and the quads the 4AG is drawing so little air at idle, and even through the lower (read non-usable) rev range (1200-4000) that there was between 8mm/Hg and 6mm/Hg in that range. So a 2mm/Hg difference over 3000rpm is extremely hard to tune for.
Once i was hitting 4200rpm it would jump to needing a hell of a lot of air, and then spike to 16mm/Hg at ~20% throttle.
I think part of the tuning difficulties were due to the sudden power ramp at 4000rpm when it came on cam (32kw to 68kw).

[TRAV-KE55]

Unless you are going forced induction i would go TPS all the way, simplicity and they still idle fine and run fine down low, i think the pulsing of map sensor will be more greif than whats it worth exspecially when it all can be done asy peasy with TPS. Generaly with the quads the moment you open them up a bit the get a big gulp of air so the need the be rich there anyway for drivability oterwise they are to sensitive, just my experiences anyway.
Trav

[Celica RA45]

yeah travs right use tps and run map sensor to atmosphere as soon as you go to big cams tps is the only way to go and most quads will idle fine ,you should be able to get a 4age with cams to idle at 750 to 1000 rpm ,you just will have to fiddle with the idle screws to get it right

[takai]

Yeah, with minimal fiddling of idle screws (far out Toyota put them in a silly spot), mine idles at about 1000-1100

[myne]

This might sound stupid, but bear with me.

What if you were to create a mini-plenum of say 1.5" pipe that ran the full length of the head and was Tee'd into eqch of the inlet runners between the head and throttle body.
This would create a volume of hopefully sufficient mass to not only smooth out the sensed vacuum but possibly even create enough to run a brake booster in the traditional way.
Tubing would have to be a bit bigger than the std vacuum hose though. Say 1cm diameter?

Something like this - where the weird thing above the throttle plate is the 'plenum'

                 ___     ________
   \            (_  _)   |
    `--------------| |----'
        ----o----
    ,---------------------,
   /                      |

[takai]

Thats the stock setup.
THe main problem is that the vacuum simply isnt enough on 4AG with agressive cams.

[myne]

Oh heh, shows how much I know - atleast my theory is sound

Only way I can think of from there is to find the worlds smallest MAF unit and rig it to one of the inlet runners, making the assumption that they're all flowing the same.

[Garth AE86]

Oh heh, shows how much I know - atleast my theory is sound

Only way I can think of from there is to find the worlds smallest MAF unit and rig it to one of the inlet runners, making the assumption that they're all flowing the same.

which they usually dont.. cyl 4 tends to starve from air all the way back in the engine bay
but takai is right, even with the greatest vac setup it seems my agressive cams/overlap will destroy any hope of decent vac

[myne]

Only other idea, aside from the tried and tested TPS method is perhaps to allow the throttles to completely close, and make an alternate idle circuit using the plenum - much like a gze does. But you'd still get dodgy vacuum readings when you're not using the idle circuit.

This is a 4-trumpet setup without a 'proper' inlet tract isnt it? If it HAD a proper inlet a MAF would probably work best.

[Garth AE86]

takai what kind of management are you running?

[ae86trueno]

Just use the TPS method, we tried with my EMS to use map but couldnt average out the pulses on each cylinder enough, that was with 280 deg cams with 9mm lift.

Ben.

[Garth AE86]

hrmm seeing as ill be using 288's and 9mm also, i guess ill just use the TPS aswell

[takai]

Im using a Haltech E6x, although considering going to a MoTeC.

[Garth AE86]

have you considered the E8 form haltech takai?