ITBs - Not Enough Vacum

[Si.G]

Hi Everyone,

I have sorted my ITBs install today (AE101s) and plumbed them up as follows

Problem is there does not seem to be enough vacum being collected, because the MAP sensor on my MS PNP seems to think that the throttle is nearly wide open, so running very very rich. I have not spent a lot of time looking into this yet, because I have only just got the engine running. Other than leaks, can anyone see any fundamental issues with the plumbing??



I will be moving to TPS at some point, but need to get the MAP working right now.

[timk]

Maybe you could do some kind of hybrid alpha-N tune? (TPS & MAP based)

[Hellmun]

From what I've seen on the forums some of the racers tune RPM * TPS for ITB's. Without the plenum it's hard to get a good vacuum reading anywhere. Similarly making the brake booster work well is also a PITA if forums are anything to go by. CT is probaly a good person to ask if he has time. He had a very well built NA with ITB's that ran in prod sports before he built a turbo NB.

[Steampunk]

is that tube just above where your 'A' circle blocked off? If not then this is where you are not getting a closed loop as its the inlet for the idle circuit bypass. Either block it off or add it to your balancing collector chamber. I'll look at it more closely later as im viewing this on my phone atm.

[Si.G]

Thanks Guys,

I think c2888 might have the answer - I am taking four points from underneath the throttle bodies, I will try the top two on body 1 and 4 and block the ones underneath. Also going to 'T' the two brake booster take offs together.

I will let you know how it goes

I will be moving to TPS, but I need to figure out the wiring on the Toyota TPS and how to wire it into my MX5 loom for the MS PNP. I don't really want to invest is dyno time until I go to TPS, but I have an event in 2 weeks, so I just want to get it running reasonable well right now.

FYI - the mx5 stock brake booster pipe has a valve or a restricter in there, so it might help with the pedel pulse, we shall see.

Si.

[Steampunk]

Just do a quick double-check on a few other things that may sound silly but doesn't hurt to check:
- throttle butterflies are fully closed
- throttle butterflies OD match runners' ID so there are no big gaps
- MAP line isn't kinked or leaking

FYI - the mx5 stock brake booster pipe has a valve or a restricter in there, so it might help with the pedel pulse, we shall see.
Si.

It's a one-way valve

BTW, that's a really nice-looking BMC brace Did you make it yourself?

Have fun tuning your fuel/ignition maps

[Si.G]

Ok - re-plumbed as follows (as suggested)

I used equal length pipes from TB1 & 4 and T'd them to try and reduce the pulsing and then hooked the fuel reg and MAP to that.

Also I check my brake booster, the MX5 has a one way valve in the pipe, but I T'd both manifold take offs into it to try and reduce the pulse, not sure how that is because I need to finish re-routing my brake lines.

Seems better now, but still not quite pulling as much vac as before, but I guess that is to be expected, however the reading is better now - more stable. I have opened the bypass valves up now so it will idle. (they are in a silly place - you have to take the stacks off to do it)

I had to take a load of fuel out of the map, it was like 10.5 AFR, I guess that is because the gas velocity is not as high at low rpm, hope it speeds up at high rpm



Thanks for all the help - check out my garage to keep up with how I am getting on with the ITBs - http://mx5cartalk.com/forum/viewtopic.php?f=57&t=40374

Si.

[Si.G]

BTW, that's a really nice-looking BMC brace Did you make it yourself?

No - I think it is a jap make - got it of ebay, its a nice piece of kit.

[clubman7]

Been having the same drama getting the low speed tuning right too.
Tuner tried TPS only but the old Microtech MT8 isn't upto it and needs a bit more computer power to overcome the wildly fluctuating mixture changes.
Good grunt after 3500rpm though.

[Si.G]

Been having the same drama getting the low speed tuning right too.
Tuner tried TPS only but the old Microtech MT8 isn't upto it and needs a bit more computer power to overcome the wildly fluctuating mixture changes.
Good grunt after 3500rpm though.

I am finding that just touching the throttle is hitting 80kpa, so there is little difference in kpa between part and WOT. Idle is only pulling around 45kpa, so I think I am going to rescale my VE table to have more resolution between 80 and 100 kpa until I can get my TPS hooked up.

I have not really had the engine above 4500rpm, but the WOT fueling looks to be similar to when I had stock manifold on.

Hoping to have a bit of a mapping session at the weekend.

Si.

[Okibi]

Simon, i'll try and chat to you on google to tee up a time tomorrow night or on the weekend you can drop around (or I can head to yours if your car isn't drivable).

[clubman7]

Spent the afternoon adjusting throttle stops on the quads.
Oh what fun.
Had number 4 body with less vacuum then the other three.
After checking closing and thinking everything was fine, spent all yesterday chasing my arce until i realised it was probably not closing fully by 0.5mm.
Enough to give it a nice miss up to 3500rpm.
Better now as all venturies equal each other.

[Si.G]

FYI - Adding more resolution above 80kpa only made matters worse. Because of the unstable signal, it meant that the ecu was constantly jumping between load sites, hence the fueling was all over the place and tuning the VE table was impossible. I lost about a day doing that, I have gone back to my original scalling and re-tuned it. Still have the large jump from 40kpa to 80+kpa at the touch of the throttle, but basically I have it drivable.

0-1/4 throttle (cruise) is ok 14.0 - 14.7 ish AFR
1/4-3/4 throttle (light / med acceleration) is not so good and typically a bit rich due to the signal jumping around - about 13.5 AFR
3/4-WOT throttle is spot on. A nice 12.5 AFR all the way to the redline.

Since it is not a daily drive WOT is the real important bit. Still looking to go to TPS asap.

I have read on this forum (I think) or another MX5 forum that ITBs make no power gains - not been on the dyno yet, but the car pulls so much better, it is obviously faster than before - enough to be very noticable from the drivers seat, so looking forward to getting to the dyno when I have TPS sorted.
Si.

[Si.G]

Spent the afternoon adjusting throttle stops on the quads.
Oh what fun.
Had number 4 body with less vacuum then the other three.
After checking closing and thinking everything was fine, spent all yesterday chasing my arce until i realised it was probably not closing fully by 0.5mm.
Enough to give it a nice miss up to 3500rpm.
Better now as all venturies equal each other.

I have not balanced my ITBs yet - I ordered a synchro things last week, so things may be better when I have that and have balanced them.

I used a pipe and my ear to try and do it, will be interesting to see how far off they are

[greenMachine]

I have read on this forum (I think) or another MX5 forum that ITBs make no power gains ...

Depends on what your baseline is, and what other mods you have made to cams, head, exhaust etc. However it is true that the principal reason for using them is throttle response (and the noise ... and the looks ... ).