Hey guys, Am new to the forum, I have owned a mx5 for quite a few years now, and last year started racing in super sprints here in australia, the tournament is over now, so i have about 3 months to make her faster. Have some huge plans that are already in motion, biggest changes are that we have bought a turbo kit, it came with a t25/28 turbo, and somewhat unconventionally have made some independant throttle bodies from gsxr1000 bodies, feeding thru a custom plenum chamber, also the conversion to e85 fuel. Just wondering if anyone else has done similar sorts of modifications? and if so, did they run into many issues?

 

https://www.miataturbo.net/general-miata-chat-9/turbo-itbs-miata-46356/

This thread discusses it at length.

 

Do you run Megasquirt?
You need to tune it with a secondary fuel load.
One mapping is responding to the TPS, the other one is trough MAP.
That's the tricky part. You might even consider the MAF route.

 

I'm confused. Are you saying there are two fuel and two spark tables, one for TPS and one for MAP?

 

Im also confused, what do u mean by secondary fuel load? yes i am using ms2, and wanting to keep away from using a maf

 

For what reason keeping away from maf?

 

Yes, ITB's are like that. There isn't enough vacuum at high revs (or was it on low, cant remember) so you need to run them hybrid alpha-n to make them work properly. Don't know how boosting changes that.

OP Check that the ITB's hold boost, there's quite many holes to blow = old blow through carb turbo's needed different carb's to run and work.
I'll promise that tuning ITB's and turbo setup will be fun Instead ITB's just go with well known intake manifold.

 

Pressure will depend on what cams, generally since a lot of N\A ITBs use hot cams there is a loss of negative pressure compared to normally and its hard to get a steady map signal. If the cams aren't to wild, stock like you probably could be fine with just using MAP only. Just have to see how much negative pressure its at idle.

 

The problem isn't necessarily the lack of vacuum, its the large change in map reading during throttle position change that is the issue. You have a cross sectional area MUCH larger with 4 2" throttles than with a single 2.5" throttle. So a 5% change in throttle position means a much larger swing in the MAP reading, making it difficult to tune with speed density.

 

so what u are saying is i should get only a mild cam, like a stage 2 cam or a custom cam with like 257 degree inlet and 262 degree exhaust? What would you guys recommend?? really appreciating all this feedback!

 

I would recommend ditching the GSXR throttle bodies until the turbo is reliable. You are going to have more than enough on your hands without dealing with the complexities of tuning an ITB system without the help of Alpha-N.

What kit did you buy?

 

alright, i'll tune the turbo on the stock manifold, I didnt buy a kit, i got a shop to make it up for me, they also ceramic coated them, its away getting the plenum chamber made up atm, will upload some pics when i get it back in the next few days

 

As asomeone who has dealt with both in the past couple years I couldnt agree more.

 

Tuning little throttle openings is hard on MAP based tables with ITB's. It starts with little vaccuum on idle, then no vaccum when throttle is opened, and more vaccuum then on idle when running. Also the ECU won't see the difference from 2/3 to 1/1 throttle. Vaccuum is zero in both situations. So running a MAP sensor based mapping just won't do it, even on mild cams.

Tuning on TPS based mapping or AlphaN would do good on ITB's but it is less precise and load not dependant. If you change the vehicle weight or circomstances alter, you need to re-tune your mapping on AlphaN.

Also when using a Turbo, the opening of the throttle does not equal the amount of air going in. You need MAP for that.

By setting up a secondairy fuel load under more engine constants (additive) you'll end up with one table for TPS based mapping and another table for MAP based mapping. The values of both tables together makes the total of injected fuel.

I think I would tune it without turbo on ITB's to set up alphaN and then setting up the second mapping with the turbo atached. But be carefull, you cannot detach the turbo. It will over-rev.

I used hybrid-AlphaN on my N/A 1.6. It made it very quick on the throttle.