MX-3.com

I'm getting the final tasks done for getting my bpt mx3 on the road, and have run into a area of question that after much searching has left me just as confused as before I started.

First, can the SOHC VAF off my 93rs be used with my BP-T engine, as in, will it read correctly all the way from idle vacuum to WOT under boost with no accuracy issues?

Second, if so, can it be used in a blow-through set-up so an atmospheric blow off valve can be used? Also, if so, do you need a straight pipe or some degree of elbow pipe before the VAF to make it read correctly?
If not, can it still be used as a draw-through with a bypass valve? Also, if not, what can be used other than having to buy a standalone or MAP conversion kit?

Third, I will be looking into using an SAFC-II, and I'm pretty sure I have a variable voltage TPS sensor. I've heard that if you have the "on/off" style TPS, it's better to replace that with at least a 2bar MAP sensor to get better blending on the fuel maps. Is this problem negated with the variable voltage TPS, and will the SAFC wire up easily to existing wires? I have no experience with SAFCs and don't know the best wires to tap into for it.

I'd appreciate anyone that really knows what they are talking about to reply, because I've seen a variety of contradicting responses to similar questions, and I really just need to get my car running asap.

To paint a picture, here's my current state:
93RS with a swapped GTX engine and g25mr transaxle. Have the stock vj20 turbo installed and a corksport intercooler ready to be installed. I've had no IC lines made yet because I don't know what to do with the VAF placement, whether to use BOV or bypass valve, and have not purchased any electronics yet (SAFC, boost-controllers, etc). If anyone has any recommended set-ups that they know work well together, please let me know. THANKS!

does anyone know how to answer any of these questions? i know the bp swap is very common, and someone has had to run into this situation, have they not?

i always had problems with fuel cut on the mx vaf, so I changed over to the Rx7 B-code Vaf. I never used the bypass BOV, it was much easier just to install my HKS SSQ.

I see you're still using it as a draw-through set-up on your car, and it looks like you're venting to the atmosphere with your ssqv. Is that causing you to run rich when the bov opens?

Nope, but I have it setup right now to run just a tad rich. But it only runs rich at wide open throttle but thats to be expected with a VAF that dumps all the fuel the injector can handle at wide open throttle.

Get the RX7 vaf and a good wideband and you can do a little tuning with it.

thanks for the info on the rx-7 vaf, looks like i may have to do that.


does anyone have their turbo set-up with a blow through vaf? i'd like to see both sides before making a decision
thanks :D

I've done a little more research on this issue, and found some real technical info on volume & mass air flow from this site:
http://www.stealth316.com/2-air-fuel-flow.htm

as well as some tidbits on VAF placement from this site:
http://www.geocities.com/MotorCity/5335/intkvaf.htm


and if i am understanding this correctly, whether the VAF is subject to anywhere from idle vacuum to WOT (14.7psi) of pressure as it is in stock location, or subject to boost pressure by placing it between the turbo and throttle body, it should not alter the readings since the volume of air does not change in any given vane position, rather the density of that volume changes under compression from the turbo?

so wouldn't it be better to place the VAF after the turbo so the IAT sensor can read the temperature of the hotter, compressed air and adjust the fuel accordingly, rather than determining the fuel needed from the colder, uncompressed air at the beginning of the intake system?


or is all that wrong, does the turbo actually increase the volume of the air and the VAF placed after turbo wont read the compressed air accurately?

The VAF has a spring loaded trapdoor that moves with the air flow to measure the volume. Dont really know how well the trapdoor and its spring are going to behave with compressed air.....

Hot wire MAF on the other hand is well suited for blow through setup provided the hot wire element is able to withstand pressure.

If you are using MegaSquirt or some other EMS then you can just use a MAP sensor.

yes, i know it would be better to use a megasquirt and map sensor setup, but i'm really looking to not spend any more money than i have to right now to get her started. maybe at some point in the future when i have some more spendable income, but right now i'm just trying to use what i have to make it work.

i'm thinking that if i mod the vaf not to peg, i shouldn't have a problem with fuel cut. plus even if more air would be going through the vaf than it thinks because of the blow-through setup, wouldn't it also be delivering more fuel than it thinks since i'm using 450cc injectors rather than the stock 280s? maybe that should even it out close enough that a little tuning could get it right?