ncider

Is the stock LT1 MAP sensor 1 BAR? Also, what's the rule about choosing a MAP sensor? Do you pick one capable of reading twice the boost you expect to see, just enough, or what? Is there any reason you shouldn't use a 3 BAR on an N/A engine?

I'm assuming the LT1 comes with a 1 BAR MAP sensor.

I appreciate the help guys.

tjwong

All of our C4s use a 1 bar sensor. You can however install a 2 bar, but you would have to recal all the fuel and timing tables to match. It would be a pain to do but it can be done. Ideally for it to really work correctly you can install a 1227749 ECM and piggy back this ECM into an earlier car, this ECM can use a 2 bar map and can be configured to run a V8 with good results. But having a LT1, it won't work for you. There has been a couple people that has installed a 2 bar sensor into a LT1 car, I beleive Bruce and Bald Turbo Freak has done this. You may PM them and ask.

If you are running say no more than 14lbs of boost a 2 bar is adequate. If you plan on running 18lbs then a 3 bar is needed. There is no reason that a 3 bar or for that matter a 2 bar sensor should ever be installed in place of a stock sensor in a NA application. It just plain won't work.

AKS Racing

As TJ stated above, the 1 Bar Map is designed to be used with the factory ECM. The 1 bar MAP represents 14.7 psia (or 1 ATM). So, a 1 bar MAP will cover from 0 psia to 14.7 psia (1 ATM) which is what wide open throttle represents on a normally aspirated engine. Vacuum readings will be something between 0 and 14.7 psia (or something less than 1 ATM). This 1 bar MAP sensor will not register above 14.7 psia, so it will not register boost above simple atmospheric. A 2 bar MAP will register the normal range, plus an additional 14.7 psi of pressure (or 2 ATM total). A 3 bar MAP will register the normal range plus 29.4 psi (or 3 ATM total). And so on...

Now, as you add the ability to read psi, the number of cells available in the ECM map stays the same (simple grid of columns and rows), thus each cell covers ~ twice the range (for a 2 bar MAP) that it had once covered for the 1 bar MAP. If you were to look at a 4 bar MAP, each cell would cover ~4 times the range vs the previous range that was covered with a 1 bar MAP.

Typically, the larger MAP sensors are reserved for the aftermarket ECMs (FAST, DFI, BS3, etc.). You will only want to run the minimum MAP size that covers the required pressure you expect to see.

You may wish to venture over to C4 FI/N2O to see more discussions on these type topics.

Aaron

69427

I don't see an advantage to using a MAP sensor with a range larger than the expected manifold pressure range. The MAP output is powered by, and referenced to, a five volt supply in the ECM. As the pressure range of the MAP sensor increases then the resolution of the output signal decreases. This just looks like reduced ability to finely control the fuel or spark values when the software is dealing with any lookup tables.
I'm open to correction on this, but it's not obvious to me what is gained by extra range MAP sensors.

AKS Racing

Your statement is correct.

There is no advantage to a MAP sensor with increased topside if the maximum pressure the engine will see is significantly less. It is like buying an oil psi guage that reads to 150 psi when the engine at redline only produces 65 psi. No advantage, just decreased resolution in being able to read the guage.

On my '87, I run a 3 bar MAP because I currently run to 16.7 psi (31.4 psia), with the hopes of reaching 22 psi at some time in the near future. I will say that I miss the resolution in the tables from when I had the 2 bar MAP previously.
Aaron

ncider

I've got a FAST system so calibration on the ecm is no big deal. I just wanted to make sure that you should only use the MAP sensor that's just enough to measure the amount of vaccuum in the intake manifold.
Just to confirm; I've got 29 psi (hypothetical), so I go with a 3 BAR.

69427

My admiration for your current boost setting (and your future target). I'm still well within one additional atmosphere, as family commitments have tied me up lately and slowed the whole tuning process dramatically. I also like your oil pressure guage analogy.
ps: Ain't boost fun!!!

AKS Racing

ncider,
Just to make sure we all understand, tell me where the measurement of 29 psi is registered. If it is on a guage in the cockpit, then you will definitely be right on the edge with a 3 bar MAP, as that represents 43.7 psia. If that value is as read in FAST (29 psia), then a 2 bar MAP will do fine as this represents 14.3 psi of boost.

If the 3 bar MAP is required, I would like to understand the combo that is generating those kind of boost levels. I hope it is a turbo combination, as it is extremely difficult to get high boost values from SC'ed applications.
Aaron

tjwong

Aaron is right, getting high boost numbers in our cars is a pain to do. Mainly because its hard for us to get enough Hp into the blower to make the boost. Especially with modified engines which requires even more flow to get to the boost levels that is in the design. If you are running a factory 6 rib serpentine you may as well forget making big boost numbers, it just isn't possible. That's why most guys with modified engines are running 8 to 10 rib belts or cogged setups like mine.

ncider

My fault, I meant to say 29 psia. 2 BAR would read up to 29.4 so I assumed that .4 was too close to the limits of the sensor. It would be fine? I guess you could always wait until you were seeing measurements of 29.4 before you decided to step up.

You guys have been a big help.

AKS Racing

Is this a blower combo that you are seeing 14.3 psi boost? Or is this a turbo application. Are you registering these values through an aftermarket ECM (DFI, BS3, FAST)?

Any way about it, it sounds like a 2 bar MAP is adequate.
Aaron

ncider

It's all in my head right now. I'm trying to map everything out and understand it as completely as possible before I start buying parts. I was planning on a supercharger though. I was thinking of the Vortech T-trim.

Almost forgot, I would be monitoring them through FAST and an analog gauge as well.

AKS Racing

You may wish to post these questions for a '93 vette in the C4 FI/N2O forum.

I believe that '93 is the absolute worst with regard to belt wrap around the crank pulley. Without a serious effort towards the drive system (10-12 rib or cog drive), I think you will have great difficulty in achieving 14+ psi boost.

FAST is a good choice for engine management but you may wish to llok at BS3 (I really like their new system).

Aaron

ncider

I actually already have the FAST system. I put it in when I built the engine last year. I'm doing all of this in little steps which is giving me time to understand everything better. I'll definitely be spending some time in the FI section. Thanks for all the help. Everyone.