In 1990, why did Chevy change the L98 TPI from a MAF system to a speed density system? Has anyone who has owned a MAF car and a speed density car noticed any significant differences in performance and reliability?

 

I think it was bean counters as the SD should be cheaper.

The SD L98s had a 5chp increase due to flat top pistons but I suspect drivability on a bone stock Vette was about the same.

 

I think they thought it was cheaper too.

Not any difference in the engines, until you mod something, then the SD cant adjust correctly.

SD is only a good thing, IMO, for radical combinations where the MAF simply cant adjust for very high cfm flow. Although a good tuner can help that considerably.

 

I have both, my '93 has the SD and my '95 has the MAF. They both performed equally when they were stock. The '93 has a gear change and a higher stall torque converter, supposedly something as simple as this can cause a change, but I haven't noticed anything yet. Now the '93 runs quicker in the 1/4 mile, but the '95 is still smoother overall and revs higher.

 

From what I have read GM claimed that they would have better control over a motors emissions and hp gains with sd over a maf system. I believe it was a bean counter move as they went back to maf. When you mod a car it is SUPPOSE to be easier to mod a maf car than a sd one, more forgiving. I AM NOT AN INJINEER nor do I play one on TV.

 

This has been something that has been debated in great lengths on the TGO. It was once spread as fact that you could not make more than 350 rwhp with the MAF by some and that substantial gains would be made with the conversion to SD. This is NOT fact.

I have shown over there on the TGO, that you can make well over 400rwhp with no hinderance with MAF. In fact I have yet to see a car converted to SD run quicker than it did with MAF.

Both Corky and I ran MAF to 10.91 & 11.02 respectively with zero problems. Another member of the TGO, ran the exact same combination as us after witnessing us run at Keystone Raceway. His car was SD from the getgo, and the best he ever got out of it was 11.2@12XMPH, in a -1000ft DA. Better than Cork and I ever raced in. I missed that weekend trying to make the SP conversion.

The ONLY advantage, having tuned both systems in a variety of setups is the fact that SD does not have a problem with higher airflow. But this leads me to my reasoning why this slight advantage is really null and void.

What motor is going to pull major amounts of air? Larger CI and more powerful motors. Here lies within the reason you do not need to worry about maxing the MAF meter. Cruising on the highway is about the only time your going to experience this "gray area" where you are not yet at WOT, and over 255g/sec (the max airflow the MAF can register). On one of these motors like my 406, that could easily max this 255 g/sec limit, you find that you very seldom on the street need more than 1/2 throttle to do ANYTHING you desire to do. Even in an autocross event, these motors would be too much for the tire that is on them. Accelerating out of a corner and giving it past 1/2 throttle would send you into a spinout surely. I have gotten the rear loose at 80+ MPH going through loose S-turns on a highway and I was no-where near full throttle or 255 g/sec. Bottom line is you do not need the throttle to make big power with these motors that challenge the MAF meter.

Now on the strip when WOT is experienced, the MAF like the SD system utilizes the parameter called PE (power enrichment). This area of the tune allows fuel to be added and subtracted via RPM. While MAF is still referenced, the addition/subtraction of fuel is done via these preset fuel settings.

There inlies a point at which MAF will become a restriction in the sense that it can't flow the air required by the motor. But I have shown to the people that said on the TGO org that my motor was being choked off by the MAF in several ways that it is not. (1) the air demand calculation show that a 406ci motor at 100% efficiency, with A/F raio of 12.5, 70*F air temp, and 6500RPM only requires 656CFM of air, we know the gutted MAF can flow 750CFM. (2) On one of my dyno tests, I attached a vacuum gage to the plenum of my intake while making WOT runs. If the vacuum gage would have registed any sort of vacuum, then we would know there was a restriction in the air track. It did not register anything, therefore its free flowing.

With the new motors we are building I do think that the conversion to SD might be necessary. But these motors should be pushing 9s on all motor and be making over 500rwhp. So it stands to reason that anything equivalent or less than our 406s should be fine with the MAF system. Converting SD from MAF requires a very good understanding of the systems functionality and a good handle on tuning, just to get back to where you were with the MAF system. This was shown this year also by the guy on the TGO that tuned his car the entire season to get a 11.2sec run, when the first time I went out I ran 11.14@123. I did nothing more major to the tune the rest of the year, added 100+ pounds to the car, and bested at 11.02sec. To me having a system that is easy to tune, and only requires minor tweaks is much better than having 30+ handles to turn and pulling my hair out all season trying to get back to square one where I would have been just leaving the system alone.

I will also point out that GM switched back to MAF in its latter years, I will not speculate on reasons, but will say that GM is about as money hungry as they come, and if they could replace the costly MAF with a 25 dollars sensor, you can bet your life it would have been replaced. Unless there is a valid reason why to keep it. ;)

:rant:


[Modified by ski_dwn_it, 9:18 AM 12/7/2003]

 

Thanks to everyone for their replies. vader86: You mentioned a speed density (SD) system's difficulty adjusting to modifications. I have asked Lingenfelter's and Howell's how many mods a stock L98 SD system can handle while retaining the stock PROM, emissions, and driveability. Lingenfelter's said that the system can handle a 15% increase in airflow, and that it can deal with a K and N airfilter assembly and cat-back exhaust. Howell's agreed.

David

 

A K&N and an exhaust wont make much difference in airflow into the engine.

Its intake/head portwork it has problems adjusting to.

 

I have a 90 and therefore SD. The approach I'm taking is to install an adjustable fuuel pressure regulator to increase fuel psi & bump up base timing a couple degrees to tune for the exhaust mods that will be done in 1-2 weeks. Then in the spring/summer with modded intake & cam, I'm having a chip custom burned which will be quite necessary by then.

 

What about the ability to program in pump shots with the 727/730 speed density that can't be programmed with the 165 maf? I have a lean miss problem that has plagued my chip burner and am going to switch to a 727 ecm to try to solve it. We shall see. You may notice by my sig that my car does have a few mods.

 

Bob,

who ever told you 165 ECM does not allow for pump shot "AE" accelerated enrichment is full of beans. 165 ECM allows and does permit pump shot. No need to convert to anything, as I said, unless your planning on putting out some major, major power. 600+ fwHP naturally aspirated. There are not too many cars running around with that much power. So most are not effected by the need to change over. Don't let anyone tell you otherwise. :cheers:

 

jesse did you notice that the people that want to change out ecms most of them have mini rams :cheers: