I've heard that speed density cars don't do as well with larger cams than cars with MAFs.

How true is this?

What's the largest cam you can go with for the LT1 on speed density before you start to see considerable losses?

I have an XE 230/236 and my car loves it.

 

you can go as far as you want to,

no problem!

 

I've heard that before, too. Personal opinion- I think when you have a cam so big that it basically generates almost no vacuum under ANY circumstances things get weird. What good is a MAP sensor if it always reads almost zero? There's not much variance to build an intake density table from.

I think beyond Speed Density for these types of applications there are systems that rely on only 2 variables: throttle position and RPM. I forget what they are called and they are certainly beyond my experience, but the full-race crowd probably can help more.

I love MAF systems on a street car. Intake restiction be damned- they are much simpler to tune.

 

I think your pretty much saying that cams that pull less vaccum effectively reduce the resoulution of the fuel tables. Which is somewhat true but not a issue of any significant importance.

And if thats the case regardless if the computer is a 93 and older or 94 and newer the smaller amount of vaccum would do the same to the spark tables. So less resoultion in spark advance??? Issue? No..

Would you agree that...

If the cam is STREETABLE it can be handled by a STOCK 93 computer?

Speed density gets a bad rap becuase with out PROPER tuning the car will be more prone to having fuel delievery problems than a MAF car.

I've tuned cars with over 250 degrees of duration at .050 yet still be "tuneable" The only real limit on the stock computer is when you exceed the fuel delievery capabilities of that computer or 7000+rpm



I'm a firm supporter of speed density system, I think its simple, accurate, and effective. In my opinion MAF systems are more of a patch for bad tuning.

Oh by TPS vs RPM tuning I think your referring to Alpha-N mode. I've messed around with a DFI setup FAST where that was an option to run in. We got the car to run so well in the normal speed density mode we didn't want to fool with it. Seems like it would be too hard to make consistant.

 

Thanx for the info.

 

All depends on how well the MAP sensor is filtered by the mechanical connection to the manifold, the time constant of the sensor's response, any electrical filtering in the ECM hardware, and then the filtering that's often present in software. If all of these factors allow the ECM to properly average-out the MAP signal, then it's likely that the engine will run well. Overaveraging (or overdamped for the systems engineers in the audience) will tend to result in transient fueling errors (lean during tip-in and rich during overrun), while underaveraging (underdamped) will result in the ECM trying to manage the AFR and timing for the widely-varying MAP levels caused by large cams, in all likelihood results in a erratic idle.

Note that all of the above can affect a MAF system, too - they're not immune by any stretch of the imagination.

My Suzuki TL1000R "solved" this problem by using an alpha-N (throttle position) strategy at idle, and then switching to a SD scheme as the revs increased and the MAP signal smoothed-out. It, uh, kinda worked.

 

On some really wild applications you can disable the computers abiltiy to compensate for wacky o2 readings at an idle. We've all seen the split BLM problem.

I know its avaible for the 92-93's not so sure about the 94+

 

FWIW, I went from getting about 170 miles per tank closed loop to 240-270 miles per tank by unplugging some fairly new O2 sensors on my 96 TA. This is with the HOT cam kit and stock manifolds and exhaust. I was considering getting a pcm tune for my gas mileage problem, but this seems to be working well. Is it safe to operate my car like this? It's been running good for the past 1500 miles. Comments?

-Harold

 

No, by unplugging your o2 sensors, your car is not going into closed loop. Your car is not reading the o2 sensors which basically means that your car is running in open loop all the time. I really dont see why your gas mileahe increased as your car will usually run pig rich and power will be down as well. I would scan yor car with the o2's plugged in and see whats going on. I have a 224/236 cam on a 114lsa and my gas mileage has not gone down that much..maybe a little but bot as much as your has and my cam is bigger than yours. Im sure some of the more experienced people will be able to chine a little bit more on this situation but thats jmo.

 

96TAWS6's Mpg when up because he has massive exhaust leaks, his fuel system is possibly running heavy due to improper injector size via, fuel pressure / program / or the injectors themselves.

As for the original question HUGE cams can be tuned for with the MAF. IMHO Open loop tunes on an NA car is a bandaid for not being able to setup the car and PCM correctly.

TAD << MAF SnoB <<

 

Maybe, maybe not.

For instance what if there is enough overlap in the cam the incoming intake charge dilutes the exhaust charge throwing off the percieved A/F ratio? In that case the tune could be spot on yet the PCM would try to pull fuel thinking the car was too rich.

Or what if the sensors are so far away from the motor they aren't getting warm enough? Even with heated sensors I had this problem with my longtube headers.

The PCM is pretty stupid sometimes. Its heavily dependant on the feedback from the O2 sensors and its perpetually locked into a 14.7:1 ratio in CL. IMO if you know what you're doing open loop is a viable alternative to letting the PCM try and "fix" things when it doesnt understand whats really going on.

Jason <--- SD/OL snob

 

That could be true... but... I have gotten more power out of tunning in closed loop than in open loop.. right now I have a fuel economy problem..

But you are refering to open loop and not speed density.. they are not the same...

 

If you say so, once you hit PE mode it doesnt matter if you're in CL or OL as the PCM drops the feedback from the O2's anyways.

Yes, yes, I know. Typo...

Jason <--- SD & OL snob

Better now?

 

Speed Density seems to surive in DFI and FAST computer pretty good for large cams Only the pricey stuff like Motec can run the MAF

 

I am not running a huge cam at 234/242 but it has no problems running with a MAF and shifting at 7,000RPM.

When ever you can use a MAF your car will run more accuately