Im trying to build a 383 stroker tpi. After filing through online articles about how map is the better system for a high performance engine I have hit a wall. Why when I contact Comp Cams tech support they tell me that I cant use a map system with a big cam and that I need to use a maf system. But every online article tells me otherwise. Anyone have an intelligent answer to why the hell there is so much confusion?

 

The guy is probably thinking about the "newer" MAF system used in the LT1/LS1 system.

Do some searching on "MAF" on the DIY Prom Board. We discuss the technical differences quite a bit. It is possible to "kludge" the ECM for the 255 gm/sec limitation by adding fuel via the "Power Enrichment vs RPM" tables. But you really need a WB O2 sensor to "dial it in".

There is also reversion issue with a "big cam" and the MAF system used in 3rd Gens. Remember, the system used on 3rd Gens bears nothing in common with the newer system. The new systems are a completely new generation of 16 bit ECM. And it actually uses BOTH SD and MAF.

FYI, guys that racing the newer systems are finding more performance by running in "SD Mode Only" opposed to "Both" or "MAF Only".

BTW, how "big" is a "big cam"? The really problem with the standard ECM system is when you start making massive HP. The high impedence injectors needed for mega-horsepower engines generally cannot produce injector pulse widths short enough for a good idle. In that case, you really need to look a DFI.

 

Because there are only 2 places in the world that people aren't scared of OBD I GM computers, here and DIY-EFI. Everyone else is full of myths and 'the fear' We've got members here running cams that pull well under 10" of vacuum at idle with speed density. One such member, traxion even passed idle emmisions that way.
Comp means well in their advice, they don't want to tell you it's possible and then have you complain that you had to do a bunch of tuning to get it to work. Others might tell you it's impossible just because they really are so uninformed and behind the times as to think that the ECM is nothing but an evil black box.
I think that about covers why you shouldn't listen to naysayers for GM ECM advice.

 

This is what Comp Cams says:

Dear Charlie: The speed density computer will not allow a larger camshaft. Maybe if you
switched to a mass air system than you could run something like (see attachment)
charlie wrote:
PART NUMBER12-268-4 
ENGINE SMALL BLOCK CHEVY 265-400
GRIND # CS XE268H-14
DESCRIPTION COMP. FLAT TAPPET CAM
INT EXH
VALVE ADJUSTMENT HYD HYD0
GROSS VALVE LIFT .479 .480
DURATION AT
.006 TAPPET LIFT 268 280
VALVE TIMING OPEN CLOSE
AT .006 INT 24 64
EXH 78 22

THESE SPECS ARE FOR CAM INSTALLED
AT 110 INT C/L
INT EXH
DUR AT .050 224 230
LOBE LIFT .3190 .3200
LOBE SEPARATION 114.0 INT MASTER ID 5443
ADVANCE 4 EXH MASTER ID 5203
RECOMMENDED CC VALVE SPRINGS 983-KIT

That was the cam I was planning on useing anyhow. I have a 3rd gen map and a maf system so either way I have what i need. Which one should I use with that cam. I have a pocket prgrammer so I will be able to fuss with the parameters on the proms.

 

That cam wont work in a MAF car without some custom reprogramming. With some fiddling you can get it to run, but its not going to be anywhere near right and the idle is going to bounce around alot. I should know, I have a cam like that with more lift, same duration, but ecm-friendlier LSA. A SD setup isnt going to like it either. Youre looking at a chip for sure to get the car running like its supposed to. Getting one from someone is more hit than miss since each car is different and its hard to guess a tune with little to no real data.

 

How much HP/torque are you targeting?

You might want to check out this thread on late 80's MAFs:
http://forums.corvetteforum.com/zerothread?id=282490

Note the reference to Beach Bum's 383 building 360rwhp. That's about 430-440hp at the crank.

From what I've read, a MAF EFI is easier to tune and you can do your own tuning so, if that is near your hp target, MAF looks like the way to go.

 

Well Mike Davis hasbeen giving me some pointers to programming both system bins so modifying a custom chip will not be a problem. Im basically just trying to figure what will handle this better a map or maf system. Im still set on the speed density since I was told it will handle the extra flow with some modification in the Ve tables.

 

WTH ?

MADMAX, please tell me an LSA thats more "ECM FRIENDLY" than 114??????

I think you only saw the Lobe center, not lobe seperation.
I've never seen a 116 cam...

 

Ummmmmmmm, yea I did read that wrong. Happens when youre at work avoiding the boss Ive never seen a 116 cam either (that I can recall) but the factory cam in my 91 engine I spec'd out at 117.5 LSA. Top that

The SD setup is 'better' but you'll end up fiddling with it more to fine tune it, mostly because you can. The MAF setup IMO is harder to tune, its more guesswork than the SD setup which is just simple tables.

 

I think the comment about COMP trying to avoid liability was nailing it. SD will run the cam just fine (with tuning) they just want the ECM thats more self adjusting.

Wussies

 

First, I have seen guys with BOTH SD and MAF (3rd Gen 165) run cams bigger than that. Again, look at Tim Sifford with his SD 7730. He's running much bigger than what that guy is suggesting and he was able to get it to pass Maryland's IM240 emissons.

Both SD and MAF will need to have the eprom reprogrammed - no doubts about that. Neither will run right "as is" with a stock eprom - NO WAY.

I personally woud give the "edge" to the SD system as a 383 is definitely going to max the MAF somewhere between 4,500 - 5,200 rpm and a large cam is probably going to want somewhere around 6,000+ (depending on your intake). I also prefer the SD's ability to be tuned for "Elevation and Ambient Air Temp" more precisely. But I personally feel I could also make it work with MAF (just not quite as "precisely"...but I'm fanatical).