Z's r Best

The new 327 is running about 15 to 16" of vacuum at idle. No SES lights in any of the idle to warm up or short trips up and down the road followed by shutting it off or letting it idle some more.

My question is about the relationship of the sensors communicating engine condition to the computer.

I don't know what is a "normal" idle vacuum reading for a TBI car but 15 to 16" seems a little low (my 305 TPI pull just over 20" at idle).

Now for the question. If the throttle position sensor is sending a "normal" signal to the ecm for an at idle condition, the engine temperature ga. is sending a normal engine temperature signal to the ecm, the tachometer shows a normal idle rpm, the inlet air temperature is normal, etc., etc. will the possibly low vacuum signal from the map sensor cause the ecm to call for more fuel? As stated above we aren't getting an SES and to me that means the O2 sensor is seeing normal exhaust.

I'm just curious. We'd really like to keep the TBI stuff on the engine if possible so we're taking all this slowly.

Thanks for any ideas/information.

dimented24x7

iTrader: (2)

Higher MAP readings will cause the ECM to deliver more fuel as it interprets it has a higher volumetric efficiency and load. Low vacuum can be a sign of a poor tune (too much fuel, not enough timing, etc). It can also point to an engine problem.

What are the specs on the motor?

Brandon2350

If you were to install a larger cam for example with a larger overlap, the vacume over stock would typically be less and the map value would be higher. The ecm sees the rise in map value as though the engine is under a load...in which case it adds more fuel when in fact it may be idling... The condition you described is pretty common and you may not get an ses light as you mentioned.

BMmonteSS

To further explain and expand on what guys have already mentioned. The "thought" process that he ECM goes through is pretty strait forward. In order to calculate how long to open the injectors the ECM starts by taking a RPM and a vaccume reading measured in KPA (100=WOT, 30-40 = idle). It then looks up the volumetric efficency from a 3D table. This value is then put through a dozen modifiers based on engine temp, air temp, baro pressure, ect. before it's finally used to calculate the PW.

What happens when you modify the engine is that the engine may not accually have the same VE at the same MAP and RPM readings, so the 3D table no longer matches up with what the enigne needs. A perfect example is that when you swap a cam the engine becomes less efficent at low engine speeds and becomes more efficent at higher engine speeds. So at say 40 map and 750 rpm the VE needs to be lower (effectively leaning the motor) and the VE at say 100 map and 5500 rpm needs to be much higher to add extra fuel up top.

Because a speed density FI setup uses a table to come up with how much air is actually entering the engine any modification will REQUIRE you to retune the VE table to provide correct fueling.

Z's r Best

Thanks for the answers. I don't pretend to understand how the computer works, I just wondered what it would do with only one of the sensors (giving it odd signals - engine speed, temp and throttle position sensor normal at idle but map signal not normal.)

Vol. eff. questions aside, has anyone attempted to modify the signal coming from the MAP sensor to the computer so that it gives a "normal" voltage signal to the computer (modified so that the difference in vacuum across the range still results in a mostly normal voltage across the range of vacuum?)

Oh yeah, what is normal vacuum on a stock TBI 305. Many years ago I remember my dad setting the timing on his trucks with a vacuum ga. and I never saw over 18 or 19" of vacuum and these engines were in good condition and the trucks ran good and got good mpg.

The engine is 4 bolt main bearing 350 block. It has a large journal 327 crank, 6" rods, RHS Pro Topline heads with the .550 lift springs and 2.02 intake valves and 1.60" exhaust valves, the cam specs:

Doug Herbert Racing Cams

P/N C6PJ (B) Hyd. Roller Cam
Adv. Dur. 284 intake - 292 exhaust
@ 050 223 intake - 233 exhaust
valve lift .500 - .500
lobe center 112
valve lash 0.010 - 0.010
center line 108
@ 050 Intake Open 3 Close 40
Exhaust Open 51 Close 1

Can't seem to find the piston info, it should be here somewhere. It's got a edelbrock air gap manifold and TES headers on it along with a TPI cat. and aftermarket TPI cat back.

Seems to have good power just running it up and down the street in front of the house. We built it for my son's 91 RS (5 speed car with an 88 Formula 3.23 pos rear end). I don't believe he will ever stop goofing off on warcraft on line to learn to program his computer and figure he'll eventually put a carb and vacuum distributor on it rather than put the effort into learning to modify a program to do his own chips.

Thanks again for the info.

Hey BMMonteSS, I'm from Greenbrier County WV so I'm wondering if you park that car all winter to keep it out of the salt? They sure don't last long if you drive 'em in that white stuff up there.

89RS_82Z

iTrader: (2)

["I don't believe he will ever stop goofing off on warcraft on line"]


WHY lol, i dont get the world of war craft stuff......

17 - 22 in hg (or whatever it is) is normal, i have 20 to 21 at idle on my 305 TBI and it has 140,000 miles

BMmonteSS

Wow that engine is way past anything the stock tuning can run, I'm surprised you even made it out of the drive way. Simply altering the MAP signal is not going to cut it, it won't even work on a mild engine.

What I was getting at before was that things aren't as simple as getting the map readings back to "stock" levels at idle. Your engine is simply way different from stock in every way imaginable. Not only do you need to rework the VE tables but the pump shot is off, the temperature compensation tables are off, and that's just the getting started on the fuel side of things. That motor is going to need a timing table that is completely different from a stock LO3.

You're only choice here is to learn to tune your ECM or swap to carb. You wouldn't run a carb that you couldn't change the jets in would you? Then why run FI that you can't tune?

Winters suck up here, rust is a constant battle.

Z's r Best

Thanks again. Like I said, he's mentioned the carb and dist. but I don't believe he'll take the time/effort to really get it right either. He'll just buy it, bolt it on and go, and complain that it's never quite right.