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Thread: TPS vs MAP AE 7427 $0E

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  1. #1
    Fuel Injected! PlayingWithTBI's Avatar
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    I have the same heads and a 46mm TB with 18LB spring on a 3704 intake (bored out to 48mm).

    AMUS 2.JPG

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    Fuel Injected! CDeeZ's Avatar
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    How's yours running? My TB measures nearly 52 MM... One of the reasons I suspect AE is what is causing this lean pop.

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    How's yours running? My TB measures nearly 52 MM... One of the reasons I suspect AE is what is causing this lean pop.
    It's getting better, Dave helped me get the Main Fuel table dialed in so my BLM is ~127 ave. Now i'm adjusting VE and PE with data logging. Of course mine's an '88 with 7747 and I'm guessing my cam (207/213 @.050, .466/.484 with 1.6 rockers) is milder than yours. Here's where I am so far;

    PE-AE bin 06.JPG

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    Well what I was saying is that increasing the Delta TPS and MAP AE seems to help lessen the lean pop when you smash the throttle open quickly, however, those same changes appear to have made it overly rich on very light throttle tip in pulling away from a stop.
    The first 25% of throttle angle change makes a greater difference in air density than the next 50%. I tend to ramp MAP AE in faster than TPS on a larger plenum. Then I'll increase the MAP difference to enable AE as a limit to MAP contribution at low throttle angles. This helps prevent rich surges when load changes without much change in throttle.

    Are those heads a fast burn style? They might require a fair amount more timing.

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    The first 25% of throttle angle change makes a greater difference in air density than the next 50%. I tend to ramp MAP AE in faster than TPS on a larger plenum. Then I'll increase the MAP difference to enable AE as a limit to MAP contribution at low throttle angles. This helps prevent rich surges when load changes without much change in throttle.

    Are those heads a fast burn style? They might require a fair amount more timing.
    Thanks 1p2m, Good info! These heads (SUM-106108) have 170cc intake runners, from what I understand "fast burn" heads are 210cc which come into power at higher revs. Although these heads have bigger valves (2.02/1.60 compared to ~2.00/1.55) I'm wondering if they are a good match for a single plane manifold with a 7.4 TB on a SBC???

    CDeeZ: do you have a 5.7 or is it stroked? A 350 @ 6,000RPM only needs a little over 600CFM (check out the "Carburetor Sizing" at the bottom")

    http://www.tciauto.com/tc/racing-calculators

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    Fuel Injected! CDeeZ's Avatar
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    I thought fast burn was just another name for the OEM vortec castings? Yeah the heads are linked in post #5.


    It's a 355, (.030" over) The Holley TB is probably a tad overkill, but I found it for $5 at a swap meet... It measures a tad larger than a BBC TB. Stock unmodified 4.3/5.0/5.7 TB probably wasn't quite enough for this engine?

    Shannen , sounds like you're saying more MAP AE might be the answer..... But you're also saying that you increase the filter table values? This is in contrast to what I had read RBob post about? He was saying to decrease the filter tables. More than one way to skin a cat I suppose?

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    Chamber design has come a long way since the days of the first smallblock. The size, shape, depth of the chamber all play a part in efficiency. Today engineers are looking for faster and more complete combustion and heads with a well designed chamber need less spark advance. Look at some of the old and new designs in this link: https://www.chevydiy.com/chevy-small...-fundamentals/ If the Summit heads are a copy of an old design head, you may need to add a bunch of spark timing. If they are a newer fast burn type chamber the increased efficiency may work to balance the heat loss through the head in a way that lets the stock L05 timing work. My comment wasn't necessarily about curing popping with timing changes, but more of a comment on how close the stock L05 tables will be to correct.

    For AE in a wet manifold you need to increase fuel delivery with an increase in air density. The amount of increase has to be greater at low rpm when the manifold air charge has more time to be exposed to the intake valves, the primary heat source responsible for popping. As rpm increases the fuel/air charge spends less time in the manifold so there's less chance for popping. And when throttle angle is greater there's less change in density with a given change in throttle angle so less fuel needs to be delivered.

    The AE tables don't care what RPM or throttle angle the engine is at. They deliver fuel based on change. So you could go from 0% to 20% TPS or 60% to 80% and both changes get the same amount of fuel from the delta TPS table, even though the 0-20% change should get more fuel.

    Decreasing the filter values makes AE respond faster. For most engines I don't feel this is the first step. For a larger plenum engine I try to shift AE fuel delivery so MAP is responsible for more of the job at low rpm. At higher rpm or lighter loads, too much AE shows up as a rich reading on O2 and possibly as soggy performance. If AE is correct when you stomp on it, but it's showing as rich with a small throttle change, then you might try increasing the MAP difference to enable AE and/or the TPS difference to enable AE. The 7.4 cal I'm looking at now (BHDC) shows a 2% difference in TPS to enable AE, an 18% difference in MAP, and a 14% difference in MAP to enable idle AE. I might try increasing the TPS difference to 4% and increasing the MAP at idle to 18%.

    If you opt to lower the filter values then you may end up redoing the AE tables. With a faster response time on AE there may be noticeable differences "all over." The filters delay AE reaction time so it may be easier to increase AE on a 0 to WOT transition, but I would expect the rich surge at light load, small throttle changes to get worse.

    Then again, I'm only on my first coffee so who knows...

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