Boost and Power Surge at high rpm at open track events

[IPPO]

I'm running a S-trim with stock pulleys in SCCA open track events. The engine has been dyno tuned for a 'safe' tune with a Superchip Custom as described below. The car exhibits a surge of power almost always near the end of a short section of straight away. What is happening is as I exit a turn, I'll be in third gear at 4000 rpm or so, I will apply the throttle as I exit the turn and the boost shoots up to 5 lbs. Then as the engine's rpm increases, usually at some point at or just over 5000rpm, the boost shoots to 10 lbs. and the car surges with power, just as I am approaching the next turn. When I say "surge", I mean the power surge is dramatic, like a hit of nitrous or spike in the horsepower curve. The engine does not exhibit this surge of high rpm power on the street, only on the track and usually at the top of 3rd or 4th gear after exiting a turn. What do you make of this and can any thing be done to smooth out the on track power surge or is this just the nature of a centrifical blower?

Background info:
306 SBF
Edelbrock Performer Heads
GT-40 intake
10 lb. S-Trim
42 lb. injectors
40 lb. fuel pressure
100 octane race gas
JMS/Superchip custom dyno tune
WOT 8% richer than stock
10 degrees base timing
26 degrees total timing
18 degrees total retarded timing (retarded while under boost)
2 degree retard at 220 degree ACT temp
11.65:1 air fuel

[fanglemeister]

You may be driving through the corners in closed loop, and it is not getting back into open loop &/or WOT mode quick enough. The stock eec programming keeps the car in closed loop for a brief moment before switching back to open loop. You also need to exceed the WOT breakpoint on the TPS sensor before it "senses high load, to go into WOT mode. Talk to your Superchip guy about making you a new chip for track use with closed loop operation disabled, and also, even more importantly IMO, set it up with the WOT TPS breakpoint reduced a good bit, so you get into that juicy WOT 11.65 a/f range at a lower TPS value. He will know what you are talking about, even if you don't. :-)

If you are dyno tuning, try to simulate this behavior on the rollers, to see what the timing and mixture are doing via some kind of datalogging.

[IPPO]

Thanks for the response Chris. I will check with my tuner on your observations and suggestions.

Initially their response was: "It's not uncommon to see something like this in the upper gears, as the engine has more time to "stretch it's legs". Usually they will build a touch more boost in the higher gears, and that is probably the surge in power you are feeling."

Nonetheless, I believe the lag time between full throttle application and power surge is between 2 - 3 seconds. I will confirm the actual time lag at my next road race in a week.

[fanglemeister]

Sounds like maybe both need a tweek then;
these are the actual values in the stock A9* processors:

TIME_AT_HIGH_LOAD_BEFORE_OPEN_LOOP 2 # (seconds)
WOT_VOLTAGE 2.71 # WOT breakpoint (TPS Voltage - 1)
OL_TP_VOLTAGE 2.71 # Throttle position to force Open Loop. (TPS Voltage - 1)

The eec won't follow the WOT fuel and timing maps until these parameters are exceeded. If your tuner people only modified the WOT parameters, then you
will definitely feel the engine "kick in" as it starts using the modified WOT values. The further from stock the WOT values are modified, the more noticeable this "kick in" effect becomes.