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Autocross Boost Question

Old 10-18-04, 09:36 PM
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Autocross Boost Question

Well the autocross season ended yesterday and I put the car away
tonight. Now I'm trying to put together the winter plan for
improvements. The car is a 13b with a single TO4B turbo with dual
Tial 38mm wastegates and a Greddy Type R BOV. Engine management is
through a Haltech 6K. 550cc primaries w/ 680cc secondaries.

Throttle response and straight line power is amazing. My concern is
that the power comes on quickly and like a hammer. While it is
wonderful launching and in a straight line it causes the car to be
difficult to drive in corners and I find I am always on and off the
gas which is not the fastest way around because it upsets the balance
of the car.

My question is: Is there a way to enjoy the power without the feeling
that my right foot is controlling a switch?

I want the straight line power, but I also want to improve the
driveability in corners. I sometimes think that limiting boost under
5k rpm would allow me to keep the car balanced in the corners and then
allow boost to build over 5k rpm would give me the straight line
power. I don't know how to set this up. I originally was running
10psi wastegate springs, but moved down to 7 psi. The car was more
driveable and my run times dropped, but I feel there is a better
answer than just limiting total boost.

A friend suggested that the problem is the way the turbo spools up.
His thought was that the turbo housing is too big for the application.
He said that having a housing that is too big will result in a
relatively high rpm spool up with an abrupt hit of boost. He
recommended using the existing turbine but go to a smaller housing.
This would give you boost at lower rpms and it will seem to spool up
slower and make it more drivable. Does this make sense?

I've looked at boost controllers and they don't appear to give me the
control I'm looking for. Or am In looking at it wrong. Should I go
to some really weak wastegate springs and then try to control building
boost vs. RPM instead of looking to limit boost?

Any help would be greatly appreciated. Thanks!!
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Old 10-19-04, 12:47 PM
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What car? How much boost?

For autocross racing, you spend very little time going in a straight line. Since that is the case, that means that you cannot use all of your power very often. Traction while cornering limits you. I suggest that you turn down your boost until you don't have an "on/off" switch when accelerating.
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Old 10-19-04, 12:58 PM
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An AVCR will give you the boost control you seek, you can control it by gear, and RPM. If it were me, I would work on driving technique. As long as your turbo is not surging at part throttle, you should be able to drive it at part throttle, giving you instant power when you need it, and relatively linear power in the turns. this technique will require that you perfect left foot breaking, but I have never seen a fast autocrosser in a turbocharged car that did not use prodigous amounts of left foot braking. I AX two cars, a 97TT Supra([email protected], [email protected]), and a ~400+ hp FC, both require left foot braking to settle the car, stay on boost, and keep the rear end in line. Another benefit is that you are less likely to spook/lift/trail throttle oversteer, if you are using this technique. Carl
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Old 10-19-04, 06:42 PM
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I have tried left foot breaking and I have struggled with it to date. I never seem to master it before I abandon it. I always find that I am spending more time braking than I need to. However, I do agree with you Carl. I believe that keeping the boost up, and controlling the front to rear weight transfer is one of the secrets to going fast. I need to commit to an honest attempt to mastering left foot braking next year.

In addition to driving technique, I also need to invest in some decent competition oriented struts.

I apprececiate the comment on the AVCR. It looks like a worthwhile investment since I am not currently running a boost controller. It looks like it will give me sgnificantly more tuneability over just swapping out wastegate springs.
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Old 10-19-04, 06:48 PM
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Carl,

I thought your name looked familar. I purchased the street ported housing from you last year. They worked out great!!

Steve Hayward
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Old 10-20-04, 01:37 PM
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Excellent, those housings would have been bridgeported by now, and I would have been much poorer. Glad they worked for you. On left foot braking, be sure to pivot off your heel, do not use your whole leg. Second, the weight of your foot+/- is about the pressure required to settle the car, ie it should not really be slowing the car, if it is you are braking too hard. Last, once the car is settled in the turn, you want to begin to trail off the brake, this will increase your exit speed, and allow the car to rotate more quickly through the second part of the turn. This is all based on my expereince, and not neccessarily "text book". Damon or one of the other more technically savvy racers could comment on text book technique. Carl
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