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At the races this afternoon there was a 4500 H.P. 632 C.I. twin turbo Mustang. Car was "de-tuned" to 2500 H.P. as he was not competing, just a crowd pleaser.

Why does the engine RPM "break-up" as he is spooling for launch? I don't see why it would be on a limiter as he is mid spool... RPM climbs, starts breaking up like mad, and as the RPM climbs higher it cleans out and then he launches.

I notice this happen on high H.P. sled pull trucks as well. As the engine loads up it runs funky and cleans up once the pedal is down.

Seems normal, just wondering why that happens?

Derek.
 

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When he's spooling up, he wants to have some boost on launch, but he does not want full boost, as it will not hook out of the hole.
Its got a pre-set boost limiter for staging, thats actually tuned into the computer timing system parameters.
The computer is alternating ignition timing in all the cylinders so it will not gain full boost or full RPM's until the clutch is engaged, or a transbrake is disengaged.

After this, the car will launch, then build more boost as it travels down track.
 
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