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Mosen won the first 10-lap heat race from the back of the 24-car grid setting a new lap record in the process and then slashed the record to 15.5542s when he started from the front and led from start to finish in heat two.
The other reason for Mosen's grin was the battle he fought with Auckland rivals Brock Maskovich and Hayden Williams in the 30-lap feature race.
The trio started from the back with Williams making the fastest run through traffic to hit the front on lap seven. By mid-race there was thrilling three-way battle on the wide and mostly smooth Baypark track that is a near-perfect stage for quality midget car racing - apart from dust issues.
"I'm stoked for those guys [Williams and Maskovich]," said Mosen.
"Every time I got in front I'd see of glimpse of them again. It was awesome racing like that.
"They've both come through from the TQs [three-quarter midget cars] in the last couple of years and they are the next wave of drivers in the midgets.
"There are a lot of good drivers in the TQs at the moment. It shows the system is working when they move up into the other classes."
Neither Maskovich nor Williams would finish on the podium. Maskovich spun away his chances with two laps to go and a last-ditch outside pass effort by Williams saw him almost spin and drop to sixth by the time be recovered.
In a side-by-side finish, Aucklander Wayne Green pipped US racer Ricky Logan to finish second by 0.02s.
The BOP Saloon Car title was dominated by Tauranga's Steve Cowling, extending an impressive string of results in the past month.
The smooth-driving Cowling started from grid two and built a big lead. He repeated the process after a mid-race caution to win by more than 3s while Tirau's Michelle Wymer finished second ahead of Chris Taylor (Tokoroa).
"We've found something with the car and built a bit of momentum and confidence," said Cowling, explaining a run that saw him win his first Baypark feature race in late January and then finish second in the NZ Champs and third at the New Zealand GP.
"I've done a lot of racing this year. Going to the South Island Series for the DHL Series was really beneficial."
It looked as if California racer Jonathan Allard would take out the BOP Sprint Car title but his race was ended by a blown right rear tyre with two laps remaining.
Dean Brindle took over the lead to clinch the 25-lapper with Auckland's Jamie McDonald taking second place ahead of Jamie Duff (Christchurch), Taranaki's Skinny Colson and midget car champ Michael Pickens (Auckland) fifth.
Meanwhile, in Christchurch on Saturday night South Island drivers swept the podium at the New Zealand Super Saloon Car Championship.
It was a Canterbury 1-2 with Mark Osborne clinching the title by one point from Ritchie Taylor, with Nelson's Ian Burson third. Baypark's Brent Emerson and Kristin Vermeulen qualified for the 20-car final with Emerson seventh and Vermeulen 16th.
Baypark Speedway - results:
Super Saloons:
Heat 1: 1 Matt Smith 46m; 2 Mark Hutchins 53m; 3 Karl Hanlon 76m. Heat 2: 1 Hutchins; 2 Smith; 3 Hanlon. Heat 3: 1 Hutchins; 2 Smith.
Sprint Cars:
BOP Champs, Heat 1: 1 Jamie Duff 19a; 2 Skinny Colson 66p; 3 Dean Brindle 2nz. Heat 2: 1 Jamie McDonald 71a; 2 Jonathan Allard 0usa; 3 Greg Kilworth 75m. Feature: 1 Brindle; 2 McDonald; 3 Duff.
Saloon Cars:
BOP Champs, Heat 1: 1 Brett Kaye 14m; 2 Chris Taylor 18m; 3 Daniel Corrin 274m. Heat 2: 1 Steve Cowling 2nz; 2 Paul Cressy 45m; 3 Brent McClymont 3nz. Feature: 1 Cowling; 2 Michelle Wymer 75m; 3 Taylor.
Midgets:
Heat 1: 1 Brad Mosen 7a; 2 Hayden Williams 27a; 3 Brock Maskovich 5a. Heat 2: 1 Mosen; 2 Williams; 3 Jared Wade 85a. Feature: 1 Mosen; 2 Wayne Green 10a; 3 Ricky Logan 81a.