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Well, if you had a high output 3.6 to 3.8L race engine with 1 3/4" headers, you would expect to find ITBs with an effective diameter of 50mm to 53mm.
My current race engine which I have not dyno'd yet, is 3.746L with 48mm AT Power "shaftless" ITBs which effectively flow the same as standard 52.8mm ITBs. The engine has 1 3/4" headers and should make over 440 HP at the crank. The 3.6L version of this engine made 431 HP with 45mm AT Power ITBs (49.5mm effective) which were too small for the engine.
I have increased displacement 4.1% and ITB size 13.8%. I would not be surprised to see 450 HP.
That is double the power you are making with the same header size.
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Scott Winders
PCA GT3 #3
2021 & 2022 PCA GT3 National Champion
2021 & 2022 PCA West Coast Series GT3 Champion
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