Why does boost increase in higher gears? ( manual trans )

Not sure if this is the correct forum but I can't seem to find a straight answer. Seems everyone has a difrent answer and none make any sense to me. In a blower application, ( vortech or ati ), why does boost pressure increase in higher gears, example 1st gear : 5.5lbs, 2nd gear 8lbs, 3rd gear 10lbs, 4th gear 12lbs ect.......... I realize the increase in engine load, but why would boost increase shifting gears?:confused:

I think you need to let us know what rpms you are running in each gear. centrifical blowers give boost in proportion to the rpms you are running, therefore, if your shift points differ, that could be the reason.

I have never observed anything that pronounced. Could you be a little more specific about what you have observed and what the conditions were?

Rich Krause

I dont know about blower stuff doing that, in a turbo car though, a 3.42 rear would often be more desireable than a 4.11 or etc because you sit in the gear longer which helps build more peak boost since the turbo gets to spin longer before shifting. When you do shift, obviously it takes awhile to build up boost again.

With a 93 M6 you get about a 30% drop in RPM for every gear change, in a 94+ you dont have a close ratio trans so it will generally get "worse" after 2nd gear, so you would be staying in each successive gear longer as a comparison to the 93, I've seen the spread somewhere but dont remember much about it.

well, 2 things I could say about this...

1: if you have a proper boost controller/wastegate setup (speaking turbo's now) your boost should NOT climb like that...if it does it means your wastegate opening isn't big enough to vent all the excess pressure your turbo is kicking out.

2: in a properly set up turbo, how long it stays in gear shouldn't matter...it should reach max boost almost instantly and hold as the wastegate vents off pressure.

I don't really know that much about how the blower setups work, but I would think since its crankshaft driven, the boost should be pretty linear with the crankshaft speed...if its not it sounds like your belt could be slipping in a lower gear (faster revving engine) and not slipping when the crankshaft acceleration speep slows down (high gears/more load)

i have heard of conditions like that but it was programmed into the computer chip (or whatever its called)

Originally posted by number77
i have heard of conditions like that but it was programmed into the computer chip (or whatever its called)

He's not talking about a factory turbo, but an aftermarket centrifugal, right?

Rich Krause

the wider gear ratios load the engine more, and like someone said the turbo has already spooled, the extra time spent in a gear will give more time fo backpressure to build up.
I'd be worried that your wastegates aren't large enough, boost creep is a result of that and unless they're not functioning properly, thats likely whats happening.