How much boost does an overdrive crank add?

[Alter Ego Trip]

I'm getting ready to install some LT's, and as per a previous post I'm looking at dropping my redline down and adding some boost.

Is there a given amount of boost a 10% overdrive crank pulley will add? I have a few different pulleys on hand but would obviously like to have a good idea when I start.

 

[stkjock]

about 3-4 lbs

 

[Alter Ego Trip]

about 3-4 lbs

Thank you. That should work out about perfect with my 3.60 pulley.

 

[bigpappy33]

Thank you. That should work out about perfect with my 3.60 pulley.

What's making you go with an OD crank and a bigger pulley when you already have a smaller one?

 

[psfracer]

Mine added 2 lbs by just changing to the 10% overdrive with no other changes.

 

[GI Joe]

with a starting point of 15 psi it adds 2 psi

 

[GT500L]

doesn't fit our engine but math is math. They break it down by PSI. http://www.lightningforceperformanc..._Lightning_99-04/Pulleys/Crank_Pulleys/grid/1

 

[Alter Ego Trip]

What's making you go with an OD crank and a bigger pulley when you already have a smaller one?

This.

 

[bigray327]

I got about 3 psi.

 

[Vapour Trails]

It adds 10%.

 

[bigray327]

It adds 10%.

Thread winner!

However, I saw more than 10%, and I think others have too. I have no explanation for why or how, though.

 

[Alter Ego Trip]

It adds 10%.

LOL. Well that's the obvious answer, but the numbers never seemed to jive, hence the question.

 

[KrisR]

It adds 10%.

Spinning the blower 10% faster does not necessarily equal 10% more boost.

10% overdrive on my friend's 99 Novi 2000 GT went from 13psi to 17psi. Obviously not the same car but an example. (Note: he still has the stock bottom end in the thing too. He's been trying to kill it and it won't die).

Alter Ego Trip: I would just drop to a 3.33 or 3.15 pulley and try that instead of dropping all the money for an overdrive crank pulley, unless you are planning on going big later. Change in ratio is similar and the pulley is only like $80. Lots easier to change too. Belt slip shouldn't be an issue as I'm using a 2.87" 6-rib on my car. I saw 16.5psi this morning.

 

[Alter Ego Trip]

Alter Ego Trip: I would just drop to a 3.33 or 3.15 pulley and try that instead of dropping all the money for an overdrive crank pulley, unless you are planning on going big later. Change in ratio is similar and the pulley is only like $80. Lots easier to change too. Belt slip shouldn't be an issue as I'm using a 2.87" 6-rib on my car. I saw 16.5psi this morning.

That crossed my mind. I am going to be dropping a JDM motor in it sometime next year though so I was planning ahead.

 

[GI Joe]

sorry but a 10% over drive pulley in and of itself will not make 5 psi more boost. Just will not happen. You can pick your pulley size 2.65, 3.0, 3.2...Dont matter... What it does do is increase the speed of the blower 10%. You can calculate the rpms of that drive and determine how much more boost that setup will make. What does happen sometimes ( and confuses people and often tricks them into believing that it directly makes more boost) is that it can often allow people to pulley down ( in conjuction with the overdrive crank) or sometimes even go one size bigger on the S/C pulley and because they see belt slip stop they may pick up huge numbers...That one reason you often see SOO many differing results. There is a sweet spot for every setup. But by the math and straight physics, it over drives the blower drive ( and everything else for that matter unless you adjust for it) by 10% or whatever the ratio is you chose...ei... 12%, 15%, or 18%.
GI Joe

 

[Brain_Mach1]

For a certrifugal, the math is not linear like with a positive displacement blower, but the math is based off pressure ratio not directly PSI.

As an example, the case of the 13 to 17 PSI, the pressure ratio went from 1.9 to 2.2. This is an increase of 15% from a 10% overdrive. Centrifugals are non-linear with respect to RPM, so you bet more boost (until you exceed the capacity of the blower).

As Roots or twin-screw should be closer to 10% increase.

 

[GI Joe]

While the boost is not linear the diameter of the crank pulley will still increased the speed of the blower a FIXED amount on any type blower...10% in speed in the case of a 10% overdrive. Having said that it is true that 10% speed does generally equate to a higher increase in boost on a centri than a roots or T/S and that its not linear....too often people think that 10% means for example that a blower making 10psi would, for example, pick up 1 psi and my point is it does not equate to a striaght 10% of the original boost....yes the ratio or step up ratio will have an effect on final numbers for sure...

 

[Alter Ego Trip]

And we're back to where we started. See post #1.

 

[GI Joe]

And we're back to where we started. See post #1.

yea...see post 6...lol

 

[bigray327]

10 print "thread over"
20 goto10
30 end