Supercharger Boost Calculations

dvk-kp
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Supercharger Boost Calculations

Postby dvk-kp » Thu May 12, 2016 8:09 pm

Gidday guys,

I'm starting work on supercharging my 3.5 pajero and was looking around the shed for some pulleys that could be suitable...

Found a couple and used a calculator online and need to know if I've used it right...

Drive pulley is 160mm OD
Driven pulley is 110mm OD.
Which makes a pulley ratio of 1.45?
Eaton M90 off a early 90s ford thunderbird.
3.5 dohc v6.

The calculator told me I'd only make about 2.84 psi of boost? Does this sound about right?

Ideally I want to run about 6psi. Won't be intercooled so I figure that would be safe.

Any help or insight would be greatly appreciated.

Thanks

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Re: Supercharger Boost Calculations

Postby sirLachlan » Fri May 13, 2016 8:04 pm

Seems about right . That blower is only 90cc isnt it ? And on a comme v6 dont they end up with the stock 5 inch ish crank pulley and like a 2 inch ish blower pulley ?

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Re: Supercharger Boost Calculations

Postby Checkerhead » Fri May 13, 2016 9:40 pm

Well, first of all, my assumptions.
1) Both the engine and the supercharger are 100% efficient at their respective sucking and pushing.
2) You want to know the answer at 6000rpm (I haven't done any volumetric efficiency stuff so not sure RPM matters yet, but I need something to work with)
3) Atmospheric pressure is 14.7 psi

I worked backwards from your 2.84 psi and I got a pulley ratio of 1.42.

I also worked forwards with a target of 7 psi and calculated that you need a pulley ratio of 1.76.

No idea if my calculations are excluding anything important, so don't quote me, but as a rationality check, your calculator seems ok.

Remember that most (all?) NA engines actually run a t a vacuum, so 2.84 psi might actually be 5 or 6 psi up from what it normally runs at.
There is a black Jeep in Auckland that does winch challenges. From memory they run 4psi through there Lexus. That thing goes hard compared to the NA lexus trucks that used to compete (most are super charged now). All I'm saying is that is one case where 4psi seems to make a massive difference. No doubt things like their tune help as well.
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Re: Supercharger Boost Calculations

Postby dvk-kp » Fri May 13, 2016 10:41 pm

I haven't looked for info on the commy set up but found a site about m90 on a Lexus using 5 inch crank pulley.

If the calculator is right I need about a 95 mm pulley to make 6-7psi.

But I Was talking with my brother today and decided 2.8psi is probly not a bad place... My idea is to suck thru the standard throttle body in the factory place thru the afm and air box.

Hopefully it should run ok. Figure it'll have some adjustment with the air flow meter and not have a map sensor. Rising rate fuel reg. And fingers crossed it goes good. :D

Have to get the whole lot mounted first. :?

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Re: Supercharger Boost Calculations

Postby Checkerhead » Sat May 14, 2016 8:36 pm

Yeah the M90 is pretty commonly used on the Lexus. I don't know of anyone running more than about 6 or 7 pound with them though. I think the volumetric efficiency goes to shit after that and your intake temps sky rocket. But I'm no guru, so don't quote me.
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Re: Supercharger Boost Calculations

Postby Big » Sat May 14, 2016 8:39 pm

im at 9spsi and runs sweet as :wink:

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Re: Supercharger Boost Calculations

Postby Madaz » Sat May 14, 2016 8:40 pm

Checkerhead wrote:Yeah the M90 is pretty commonly used on the Lexus. I don't know of anyone running more than about 6 or 7 pound with them though. I think the volumetric efficiency goes to shit after that and your intake temps sky rocket. But I'm no guru, so don't quote me.


Except for me.............. :lol: :lol:

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Re: Supercharger Boost Calculations

Postby Madaz » Sat May 14, 2016 8:49 pm

M90s do seem to fluctuate in boost pressures depending if your model runs coated rotors or not, also later model thunderbird ones have bigger and better inlet ports, then if you use a commodore or buick one they are different again, also different inlet manifolds and intercooling tend to give vastly different boost readings on similar pulley sizes. Your safest bet would be to run it up with what you have then make a careful guess and try a different size to give you something to work off. Unfortunantly the maths can be way off in this case. On my 1uz i use a alternator pulley for my supercharger pulley, I cant remember the size, but im running the blower at the max continuous rpm that the bearings are rated too, which is 16000rpm at the 6500rpm the motor runs to. That gets me 12lbs boost.

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Re: Supercharger Boost Calculations

Postby dvk-kp » Sat May 14, 2016 9:16 pm

Madaz wrote:M90s do seem to fluctuate in boost pressures depending if your model runs coated rotors or not, also later model thunderbird ones have bigger and better inlet ports, then if you use a commodore or buick one they are different again, also different inlet manifolds and intercooling tend to give vastly different boost readings on similar pulley sizes. Your safest bet would be to run it up with what you have then make a careful guess and try a different size to give you something to work off. Unfortunantly the maths can be way off in this case. On my 1uz i use a alternator pulley for my supercharger pulley, I cant remember the size, but im running the blower at the max continuous rpm that the bearings are rated too, which is 16000rpm at the 6500rpm the motor runs to. That gets me 12lbs boost.


That's high boost. Take it you've got an intercooler?

The pulley I've found is a 4 bolt idler pulley... I'm sure I'll be able to buy the same style in a few different sizes.

Thanks for your input guys, it's good to read real world results.

Since it's a bit of an experiment I'll start with what I've got and move from there, probly not ideal with factory ECU but I'll start there and possibly go to an SAFC and dyno tune if it causes an issue.

Another thing is my truck rev limits at 7200ish rpm, but torque drops off hugely after 6000 rpm. I don't rev past 6 where there's plenty of traction because there's no point, it's struggling. But in the mud the extra 1200rpm really helps to spin the wheels hard and get them clearing the mud. But the supercharger will be spinning pretty hard up there? :? Will see how it goes at low boost first :D

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Re: Supercharger Boost Calculations

Postby dvk-kp » Sat May 14, 2016 9:18 pm

I'm also guessing the M90 will be a bit more efficient for a wider range on a 3.5 as opposed to a 4 litre being a smaller engine? Or is my logic wrong there?

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Re: Supercharger Boost Calculations

Postby Madaz » Sat May 14, 2016 9:42 pm

Yeah, the smaller the engine, the better it will be.
Yes mine is intercooled, and it manages to keep the inlet charge around 50 deg at full noise on the dyno.
Mine ran on standard computer at 6lb for a few years with just a rising rate reg and was thrashed pretty hard with no issues, the only reason for changing to aftermarket ecu was to let us change the timing down low, which got us a lot more off the line grunt, also pulling 6lbs boost at 1000rpm helps too, mine runs out of puff just over 5000rpm and boost has dropped to 10lb by the 6500 limiter, which is just the blower running out of puff and a combo of the exhaust choking it and the cam profiles in the motor
Last edited by Madaz on Sat May 14, 2016 10:16 pm, edited 1 time in total.

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Re: Supercharger Boost Calculations

Postby dvk-kp » Sat May 14, 2016 9:52 pm

Madaz wrote:Yeah, the smaller the engine, the better it will be.
Yes mine is intercooled, and it manages to keep the inlet charge around 50 deg at full noise on the dyno.
Mine ran on standard computer at 6lb for a few years with just a rising rate reg and was thrashed pretty hard with no issues, the only reason for changing to aftermarket ecu was to let us change the timing down low, which got us a lot more off the line grunt, also pulling 12lbs boost at 1000rpm helps too, mine runs out of puff just over 5000rpm and boost has dropped to 10lb by the 6500 limiter, which is just the blower running out of puff and a combo of the exhaust choking it and the cam profiles in the motor


That's awesome mate, sounds like I'm heading down the right track! Got any videos on YouTube of your truck?

Good news about the revs... Nothing too worry about up high :D it's more low down grunt that I'm after.

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Re: Supercharger Boost Calculations

Postby Big » Sat May 14, 2016 11:24 pm

Madaz wrote:Yeah, the smaller the engine, the better it will be.
Yes mine is intercooled, and it manages to keep the inlet charge around 50 deg at full noise on the dyno.
Mine ran on standard computer at 6lb for a few years with just a rising rate reg and was thrashed pretty hard with no issues, the only reason for changing to aftermarket ecu was to let us change the timing down low, which got us a lot more off the line grunt, also pulling 6lbs boost at 1000rpm helps too, mine runs out of puff just over 5000rpm and boost has dropped to 10lb by the 6500 limiter, which is just the blower running out of puff and a combo of the exhaust choking it and the cam profiles in the motor

what size pulley there Madaz ? im on a 60mm so are you using a 50mm ? Just got me thinking about the rev limit on mine.. should get to 7000 with 60mm ( max 90 speed give or take a bit)not that I need it but have gone to 6800 and its awesome :lol:

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Re: Supercharger Boost Calculations

Postby Madaz » Sun May 15, 2016 7:59 am

Big wrote:
Madaz wrote:Yeah, the smaller the engine, the better it will be.
Yes mine is intercooled, and it manages to keep the inlet charge around 50 deg at full noise on the dyno.
Mine ran on standard computer at 6lb for a few years with just a rising rate reg and was thrashed pretty hard with no issues, the only reason for changing to aftermarket ecu was to let us change the timing down low, which got us a lot more off the line grunt, also pulling 6lbs boost at 1000rpm helps too, mine runs out of puff just over 5000rpm and boost has dropped to 10lb by the 6500 limiter, which is just the blower running out of puff and a combo of the exhaust choking it and the cam profiles in the motor

what size pulley there Madaz ? im on a 60mm so are you using a 50mm ? Just got me thinking about the rev limit on mine.. should get to 7000 with 60mm ( max 90 speed give or take a bit)not that I need it but have gone to 6800 and its awesome :lol:


Same size as the alternator pulley I think, I'll check later on today.

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Re: Supercharger Boost Calculations

Postby Checkerhead » Sun May 15, 2016 9:07 am

Madaz wrote:
Checkerhead wrote:Yeah the M90 is pretty commonly used on the Lexus. I don't know of anyone running more than about 6 or 7 pound with them though. I think the volumetric efficiency goes to shit after that and your intake temps sky rocket. But I'm no guru, so don't quote me.


Except for me.............. :lol: :lol:


Was waiting for you to ruin my story. I thought you were getting around 10 but wasn't sure so didn't want to write it down.

I have read that the post 94 (i think) M90's have a significantly improved flow and hence can handle higher rev with out the same temp spike. But like I said, I'm no guru.

What's Klem running Madaz?
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Re: Supercharger Boost Calculations

Postby Madaz » Sun May 15, 2016 10:55 am

Pretty sure Klems got the later model blower, his boost is up around mine too i think, maybe even a bit higher

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