Supercharger Calculations
Discussion
How do I go about choosing the right supercharger for a particular application ? The supercharger manufactures provide air flow maps which at the moment mean nothing to me. I understand that there is a lower level below which the air flow stalls and a higher level above which surging can occur but for a simple person like me how can I choose the correct charger and the correct gearing. Is there a rule of thumb ?
Obviously I can calculate the theoretical engine air requirements between max RPM and tick over and presumably being N/A at the moment it will only have a V/E of 70% or so but where do I go from there ?
Also the other bit I can’t figure out from data is how much power does the supercharger take from the engine.
Obviously I can calculate the theoretical engine air requirements between max RPM and tick over and presumably being N/A at the moment it will only have a V/E of 70% or so but where do I go from there ?
Also the other bit I can’t figure out from data is how much power does the supercharger take from the engine.
You could ask the people selling the superchargers ?
What blower were you intending to use ? And as with most compressors, you do have the option of altering gearing to spin the unit faster or slower to meet your airflow/power requirements.
When buying, I'd sooner buy a slightly larger unit and spin it slower, than a smaller one and spin the crap out of it.
What blower were you intending to use ? And as with most compressors, you do have the option of altering gearing to spin the unit faster or slower to meet your airflow/power requirements.
When buying, I'd sooner buy a slightly larger unit and spin it slower, than a smaller one and spin the crap out of it.
eliotrw said:
What car is it?
It is a 5.2 rover V8 fitted in a Hill climb car. The engine is not ideally suited to forced induction as It has a fairly radical cam and was built for reasonably high compression. The problem I have is fitting some non restrictive plumbing for the induction air. Allowing the throttle bodies to breathe directly from the outside (via filters) the engine makes reasonable power although a bit more is generally good. Adding the rather torturous induction plumbing rather takes the edge off.
My perhaps ill advised thought was to bolt on a blower that at least cancelled the induction plumbing losses and provided just enough additional boost to cover the additional loading plus a small bit.
At the moment I am considering the Rotrex C30
The issue I see here, is the C30 is only a 400hp unit. How much power are you currently making ?
You'll be spinning the crap out of that little blower and gaining very little in return.
With blow through, the intake tract from blower to intake valves really doesnt need to be pretty. Yes it's better if it is, but it really isnt essential.
I can see what you are thinking, but really given the cost and work involved it really would not make any sense at all unless the parts were free.
Any photos of the vehicle and engine compartment ?
You'll be spinning the crap out of that little blower and gaining very little in return.
With blow through, the intake tract from blower to intake valves really doesnt need to be pretty. Yes it's better if it is, but it really isnt essential.
I can see what you are thinking, but really given the cost and work involved it really would not make any sense at all unless the parts were free.
Any photos of the vehicle and engine compartment ?
I'd also be a little concerned about utilising a really big centrifugal charger, it'll be a bit peaky as there's a (quite high) boost threshold on them meaning it'll be "no power...no power...BOOOOOOST-redline" unless you are very careful with induction routing, cooling and so forth.
A Roots type blower will be more useful IMO - essentially it'll be the same dyno plot as you have now, just further up the chart. Also the supercharger whine will be awesome.
A Roots type blower will be more useful IMO - essentially it'll be the same dyno plot as you have now, just further up the chart. Also the supercharger whine will be awesome.
davepoth said:
I'd also be a little concerned about utilising a really big centrifugal charger, it'll be a bit peaky as there's a (quite high) boost threshold on them meaning it'll be "no power...no power...BOOOOOOST-redline" unless you are very careful with induction routing, cooling and so forth.
A Roots type blower will be more useful IMO - essentially it'll be the same dyno plot as you have now, just further up the chart. Also the supercharger whine will be awesome.
That makes no sense. A big centrifugal geared correctly will give smooth linear power delivery.A Roots type blower will be more useful IMO - essentially it'll be the same dyno plot as you have now, just further up the chart. Also the supercharger whine will be awesome.
A small CF blower like the CF30 in this instance will be pointless IMO.
And a Rootes style will mostly boost low rpm power with less up top. The opposite of what he is trying to achieve.
It's not the gearing its the difference between a centrifugal "dynamic" compressor and a positive displacement compressor.
The Rotrex type units that use a high speed turbocharger compressor style wheel rely on recovering the dynamic pressure imparted to the air flow as that airflow is accelerated through the compressor blades. The high "static" pressure is generated in the compressor outlet scroll as that high speed air is slowed back down again.
In a conventional roots type supercharger, a fixed "positive" type volume of air is simple moved (at a low speed and hence low dynamic pressure component) to the outlet side of the compressor.
The key point being that dynamic pressure is the square of velocity. So, a Rotrex type compressor makes boost/airflow as a square of it's rotational velocity. A roots type compressor makes boost/airflow as a linear function of it's velocity
hence a Rotrex type compress will always have a "peaky" delivery compared to the roots/conventional type
The Rotrex type units that use a high speed turbocharger compressor style wheel rely on recovering the dynamic pressure imparted to the air flow as that airflow is accelerated through the compressor blades. The high "static" pressure is generated in the compressor outlet scroll as that high speed air is slowed back down again.
In a conventional roots type supercharger, a fixed "positive" type volume of air is simple moved (at a low speed and hence low dynamic pressure component) to the outlet side of the compressor.
The key point being that dynamic pressure is the square of velocity. So, a Rotrex type compressor makes boost/airflow as a square of it's rotational velocity. A roots type compressor makes boost/airflow as a linear function of it's velocity
hence a Rotrex type compress will always have a "peaky" delivery compared to the roots/conventional type
Max_Torque said:
hence a Rotrex type compress will always have a "peaky" delivery compared to the roots/conventional type
A power graph with my Vortech would show almost a straight diagnol line. Certainly not peaky at all. Although obviously as boost rises and everything starts to work nicely, the extra oomph is most certainly felt.I guess the term "peaky" open to interpretation. I call mine smooth, but mildly powerful.
"Peaky" by my definition would be seeing no boost at all below 4000rpm, 5psi by 4500rpm, 15psi by 5500rpm and then still climbing to the limiter. This was using a Rotrex C30-94 on a VR6 engine. Saw very similar behaviour from a Vortech V9-F trim (a really terrible blower).
We also played with an Eaton M110 (IIRC)on the same engine and that gave a totally differnt torque curve, being a PD blower.
And then a GT3582R turbo. 5psi at 2500rpm, 10psi at 3000rpm, 15psi at 3500rpm then held to the redline. Even that looked more linear and progressive than the Rotrex plot.
So yes, centrifugals are generally peaky on anything but massive V8s it would seem.
We also played with an Eaton M110 (IIRC)on the same engine and that gave a totally differnt torque curve, being a PD blower.
And then a GT3582R turbo. 5psi at 2500rpm, 10psi at 3000rpm, 15psi at 3500rpm then held to the redline. Even that looked more linear and progressive than the Rotrex plot.
So yes, centrifugals are generally peaky on anything but massive V8s it would seem.
Edited by SuperchargedVR6 on Thursday 15th November 12:09
SuperchargedVR6 said:
"Peaky" by my definition would be seeing no boost at all below 4000rpm, 5psi by 4500rpm, 15psi by 5500rpm and then still climbing to the limiter. This was using a Rotrex C30-94 on a VR6 engine. Saw very similar behaviour from a Vortech V9-F trim (a really terrible blower).
We also played with an Eaton M110 (IIRC)on the same engine and that gave a totally differnt torque curve, being a PD blower.
And then a GT3582R turbo. 5psi at 2500rpm, 10psi at 3000rpm, 15psi at 3500rpm then held to the redline. Even that looked more linear and progressive than the Rotrex plot.
So yes, centrifugals are generally peaky on anything but massive V8s it would seem.
Peaky ? That sounds smooth to me. Peaky to me would mean unstable or unpredictable.We also played with an Eaton M110 (IIRC)on the same engine and that gave a totally differnt torque curve, being a PD blower.
And then a GT3582R turbo. 5psi at 2500rpm, 10psi at 3000rpm, 15psi at 3500rpm then held to the redline. Even that looked more linear and progressive than the Rotrex plot.
So yes, centrifugals are generally peaky on anything but massive V8s it would seem.
And a linear boost curve is not a linear power curve. It isnt boost we are trying to make, it is power/torque.
Having boost rise with rpm has lots of benefits for drivetrain longevity and traction, which ultimately can result in a faster and more reliable car.
It's easy to get fixated on the huge rush of torque from a turbo setup often gives. It doesnt always make for a faster car though.
I dont think my Vortech centrifugal supercharger has a peaky delivery look at the graphs. There are so many factors that account for how any engine will respond to a forced induction such as cams, plenums, throttle bodies, induction length, piston design, combustion chamber design, compression ration etc etc.
Unless someone has supercharged an identical engine before then I would ask the supercharger manufacturer for an approximate guide of pulley based on bhp and engine size. You will then have to run the engine and see what boost it produces it is then easy to calculate a new pulley based on the figures of the first pulley.
I would cosider your options very carefully, if you are running high CR on the Rover engine then you may be disappointed with your results as to stop this type of high CR engine pinking you sometimes need to retard the ignition so much it negates the benifit the supercharger! My results are shown below.This is not the Rover engine.
Unless someone has supercharged an identical engine before then I would ask the supercharger manufacturer for an approximate guide of pulley based on bhp and engine size. You will then have to run the engine and see what boost it produces it is then easy to calculate a new pulley based on the figures of the first pulley.
I would cosider your options very carefully, if you are running high CR on the Rover engine then you may be disappointed with your results as to stop this type of high CR engine pinking you sometimes need to retard the ignition so much it negates the benifit the supercharger! My results are shown below.This is not the Rover engine.
a1rak said:
Figures are as folllows: 535bhp at 6250rpm. 460ftlbs torque at 5354rpm didnt want to take it any higher as the engine h as only done 500miles and only has semisynthetic oil at present. The supercharger is producing 6psi of boost. No detonation problems or overheating even though the car was on the rollers for 8hrs. I think with some differnt pullies careful mapping at the higher RPM maybe 600bhp could be possible. One unexpected plus point is the throttle responce, its instant no lag it picks up like a bike engine (Emeralds comments not mine)
Once its run in I will try and get some performance figures. So if it all holds together then I think its been a worthwhile project.
Once its run in I will try and get some performance figures. So if it all holds together then I think its been a worthwhile project.
Thom said:
Was that on a 2.8 VR6 or bigger? What turbine A/R on the GT35?
Standard Corrado 2.9 with an 8.5:1 decompression plate. The back housing on the 35R was 0.82 AR. The 1.06 housing gives a better top end but the onset of boost suffers a little, by approx 500rpm.stevieturbo said:
Peaky ? That sounds smooth to me. Peaky to me would mean unstable or unpredictable.
And a linear boost curve is not a linear power curve. It isnt boost we are trying to make, it is power/torque.
Having boost rise with rpm has lots of benefits for drivetrain longevity and traction, which ultimately can result in a faster and more reliable car.
It's easy to get fixated on the huge rush of torque from a turbo setup often gives. It doesnt always make for a faster car though.
But unfortunately the boost curve was translated to the torque curve in this instance. It was far from smooth to drive. 5 psi jumping to 15psi over such a narrow rpm band wasn't too pleasant. A proper nothing, nothing, nothing, BANG car to drive on the road.And a linear boost curve is not a linear power curve. It isnt boost we are trying to make, it is power/torque.
Having boost rise with rpm has lots of benefits for drivetrain longevity and traction, which ultimately can result in a faster and more reliable car.
It's easy to get fixated on the huge rush of torque from a turbo setup often gives. It doesnt always make for a faster car though.
Agreed on the turbo. I chose a larger turbo than is the norm for my application to give the gearbox a fighting chance of survival and I hate modern turbo engines where you get a massive spike in the midrange and then the party is over soon after.
The turbo was more reliable in the end. The amount of belt tension and wrap required to run 12psi from the Vortech wrecked water pump and PAS pump bearings quickly. The Rotrex is a lot better in this respect though, thanks to it's planetry gears.
What I have failed to fully understand are the air flow maps. I have spoken to several suppliers but not got an answer I have understood yet. The maps show what looks like contour lines and I understand that shows the blower efficiency so ideally you want to choose the blower and gearing so your airflow sits in the highest contour area.
None of the maps however seem to start from zero pressure differential and I don’t understand why that is, they all seem to start at around 1.5 they would more sense to me if the air flow was shown from 0 pressure differential which would presumably be 1 atmosphere on the map. Perhaps the point from 0 and wherever the maps start is unstable or difficult to map but you would think it would simply be a linear increase relative to RPM.
None of the maps however seem to start from zero pressure differential and I don’t understand why that is, they all seem to start at around 1.5 they would more sense to me if the air flow was shown from 0 pressure differential which would presumably be 1 atmosphere on the map. Perhaps the point from 0 and wherever the maps start is unstable or difficult to map but you would think it would simply be a linear increase relative to RPM.
CF blowers dont flow much air at very low speeds. And yes, 1 bar is atmospheric.
You need to establish how much air your engine will consume, then you can make an estimate as to which blower is most suitable and what sort of speeds you need to turn it to achieve airflow/PR etc
Or in more simple terms. Buy a 600hp blower and spin it at max rpm to try and achieve close to 600hp
Or spin it slower to achieve less. Then just deal with whatever boost etc it makes or doesnt make
You need to establish how much air your engine will consume, then you can make an estimate as to which blower is most suitable and what sort of speeds you need to turn it to achieve airflow/PR etc
Or in more simple terms. Buy a 600hp blower and spin it at max rpm to try and achieve close to 600hp
Or spin it slower to achieve less. Then just deal with whatever boost etc it makes or doesnt make
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