Roots vs screw? And blower vs turbo questions

[skav]

How do roots sizes compare to screw sizes?

For example, how many liters is a 6-71?

What is the advantage of one over the other?

Last blower question, how much boost is realistic with each and what's the 91 pump gas limit?

Now turbos...

During the monster bash poker run, there were a ton of blowers, but the big merc power was twin turbo.

At what point is a turbo preferred over a blower and why aren't they used more often?

Seems like most of the turbo boat aside from merc are older ray jay or banks systems.

 

[AzGeo]

HA HA HA HA HA with differing 'modes of attack', the number of questions don't reflect the number of possible correct answers .

How about 'more focus' or 'categorizing' money VS power ?

Your questions seem to put no 'lid' on the amount of money to spend on any system ......

 

[skav]

I thought the shotgun approach would be best[emoji6]

Let's just focus on the blower portion for now.

Like how you roots numbers compare to screw.

I.e. 6-71 is 3.0 liters or an 8-71 is the same as a 4.0...

 

[PLACECRAFT20]

I am a Whipple guy, but after driving and riding in a m31 with twin 1350. I think turbo is the way to go if money is not a question.
I would love to drive one of Carsons motors, and maybe one day I will own one!

 

[Outdrive1]

I thought the shotgun approach would be best[emoji6]

Let's just focus on the blower portion for now.

Like how you roots numbers compare to screw.

I.e. 6-71 is 3.0 liters or an 8-71 is the same as a 4.0...

So let's say. Most screw type setups are EFI. Not all but most. Most roots style setups are carb. The screw type is more efficient, more expensive and usually include an EFI setup which makes them really tame at idle and reliable. Easy to start. Low idle for shifting etc. Just my opinion or what I see. EFI whipple is way more marketable to a lot of people also. It's also a smaller footprint and easier to fit under a hatch. Cog style belts on roots blowers are also messy.


Turbos are expensive. Plus your not going to waste the money building an 800 hp turbo motor. You can do a Whipple with off the shelve parts much cheaper. If you want big hp turbos become an option.

 

[skav]

Thanks guys. What about displacement? What Whipple would compare to what blower shop?

Wh

 

[LargeOrangeFont]

So let's say. Most screw type setups are EFI. Not all but most. Most roots style setups are carb. The screw type is more efficient, more expensive and usually include an EFI setup which makes them really tame at idle and reliable. Easy to start. Low idle for shifting etc. Just my opinion or what I see. EFI whipple is way more marketable to a lot of people also. It's also a smaller footprint and easier to fit under a hatch. Cog style belts on roots blowers are also messy.


Turbos are expensive. Plus your not going to waste the money building an 800 hp turbo motor. You can do a Whipple with off the shelve parts much cheaper. If you want big hp turbos become an option.

Yep, yep and yep.

I do wonder why everyone does twin turbos.. I assume it is for aesthetics, but a modern large single turbo can spool just as fast as twins, and cut down on price.

The ability to run 91 is completely setup dependent. Generally speaking the larger the engine, the further you will be able to go on 91, but that is not always the case. When forced induction is thrown in the mix, the most efficient blower or turbo will heat the air less and allow you to make the most power on 91 before detonation.

 

[Outdrive1]

One more thought on big hp motors. You aren't going to lose a belt on a Turbo motor.

 

[GMD]

So let's say. Most screw type setups are EFI. Not all but most. Most roots style setups are carb. The screw type is more efficient, more expensive and usually include an EFI setup which makes them really tame at idle and reliable. Easy to start. Low idle for shifting etc. Just my opinion or what I see. EFI whipple is way more marketable to a lot of people also. It's also a smaller footprint and easier to fit under a hatch. Cog style belts on roots blowers are also messy.


Turbos are expensive. Plus your not going to waste the money building an 800 hp turbo motor. You can do a Whipple with off the shelve parts much cheaper. If you want big hp turbos become an option.

EXACTLY what he ^^^^^^^^^said.

 

[skav]

So let's say. Most screw type setups are EFI. Not all but most. Most roots style setups are carb. The screw type is more efficient, more expensive and usually include an EFI setup which makes them really tame at idle and reliable. Easy to start. Low idle for shifting etc. Just my opinion or what I see. EFI whipple is way more marketable to a lot of people also. It's also a smaller footprint and easier to fit under a hatch. Cog style belts on roots blowers are also messy.

What makes the Whipple/screw more efficient than a roots straight lobe? Is it just a newer, better design? Both are available with intercoolers so heat shouldn't be an issue. Would it be correct to call a roots setup "old school"?

I get the whole efi computer deal. My Whipple and mefi is super mellow-if that's the right way to describe it.

It always fires right up and idles at 850 with no surge.

 

[pronstar]

What makes the Whipple/screw more efficient than a roots straight lobe? Is it just a newer, better design? Both are available with intercoolers so heat shouldn't be an issue. Would it be correct to call a roots setup "old school"?

The twin-screw compressor itself is a much more efficient design.

 

[BamBam]

Yep, yep and yep.

I do wonder why everyone does twin turbos.. I assume it is for aesthetics, but a modern large single turbo can spool just as fast as twins, and cut down on price.

The ability to run 91 is completely setup dependent. Generally speaking the larger the engine, the further you will be able to go on 91, but that is not always the case. When forced induction is thrown in the mix, the most efficient blower or turbo will heat the air less and allow you to make the most power on 91 before detonation.

Twins look cool. It is also much more difficult to do a big single in a boat as all of the exhaust tubing crossing over needs to be stainless double jacket and stay full of water. By the time you pay a fabricator to build it and buy all the tubing etc. you could just buy a twin setup.

 

[Outdrive1]

What makes the Whipple/screw more efficient than a roots straight lobe? Is it just a newer, better design? Both are available with intercoolers so heat shouldn't be an issue. Would it be correct to call a roots setup "old school"?

I get the whole efi computer deal. My Whipple and mefi is super mellow-if that's the right way to describe it.

It always fires right up and idles at 850 with no surge.

It's a little above my pay grade. From what I understood a roots type blower pushes air a screw type compresses air. Not sure why or how that's more efficient. Lol.

And yes roots style are old school. They came from old school two stroke diesels.

 

[skav]

The twin-screw compressor itself is a much more efficient design.

Isn't a twin screw just a twisted three lobe? They both have two rotors with three lobes.

 

[thetub]

prochargers are another good way to make power. I dont know why they dont run more of these

 

[pronstar]

Isn't a twin screw just a twisted three lobe? They both have two rotors with three lobes.

A roots blower basically pumps air into the engine, and that's where pressure builds...and heat.

A twin-screw builds pressure inside the blower housing, where it can be cooled before being introduced to the engine.

The rotors themselves are also quite different if you google some pics.

The size difference alone is quite significant for a given boost range.

prochargers are another good way to make power. I dont know why they dont run more of these

They're more dependent on pulley size and rpm, it's basically a belt-driven turbo. They're very efficient, but can't make usable boost at all rpm - especially lower engine speeds - like roots or twin-screw.

 

[skav]

A roots blower basically pumps air into the engine, and that's where pressure builds...and heat.

A twin-screw builds pressure inside the blower housing, where it can be cooled before being introduced to the engine.

The rotors themselves are also quite different if you google some pics.

The size difference alone is quite significant for a given boost range.




They're more dependent on pulley size and rpm, it's basically a belt-driven turbo. They're very efficient, but can't make usable boost at all rpm - especially lower engine speeds - like roots or twin-screw.

It's a little above my pay grade. From what I understood a roots type blower pushes air a screw type compresses air. Not sure why or how that's more efficient. Lol.

And yes roots style are old school. They came from old school two stroke diesels.

Ok that makes sense on the difference between the two and it's easy to see how the screw will compress the air as it enters the rear of the blower.

How would you compare sizes of roots to sizes of screws?

 

[rrrr]

The Roots supercharger was patented in 1860. It's been around for a while.

The common size numbers were derived from Detroit Diesel's adaptation of the Roots blower. The common Detroit two stroke engine had standardized 71 cubic inch cylinders, and the number of cylinders was added as a prefix; thus 6-71, 8-71, 12-71.

The Detroit's two stroke combustion cycle meant there was no intake stroke, the blower provided an intake charge through a cylinder port. The blower charged the cylinder as the piston traveled upward.

Then the piston passed the port opening as it approached TDC, the fuel was injected, and the charge was ignited.

It was dirty, noisy, and inefficient.

Size comparison is simply volume of air moved in lbs/hr at a given RPM. A pound of air at standard density and at sea level weighs .0748 lbs/cu ft.

 

[pronstar]

Ok that makes sense on the difference between the two and it's easy to see how the screw will compress the air as it enters the rear of the blower.

How would you compare sizes of roots to sizes of screws?

Screws are much smaller, with tighter tolerances, than the large lobes used in roots blowers.

Just look at the packaging of the blowers themselves - you can usually put a whipple under a normal-size hatch. You usually aren't able to do this with a roots blower.

 

[Instigator]

What makes the Whipple/screw more efficient than a roots straight lobe? Is it just a newer, better design? Both are available with intercoolers so heat shouldn't be an issue. Would it be correct to call a roots setup "old school"?

I get the whole efi computer deal. My Whipple and mefi is super mellow-if that's the right way to describe it.

It always fires right up and idles at 850 with no surge.

Screw Superchargers are actually an air compressor which is more adiabatic efficient than a Roots Supercharger. A Roots supercharger, which most originated from a 2 stroke diesel engine, typically a Detroit ie; 8-71 (8 cylinder, 71 cubic inches per cylinder). Centrifugal superchargers are kind of a cross breed between turbos and Screw or Roots Superchargers. All of these are mechanically driven.

Turbo Chargers work primarily on the Radiant Heat/Expansion of exhaust gases not so much exhaust pressure or back pressure. In a Diesel engine they work perfectly. However with a gasoline engine they can cause a snowball effect once the engine fuel/air ratio goes lean as it will case more radiant exhaust heat which will spool the turbo to make more boost which will only further lean the engine, and so on.

All types of supercharging used to increase the volumetric efficiency of the engine which will raise the power output or Brake Means Effective Pressure.

Here are a couple of links that you should find interesting. All of them are pretty basic and short reads but will give you a better understanding of what you are asking about.
Hope this helps.

http://www.superchargersonline.com/index.php?main_page=page&id=12

https://en.wikipedia.org/wiki/Turbocharger

http://kennebell.net/KBWebsite/Common/pdfs/SC_efficiency.pdf

 

[SoCalZero]

 

[LargeOrangeFont]

prochargers are another good way to make power. I dont know why they dont run more of these

They are less efficient than a turbo and add boost by RPM, so power is just added on the mid-top end. You don't have instant torque like you do with a roots or twin screw. The torque curve of a roots or twin screw looks like a mesa.. it will stay within 10-15% of maximum torque across the entire RPM range.

Pluses of centrifugal blowers like Prochargers are compact, low profile packaging and relatively simple install, but again you are giving up some low and midrange power.

 

[Racey]

What makes the Whipple/screw more efficient than a roots straight lobe? Is it just a newer, better design? Both are available with intercoolers so heat shouldn't be an issue. Would it be correct to call a roots setup "old school"?

I get the whole efi computer deal. My Whipple and mefi is super mellow-if that's the right way to describe it.

It always fires right up and idles at 850 with no surge.

It takes less input hp to compress the charge by a given amount, so it robs less power from the crank. It also doesn't build as much friction heat, so your inlet temps are better, the air doesn't spend a whole lot of time in contact with the intercooler, so the lower the temp is pre-intercooler the better the intercooler can work. I've seen roots blower motors that build over 200F of intake temp at idle, that is an incredible waste of energy.

Roots blowers weren't designed to actually build boost in the first place, they were designed to move intake air into a 2 stroke diesel motor that doesn't have an intake stroke. They were only adapted to compress air after the fact on gasoline engines. Screw compressors (Whipple, etc) on the other hand are industrial compressors, and were designed specifically to compress air efficiently.

 

[plaster dave]

Can any give a reference on the size question? Is a 871 equal to a 3.1 whipple or would you say a 871 is equal to a 2.3? Thx

 

[Bigbore500r]

I found this interesting -

Nelson Racing Engines stated that when he pulls his 1200hp SBC turbo motors apart after 100 dyno pulls, the main bearings look brand new. When he pulls apart his SBC blower motors after 100 dyno runs @ the same power levels, it looks like somebody beat the shit out of the bearings.

He attributes it to the load and harmonics of the belt being driven off the crank @ those horsepower and boost levels. I'd be curious if any other builders have seen similar results? Obviously most all big boat motors are supercharged and expected to run without teardown for 100's of hours - But they usually are running much lower boost levels than a car engine, and less RPM. (Nelson regularly runs 25 psi and 6800+ Rpm on big HP small CI builds).

 

[ToMorrow44]

Can any give a reference on the size question? Is a 871 equal to a 3.1 whipple or would you say a 871 is equal to a 2.3? Thx

Its hard to compare the two apples to apples because the screw type is so much more efficient and spins at a much faster speed. But heres a chart I stole, you can convert the numbers to liters for comparison to a whipple. The roots-type blowers spin closer to a 1:1 ratio with motor RPM, the Whipples are spun at 2-3xs faster than the motor (hence the huge crank pulley and tiny blower pulley). So while a 10-71 blower per this chart is about 7 liters per revolution, a 3.3Lwhipple can make the same power.

For comparison, Teague used 10-71 blowers on their 800s and now use 3.3L Whipples, 14-71 on their 1000s and now 4.0L whipple on their 1025.

 

[plaster dave]

Its hard to compare the two apples to apples because the screw type is so much more efficient and spins at a much faster speed. But heres a chart I stole, you can convert the numbers to liters for comparison to a whipple. The roots-type blowers spin closer to a 1:1 ratio with motor RPM, the Whipples are spun at 2-3xs faster than the motor (hence the huge crank pulley and tiny blower pulley). So while a 10-71 blower per this chart is about 7 liters per revolution, a 3.3Lwhipple can make the same power.

For comparison, Teague used 10-71 blowers on their 800s and now use 3.3L Whipples, 14-71 on their 1000s and now 4.0L whipple on their 1025.

View attachment 443307

Cool thx. I'm swapping out a 871 for a quad 2.3 setup this winter and was curious like Skav was.

 

[ToMorrow44]

Oh and for comparison, heres what the Whipple rotors look like, one 3 lobe, one 5 lobe. The tolerances are much much tighter which means more efficiency. Any air pump (like turbine engines) the gap between the case and rotors = inefficiency, so the tighter the better. Case in point, I had a set of bearings go out in my Whipple (old style made by Lysholm) which caused the rotors to contact the case after a hard run. After the motor cooled down, the rotors no longer hit the case, the slight metal expansion due to heat was all it took to make that contact.

 

[ToMorrow44]

Cool thx. I'm swapping out a 871 for a quad 2.3 setup this winter and was curious like Skav was.

I'm sure you'll be able to make the same power with less boost and less heat with the quadrotors.

 

[HBCraig]

I can only chime in here from buddies that have had both. One buddy had a procharged 454 Mag back in the day. He ran the piss out of it. well over 400hours and it was great.

one buddy had a Weiand that he constantly changed pulley sizes, boost, etc and he chased problems non stop. It never ran great for more than a day.

 

[LargeOrangeFont]

I found this interesting -

Nelson Racing Engines stated that when he pulls his 1200hp SBC turbo motors apart after 100 dyno pulls, the main bearings look brand new. When he pulls apart his SBC blower motors after 100 dyno runs @ the same power levels, it looks like somebody beat the shit out of the bearings.

He attributes it to the load and harmonics of the belt being driven off the crank @ those horsepower and boost levels. I'd be curious if any other builders have seen similar results? Obviously most all big boat motors are supercharged and expected to run without teardown for 100's of hours - But they usually are running much lower boost levels than a car engine, and less RPM. (Nelson regularly runs 25 psi and 6800+ Rpm on big HP small CI builds).

That is interesting and makes a lot of sense..

 

[rrrr]

Can any give a reference on the size question? Is a 871 equal to a 3.1 whipple or would you say a 871 is equal to a 2.3? Thx

As I said above its a function of lbs of air moved per minute and that depends on blower RPM and pulley ratios. You have to get specs on each blower running at a typical speed.

There's not just one answer.

 

[Racey]

As I said above its a function of lbs of air moved per minute and that depends on blower RPM and pulley ratios. You have to get specs on each blower running at a typical speed.

There's not just one answer.

Compressor maps :thumbup:

 

[DLow]

View attachment 443266

You know, I was looking at this picture wondering how the gear set was different. Didn't make sense that the rotors spun at two speeds. Then you guys posted this...

View attachment 443309

And now I realize they do... In this case would it be a 3:5 ratio, correct? 5 rotor spinning 3 times, to the 3 rotor spinning 5? I've spent all my time around a roots type, which are 1:1.

 

[AEA]

I found this interesting -

Nelson Racing Engines stated that when he pulls his 1200hp SBC turbo motors apart after 100 dyno pulls, the main bearings look brand new. When he pulls apart his SBC blower motors after 100 dyno runs @ the same power levels, it looks like somebody beat the shit out of the bearings.

He attributes it to the load and harmonics of the belt being driven off the crank @ those horsepower and boost levels. I'd be curious if any other builders have seen similar results? Obviously most all big boat motors are supercharged and expected to run without teardown for 100's of hours - But they usually are running much lower boost levels than a car engine, and less RPM. (Nelson regularly runs 25 psi and 6800+ Rpm on big HP small CI builds).

Does Nelson build marine engines?

 

[Bigbore500r]

Does Nelson build marine engines?

I dont think so, at least not that I have seen.

 

[skav]

So here's another question.... Just using Whipples for this example.

Let's say you have a 4.0 with a 4" pulley and it makes 10psi at 5500. This should be a solid 1100hp on a 565ci motor.

Now you move from a 4.0 to a 5.0 with the same 4" pulley. Everything else remains the same.

Will boost stay at 10psi, drop or increase? And why?

 

[ToMorrow44]

And now I realize they do... In this case would it be a 3:5 ratio, correct? 5 rotor spinning 3 times, to the 3 rotor spinning 5? I've spent all my time around a roots type, which are 1:1.

I was talking purely the blower speed vs engine RPM (i.e. the pulley sizes) but you're right, the blower has its own gearcase as each rotor turns a different rpm. I believe the pulley drives the 5 lobe rotor, with the exception of the 8.3L whipple where they drive the 3 lobe rotor.

So if the motor is turning 6000rpm, the whipple may be spinning 15,000 rpm, but the 3 lobe rotor may be spinning even faster. Thats part of what gives screw superchargers that distinctive whine.

 

[ToMorrow44]

So here's another question.... Just using Whipples for this example.

Let's say you have a 4.0 with a 4" pulley and it makes 10psi at 5500. This should be a solid 1100hp on a 565ci motor.

Now you move from a 4.0 to a 5.0 with the same 4" pulley. Everything else remains the same.

Will boost stay at 10psi, drop or increase? And why?

I would guess boost would increase, just because the 5L blower compresses more air per revolution, and it would be spinning the same speed, so more air = more boost.

 

[skav]

I would guess boost would increase, just because the 5L blower compresses more air per revolution, and it would be spinning the same speed, so more air = more boost.

Ok, that seems right. let's say it takes a 5" pulley to bring the 5.0 back to 10psi. Would the 5.0 make more HP at the same psi as the 4.0? It's spinning slower but displaces more air.

 

[ToMorrow44]

Ok, that seems right. let's say it takes a 5" pulley to bring the 5.0 back to 10psi. Would the 5.0 make more HP at the same psi as the 4.0? It's spinning slower but displaces more air.

I asked Dustin Whipple a similar question about upgrading my 3.3 to either a 4.0 or 5.0. He said the 5.0 would make a tad more power on the top end, but less down low so the 4.0 was the way to go. That was with my motor being the limiting factor, not a general rule or anything. I think the technical answer is "it depends" lol.

 

[pronstar]

All compressors have efficiency ranges, I think you have to overlay the efficiency maps to determine the correct answer to your question.

 

[skav]

I asked Dustin Whipple a similar question about upgrading my 3.3 to either a 4.0 or 5.0. He said the 5.0 would make a tad more power on the top end, but less down low so the 4.0 was the way to go. That was with my motor being the limiting factor, not a general rule or anything. I think the technical answer is "it depends" lol.

At what point does cylinder pressure become an issue? You go from 4l at 10psi to 5l at the same psi. Your not increasing the boost but adding more volume.

Also, would the 5l be more efficient at the same psi? It is spinning slower so should have a cooler charge.

 

[BamBam]

At what point does cylinder pressure become an issue? You go from 4l at 10psi to 5l at the same psi. Your not increasing the boost but adding more volume.

Also, would the 5l be more efficient at the same psi? It is spinning slower so should have a cooler charge.

I'm not quite sure that you are correct. 10psi is 10psi for a given engine (that is equal volume). A 4L will make the 10psi with less heat than the 3.3L because its not working as hard but it will ultimately make the same HP with less overdrive (pulley ratio) The cylinder pressure is a function of the boost pressure no matter what type of compressor (turbo, roots or screw blower) adds the power.

 

[Bigbore500r]

Ok, that seems right. let's say it takes a 5" pulley to bring the 5.0 back to 10psi. Would the 5.0 make more HP at the same psi as the 4.0? It's spinning slower but displaces more air.

If the 4.0 was spinning fast enough that IAT's were becoming an issue, and switching to the a slower spinning 5.0L unit resulted in a cooler IAT's, then yes it would make more power at the same boost level

 

[Bigbore500r]

At what point does cylinder pressure become an issue? You go from 4l at 10psi to 5l at the same psi. Your not increasing the boost but adding more volume.

Also, would the 5l be more efficient at the same psi? It is spinning slower so should have a cooler charge.

You are not increasing volume because you are spinning the bigger compressor slower....resulting in the same net volume and pressure. The cooler intake charge as a result would increase power....how much would depend on how much improvement in temps you get.

 

[skav]

I asked Dustin Whipple a similar question about upgrading my 3.3 to either a 4.0 or 5.0. He said the 5.0 would make a tad more power on the top end, but less down low so the 4.0 was the way to go. That was with my motor being the limiting factor, not a general rule or anything. I think the technical answer is "it depends" lol.

ToMorrow,

Did you increase boost when you went with the 4l or keep it the same?

What were your results? More power? Did the boost start later than it did with the 3.3?

I.e. The 3.3 started making boost at 3500 where the 4 starts making boost at 4000?

 

[skav]

You are not increasing volume because you are spinning the bigger compressor slower....resulting in the same net volume and pressure. The cooler intake charge as a result would increase power....how much would depend on how much improvement in temps you get.

So the only reason to go with a larger compressor is to increase boost and keep it at roughly the same iat as older compressor?

If that's the case, could you use a larger intercooler and spin a 3.3 faster? This seems like it would make more power at lower rpm than a 4.0.

I remember when I was playing with my 7.3l diesel-if we swapped the stock turbo for a gtrx4292 there was a noticeable difference in power at the same boost and rpm because of the increased volume that the 4294 provided over the stock turbo.

But I know turbos are different than Superchargers.

 

[pronstar]

So the only reason to go with a larger compressor is to increase boost and keep it at roughly the same iat as older compressor?

If that's the case, could you use a larger intercooler and spin a 3.3 faster? This seems like it would make more power at lower rpm than a 4.0.

I remember when I was playing with my 7.3l diesel-if we swapped the stock turbo for a gtrx4292 there was a noticeable difference in power at the same boost and rpm because of the increased volume that the 4294 provided over the stock turbo.

But I know turbos are different than Superchargers.

You have to work within the compressor's efficiency map.

You can't just put a giant compressor on and spin it slowly, anymore than you can put a tiny compressor on and spin it to the moon.

 

[ToMorrow44]

ToMorrow,

Did you increase boost when you went with the 4l or keep it the same?

What were your results? More power? Did the boost start later than it did with the 3.3?

I.e. The 3.3 started making boost at 3500 where the 4 starts making boost at 4000?

I didn't make the switch (someday want to). What I was referencing when talking to Dustin was a motor he built. He took a Teague 800, changed the cam, blower, innercooler, and tune and make just over 1000hp. He also spun the motor to 6400 vice 6000. Not sure what boost level he used.