Turbo Versus Roots Net Horsepower

[38flattie]

I'm looking for a little info. I've called several manufactures of 6-71 style blowers, and haven't received a definitive answer.

I know it takes HP to turn the blower-let's call that X. Just for the sake of this discussion, let's say X=100HP

The FlatCad made 570HP at 14.2lbs boost. If I took the blower off, and added a turbo, at 14.2 lbs boost would I then see that 100HP in my net numbers-670HP?

...or am I looking at this wrong?



Yes, I know the turbo isn't legal in Vintage class!

[dw230]

If you are thinking of swapping the roots for the turbo you will have to change classes. Page 18, last paragraph. You will run heads up cubic inches in the new class.

DW

[RansomT]

Since we are just using "wag" numbers .... The answer is not really.  You lose some exhaust efficiency to turn the turbo.  But, there is so much in play there; you may gain more than that” X” back especially on the top end of the RPM band.

[Bob Drury]

  Let's not forget that unless you run a intercooler under the Root's blower that they inherantly build heat while compressing air and therefore tend to lean out the motor which will be felt mostly by the rear cylinders as a Root's blower pushes air (and fuel) to the front of the case.
  This is not as bad with alcohol, but with gas it is almost imperitive to run a intercooler if you want the maximum in power, and who doesn't.
  In the old days before port nossles, most of the top fuel "hero's" ran two and sometimes three different length rods to compensate for the cylinders getting less fuel (and air) from front to rear.  Now days, if the fuel ports are also used in the injector hat, that is mainly to cool the rotors and hopefully the compressed air below the rotors.
  Unfortunatly, the hotter the air fuel mixture is, the leaner it gets.
  With the Turbo you almost allways run a large ice box type intercooler (as do the Langlo's with their centrifical supercharger) which in my book would be better than a Root's.
  On the other hand, a "screw type" like a PSI or Whipple (Lysolm) produces massive amounts of boost {depending on the amount of overdrive) with less heat as they have much more clearance between the rotors (screws) than a Root's type.         Bob

[38flattie]

DW, I was thinking special construction!

Bob, and RansomT, thanks!

The roots blower has a chiller intercooler, and so will the turbo. The turbo will have both the existing chiller, and an intercooler like Tony has on his REMR.

Manufacturers will not tell me an approximation of the HP need to produce 14.2 lbs boost on my 366 CID engine, so I used a 'wag' number, that I think is probably conservative. My goal is to try to 'free' up the HP I have, and not have to hammer the engine any harder. That is to say, I don't want to add RPM's or more boost at the moment.

My goal is to run the engine this year, then go back on the dyno with a turbo, engine management, and fuel. Unless I can mathematically come up close to the numbers I'm looking for, though, it will be a waste of time.

So, does anyone know how to calculate, or does anyone have results, showing the gains of a turbo over a roots blower??

[RichFox]

Try asking turbo kit manufactures. I bet they are more willing to talk about parasitic loss with roots blowers.

[Tman]

Coincidentally Buddy I have a hair dryer coming my way also!

[38flattie]

Coincidentally Buddy I have a hair dryer coming my way also!

Cool! I don't-I'm still in the research stage, to see if I can get the HP I think I need.

I did find this, from the 2003-Interesting article in the August issue of Hot Rod.
Take a stroked 327 ci SBF with common aftermarket parts, and compare the performance of the engine with commonly available Centrifugal (Paxton Novi 1200), Turbo (HP Performance T62-1), and Roots (Holley 174) supercharger kits. Boost was limited to 9.5psi, non-intercooled.

-------------------------Baseline------Centrifugal------Turbo--------Roots
Peak HP------------------392@6000------617@6000---------600@6000-----535@6000
Peak TQ------------------386@5200------561@5200---------617@4200-----513@4600
Min Boost----------------1.7@2500------5.7@2500---------4.8@2500
Max Boost----------------9.5@6000------9.5@5100---------8.0@6000
Ave HP(2500-6000rpm)-----310-------------412------------460---------394
Ave TQ(2500-6000rpm)-----365-------------494------------564---------483
Ave HP(4000-6000rpm)-----352-------------518------------555---------472
Ave TQ(4000-6000rpm)-----371-------------542------------585---------497

TQ@2500----------------------------------360------------490---------440
TQ@3000------------------340-------------405------------500---------450
TQ@3500------------------355-------------450------------560---------475
TQ@4000------------------365-------------500------------610---------500
TQ@4500------------------380-------------525------------610---------505
TQ@5000------------------375-------------555------------600---------505
TQ@5500------------------355-------------555------------560---------485
TQ@6000------------------354-------------540------------530---------475

HP@2500----------------------------------170------------235---------210
HP@3000------------------190-------------235------------290---------250
HP@3500------------------240-------------300------------375---------325
HP@4000------------------275-------------375------------455---------375
HP@4500------------------325-------------450------------525---------445
HP@5000------------------360-------------525------------575---------485
HP@5500------------------380-------------575------------600---------510
HP@6000------------------395-------------617------------600---------535

This makes me think I can gain HP, without adding stress from higher boost, or higher RPM's.

Surely, with all the knowledge on this board, someone knows the answer?

[Tman]

I am learning as well. Elpolacko sent me this link.

http://www.theturboforums.com/smf/index.php

[Bob Drury]

  One more thing, the more overdrive or bigger the blower is with a Root's type, the more horsepower it takes to turn it.
  I have heard but don't quote me, that a Top Fuel 1471 set back blower with a fair amount (?) of overdrive takes over 500 horsepower to turn.  Most large teams have their own blower dyno's and restrip them after every run if they show any signs of wear on the Teflon and Nylatron seals.
  They hesitate to install a different blower between runs because it can change the whole tune up from the previous run.
  For those not familiar with Roots type (GMC) superchargers, the rotors and case excluding the end covers is 16" on a 671, 17" on a 871, etc.
  Also, with a supercharger you can "feather" into the throttle although it ain't easy with a hat on it.  With the turbo (depending on size and number of turbo's) along with engine size, you better have your belts on tight when they come in or you may be going down the course in a 360 degree angle of attack..................     Bob

[wheelrdealer]

Interesting, I am in the middle of the research stage as well. I am thinking about switching from CGALT to CBGALT and/or CBFALT with a D motor for 2013. Matt my engine guy is a big turbo guy so that is the way he wants me to go. I like a roots for the look in a Classic catagory. But looking at the Hot Rod chart a centrifical may be worth looking at. I have already started down the turbo route. I had my spare heads rebuilt with valves and springs ready for a turbo. We built a set of forward facing headers. Still dont have the turbo, manifold, or intercooler so I can esily adjust now.

In 2011 the car ran ann top speed of 204 at the 3 mile, with a 3:50 gear at 8,450 RPM. Need to go mid 230's for the CGALT or mid 220's for the CBGALT. The chassis dyno in Tampa said we made 448 hp at the rear wheels with the carburated motor. Probably need 700 plus at the flywheel to get in the mid 220's.

With that said, the turbo set up is probably the most cost effective for my situation...just really would like to have a bug or bird catcher poping out of the hood.



Bill
 

[WOODY@DDLLC]

With that said, the turbo set up is probably the most cost effective for my situation...just really would like to have a bug or bird catcher poping out of the hood.

Bill

Go with the turbo and make an intercooler that looks like a bird catcher? 

[Milwaukee Midget]

Just thinking out loud on this, and correct me if I'm wrong, because I genuinely don't know for sure.   

With the exhaust backpressure that a turbo creates, I think you would have a tougher time dissipating heat from the exhaust side of the block than you would with open headers.  If that logic proves true, bear in mind that your exhaust port is through the block - a block that is 75 years old, and already stressed to a point it was never intended to be.

So to my mind, a question comes up as to whether or not the block, valve seats area would hold up to the potential heat a turbo might create.  You know the engine better than I do.

What kind of exhaust temps were you seeing on the dyno?  What kind of increase in exhaust temperature is usual for a turbocharged engine?

These are the questions I’d be asking.  Am I off base?

The upside is that with the exhaust exiting the top of the engine, it could be a very compact packaging situation.

Again, thinking out loud.

[hotrod]

This makes me think I can gain HP, without adding stress from higher boost, or higher RPM's.

It all depends on the thermal efficiency of the blower/turbo, ie how much of the work input to the supercharger gets turned into useful manifold pressure.

The problem is not all manufactures are willing to give you useful information regarding the thermal efficiency of their blower/supercharger.

If you look on a turbocharge compressor map it shows you the thermal efficiency at certain pressure ratios and flow rates.

http://www.automotivearticles.com/uploads/flow_map.jpg

As you can see in the above (for this particular turbo) there is a sweet spot in the map ranging from a pressure ratio of 1.5 - 2.0 and flow rates from 15 - 22 pounds of air per minute flow rate where the turbo is operating at its maximum thermal efficiency of about 73%. If you can size the turbo to operate near its sweet spot at your design boost pressure and the cfm air flow demand of your engine at that rpm it will usually give you more boost for the cost in power it takes to drive the turbo (or centrifugal blower) than a roots blower.

http://www.automotivearticles.com/Turbo_Selection.shtml

https://en.wikipedia.org/wiki/Roots-type_supercharger
https://en.wikipedia.org/wiki/File:Roots_Supercharger_efficiency_map.jpg

Roots blowers at high boost typically only manage about 50% - 60% thermal efficiency where a centrifugal/turbo can if properly sized hit thermal efficiencies of about 70%. You have to simply work through the numbers for your particular combination to determine which combination will give you the best total cylinder pressure.

In the two compressor maps above the roots will deliver air at a pressure ratio of 2:1 but only at a thermal efficiency of 58% where if sized properly the turbo map shows you could get the same pressure ratio of 2:1 at about 73% thermal efficiency for the right sized engine/turbo combination.

Due to how internal combustion engines work not all pressure is useful. Horsepower goes up almost in direct proportion to absolute manifold pressure if the air temperature is held constant. Unfortunately power varies at one over the square root of the absolute temperature. (cooler temps give more power)

Some of the boost pressure in a real compressor is due to heating of the air charge by the work of compressing the air, so bottom line the blower/turbo/centrifugal supercharger that delivers the coolest air at the boost pressure you need at the flow rate you need will be doing the least wasted work heating the air charge and will use the least power.

You can recover part of that power by force cooling the air charge with a heat exchanger but you are still losing the excess power wasted heating the air, and you are losing boost pressure as the air cools so your initial boost pressure has to be higher to compensate for this cooling pressure loss. Intercoolers also cause their own pressure loss due to their inlet restriction to the flow. The final manifold pressure you see in the engine will always be lower than the actual outlet air pressure from the supercharger before intercooling.

Look for the setup that will deliver the coolest air charge at the blower discharge for the boost and flow rate you need.

http://www.stealth316.com/0-frames.htm
http://www.stealth316.com/2-adiabat1.htm

Larry

[38flattie]

Bill, I agree that the 'cool factor' of the bug/bird catcher is immense, but at the end of the day, probably cost HP.

This winter, I'm going to switch to EFI, with port injection, and engine management.  Looking at the turbo curve, keeping RPM's to about 5250 would be perfect for this engine. Lot's of HP, equal torque, and I would not have to hammer it so hard.

Your engine revs a lot higher, but I'm sure you get raise the HP peak on the turbos with a different setup than what was tested.

Good luck!

Chris, you bring up an interesting point, but I don't think it will hurt us much. Since the ports are on top of the block, they don't heat the engine block as much as on most engines. They may, however, have an effect on intake temps, but hopefully we can combat some of that with the fuel mixture, and intercoolers. Also, I run a lot of overlap, which also helps cool the exhaust side.

Larry, that's good info-Just the info I need! It's also the reason I'm asking questions and researching this- there is an efficient way, and a fubar way!