9 psi Vs. 9 psi

[Lextreme II]

Ok guys,

This is for your gearheads. Here is the question. 9 psi from a Centrifugal Vs. 9 psi from a single turbo. Assume sizing is pretty optimal for the given engine size and hp goal. Which one will win in:

* 1/8 mile
* 1/4 mile

and Why?

 

[Jake Breyck]

adding in twin screw as well, and grip isnt a problem and no launching tricks like break boosting or two steps, I would rate it like this..

twin screw would win to 60ft and maybe even 1/8,
Centrifugal would win to just around 1/8 but maybe even after,
Turbo would win to 1/4 and beond into the highway battle.

Reasons, twinscrew can make boost almost instantly after the trottle is pressed enough to get the bypass to close. but has power draw.. so one or two of its psi is used to make the supercherger turn

the centrifugal will make boost faster then the turbo but still has a little lag, also uses more power to turn then a turbo

Despite common knologe turbos accually do draw power from the motor to make pressure, although much less then superchargers, dont be fooled into thinking turbos are free power.

If someone really cared enough to do this as a test. If you where to install a second engine (doent have to be the same size could just be a 100cc 2stroke) and have this second engine just power a supercharger and set it to 9psi for the sake of this thread, it would make more peak power then the turbo setup at also 9psi.

In the real world tho cars are driven in such a way to get the best out of there setup, so a skilled driver of a turbo car wouldnt just lauch the car from a low rpm, he/she would lauch the car in such a way that there is no lag. back in my dragracing my turbo supra days, i would be at the lights with 10psi and as soon as i went it would jump to my 25psi full boost and hold that all the way down the track.

but thats my just 2c

 

[cjsupra90]

In the 1/4, turbo hands down.
In the 1/8, turbo still but it would be a lot closer.

The reasoning behind all this is first off the turbo is gonna most likely make more power for the the given boost pressure cause it for one is a more effenient throughout its operating range plus the turbo is going to hit the given boost sooner in the RPM range and sustain it through each gear. The centrifical charger being RPM is going to fall out with each gear change. The other thing is that depending on how much boost change accures with gear change, you are more likely to fall out of the "sweet spot" of the compressor's effeniency where as with the turbo, it is less likely to happen, but this change shouldn't have much effect.

Another thing to concider is parasitic loses accociated with the two different forms. Typically, the supercharger is going use up more power to produce the boost then what a properly setup turbo with its induced back pressure on the exhaust pre turbo.....

There is more to it but the rest of the reasonings will have very small effect if any at all on the final outcome.

 

[mshawari1]

Turbo will win in both… why?

At this level of boost the supercharger will take more power to reaches the efficiency region of boost.
You can design the turbo to reach full boost before the supercharge do with less "lose"
​

 

[spf_lexus]

There's no way the centrifugal will "fall out" between gear changes.... once I enter my powerband the only thing that will cause the boost climb to slow down would be laying off throttle... Once the engine is running balls out it only takes the spool time needed to initially spool the charger to 3000rpm+ and over that there is no weakness... just linear climb until i decide to stop. Each gear change has absolutely no effect on boost w/ the prochargers... don't know how the step-up on others affect this but I know the procharger has the largest internal step-up of the group.

As for 1/8 mile i give the turbo and 1/4 I say even draw. The efficiency of the the turbine in a turbo is relative to the gear step-up on centri... a more aggressive step up will spool faster. Same scenario when sourcing a turbo for a specific engine. I still stand strong on "throttling" the centri w/ gate. stone for stone the turbo will come out on top unmodified but the same turbo vs. a throttled centri is no comparison... best of both worlds.

 

[stevechumo]

I know right away we'll run into great debate here. I'd say we must have the correct sizing of the supercharger and the turbocharger must be equal. That's to say both setups will max out at around 11-12 psi while they're still in their efficiency ranges. So 9 psi is in their great efficiency.

Then, I strongly believe the turbo will make more rwhp if the manifold structure is properly designed with straight flow and fast velocity. It'll have backpressure, but it won't lose power as much as the supercharger driven by the belt. As a rule of thumb from all FI books that I read, a supercharger will lose 20% power to the parasitic drag and the turbo will lose up to 5% power of backpressure. More power makes the car faster.

For 1/4 mile, the turbo will win the centrifugal because it can start boosting at 2000 rpm ( as in my case) and reach full boost at around 4500 rpm (at this rpm point I don't remember exactly), and hold the peak boost constanly untill I release the gas pedal. The centrifugal might have some lag because it takes time to build boost from the belt and the somewhat same distance of piping as the turbo. However, the turbo might lose to the twin screw. It's because I saw Cribbj's dyno graph that shoots the power straight up.

Well, I'll let you gents know after my car is dynoed and how it reacts at the track soon. :slomo:

 

[cjsupra90]

Steve, very correct..... The only down fall to the twin screw is that it would make a lot less power for the same boost as it takes up even more power then the centrifical in parasitic loose. IIRC, John HP at 17PSI is barely hitting 500HP flywheel where as Justens car is getting over 500RWHP for the same boost, so assuming a realistically 12% drivetrain loss, Justens engine at 17PSI is making nearly 600HP flywheel....

Yes the roots or twin screw does have power instanly, but all that power at the launch makes the launch far more difficult so PSI for PSI, all else being equal, My money is still on turbo except for an 1/8 mile then I say twin screw without issues.....

 

[cjsupra90]

ok, just went back and dubble checked everything, and Johns engine was actually running 18 to 20 psi and made a just shy of 500 hp (Flywheel) Justen's car made 400 rwkw which is 536 rwhp at only 17psi so engine wise again, assuming a realistic 12% drivetrain lose, thats 610 hp (flywheel).... That is more then a 100 hp difference at less psi.... One thing though is Johns setup might not be fully optimised yet, but still dont see another 100hp coming from it if being that the AFR and the BSFC is pretty darn good as to being well tuned... So that lack of 100 hp is going to make a significant diff in a 1/4 but not so much in the 1/8 cause of the power being there instantly....

 

[Zuffen]

I'm not sure you can compare John and Justins engines to each other as Justen's is an unopened engine running standard comp and John's is a lower comp built engine.

Drag racing is one thing but drivability must enter the equation somewhere as we do tend to drive a lot more distance on the road than the track.

 

[Death By Bass]

did anyone actually ask auto or manual?

a small turbo, doing 9psi, would pretty much be upto boost right off the line.

however, if they're both manuals, then you're going to have an advantage with the supercharger on gear changes.

 

[stevechumo]

My turbo still has peak & constant boost between gear change. I don't see any disadvantage. I used to drive manual turbo Honda before, but I never saw any boost loss during shifting.

 

[XR8tt]

Turbo and auto takes some beating as spool is made while on converter..
In most cases the blower requires higher stall as turbo's respond to load and are more streetable depending on set up..
It all comes down to set up, traction and tyres etc...
No point in having all your power off the line if going to over power the tyres..
In the past drag racing RULES limit turbo cars..
As in no ECU to control engine..
How head in the sand is that ??
Turbo rotors and jap motors [non ANDRA] are 2 seconds slower than the funny cars..

 

[Zuffen]

I'm afraid people like ANDRA and NAHRA still live in the era of twin valve push rod engines so they select against everything else.

 

[JBrady]

Let's clarify further.

400rwhp PERIOD.

Supercharged with centrifugal, roots, screw, vane, ect...
Turbocharged with wastegate.

Each of the above would require a different boost "peak" to acheive the horsepower peak.

From there you MUST consider the torque/power CURVE also known as torque/power under the curve.

ALL engines have a changing volumetric efficiency (VE) as the RPM moves through its range. Peak torque occurs at or very near the peak VE.

Peak VE is when the engine MOVES the most air mass PER event.

On a belt or gear driven supercharger this increased air flow will reduce the boost. On a wastegated turbocharged engine the wastegate will maintain the boost set point through the VE peak. The resulting increase in flow will further boost the torque at VE and throughout the curve.

Even on a positive displacement high efficiency supercharger like a screw/lysolm you will have a depressed boost line from below, through and past peak VE. The wastegated turbo does not have this problem and will return a stronger torque/power curve.

Of course the wastegate also compensates for changes in density altitude (DA) whether occuring with barometric changes and or actual altitude changes (driving into the mountains or from mountains to lower elevation).

Supercharges are wonderful and fun. That said... turbos RULE :fing02:

 

[JustenGT8]

^^^ what he said