Force feeding a SC14

[jabbatron]

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[samkaos]

Been there done that lol
I had a SC14 under driven to 5psi, and a GT2860RS pumping into it (after intercooler), then a smaller cooler after the SC14.

1. The boost doesnt add up, the SC multiplies the boost not adds to it eg. 12psi from turbo + 5psi from SC = 25psi! Because the SC is multiplying the air going into it, not adding to it.

2. The SC creates a lot of heat and power wastage because of the pressure differential accross it (13psi in my case), so it is great to drive but makes less power then turbo only setup.

3. The SC does make the turbo spool earlier, about 1500rpm earlier in my case.

Ultimately you need to bypass the SC when the turbo kicks in, so you dont get the pumping losses.

[NME308]

Hi Damien,
Did I somehow miss you at Toyota Nationals just gone or just simply not join the dots and acknowledge who the hell you were...??

Anyhow as to the subject matter I say go for it! The supercharger will not care where its air comes from its job in life is to compress it by a certain pressure ratio.

Just keep in mind the system will need to be calculated on pressure ratios. If the supercharger can theoretically pump up the engine to 12psi boost and you want a total of 25psi with the turbo at full song it will need to look like follows:
Supercharger 12 + 14.7 / 14.7 = 1.82PR (pressure ratio).
Total required boost from turbo 25 + 14.7 / 14.7 /1.82 = 1.48PR x 14.7 - 14.7 = 7psi
Check our math 1.82PR from the supercharger multiplied by 1.48PR from the turbo and convert back to PSI gauge - 1.82 x 1.48 x 14.7 - 14.7 = 24.9psi if you will forgive the 0.1psi missing in the math...

So the question is will the turbo be happy putting out 7psi pumping the required mass flow to support the HP goal in the theoretical scenario I posed? Where will it sit in the compressor map?

Now the above is just an example I pulled from thin air but it is how to determine what each of your boosting devices is going to be doing.

If you already are educated on all the above then my apologies for boring you!!

Cheers,
Jason

Edit - beaten by samkaos!
Samkaos are you saying there was a deficit in the total boost as calculated on paper once operating in an actual engine?

[jabbatron]

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[jabbatron]

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[JustenGT8]

You will always make less power psi for psi with a twin charge setup. Compound boost setups also compound the inefficiencies.....but that's not the point.

Compounded setup won't mske the turbo work less either? The trick is to have the flow you want from the turbo at a low enough pressure ratio that you don't compound the setup into a hand grenade.

Run the SC as slow as you can to give you the bottom end power you want. Run the turbo to get the maximum mid to top end power at a boost level your engine will survive.

On the UZ I ran the M90 to 6 psi and the twin turbos to 12psi....giving near enough the compound boost that Jase calculated of 25psi.

To give you an idea of how inefficient a twin charge setup is, the twincharge made 377rwkw on 25 psi, compared to the TT which made 411rwkw on only 17psi!! With bigger turbine a/r I could have made the setup much more efficient but it would never come close to a turbo only.

It did make 250rwkw by 2500rpm though and the mid range torque was mind blowing!!

Do it, but just be aware of the limitations

[jabbatron]

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[JustenGT8]

No the M90 isn't clutched.

Tricky bits will be getting a decent enough bypass around the SC and obviously the transition from SC to T. Doable but fiddly and hard to get right in practise....which is why you rarely see it done.

[jabbatron]

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[NME308]

You've probably thought the sequential bit through by now but I have a head full of questions... I think Audi or one of the group b rally cars of awesome used that configuration very successfully however!

- The suoercharger 'shows' the turbo a much larger engine so it comes on boost sooner. What happens when you divert around the supercharger? Does the turbo drop off boost?
- If the above is a concern how do you control the switching point & avoid overboosting until high enough rpm's to adequately drive the turbo without life support from the supercharger?
- Does the turbo compound the supercharger up to a certain point then the diverter is activated & supercharger is turned off at a given point?
- Does the turbo have its own entry into the plumbing pre throttle body? That is to say no compounding at all.
- If so how do we stop the superchargers boost being lost blowing back through the turbo before it is on boost?

I can think of a couple of possibly over complicated solutions but hopefully it will be simple enough...
Maybe compounding it with the turbo's wastegate large enough to successfully hold only the peak boost required & the suoercharger bypass activated at say 4500rpm where it is certain the turbo is fully capable of holding the boost on its own?

I'm just thinking on the keyboard here. I drew everything out & studied it carefully when doing the compound turbo thing. Then I'd bounce it all off my brother to see if he thought it was sound theory etc.

Cheers,
Jasonn

[jabbatron]

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[jabbatron]

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[AE25]

*Lancia delta S4 or Nissan march superturbo.
also researched lots/never completed. and wrote ideas on rollaclub thread.
the sequential T into SC bypass thing.. To avoid the boost drop, do not disengage SC until the bypass is fully open (or don't disengage at all except cruise and idle situations). To avoid the compounding boost spike, the bypass needs to be full open before the turbo reaches max boost. Thus the bypass needs to start opening after turbo has equalised the set SC output and be fully open well before turbo reaches max boost to have a smooth progressive transition.
You'll hopefully have enough programmability and control to adjust when it happens with the stepper/servo butterfly valve and suitable ecu. ie if SC max is 5psi, turbo max 20 (non compounding), start opening bypass at 6 and fully open by 10. (guessed figures, tune to suit).
Or if it's a basic open/shut control, may need to be some hysteresis built into it so the bypass doesn't spazz out. Be curious to see if that 3" valve has enough control at partial open position.
Turning off the SC at set rev or set boost doesn't account for all load situations.

[NME308]

I recon you will make a killing - go for gold!

Cheers,
Jason

[JustenGT8]

I still think you can get an acceptable result twincharge, you just use a much larger turbo than you'd think necessary. Big turbine with large a/r to avoid back pressure. You can go much bigger than even what you would for the boosted capacity to turbo sees as response isn't an issue...the SC is doing that. You want a big compressor as you want the flow at a low PR. Now you can't do this normally as you'd get surge, but here you won't as the SC essentially sucks the turbo along at the lower rpm.

How to calc the required size is the tricky big but minimum would be a big GT3582...., more likely something GT40ish or an in betweener from Precision.

It would be stout an eleventy times simpler than the seq setup....so what it's not as efficient and needs more psi per hp. The 377rwkw I had was way more fun than the 411rwkw 98% of the time