Hi,

I have a 1999 Mazda Miata with a 1.9L I4. It has a 1.6L Whipple Supercharger pullied to make 30 PSI boost with the 2.59:1 pulley ratio, air/air intercooled on E85.

I am planning to drop the pulley ratio to 2.00 which puts the boost around 20-22 PSI from the Whipple, then use a large rear mounted turbo to compound into it. Total Target boost at the intake manifold would be in the 40-45 PSI range, 70-75lbs/min.

Question 1: If the engine is 1.9L and breathing in 15 PSI boost from the SC at say 4,500 RPMs, would the exhaust mass flow of this setup mirror an engine of twice the displacement? Would it spool the turbo as if it were an engine of twice the displacement?

Question 2: I'm considering running an ebay GT45 in the rear, it's a 68.7mm turbo and has a 2.25" diameter compressor discharge. Would running a 2.25" diameter pipe from the rear of the car to the front be acceptable regarding the pressure drop inside a pipe this small? Due to packaging, running a 2.5" pipe would be much more difficult.

Question 3: Due to packaging, installing an intercooler for the turbo would mean sacrificing what is left of my trunk. I have a larger intercooler after the Whipple. What level of boost could I run from the turbo before I need to intercool it before going into the Whipple?

Your thoughts and advice are much appreciated! I've never compound boosted anything before so this is new to me.

Pat

 

The Whipple must have the capability to flow the cfm flow for your new HP target.

Even though it is a screw supercharger and can turn more SC rpm vs the Eaton or Roots stuff it still needs to be able to move the cfm
to supply the engine at your new target HP.

The turbocharger will increase the boost before the supercharger and increase the mass flow (allowing you to add more fuel to the engine
but the supercharger will be the cork as far as overall flow thru the engine.

What is the capability of the Whipple that you are looking at?

Tom V.

 

The whipple is rated to flow up to 1,100 CFM/ ~69 lbs/min of air. But in my experience so far, it robs so much HP from the crankshaft to do so that running boost over 20 PSI, I hit the point of diminishing returns where I pump more air, and burn more fuel, but only make a little bit more power.

Thus I believe using a compound setup, and dropping the boost level on the SC, would result in making more hp, and allow me to increase the mass flow through the system.

Wouldn't the volumetric flow rate through the SC stay the same at a given pulley ratio though? And the mass flow rate would vary depending on how much boost the turbo supplies to the inlet of the supercharger?

From what you wrote, you are saying the increase in mass flow due to the turbo would cause the supercharger to pose an additional restriction, even at the same volumetric flow rate?

 

Doesn't going a smaller blower pulley (2.59 to 2.0), spin the blower faster and makes more boost? You would need a larger diameter blower pulley to make less boost.

I think it would work and like the idea for your setup, you may have to play around with the whipple boost level to see what works best, you want the blower only spinning fast enough to get the turbo going properly and you want it taking over before you hit the max boost of the supercharger. The twin screw is different than a roots as you will be limited by the displacement of the blower since the air is trapped/compressed between the vanes of the screw. Also you will need a larger wastegate to keep boost level down.

 

I was referring to the pulley ratio, not the pulley size.

For the wastegate, I was thinking of using a 50mm or 60mm external gate. Probably just use a 60mm since I don't see any downside to having too much valve.

I agree on the SC pulley ratio. I'm going to try the 2.0 ratio first (5.5 lower, 2.75 upper) and go from there.


Regarding my original 3 questions, what do you think? I'm thinking the blower boost will more or less act like a bigger engine regarding spoolage of a large turbo, a 2.25" pipe will probably be a 2-4 PSI pressure drop but acceptable, and no intercooler would probably be ok up to 8-10 psi from the turbo.

 

Hello Pat!

My thoughts are above.

Edit: I've included them below for easier quoting and response.

Answer 1: In a perfect world the exhaust mass flow will double if your DENSITY ratio doubles. (If holding the same AFR.) Don't look at it as boost pressure, look at it as density or mass flow.

Answer 2: I wouldn't really be afraid of the 2.25" pipe at first. I say, give it a shot and see how it goes. If anything, place two pressure transmitters, errrr pressure gauges for those without tens of thousands of dollars in research data aq equipment, at the compressor discharge and at the inlet of the supercharger to see where you are really at.


Answer 3: I'm all about some interstage intercooling. But right now my car (compounded 3000GT VR4) doesn't have one, just a big 4" thick core after the high pressure turbo (small turbo). It is still tons of fun but its definitely on my list of improvements to do next.

 

Thanks VR4drive92. I do have a second MAP sensor I use to measure pressure drops in various things on the car. I will measure the drop in the 2.25" pipe. I think it will be acceptable, so I'm going to build it that way. Just wanted to make sure that's not a huge problem before building it and finding out on my own that it won't work.

Would you mind if I PM'd you a few questions regarding compounding? I've never done it before.

 

Agree with VR4drive92 that you are interested in the MASS FLOW change going thru the engine.

If you kept the Supercharger pulley ratio exactly the same in both cases, the blower hp required from the engine would basically be the same but the density of the charge,
(mass flow) would double if the turbo was supplying the proper additional air required to double that density.

I have been told that you would like for the pressure ratio to be similar in both units, say 1.8 PR in both cases.

Does that make sense?

Tom V.

 

Hey patsmx5, sure thing. I'm open to helping people out! Especially on this Forum.

 

Hey patsmx5, I tried to send a couple of PMs but they aren't showing up in my sent items box.

Let me know if you haven't received anything from me.

 

Annnnnnnnnnnnnnnnnnd I should have had more coffee this morning.

Thanks Ken! HAHA

 

As an update, I ended up pulling the whipple and building a compound setup with two turbos instead. It's working great so far. Still tuning it and upping the boost. At 29 lbs of boost so far. Plan is to keep the turbos at equal boost, and raise them together.

 

E-85 fuel will only buy you a given amount of boost from the turbo (going to the supercharger) before you get into a over-temp issue on the supercharger. The Whipple will not be happy at some point. All screw type superchargers make heat thru compression of the air.

A Roots S/C unit is a "positive displacement" supercharger and it gets HOT.

A Whipple (internal compression supercharger) will get even hotter with non cooled air from a turbo blowing into the inlet at any real boost level. Without that inter-stage inter-cooler I think you will have a flawed system from the get-go.

Tom V.

 

I ended up building it compound turbos, no whipple. Also the whipple had an air/air intercooler after it.

Right now it's Big turbo, small turbo, intercooler, engine. No room for interstage cooling.

 

Probably a better combination of parts at the end of the day without the inter-stage inter-cooler.

Whipple and Roots superchargers run hot enought as it is.

I have had compression superchargers touch the rotors to the case under high load/temp conditions even though
the manufacturer said the unit would be fine.

Tom V.

 

YIKES! Sounds like their team underestimated their thermal growth. That's an odd oversight!

 

My whipple would get REALLY loud at the end of an 1/8th mile pass when hot lapping the car at 30 PSI. Upon teardown, rotors touched.

Good thing is turbos don't have this problem.

 

That is true! Turbos have their own set of potential issues.

1) Turbo Installation Angle "too high" which causes oil to migrate past the piston ring seal and into the exhaust Turbine Housing where the oil either burns, (Great mosquito killer),
or it oils down the underside of your vehicle and oil on the drive way.

2) Too much crankcase pressure which causes the oil to find its way past the compressor seal and into the compressor housing. (Same mosquito killer), but now you are town on
power due to the excessive oil in the combustion chamber. First thing the people say is "Your Valve Guides are Shot". PCV systems and crankcase evac systems help on that deal
but a properly designed Engine Crankcase/Valve Cover design (to relieve the pressure in the pan) works well too.

3) Too much oil to the turbo. A proper amount of oil to the turbo(s) is a good thing but most manufacturers have a oil pressure and line size recommendation where a "fire hose"
supply system is not required. DO NOT UNDER ANY CIRCUMSTANCES REMOVE THE OIL FEED LINE TO THE TURBO TO 'SEE' IF YOU HAVE OIL FEEDING THE TURBO. Remove the drain line and catch the oil (and a lot of bubbles) in a large bucket/drain pan for your test and then hook the line back up to the oil pan, etc.

4) No dips and "rolling hills" in your oil drain hose down to the oil pan. Those "rolling hills" trap the oil and bubbles, which builds up pressure in the drain line and then the oil goes
an easier path past the turbine seal or compressor seal and you are back "killing mosquitos".

5) Very high "pounds per minute" compressors move a lot of air (which does not want to be moved) and the load on the turbo thrust bearing (even with oil) over time will hurt the
turbo, (AND THE ENGINE). THE THRUST BEARING WEARS THE METAL COMES OFF THE THRUST BEARING MIXES WITH THE OIL AND BUBBLES AND AT SOME POINT WILL GET INTO
THE ENGINE OIL SYSTEM AND ENGINE BEARINGS. If you keep after the engine with proper oil changes the material does not hurt the bearings. At some point moving to a 'Ceramic
Ball Bearing Thrust System' makes sense for Go For Broke race set-ups. Justin, on Street Outlaws, eventually went from journal bearings to the ceramic ball bearing stuff on Harry's
units.

So sorry for the long post with a mix of different potential issues that can happen with combustion, boosting, plumbing, etc.

Tom V.