Twin Charging a Honda CRX 3

[patprimmer]

I having a moment of madness and considering twin charging a 1988 Aussie model CRX.

Aussie model has the DOHC D16A8 engine and factory A/C and P/S both of which I must retain.

The plan would involve a second hand Toyota SC14 roots type blower.

I already have a turbo being a brand new Garrett GT2859R-707160-9. M24 cast on compressor.
Ball bearing, water cooled bearings.
Compressor is 44.5 inducer, 59.4 exducer, 56 trim, 0.42 A/R
Turbine is 53.8 wheel dia, 62 trim, 0.64 A/R
It has an internal waste gate but I also have a 41mm Chinese external wastegate.

The turbo is correct size for my ambitions turbo only, but probably a bit small for twin charged.





Regards
Pat
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[70btdc]

Yes, but blower boost does not mean turbo boost and therefore max boost. That will still take 'some' time to get the shaft back to speed once you have snapped the throttle closed and opened it again. Having the standard dump-valve or recirc valve helps the turbo shaft speed deteriorate less quickly (and reduces shock), but does not maintain the speed. Keeping the SC flow through the TC during zero and part-throttle just acts like an anti-lag.

Like you say though, maybe response is good enough without that benefit, or maybe you are never off throttle whith this engine? Drag racing?

 

[patprimmer]

It's a daily drive and will be used for all sorts of duties from city traffic to drifting through windy and even dirt outback roads to street class drag strip to auto X

It will weigh in at 1900# I expect and have 250ish hp and around 180 ft lb with 4WD. 1.8 litre 4 valve twin cam at 7# boost from the blower with no turbo at all should respond well enough. I expect the turbo boost will just be extra party time.

Regards
Pat
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[TDIMeister]

Quotes of horsepower are still premature in the absence of an operating map for your SC. If you lay out your SC and TC in the order you propose (TC blows into SC - in reverse of what I suggested), your power will be limited by the mass flow rate limit of your TC map, which is already well established, regardless of how much boost you may be able to generate on paper.

On the other hand, if you lay out the SC to blow into the TC and the SC is too small, then IT will be your limiting factor.

In either case, if you generate much the boost in the first stage without intercooling between stages, you generate conditions of high compressor work in the second stage, and this is more grave for the SC in the second stage in terms of thermal considerations and drive-belt skipping because you're asking for too much power transmission upon the belt.

The higher mass flow-rated device should ALWAYS be in the first stage.

 

[Warpspeed]

The mass flow will be the same, but the turbo is far better able to cope with the high flow volume, and low pressure of the first stage.

The second stage compressor work will be higher, (without interstage cooling), but the disadvantage is more theoretical than practical.

The only real limitation might be supercharger bearing and lubricant temperature. But if the interstage temperatures are objectionably high, it does not take much to pull down that temperature significantly, because the temperature differential across an interstage heat exchanger will be so high.

In any case, the supercharger will be well heat soaked, and running at under hood ambient temperatures anyway, so cooling the supercharger intake air temperature down below that, is a complete waste of time.

In practice, the blower is going to run hot, and a short burst of acceleration (say ten seconds worth) is not going to have any significant effect on the supercharger bearing temperatures.

Running it for several minutes flat out on a dyno might be different.
But on the road, cooking the supercharger bearings has never in my experience been a problem for anyone.

If you start to think about water pump and alternator bearings, and how reliable they are at similar operating temperatures, supercharger bearings will be just as reliable.

 

[TDIMeister]

I have a map of the compressor which Pat describes: "Compressor is 44.5 inducer, 59.4 exducer, 56 trim, 0.42 A/R"

It is capable of flowing 26 lb/min at PR=2.3 with 70% efficiency. Efficiency drops off rapidly without a significant increase in mass flow beyond this point.

The order of which device is put in which stage does not materially affect the attainable power output; the output will be determined by which ever device has the limiting *corrected* mass flow in the system -- mass flow remains constant because of the continuity equation but *corrected* mass flow across the device depends on inlet temperature and pressure doesn't have to be the same across both devices. A device in the first stage draws from ambient so that actual mass flow is approximately equal to corrected mass flow. In contrast, a device in the second stage has its inlet charge pre-compressed by the first stage, thus having the appearance of a "larger" device - meaning that the corrected mass flow is less than the actual mass flow by the correction factor that's proportional to T^0.5/P, i.e. the plotted operating points of corrected mass flow vs. pressure ratio on a compressor map will effectively shift toward the left.

 

[Warpspeed]

What happens up stream of any compressor is vitally important.

The flow of a turbo is largely influenced by any up stream flow restriction, they tend towards being constant pressure differential devices, with variable flow.

A positive displacement supercharger tends towards being a constant flow device. Restrict the outlet significantly, and discharge pressure can rise hugely.

 

[patprimmer]

I fully understand that the supercharger will not really increase power other than by limiting detonation and freeing up the tune a little.

The purpose of the supercharger is twofold.

1) To improve response while the turbo is off boost.

2) To scavenge residual hot exhaust gas from the chamber during overlap.

As I understand it, a turbo is not positive displacement and it can allow air to flow past it with moderate resistance even if the wheel is stationary. It obviously offers less resistance as it increases rpm up to the point that it contributes to airflow.

A roots blower is positive displacement and it's displacement is absolutely tied to rpm. If it is stationary, no air flows. Although it cannot displace extra air from the turbo under boost, that air can have a higher density so mas air flow is maintained at the level of the turbo.

This means to a reasonable degree you get the mass air flow of the highest device. I know the turbo airflow depends on the head of pressure it pumps against as it is not positive displacement.

Regards
Pat
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[naturalextraction]

Hi Pat, it's been a few weeks since the last entry here and was wondering what you ended up doing/deciding? I had thoughts on this subject. Sometimes the threads just end!

 

[patprimmer]

I am searching for suitable superchargers so I can assess the space available.

Also car is stripped back to a bare shell and I need a few parts back on before I can do more measurements. It will take some time if it ever happens.

Regards
Pat
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[naturalextraction]

Your two reasons for utilizing the belt driven supercharger is well understood but was wondering why not hybrid a turbo or utilizing design changes for the turbine side to improve boost response. We mated a Holset turbine and a Schwitzer compressor housing and impeller to achieve the same for my ol 72 Duster 318. I had great low rpm boost while not loosing top end pressure. Also kept the efficiency of the turbo within a ~70% margin. Hybriding turbos or modifying them takes a bit of doing but the results, if calculated appropriately, can make an old stone dance nicely. I would also think your reason #2 could still be achieved with some additional creative engineering. Just a thought.

 

[1gibson]

Similarly off topic, I know that Saab used some electric turbos that had an alternator/motor on a common shaft. Was able to assist spool using battery power, and charge the battery as well.

This is not a bilge pump "electric turbo" so I hope you read past the first sentence!

 

[patprimmer]

I already have a properly sized turbo, cam and manifold selections for minimal lag.

ANY turbo system has some lag.

A roots blower has virtually zero lag and that is the only reason I am considering the extra complication.

Regards
Pat
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[Warpspeed]

Pat, size up the available space and then make up a complete dummy blower out of any suitable material, from the engineering drawings.

Bodge it all up, and you will then have an excellent idea of how to mount it, required drive snout length, and how to drive it, and available clearance.

When you know EXACTLY what you want, your supreme patience will eventually be rewarded on e-bay.

 

[izzmus]

"why not hybrid a turbo or utilizing design changes for the turbine side to improve boost response."

That would seem to do the exact opposite of twincharging. Choking the turbine side down would increase exhaust manifold pressure, while twincharging would decrease it.

 

[naturalextraction]

Who said anything about "choking the turbine side down"? That was not what I implied and not the only way to gain faster spool up while not changing the A/R of the turbine side.

 

[Warpspeed]

It has all been tried before.
I have spent around eighteen months of totally wasted effort trying to get a VNT turbo to work on a gasoline engine.

Forget it !!!!!

Just build a twincharge system using a mundane garden variety roots blower, and a suitable modern ball bearing turbo, and it will absolutely KILL any straight turbo engine in every single respect.

Strong words, but anyone that has ever tried twincharging, or driven a twincharged vehicle knows exactly what I am talking about.

 

[70btdc]

Hey Warpspeed, interesting that you tried the VGT on a SI engine, I also had some experience of doing just that on a motorbike engine. What were the difficulties that you experienced? Vanes jamming? Poor boost control at high rpm?

As far as I am aware the only (current) production SI engine that uses VGT turbine technology is Porsche's Flat-6 Turbo. They changed some materials to give lower thermal expansion, and if you look at the turbine side a/r, it is quite large....

 

[Warpspeed]

The actual turbo itself never had a single problem. They are totally reliable on diesels, reliability was never an issue.

Now the theory goes, that you can close the vanes right up and get an incredibly small a/r and the turbo will spool up at very low rpm.

The engine simply does not like seeing 80psi+ exhaust back pressure at wide open throttle low rpm.

The theory also goes that you can open the vanes right up and get massive low restriction exhaust flow.

Again all you get is massive exhaust back pressure at the top end. The reason being you are trying to force ALL of the exhaust gas through the turbine. It just goes into sonic choke and the power curve hits a brick wall.

Not even Garrett could make it work on a gasoline engine.
If it did work well, Garrett would be selling them instead of their excellent ball bearing turbos.

These VNT turbos have been around for decades on diesels, everyone has tried, only Porsche have been able to get it to work after massive complexity, and adding twin wastegates and a lot of electronics to control the vanes.

Twincharging is vastly simpler to control and get working, the results will be infinitely better, and anyone can do it.

 

[patprimmer]

Once again. Thank you Warpspeed. You have fully answered my questions and have been most helpful. I wish there was a way I could close this thread as I certainly have no interest in alternatives to a rootes blower and turbo in series with the rootes blower being between the turbo and the inlet manifold.

Regards
Pat
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[70btdc]

Hmm, the reason Garrett dont make the VGT tech for SI engines is preturbine temps above the material limit causing vanes to expand and seize; not due to the VGT mechanism causing over-restriction like you say. In fact the VGT mechanism has the opposite effect, that is why large Turbine'd VGT's are more efficient at full load compared to a similar FGT that would be fitted to the same engine. You can run a much larger Turbine a/r but still maintain low flow response.

What type of vane position control did you use? What type of VGT mechanism did the turbo have?

Sorry Pat, if this is a bit OT, I will start another thread.