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Supercharger Calculations

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jkdmag

Automotive
Sep 9, 2009
40
Have questions regarding supercharger calculations including intercooler type selection.

First part, my calculations indicate that an SC14 supercharger is capable of supplying at least 10 psi boost to a 3.8 litre engine, notwithstanding many comments to the contrary on the internet. The prevailing view seems to be that because this model of supercharger came from a 2 litre engine it is incapable of boosting such a large engine.

Given that the SC14 swept volume is 1420 cc & assuming it has a max rpm approaching 10,000 rpm, I do not see why it cannot provide enough air flow & density, with an intercooler, for 3.8 litre engine.

Understand that the rotors are teflon coated & do not like continual high speed/high load and therefore intend to use it with a clutched pulley, thereby only engaging the supercharger when required.

Given that my air flow numbers are correct & the supercharger is subject to only intermittent use, is there some engineering issue I have missed that makes the idea unworkable.

Second part, installation difficulties of front mounted air/air intercooler, including engine bay space limitations for the plumbing, make air/water intercooler attractive. Easier to route water hoses through engine bay & small size air/water intercooler can be accommodated within the engine bay.

However, concerned that air/water intercooler can only reduce supercharger outlet temperature to approx. 35 deg F above ambient, two stages of imperfect heat transfer as opposed to air/air intercooler's one. I can see that the air/water intercooler's 35 deg above ambient issue could be acceptable if the supercharger outlet temperature was significantly higher than ambient, e.g. 250/300 deg, but not for temperatures less than 200 deg.

Again is this a sound assessment of the air/air vs air/water intercooler quandary.
 
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Just at a glance i'd say the SC you plan to use is too small. The give any decent boost on a 3.8l engine you'd have to gear it at above 2:1 so if the rpm limit of the SC is 10,000rpm then the engine would only be able to rev to 4-5000rpm then you're at risk or overspinning the SC.

There are many others on the forum with more experience than me who would better advise you but in my opinion the swept volume is too low for that engine

As for the intercooler. I'd be fitting the largest and deepest core you can into the available space
 
If it is the supercharger I think it is, it is fairly small rotor diameter. Unless something is fairly fragile, it should easily stand a fair bit more than 10,000rpm.

I have spun GM blowers to 15,000rpm regularly and not had one fail and they are a lot bigger.

To get any useful boost you would need about twice engine speed.

Temperature also has an influence on boost readings.

Regards
Pat
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To assist in commenting on my enquiry, I include brief of my calcs.

Desired power vs present from a 3.8 litre engine would require a pressure ratio of 200kW/125kW = 1.6. Assuming an intercooler keeps the temperature rise to acceptable value, the necessary airflow to acheive the desired power would become (1.6 x cid x rpm x 0.5 x VE)/1728 = 427 cfm. VE assumed 80% and rpm 5000. The supercharger with swept volume of 85 ci (1400cc) would need to run at approx. 8700rpm to achieve this air flow rate, well short of my max 10,000 rpm. If this reasoning is incorrect, where have I gone wrong.
 
I just read the SC14 has weak plastic rotors. That may reduce the potential enough to make this non-viable.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
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The rotors a teflon coated to assist in maintaining a tight seal for good low speed boost. That is why it uses a clutched drive so it only runs when boost is required. Intend to control the clutch from the ecu & use vacuum operated bypass valve.
 
jkdmag

To start---I am not particularly well versed in supercharger installations aside the occasional side trip with a turbocharger or two. However, Pat IS an expert in this area. I would pay close attention to his comments.

For my part, I think using such a timid little blower for what you seem bent on building is a fools errand. Do you have any idea the expense, the havoc that a broken rotor can cause? I'll bet there are better choices about!

Rod
 
Rod

I only ever played with GM type blowers.

I never actually saw a SC14 and I only repeated hearsay.

It is not clear in my mind if the SC14 has plastic rotors or if it has plastic coated (Teflon) aluminium rotors.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
The SC14 is on the small side, and will have to be spun fairly fast to feed a 3.8 litre engine. It will certainly do it, but it is in my opinion an extremely poor choice.

The SC14 has plastic coated rotors which swell with heat, and excessive heat and rpm are well known causes of the rotors shedding the plastic coating. These blowers have a very crude straight two lobe rotor design which pulses badly, the efficiency will be low, and internal heat buildup high.

On the original Toyota vehicles, these blowers are clutched, and only used intermittently, which means the life of the blower is acceptable. Driven at 2.0 to 2.5 the original designed rpm, it will not last long, especially if the vehicle is driven hard. I speak from personal experience here. Having worn out several. And with glued in bearings of unique size (I am not kidding) they are not repairable or rebuildable.

Much better choice would be an Eaton M90. They are about the same size, 90 CID per revolution (1470cc) Much more solid construction, better designed three lobe rotors, a far more serious blower that will have a far longer working life, and will work far better.

Even better again would be an Eaton TVS, maybe an R1650. In the end you will get what you pay for.
 
I just fitted the s.c.14 to my 2.4 diesel, using a 77m.m. v belt pulley that I purhased off ebay. This gives me 8-9 pounds of boost once the charger is up to temp,...when the engine/charger is cold I get 4 pounds. Without an intercooler the intake air temps were sitting on 90.c, so until I get around to fitting one I'm running a passive water injection system which keeps intake temps around the 60 to 70 degree mark. There is a company in castlemaine , Australia, who sell the sc 14 as a kit to fit the 3.8, or you can have them fit one for you at extra cost.
 
Warpspeed, your comment that I would need to drive the SC some 2 to 2.5 times its original design rpm has me perplexed. I refer to my calculations, included above, that indicate that I need only need run the SC to its claimed max speed of 10,000 rpm to acheive the necessary flow. Will accept that from the rpm vs flow data that I have that 10 psi boost is all that I can expect before the pumping efficiency suffers but this is all I need.

Most of the hardware I have is from the Castlemaine kit as described by Winfieldblue. However, the Castlemaine kit runs only 4 psi boost, the original 1 bar MAP, no ecu remapping & no bypass valve. It is clutched, but it's a wonder that it works at all.

I will be running either an air/air or air/water intercooler, 2 bar MAP, remapped ecu & a bypass valve.
 
A similar setup used on the Australian VT Commodore uses an Eaton M90 on a 3.8 litre V6 engine. The factory blower drive ratio is 1.8:1 and they claim 7 psi boost spinning the blower to 10,500 rpm.

Now the SC14 is a tad smaller in capacity than an M90, and has a much lower airflow capacity and adiabatic efficiency at higher rpm.

I cannot see why you find turning the blower at twice crank speed to get 10 psi so perplexing.

In fact, if you fit an intercooler and a free flowing exhaust, engine airflow will increase, and boost pressure will definitely fall.
A supercharger drive ratio increase to 2.5:1 will probably be required to reach the original target 10 psi boost pressure.

It may also surprise you that the original Jaguar XJR supercharged 4.0 litre six cylinder engine also used an Eaton M90, and the factory blower drive ratio was 2.5:1 to reach 10 psi at 13,750 blower rpm. But the M90 is a much stouter unit than an SC14.

The plastic two lobe rotor SC14 will just not survive such harsh operating conditions for any length of time, whereas the aluminium three lobe rotor M90 will.

 
I was perplexed by your comment "Driven at 2.0 to 2.5 the original designed rpm". I now see you were simply referring to the drive ratio between the SC & engine. My calculations indicate that I can achieve my desired boost level with an SC speed of no more than 9,000 rpm, which would require a ratio of 1 to 2 for my engine.

In practice my theoretical calculations may proof optimistic but hopefully not too far from reality. I am looking for primarily a low to mid speed engine speed torque increase, not outright power.

The basis for the performance of the SC14 has been the Ogura TX15 SC which appears to be the commercial equivalent of the SC14 as used by Toyota. The TX15 can run 10000 rpm continuous & 13000 "short time", short time is not defined, & has Teflon coated rotors.

Items I forgot to include in my hardware setup listed earlier are: extractor exhaust manifolds & clutched drive to the SC controlled by the engine ecu.
 
The Toyota blowers in the original Toyota applications are typically driven at factory crank ratios of around 1.1 to 1.15:1.
These are for DOHC four valve engines in the 1.6 to 2.0 litre capacity class that might be expected to peak around 5,800 to 6,000 engine rpm.

Placed on an engine of twice the swept capacity, it would not be at all unreasonable to expect the required blower drive ratio and blower drive speed requirements to double.

A drive ratio of 1.1 will provide around 1400cc of air per revolution with an SC14, where a 3.8 litre engine will require 1900cc of air per revolution just to reach atmospheric induction pressure flat out.
Your 1:1 minimum drive ratio will actually create a massive flow blockage and negative boost pressure at full throttle.

By all means try your own ideas, but make provision for fitting drive parts that will allow a drive ratio of at least 2.5:1 without running into problems of maximum or minimum pulley size limitations.

Nobody gets this right the first attempt, including me.

The first test drive, the actual measured boost pressure is always a huge surprise (both ways). But it is always an excellent basis for pulley drive ratio attempt number two.

The problem will be, the Toyota blowers will need to run at extreme speeds to feed such a large engine. If you don't mind replacing worn out, noisy, or seized superchargers on a regular basis, maybe every six to nine months in an everyday road car, then there is no problem.

You are talking to someone that has now worn out or broken THREE of these Toyota SC14 blowers. I am not making any of this up.
 
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