Joel mounted a Toyota SC12 supercharger to his 1776cc aircooled VW engine with fabulous results. The larger SC14 (read longer by 50mm) doesn’t fit in the engine bay as the doghouse gets in the way. Regardless the SC12 fits well.
For the novice (as I was 8 weeks ago) let me explain in laymen terms, what this is all about. Turbo chargers run on exhaust gasses, superchargers run off the crank pulley. Superchargers boost is calculated by a formula…I’ve included the formula later. Such formula depends on pulley sizes, the cc of the engine, the size of the SC etc. All basic stuff. Turbos and SC’s have positives and negatives. The supercharger has its boost virtually throughout its rev range. Perfect for towing and enhancing the engines torque, indeed lowering its max torque point a few hundred revs.
Superchargers can be expensive. However many people are buying the SC12 or SC14 for less than $400. Total package though is still less than $1500. For around 30% increase in torque and power (say 6-7 psi boost) it is the cheapest bang for buck around. You can buy a sprintex supercharger that have even managed to find their way into new Harleys. These are “twin screw” superchargers meaning the air is compressed internally in the unit unlike the SC12 Toyota units that are called “Roots type” that pressurise the air on the outside of the blower. These are Australian Sc’s and are very efficient. There are centrifugal SC’s also. But I wasn’t prepared to fork out many thousands of dollars on this project.
Electric superchargers are on ebay and other places. They, in my view are a waste of time and money.
There are two methods in connecting up a SC. First is the method that the blower was originally designed- to compress air only through a carbie and into the engine. This has a few issues with the home built conversion. The carbie has to be built/rebuilt to withstand compressed air for example. This is called “blow through”. But there is a simpler set up.
“Draw through” means the carbie works in its conventional manner to inject an air/fuel mixture into the blower. The blower compresses the mixture before it is ingested into the intake. No carbie mods are required though there are good and not so good carbies to select from for this purpose. It is commonly known that SU’s are good as are side draught webers and Dell’orto’s. Some are more efficient than others so there is some homework there to do.
So onto Tweety, the Panther trike (Oz). I wont go into why I selected the ea81 Brumby/brat OHV engine in the first place in detail except to say that I wanted simplicity in that I wanted to service and maintain my trike myself. I know my limitations and DOHC’s and wiring looms a ,mile long scared me off. Having said that I also carried out modifications on the standard Panther tow bar to make it run the full length of the trike for it to tow a small caravan weighing some 450kgms wet. I didn’t count on the automatic zapping 15hp out of the engine though. On a dyno the engine, correctly recoed 60,000kms earlier by a Subaru mechanic produced a healthy 75hp. It tows the van well but hills see it die a little too much. A 4 speed manual would have more usable power.
So I decided to install an SC12. I purchased one and also purchased a Lynx twin carb manifold suitable for a Datsun 1600-2 litre engine. I cut the manifold in two parts. This was suitable for my needs because the ports, as they meet with the blowers intake, are close together as are the square SC intakes. I made up a small adapter plate. Then I purchased a Dellorto 40mm DHLA carbie fully refurbished for $210 on ebay with a spare carbie for $60.
I needed to mount the SC and did so between the alternator and the engine intake by utilising the cast iron mount.. I made a manifold from two 75mm steel pipes welded together sideways, added a blow off valve and I thought this was going to be easy. It wasn’t. I would recommend anyone doing this set up to make two separate mounts one on the SC exit and the other on the engines manifold intake both with 75mm pipes and connect them with a short length of 75mm silicone tube. This allows for imperfections in the joining of them. If the SC is hard mounted like mine you wont need a top mount. If you use the silicone tube then you’ll need a top mount which isn’t hard to make.
When making up the manifolds/adapter plates I finished the joins both externally and internally with Devcon plastic steel. A more common material is JB weld. This epoxy can line the inside with little chance of breaking off, is sandable etc.
There is usually a pulley on the SC12 that has an electric clutch. This is not suitable for a draw through set up. You need the blower operating all the time with draw through. So I got my engineer to fix it with 3 bolts through it. At the same time he fixed a 5 ribbed pulley to the crank pulley. Then I purchased a Ford tensioner pulley and mount. I used a 5 ribbed belt 5pk-0965 which is a common size.
The specs of the set up are: cranks pulley 135mm, SC pulley 120mm. Calculated boost around 6-8psi.
Below is a calculation using my Ea81 engine. Simply calculate your own by replacing my figures with yours.
When fitting a supercharger, you should match the swept volume of the supercharger to the size of the engine. If the choice is made carefully, problems from overboosting and the required paraphenalia to solve these problems will be minimised.
The supercharger will also be kept in its safe operating speed with correct selection.
To choose a setup you need to know :-
1. Engine capacity 1.8
2. Maximum engine speed you will be using. 5500rpm
3. Boost level desired 6-8 psi
I am setting out the calcs needed for a 1800cc engine in the steps below.
FIRST CALCULATION (Engine Litres/min @ 0 Psi )
Multiply engine capacity (in litres) times maximum engine Rpm. E.g. 1.8 litres x 5500 rpm = 9900 litres/minute. Divide this figure by two as engine only fills every second stroke.
9900/2 = 4950 litres/min.
This is the engines air requirements in litres/minute at 0 Psi boost.
SECOND CALCULATION (boost ratio)
Add the boost pressure desired (8 Psi) for the engine to 14.7 Psi (atmospheric pressure).
(8 psi boost desired +14.7 =22.7 psi)
Divide this answer by 14.7 and this gives the boost pressure ratio.
This is the boost pressure ratio above atmospheric pressure.
THIRD CALCULATION (Actual air requirements @ desired boost)
Multiply the boost ratio by the litres/minute obtained for 0 Psi and you get the actual air requirements in Litres/min for the engine at that boost. In our example this is 4950 litres/min X 1.544 = 7642.8 litres/min for 8Psi boost.
To decide on the correct size of supercharger you need to know :-
1. The swept volume per revolution of the supercharger. ( SC12- 1.2 L/rev)
2. The maximum continuous safe operating speed for the supercharger. (Toyota SC12 11000 rpm??)
3. The maximum pressure that can be safely produced by the supercharger continuously. (Eaton M62 12 psi, SC14 10-12Psi)
CALCULATION (Supercharger rotor speed)
Divide the desired air flow (7643 L/min) by the swept volume of the supercharger (SC12 from the is 1.2 litres per revolution). This will tell you the maximum speed the supercharger rotors must be run at to produce the volume required.
7643/1.2 litres = 6369 rpm for the SC12 well within its capabilities.
CALCULATION (Pulley size ratio)
Divide the rotor Rpm by maximum desired engine rpm to get the drive ratio of the pulleys. For an SC12 on a 1800cc @ 8psi boost the desired supercharger pulley ratio is
6369 /5500 rpm = say 1.1:1
My Crank pulley is 135mm diameter and SC pulley is 120mm. About 1.1:1 =spot on.
Ignition and Fuel.
Commonly and especially with aircooled engines the ignition should be retarded or the advanced limited. As you are forcing air into the combustion chamber more fuel is needed to balance the mix. Hence larger jets. In both cases a dyno should sort this out. Be warned however that a rich or lean mixture can cause harm to your internals.
Intercoolers. Read up on them. They are not however suitable for draw through set ups.
Tweety’s Dellorto was rich and the engine bogged down a lot. Eventually I feathered the throttle to enable him to pick up speed. Then going up a small hill I let him kickdown from 3rd to 2nd and the front wheel lifted. The trike has more kick in the back and I’m impressed. Tuning will result in a better response.
Supercharging is addictive and some say eventually I’ll want more boost. Trouble is when you go over about 6psi reliability becomes an issue. The SC12 in my case is spinning at around half its capability. I am happy enough and don’t want a screamer.
Lining up the belt correctly is critical.
Finally, tuning is critical. get it right sooner than later.
Edited by tweety, 27 June 2012 - 03:11 PM.