Tech Talk
18 JunThose of you who have read our charity page, would have noticed that the official SWORDE-Teppa page was not available. That has now been rectified, and you can visit their website at www.sworde-teppa.org.uk
A cautious two-thumbs up for our Suzuki’s engine today. Topserv Automotive in Brackley have checked its vitals and come up with the following numbers.
Compression (How well the pistons and valves seal inside the engine)
| Cylinder | Measured (psi) | Specification (psi) |
|---|---|---|
| 1 | 160 | 156-199 |
| 2 | 168 | 156-199 |
| 3 | 175 | 156-199 |
| 4 | 165 | 156-199 |
Vacuum (How well the engine sucks air from the atmosphere. Indicates how well the pistons, valves and carburetor seal)
18inHg @ 750rpm (specification 17.7 - 21.6 inHg)
Oil Pressure (Indicates condition of oil pump, and of the engine’s bearing surfaces)
55psi @ 3000rpm (specification 42.7 - 59.7 psi)
All of these numbers are okay. The compression is on the lower side of the specification limit, and this will be costing us some precious horsepower and a few mph off our terminal velocity, but for 700 pounds it’s not so bad
Topserv were also successful in changing the oil in the rear differential.
Since yesterday’s post, I’ve had a question about what the transfer box does, so I’ve provided some information about the 4wd system in our Suzuki. Read it if you wish.
Suzuki 4wd System
On the Suzuki SJ, the engine drives a standard style gearbox, which then drives the input of the transfer box through a short driveshaft. The transfer box has two output driveshafts, one which connects to the rear differential, and one which connects to the front differential.
There is an extra gear lever in the car, which controls what is happening inside the transfer box. It can be set so that the gearbox drives only the rear differential (2wd), or both front and rear differentials (4wd). When in 4wd the transfer box has two settings, called 4wd high and 4wd low. 4wd low is for heavy 4wd work. In this setting, the top speed in each gear is much less, which means there is more turning force available at the wheels. The engine is less likely to stall, as for a given car speed, the engine speed is much higher.
This conventional style 4wd setup has one important limitation. When in 4wd there is no provision for the front and rear wheels to rotate at different speeds. They are directly coupled. When you turn a tight corner, the front wheels follow a longer path than the rear wheels. Rather than rotate at different speeds, the rear wheels must slip a small amount. This is okay in the mud, or on dirt, but on high friction road surfaces, like bitumen, this is not possible. There is just too much grip for the wheels to slip. In this circumstance something in the system must give (they call it wind-up), and eventually either a differential, a driveshaft, or the transfer box will fail. Modern constant 4wd systems utilize a centre differential instead of a transfer box, which allows a differential in speed between front and rear wheels.
The other thing about our Suzuki’s drivetrain is that when in 2wd, the transfer box uncouples the gearbox from the front driveshaft. However the front driveshaft is still coupled to the front wheels through the front differential. Thus even in 2wd the front driveshaft still rotates at road speed, which wastes a lot of energy! This is why a lot of people fit free wheeling front hubs to their 4wd, which uncouples the front wheels from the front differential when not in 4wd. In this case to select 4wd you must select it in the transfer box, and lock the front hubs.
