16V Tuning

Increasing Cam Timing

The XU9J4 and XU10J4 engines are fitted with hydraulic lifters, single valve springs and very mild cam timing in standard trim. This all makes for a quiet, free reving engine, that is both economical, reasonably powerful and fun to drive. It’s a package that works very well with the standard rev limit of 7100 RPM. There is little point going beyond this as the power drops off quickly. The exception is the XU9J4 engine fitted to the Mi16x4. It has slightly more duration and a 7400 limit.

In the quest for more power, people often change the cam timing. The Mi16 is very responsive to cam timing but it all has to work together as a package. Long duration, high lift race cams are useless in a street engine. All the components of the engine must complement each other to get the intended performance gain. In fact, you don’t need a lot of duration to make good power in a 16V engine.

As stated before, compression is by far the most important for a change in cam timing to be effective. The XU9J4Z engine has 9.7:1 CR. This limits the cam timing that can be used in this engine. It’s important to remember that the compression stroke doesn’t start until the inlet valve closes. So an engine with low static compression and long duration isn’t going to perform well. It might sound “camy” but it won’t go. Dynamic compression is the key to an engine’s expected performance. Dynamic compression is a lot more difficult to calculate as it takes into account stroke, rod length and inlet valve closing. Thankfully, calculators such as the DCR make the task a lot easier.

Offset Keys and Verniers

Offset Key Fitting Instructions

 

Stage I Inlet Cam
Everything has to work together as a package. It is possible however, to increase the power and torque of the XU9J4Z by a useful 15% by increasing duration. My Stage I cam has been developed to work with standard valve train, compression and ECU. It has the same lift as standard but has more duration, 226 deg. @ 0.050” lift. Because it has the same lift it doesn’t place any extra load on the valve springs. Most aftermarket cams have more lift than standard. If you consider the cylinder head flow chart, there should be a benefit in opening the inlet valve further. But what if you don’t have to? The inlet flow is already comparatively high and opening the valve springs further requires more energy (hp).

The next question you should be asking is dynamic compression. Won’t longer duration and standard compression mean less dynamic compression? Yes. But what if the camshaft is advanced slightly? This restores some of the dynamic compression resulting in very pleasing results. The engine still has a smooth idle but has a broad increase in torque and power.

The results below are for a freshly rebuilt XU9J4Z engine. It had new pistons and liners, a fully reconditioned standard cylinder head. About as good as it gets. In the first run it was fitted with an offset key on the inlet camshaft to advance the inlet timing 4 degrees. This effectively takes the inlet lobe centre line from 111 deg. ATDC to 107 deg. ATDC. Many achieve the same by fitting vernier pulleys. This change makes the engine a lot stronger in the mid range , moving top end horsepower peak down a tad. With this modification the car is a joy to drive at 100-110km/h. It will overtake with ease in 5th gear.

 

In the second run, a Stage I cam was fitted with the standard key. This gives an inlet lobe centre line of 109 degrees. The results speak for themselves. There is little change below 4500 to the offset key, above though, there’s a nice increase in power and torque. You get the best of both.

Please note that the Nm quoted in this table is "raw", straight off the rollers. It is flwheel torque multipied by the gear ratio, diff ratio and tyre rolling radius.

Stage II Inlet Cam
The Stage II inlet cam has more duration again, 236 deg. @ 0.050”, but again the same lift as standard. The extra duration means it definitely doesn’t work with the XU9J4Z engine. It needs 10:1 (or better) to make things happen. It works well with the standard valve springs however, making strong power right up to 7300. The rev. limit should be kept to 7400. With the right package, this cam will make 200+hp on throttle bodies. On the same dyno as the Stage I comparison above, it made 113kW at the wheels. You’ll definitely need mapped fuel and ignition, together with larger injectors to get the best out of this cam.

Beyond Stage II
With duration of 240 deg. and above, you’re really pushing the limits of the standard valve train. The hydraulic lifters are known to float at a sustained 7300, so go beyond that limit at your own risk. If you’ve got the need to be at 7500 regularly you should be considering solid lifters. With higher lift cams you’ll also push the valve springs to their limit. You need to check the installed and open lift heights with any cam change that has more lift than standard. It would be a shame to take off the nose of a billet cam through carelessness. There’s plenty of double springs and light weight retainers on the market at reasonable prices that will make 7500+ reliable territory.

Exhaust Timing
As stated in the head/valve section, the exhaust port is very efficient. It flows better than a 5.7L V8 Chev! To maintain intake/exhaust flow ratio of approx.75- 80%, the standard exhaust cam is more than adequate for Stage I and Stage II type intake grinds. Generally the exhaust duration can be approx. 10 deg. less than the inlet (at 0.050” lift).

You’ll also notice from the flow chart that the exhaust flow goes flat after the standard lift point. So there’s little point opening the valve past 0.360”. It doesn’t get any better. More duration is necessary, not lift. You’re definitely wasting valuable energy with high lift exhaust cams.

So don’t get ripped off buying a pair of billet cams when your standard exhaust will suffice! Most cam companies will sell you one cam.

 

REGROUND AND BUILT UP CAM PROFILES

 

Main Index