Before tearing out the original cams, we put a dial indicator to the top of the intake and exhaust buckets and measured the valve lift profile of the stock and Toda cams.
In the case of the Toda cam, it has an asymmetrical lobe profile--valve opening is fast and smooth while closing is slower and longer, so as to not smack the valve too much against the valve seat. The profile also has to be a compromise between airflow and the physics of reciprocating masses. If a valve opens to full lift too quickly, acceleration forces mean it will maintain the momentum of opening, even after the acceleration rate has peeled off from the profile. In excessive amounts, this is valve float. Most tuners will upgrade the valve spring rates to compensate, but this has the drawback of additional wear on the lobe's surface.
If you really want to be fancy and impress the girls with your ber-geekness, take derivatives of the lobe or position profile and figure out the first, second and third order derivatives. In other words, now you have the valve's velocity, acceleration, and jerk profiles. This is fancy math and real-world cam design, since the smoothness of the acceleration and jerk profiles ultimately affect valve harmonics and spring surge. A lobe that accounts for the natural and operating frequencies of the valvetrain can potentially use much lower valve spring rates while avoiding valve float and spring surge.
The shim-over-bucket design of the stock 4AGE. Valve shims sit on top of the buckets, which push down on the valves. As the cam lobe swings by and pushes down the buck, the valve will open. If the lobe profile is too aggressive, there is a chance of flicking the shim out of the top of the bucket. For lifts beyond 9mm, Toda recommends switching to a shim-under-bucket design, where the cam acts directly on the bucket.
For the purposes of this article, we'll settle for plotting the cam opening and closing through the 720 degrees of crank rotation for one complete four-stroke combustion cycle. This will help visualize the combustion process, since we've also plotted the number-one piston position (in grey) using the piston motion kinematic equation based on stock stroke and rod length dimensions, as well as the ignition timing at wide-open throttle (orange line) and the charge expansion process, to see where the valves are opening relative to piston position.
So now we have a pretty graph that shows what the valves are doing at a given point in the crank's rotation, based on stock cam timing. This is where the adjustable Toda cam gears come in. The high-grade T6 7075 aluminum pulleys use a Vernier timing degree system that indicates changes in cam timing in crank degrees, offering twice the resolution as opposed to the conventional cam degrees of many other adjustable cam gears. We can now advance and retard the intake and exhaust cam timing independently, dyno the car and see how cam timing affects overall power and power delivery. The amount of valve overlap can be determined by shifting the valve position curves (by the degrees that the timing has been advanced or retarded) and measuring the duration where both valves are open at the same time.
We started with back-to-back dyno runs between the stock and Toda cams, which gained 10 whp overall, but changed power delivery drastically. Over 20lb-ft of lower-end torque was sacrificed for power gains above 6000rpm (where the stock cam fell off). If we weren't rev-limited, the Toda cam would have made more power as the power curve was still climbing at rev cut.
Having 10hp more up top isn't worth the huge losses down low, so we played with the cam timing, hoping to regain some of that lost low-end performance as well as bringing peak power down to 500rpm before redline. Having no idea which way to adjust the cams, we consulted several local 4AGE experts who have used this cam combination before.
Unfortunately, everyone had a different story and no one used the same low-compression, big-port engine we had. So we started by systematically advancing and retarding each gear separately, by four crank degrees. Even though ours isn't an interference engine where the valves can smack the piston if timed wrong, too much overlap in valve timing might cause the valves to clip each other and open a whole other can of worms.
Of the few headers available for the 4AGE, the ceramic coated TRD four-into on pice is probably the only accessible unit that will clear a left-hand-drive-steering column
Even though we could narrow down what changes occurred when each cam was timed individually, we expected different results when both cams were retarded by eight degrees. We were lucky that wasn't the case, because we ended up with the best of both worlds, near-stock low-end torque and high-end flow/power that stretched to redline. Even with further combinations of retarded cam timing, we reverted to our optimum configuration.
From what we could tell, advancing either the intake or exhaust cam resulted in a loss of power while retarding the cam timing would increase power and torque. We kept retarding the exhaust cam in increments of four crank degrees, which increased low-end performance with minimal sacrifices to peak power. By 12 crank or six cam degrees of exhaust cam retardation, we lost a few horses at the top and regained most of the low-end torque of the stock cams. After fiddling around with smaller increments of exhaust timing, we settled back on eight degrees of exhaust retardation.
The process was repeated for the intake cam with the exhaust set back to zero. As intake valve timing changed, we found the peak power rpm would change, without affecting low-end performance. Retarding the intake would bring peak power down below rev cut while advancing would raise it, which would explain the drop in peak power at our stock rev limit. We decided on eight degrees of intake retardation, putting us at 111 wheel-hp at 7000rpm, just 500 rpm short of rev cut.