With almost ¾ inch of offset on the T&D shaft rockers, it’s easy to see that a stock rocke
While the camshaft controls the attitude of the engine, controlling that cam can be a problem. “The blocks are almost 9.6 inches tall from the factory, and I think the worst thing about them is probably that the timing chain length is quite a bit longer [than other small-blocks]. So the LA series engines beat the hell out of the timing chain pretty quick.” Robinson preempted that issue, buying a geardrive to run the cam. In true hot rodder fashion, he found the best deal for one on the web. “It was like $99 off eBay. Of course, I questioned its quality but I have a hardness tester here [Naturally. What? You don’t have that?] so I tested all of the components, and the gears were like high 50s Rockwell C-scale. Extremely hard, and the bearings looked like good quality. Yeah, there were some fitment issues, but other than that it worked great.” Score one point for the Internet! “The lower gear was one of those eight- or nine-tooth gears to adjust timing, but there wasn’t a 2- or a 4-degree spread. One would be 2 degrees and the other would be five and a half. So it took a little head scratching and careful marking to make it work.” Internet point now removed!
Robinson found the Eagle crank sporting their Armor Shield coating did a great job of prot
Once he had the geardrive sorted out, Robinson set the intake centerline to 101.5 degrees. Combined with a tight 104 lobe separation, the valves tricked the engine into thinking it has more compression than it really did. Here’s how.
As the engine rotates over after firing and the piston is being pushed down from the rapidly expanding flame front, the exhaust valve starts to open up a bit before the piston reaches bottom dead center, or BDC. That period of time is referred to as the blowdown period and fiddling with the exhaust opening point is a balancing act between extracting the maximum benefit of the still-expanding flame and making sure the valve will be open wide and long enough to get all that exhaust out. Of course as the piston passes BDC it can help shove that exhaust out. When the piston starts to approach top dead center (TDC), again is when things start getting tricky. The exhaust valve is on its way to closing and now the intake valve gets a head start by opening up. When both valves are open at the same time it is called overlap, and it’s the second balancing act that the camshaft has to control.
Even with a long rod and a 1.7 rod-to-stroke ratio, the tall deck height of the LA engine
If this overlap period is short (wide lobe separation), that means the intake valve is opening late and so, especially at low rpm, the exhaust tends to keep going out the tailpipe and not back up the intake valve—good for emissions and a smoother running engine. Nevertheless, by opening up the intake valve early and closing the exhaust valve later, the inertia of the exhaust going out the header can actually pull some intake charge into the engine and increase its efficiency at higher rpm, but it could also contaminate the intake with reversion at low rpm. Definitely a balancing act.
The final piece in the bumpstick puzzle is determining when to close the intake valve. The piston has now gone past TDC, all the way down the cylinder and is now moving past BDC. Since the piston can’t compress the air it just gulped until the valve is closed, it would make sense to shut it right at BDC to get the maximum compression. But wait, there’s more! Since the air and fuel coming in have mass and inertia, they are still pouring in even when the piston is back on its way up again. So, that last trick is to find the exact point to close the valve that is a perfect balance between the last few tidbits of air and fuel coming in versus shutting it and compressing what you got. That mother-of-all balancing acts is what top engine builders like the guys at SKMFX deal with in every engine they build.