[BogiesAnnex1]

What is the valve train? High lift cams need a better that average supporting cast that includes for flat tappet cams the use of hard face lifters, sturdy pushrods, 7/16th studs with for aluminum head’s must have a 3/4 inch long bottom thread to grab as much of the head threads as possible when they project into the port the length is correct, probably even 7/16th studs need a girdle. Getting into these high lift flat tappet cams with fast lobes using a roller can thrust bumper is a damn good idea it greatly relieves the lifter interface from having to also manage the thrust moments that high lifts and high spring pressures engender. This thrust problem is also enhanced by running roller or gear driven timing sets. These remove a flex absorption point that a link belt style timing chain provides at the cost of a short life but the crankshaft has its vibratory modes as does the cam unlike a turbine which at least in theory has a smooth rotation the piston engine is introducing discontinuous rotation moments on the crankshaft and does a similar thing on the camshaft albeit for differing reasons but the result is a shaky motion on both shafts that meet at the timing set. If the timing set pretty rigid as dictated by the need to keep the timing in a tight circle of error probability but the downside is a whole lot of shaking goin-on between these shafts. This tends to tear up lifters and lobes pretty fast. This might grab you where you’re not expecting it but street driving is much more severe than racing in this regard because at lower RPM the forces are present over longer time periods. But transferring this to high levels of competition builds over the years you see top top end competition builders try to deal with this by the use of thrust buttons on flat tappet cams often using silent link chain drives with frequent replacement or belt drives and the really radical flat tappet guys go to Chrysler (.904) or Ford larger (.875) diameter lifters which wear better than the Chevy .842 and a matching contour cam lobe. Another thing is keeping track of the lifter bore diameter and clearance. The lifter bore wears in an egg shape and clearance between lifter and bore should be .0015 to .0021 inch. The question here is how many guys actually measure these things? The lifter’s motion is much more complicated that just up and down abs round and round in its bore, there are substantial lateral loads that push the lifter pretty hard against its bore wall in a motion that would like to bind it across to bore diameter.

So the message I’m trying to put through is on the Chevy and GM engines in particular that use the .842 dia lifter are subject to motions that want to restrict vertical motion, that the lifter has a lot to do in order to maintain lateral thrust control of the vibrating and rotating cam. The Chevrolet in particular snd GM in general is one of few if not the only company that doesn’t use a thrust plate on flat tappet engines. And Chevrolet SBC’s in particular suffer abnormal cam and lifter wear some of that can be directed at the smallish root diameter of the cam as well, but that rather than an excuse is another reason to beef up the thrust control efforts on these engines even with flat tappets. Now there’s a thought for the aftermarket to make a flat tappet cam for Gen1 roller blocks using the thrust plate and the OEM roller cam timing set. This would be pretty simple mod to standard flat tappet blanks by just machining the thrust step on the nose and drilling the roller timing set bolt holes.

Bogie

 

[BogiesAnnex1]

I agree on home porting best being confined to cleaning up the valve pocket. This is 80% of the problem on older heads. Starting at the L31Vortec and certainly most but not all aftermarket head’s you will notice that the guide sail is already much improved to old factory head’s. So there isn’t so much to be gained on modern heads in this area beyond clean up. A caution is not to make the pocket wall a straight bore to the seat i.d. Here a little throating approaching the seat will aid the transition across the seat. An issue with flows is a thing called vena-contracta. This works in both directions of a flow across a change in area. Essentially without guidance a flow crossing an abrupt edge naturally contracts reducing flow area, dropping pressure in the remaining flow and accelerating it. The take away here is less area, lower molecular density (weight) at a higher velocity. Power is the result of the weights of reactants less fuel and air is less reactants is less power.

Port matching doesn’t net a lot unless you have the intake your going to use snd a flow bench and are prepared to chop at both to do a better job of controlling where the flows are in the manifold passage and matching the head port to receive that flow. This is tricky to do and the resultant cutting can look pretty ugly and counter intuitive, i.e. forget having 4 identical intake ports to a V8 head.

Back cutting the valves is very effective at improving low through mid lift flow and is pretty harmless to flows above .4 inch. Larger valves most aid high lift flow starting at .4 and going up. One piece valves that do not have the big old time weld between head and stem flow much better than the old welded two piece valves. Reduced stem diameter approaching the valve head is better still but you need to bring more money to up the material quality as this feature reduces the structural area carrying loads and transmitting heat into the stem so such valves are more subject to failure without improving the material properties.

Much is written about how lousy the SBC exhaust port is, not that there isn’t room for improvement but I really wonder if the authors of such articles ever tried to get port flow up on a Ford Cleveland head to something reasonably close to the intake, that takes more than a die grinder. For any street vehicle with an exhaust system that doesn’t get you pulled over or drums your brain to mush in 5 miles really can use the extra open exhaust valve time afforded by a split duration cam.

Bogie