There are no "humble" questions. You're correct in your assumption that
you'd want to have your ports work best at crank angles where piston velocity
is highest. Since the movement of the piston is what sucks the air/fuel
mixture into the cylinder, the higher the piston speed, the greater the
suction (pressure drop). Yes, you'd want to perform flow tests with the
valves in that general lift range. As far as timing pulleys go you can't
really move the areas of greatest lift and flow enough to make any difference,
however, when you open and close the valves is also important relative
to induction timing. You can shift the torque range slightly by advancing
or retarding cam timing, and if you have dual cams, you can independently
move each to not only affect charge timing, but also altering lobe centers
will affect the overlap cycle, and there's positive and negative power
to be found there.
Any flow work should be done with a cylinder of the exact bore diameter as the engine, and idealy the length of the "dummy cylinder" should be 1.5 x length of the engine's stroke. A sealed engine block with some aerodynamic removal of some material in the main bearing webbing area will work well. To open the valves you can use the cam. Put light springs on the valves you're testing, locate a dial indicator on the retainer to let you read actual valve lift, and simply rotate the cam to check flow at different lifts. You can purchase shim stock or cut up some feeler guages and place them between the rocker/ cam to simulate higher lifts.
Your question regarding "lag" time is dealing with some verbal terminology that I'm not familiar with. If you are discussing the amount of timing variation due to belt stretch, that'd be the same as retarding the cam(s)= more top end hp. If you are talking about valve lash, that's a different deal all together. Some amount of lash is necessary so the head of the valve can spend some time physically resting on the seat in the head. This is how heat is transferred from the valve to the head and finally to the coolant. Valve lash is a constant, and unless you haven't set the lash properly it's not worth the worry. Now, you can reduce the valve lash by 50% and see if your car runs better, and if so it's telling you that it would respond favorably to slightly longer duration with a different cam. You'll want to do this with both intake and exhaust, and both together as well. Make sure you know what the cold lash equates to if the engine is still warm when you reset the valve lash, because due to the various expansion coefficients with aluminum and steel, the clearances will never be the same as temperatures change.
Main bolts have absolutely no effect on air flow, unless they break. If that happens, you're dealing with more than air flowing.
Temperature does indeed affect air flow. The higher the temperature, the less effort is required to move the air. This is one reason that ceramic header coating is so popular. On the other end, cold air is denser, and will therefore produce more hp per given volume, same holds true for the barometer, the higher the pressure, the denser the mixture. That's why a car will always run well at sea level and as altitude increases, pressure drops and you lose power.
As far as books go, I'd head to a Borders or another large book store. All the books written on cylinder head modification are pure trash, however, the books about modifying the small block Chevy heads are not bad, even if your dealing with a two valve application.......hope this helps you some..........................T.O.O. .............................
Posted by T.O.O. on June 21, 1998 at 11:48:23:
In Reply to: Never mind, I was being stupid. posted by body on June 21, 1998 at 10:21:12:
Your notion of having truely optimized cam timing is a good one, and
someday the experimental "totally variable" valve events will be common.
I didn't mean to make it seem a stupid idea....we're all always looking for "more". My comments were based on the hundreds of people wanting to do the same thing, and also the knowledge that too much power in the wrong part of the rpm range can indeed be a bad thing. FWD makes the situation even worse. If I were dishonest, I'd grind some cams that'd make some difference on the dyno, but impressive dyno numbers don't mean a thing if you can't hook it up.
We've been fighting this low rpm torque thing for three years, and everything we're currently doing is an attempt to kill torque down low, and midrange as well. Your idea is something we've given a lot of thought to, except the cam I've designed for low rpm is a very low lift and longer duration piece with altered lobe centers, and it would actually cut low rpm power significantly to help drivability. We'll finally find something that will make everyone happy, but, you have to admit that if I sold you a cam and it produced less power on the dyno, but was quicker in the quarter, you'd feel like I'd done something wrong, because people would be ragging you for spending money for a cam that lost power on the dyno.
I've had NASCAR customers who "had to have" an engine identical to the one that another team was winning with, and we'd build it, ship it, and instruct the team to simply put it in the car, and run the gear we recomend. Instead, the first thing they'd do is put it on their dyno, and when it made less power than engines they were currently running (poorly) with. Then the phone starts ringing and they tell you that they've been screwed etc, etc. I'll tell them that we sold them what they asked for, and that's that. I'll call another customer who's on our waiting list and tell them to call (the other team) as they may have a good engine we built for sale. The team that originally bought the engine sells the engine at a "bargain" price, and at the next race the team that bought the engine is running up front with ease, and the first team is now pissed at me. I'll say this, I've never done direct business with that team again, regardless of what they offer.
Think about this some and I think that you'll better understand why I'm currently taking thid approach, and I'm sorry for misunderstanding you..................T.O.O. ............