Your Brabham-assembly friend was right. The more critical you get with engine parameters the more you have to make sure you are on the safe side of things and the margins are very close. Cam timing is something that must be researched with the engine on the dyno. There is
no ideal 'recipe' here, what works for me in my engine will not necessarily work in yours even if they have similar spec, they can't be exactly the same (ports, valve seats etc). When you arrive at the perfect dyno setting, write everything down and from then on those are your settings for the
particular engine. Supposedly this is the ideal way, but think about this: is the temperature and pressure in the dyno room the same as the underhood temperature at the track you will happen to be in later?? This is one variable, another is water temperature which can range from a nice 78*C to more than 100*C at the track! What will happen to your carefully mapped advance curve then? Engines in the dyno room make more power than on the track at least where mere mortals are concerned,
not F1 teams (where individual track conditions can be replicated).
I wrote the above to show that no matter what engine you are building you must have sufficient safety margins. Surely you must have
at least 1.5-2mm travel left in valve springs at full cam lift and 2-3* more advance margin before it starts 'pinging'. Cold clearances are tighter than hot ones, they usually increase by about 0.05mm (.002"). This is not important in road cams but it does make a difference in high acceleration ramps used in modern cams (JK303 included)
If you have time to play when assembling the engine, try to find out various mechanical limits like valve-piston contact points. This will give you the cam timing envelope limits. When people talk 102*, 106* or 110* lobe centers, you'll know if you can safely try these values out.
I don't know if I answered your question here but there's one parameter many underestimate:
driver preferences. Since we all drive on the street, we
do like low speed torque and response, the cam timing for which is
not suitable for maximum power! To me therefore there is
no ideal timing, just a grey area in which you must find your optimum. This means you will ideally need a degreed crank pulley and a friend to slowly rotate the engine (or push the car in 4th) while you carefully check valve opening/closing points by rotating the followers with your fingers...when the lobe jams them, thats the opening point, when they are released, thats the closing point! Verify this a number of times (I hope your chains are both tight! You must turn the engine only in the direction of rotation when measuring) and you can accurately find lobe centers. If some think of this as utter BS, so be it, it works for me as the saying goes! (With belt drive engines you must subtract about 2* due to belt stretch).
End of long-winded Sunday speech!
Jim K.
Quote:
That is the expensive way of finding out about things
Linear Mode
