In theoretical terms, will changing to a different sized carburetor change the engine's required total timing for max performance/horsepower? In other words will going from a 650 cfm carb on a 350 ci engine to an 850 cfm carb cause it to run better with more or less total timing? In practical terms, I know to advance or retard total timing to whatever the engine performs best at. I'm just talking theoretically here.
Yes it can because the mixtures droplet size can change based on the venturi/booster relationship above the CID air pump it resides on. In extreme cases (not the case here) it can even affect manifold vacuum to the point of changing the fuel and ignition curve. IE; 500 Holley on big block or dominator on small block. To clarify.. smaller droplet size improves mixture homogeneity so that it could POTENTIALLY call for less timing advance to achieve same cylinder pressure. This improves combustion efficiency and leads to less pumping losses. And then you'd need to factor in for any increase in the engines acceleration rate based off that added booster response and also potential power gains that might have occurred as a result of improved mixture homogeneity. Faster through the gears wether it comes from added power or lighter vehicle weight or increased gear multiplication factor allows more aggressive timing curves. Would be akin to the engine coming up on the cam quicker and also improves transient throttle response. So it's very much a cumulative result where one change allows another.
That's pretty much what I believed to be the case. I removed the 750 vacuum secondary and replaced it with QFT 650 DP on my 302. The car went a tenth faster, and it sounds different which leads me to think I will move the timing around to see what effect that has on my ETs. The way it sounds (sharper) makes me think I may need less total timing.
Theory is sometimes bassackwards of what really happens. I doubt you'll need to change timing any based on my own experience in doing the same thing....that is...UNLESS something else is changed too. Case in point, when I converted over to methanol, "everyone" said I would need more lead (timing). In reality, it wanted less. Not trying to start an arguement, just saying that testing and tuning is the only way to know. That or a dyno.
Far past theory and boils down to well proven physics. So just because the engine doesn't respond as anticipated doesn't change the underlying physics that are involved in optimizing the combustion process. If that were the case then EFI and annular boosters would be clear power winners. This is not to say that having a purely atomized gaseous state mixture will make more power. That has been proven to make an engine more efficient towards an adiobatic state.. but it doesn't make more power. The reason for this is based on the physical fact that a performance engine is all about controlling the length of the burn within the chamber(combustion space) which requires different size fuel droplets to ultimately spread out and extend the push on the crankshaft to a later moment when the rod is at a higher degree of right angle. Basically more leverage over a greater length of time allows the combustion event to remain under control at higher rpm when there is less time to extract the power from the fuel. The finer gaseous state fuel gets the party started.. and the medium droplets build the flame front to high enough temps to finally allow the larger droplets (even coagulated strings) to break down and be burned. It's like this. Better fueling(for whatever reason and however it's achieved) in the COMBUSTION SPACE = potential timing changes. And what's best for low speed is not as effective at higher speeds. In essence all you are attempting to do here is to time peak combustion pressure to a point where the rod angle can apply the greatest leverage to more easily turn the crank. Spark advance affects wasted fuel and power going out the exhaust pipe just like excessive fueling into the manifold can push it's way out the same pipe. As for the methanol experiment that broke the laws of physics. Keep in mind that while it's true that methanol burns more quickly in direct comparison to gas.. it takes considerably more of it to reach proper AFR for best power production. So.. it stands to reason.. and physics.. that it needs an added head start in the early stages of the combustion process due to greater liquid fuel density within the chamber. Plus its MUCH larger droplet sizing and considerable puddling needs greater ignition lead to enable larger percentages of burnt fuel by the time the piston moves down to expand the combustion space to kill pressure into the blowdown stage. Not entirely different from e85 in that same respect either. Then there's the fact that the higher octane rating of methanol tolerates.. more like permits.. more ignition advance to be used in search for even more power. Just because you start ignition earlier doesn't mean it will drastically change the point of peak combustion pressure. Its just more domino's added into the mix to topple the whole thing down more efficiently. Done rambling on this stoopid auto correcting smart phone for now.
A basic rule of thumb,if your mtr is 10 & 1/2 to 1 or less & you don`t rev over 7000 rpm,set it at a max of 34 degrees.If you have a high comp. mtr & you are going to rev 7 to 12000 rpm then go to 38 to 42 degrees of total timing.Then adjust your ignition curve as to how fast or how slow you want the mtr to reach total timing,through trial & error this is how you will achieve the best results from your car!
From an engine speed "time to get the job done" standpoint that makes perfect sense.. BUT.. actually it's just the opposite, Dan. 9:1 motor has more combustion space volume and therefore less densely packed and slower mixture and can use more total advance to make same power. Increases pumping losses slightly but the net gain still increases due to the better overall timed peak cylinder pressure. This is precisely why big cams NEED more timing advance at low engine speeds. Conversely, smaller wedge style combustion chambers/spaces have greater fuel density levels being compressed together which makes them easier to speed up the chain reaction(needs more spark energy to get the party started but gets hopping quicker too). Plus you will typically have smaller squish heights and extra added mixture motion from the higher induction and piston speeds too.. so you would need less total timing advance. Combustion space design and spark plug location of a stock or aftermarket head also plays an important role as well. Even the valve angles can change that overall design and impact optimum spark lead. So, rules of thumb are always directly related to the types of hands being compared.
Combustion chamber SHAPE, piston dome shape, spark plug location in the chamber have a bigger effect on ignition lead than any other variable. I am not saying that other variables don't have an effect, because they do, but they are much smaller in comparison to the former. At least from a maximum horsepower standpoint. Let's leave emissions, NVH, and MPG out of the equation, for the sake of this being a "drag racing" forum.
True.. true.. domes are typically always detrimental to flame front propagation but end up being the lessor of two evils.. lower compression or hurt the propagation/burn efficiency and overall flame speed? Increased compression wins up to a certian level but there's always a tipping point where you went too far. In response to the rest of that.. I would agree up to the point until VOLUME and SQUISH HEIGHTS enter the equation. They are the great equalizers.. even when used with crappily designed OEM open chamber style heads. IOW, mill the bejesus out of some large open chamber heads with poorly placed spark plug location and reduce combustion space volume substantially(say 11:1 SCR or so) while also tightening up the piston to head squish height in a big way(low .030"s).. as compared to a fairly well developed larger volumed closed chamber(deeper/higher roof/larger combustion space equating to around 9:1 SCR) with optimum plug location and overly large .080" squish height?.. and they are no longer worlds apart for total ignition lead requirements. Chamber/spark plug location design becomes more moot in the scheme of things as the spacing between the molecules gets tighter with higher SCR and squish speeds increase substantially to improve homogeneity and flame propagation speed. Less ignition timing needed to hit peak pressure where needed.. around 15 - 20° ATDC.. and extend it out far enough.. around 70 - 75° ATDC.. to make best use of the fuel and improve blowdown efficiency. Although there are also other benefits to doing it(reducing valve angle).. angle milling to shrink the chamber volume would be a perfect example of how far some will go to improve the older outdated designs. Old time racers have been doing it for decades because it works to close the gap between the aftermarket and the older iron every single time. Angle and excessive flat milling also has the secondary benefit of creating or increasing the size of the dual squish pads on both sides of the chamber. This alone helps mixture motion in a big way as well, especially if the squish is kept tighter for the end result. Interesting discussion.. I love talking about this kind of stuff. And hey, before you go bashing me for getting too techy.. at least I didn't go into how the mixture quality/homogeneity coming through the port and flowing around the valve curtain area affects things before any of this other stuff even enters the equation.
Went to Petersburg yesterday(June 18),talked to many drivers running everything under the sun,average total timing was from 34 to 36 degrees.For me,discussion over.
Dan, with all due respect, why not give the engine what it wants for total timing and what the dragstrip proves makes the most power? For example, one of the guys I park next to at Firebird is running a 427 Windsor with Kaase heads and .800 roller cam. He uses 28* timing on that engine. He said he was surprised how little total timing it needed to run best. He credits the Kasse heads. He was running 9.50s yesterday in a 2700 lb early Studebaker. In my case, for my engine, it made the most power at 40* on the dyno. It ran 12.52 out of the box, first run. HOWEVER, I ran a best of 12.323 yesterday with 37* total timing. That's an improvement from 12.359 last week. It may run better with less. Time will tell. I'm not sure the reduction in timing helped because I broke one of my rules by changing two things at one: I changed the timing and also set the lifter pre-load at 1/4 turn from their original .000 pre-load. Don't know which one helped.
Not significantly. Temps both days in mid 70s and humidity in mid 30s. The only big variable this week was a 20mph headwind. That caused my trap speeds to be about 1 mph less.
Just my 2 cents, my 427 runs peak dead on 32 degrees. Been everywhere from 28-38, 32's the sweet spot for me. Variables will dictate where the sweet spot is, you just have to do some test n tunes and make a lot of passes at the track.
