WhizzMan wrote:- Set timing to something conservative, so you won't be detonating for certain
- Get fueling right. Start in the middle of the throttle and rev range and work your way to the extremes. (for carbs, other rules probably apply)
- once you have fueling right, set your ignition timing.
- check fueling again, do adjustments to both if required
WhizzMan wrote:Once you have those done, you have a decent base line, where you can experiment with things like overlap, and individual cam timing. Those mostly will get you another rev range and possibly a better flow. They will probably not influence timing or mixture settings significantly, unless you were way out at base setting. A recent discussion on another forum suggested that most hot 2-valve engines prefer an LSA of around 108 degrees. It also suggested that going for one or two degrees less than optimal, would be better than going for too high LSE, if you're building a racing engine. You may want to start your base setup tighter than 112 degrees LSA and go up from for example 107 to 112 in steps to see where the drop-off is. Once you hit that, go back one or two degrees and leave it there.
WhizzMan wrote:Once you have those set, you can try and see if you can get more power with bigger chokes, ram pipe length and all that. I'd be careful with going for bigger chokes than 34 in a 40 carb. It will probably work, but velocity at lower flow rates will suffer and your engine may very well run significantly worse at lower revs as a result. I see people build an entire car and not experiment with bigger chokes until they're done trying a lot of other things, on track, setting lap times. It may be the thing for you, to go for bigger, but don't rule out the benefits of a larger rev range with more torque in the mid section. It depends on your skill, driving style and the tracks you race at which will work best for your application. Those are hard to measure at a dyno session, so I'd leave the chokes out of the equation until you have some lap time done with the car. Especially if you're not a very experienced racing driver, you're probably going to set better times with a car that is a bit more forgiving, so I'd try a 34 or maybe a 35 choke and leave it at that for now.
WhizzMan wrote:I personally wouldn't mess with valve clearance. Set it once at where it should be and only derive if you have clear indications you are damaging parts because your information on where it should be is wrong. When in doubt, make sure that a warm engine still has some form of clearance when the cam nose is opposite of the tappet and that the ramp is actually being used to start opening the cam. If the cam doesn't contact the tappet until the actual nose, it will wear very quickly. I'm sure Guy will be able to give you very precise instructions on how to set this up perfectly with your cam and the head he provided.
WhizzMan wrote:Keep in mind that fueling with carbs tends to be influenced mostly by flow rate, with the resonance and blow-back making it difficult to tune perfectly for each possibility of rpm, throttle position and engine load. If you have your basic flow rate set up correctly, you can get most other parameters determined without having to adjust fueling. That makes it beneficial to get that set up as one of the earliest stages. Changing chokes will make you have to do that all over again. Changing chokes might only work if the rest of the setup is already proper, because you won't benefit much from bigger chokes if the engine isn't pumping it's full potential yet. Therefor, I'd choose to do that as one of the last things. Once you've set all other parameters, double check your fueling to see if it's still where you want it to be and of course keep the mixture/O2 probe in during all tests for safety.
WhizzMan wrote:Ignition timing is primarily set by determining the "load" of the engine and the RPM. As soon as you know how much mixture is in the engine, you can determine what ignition timing is giving the optimal power without detonation at the rpm it's running. You will only need to readjust your timing if your fill rate changes significantly. By staying on the conservative side of timing while doing other adjustments, you can judge the fill rate improvements without having to tinker with your ignition timing and having more than one factor to consider. Setting ignition timing proper for partial load on race cars is only done when people have spare time on the dyno, or if fuel economy is a factor in the race. Just setting advance proper for full throttle and not advancing more at partial loads is a common practice. You can gain "throttle response at part throttle" and "fuel economy" by setting those too, but it will be expensive to do so if you're not done experimenting with other parameters.
WhizzMan wrote:If you have the option to do rampipe length tuning, I'd do that as one of the very last things. Usually, you can calculate more or less what the optimal pipe would be from the alternatives you have available. Don't expect big horsepower changes if you change that length by only a few centimeters. Things like these, when prepared and calculated correctly, are details that will help you gain horsepower, but should not be perfected until you have the basics worked out. A "ballpark figure" is probably going to give you a very close to optimal result from the start. If you have to choose what tests to do on the dyno, I wouldn't put my priority on these.
WhizzMan wrote:Once you're happy, make sure you do a final check for mixture and detonation (ignition timing). Things tend to creep in without you noticing because you're focusing on other areas. Don't leave the dyno place without actual proof your mixture and timing are "safe" for the engine.
WhizzMan wrote:Don't overestimate the time you have at the dyno. Things go wrong, tinkering takes time, you get tired. Establish a list of things you want to do with the order in which to do them. Do not take shortcuts you haven't planned in before. For example, set yourself rules before you go to the dyno like "If I run out of time, I'll only do 3 LSA angles and no individual cam angle advance tests" or "I'll only do idle and full throttle rev range timing setup, I'll disable vacuum advance". If you make on the fly decisions to skip things, you may oversee consequences your decision may have. If you make them in advance, you can make rational decisions and think them over for a while, without wasting valuable dyno time or breaking your engine. You seem to have this part already figured out. You asked others about their opinion here, so you'd learn from their mistakes and experience as well.
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