Dominic Magrini; Manalapan, NJ: Regarding your tech answer on timing in the Jan, ’12 issue, I, too, am having a similar problem with the initial timing on my stroked 340 Mopar motor. It is now 416 ci with a HR226/345-251-10IG Crane Cam, 10.5:1 flat-top pistons, an Edelbrock 800-cfm carb, an Edelbrock RPM cylinder heads, and a Procomp Electronics ignition. I was told by a mechanic to power-time the motor to 35 degrees at 3,000 rpm. This leaves my initial timing at 8 degrees, and the car idles very poorly and is very hard to start. Where should I go from here?
Jeff Smith: There is a fairly simple remedy to your problem, Dominic. Depending on the distributor you are using, it might take only a few minutes. The information you received about power-timing the engine is correct. What you are doing is setting the total ignition timing at the rpm at which the engine should have achieved its maximum advance. Just make sure to set this with the vacuum-advance hose disconnected from the distributor. The fact that you have only 8 degrees of initial timing means that the distributor is delivering 27 degrees of mechanical advance (8 initial + 27 mechanical = 35 degrees total). The engine is hard to start and lazy at low engine speeds because the longer-duration camshaft (226/230 degrees at 0.050) wants more timing at low engine speeds to compensate for weak cylinder pressure (dynamic compression). A good place to start would be 12 to 16 degrees of initial timing. Unfortunately, 16 degrees of initial timing would put your total advance at 43 degrees, which is way too much. I would suggest setting the initial timing at 14 degrees and driving the car around for a short time at light throttle to see if it helps driveability and restarts. Do not go to WOT during this drive, as the engine will almost certainly detonate. What you’re trying to determine is how the engine responds to part throttle and if it feels crisp. If it does, you know you will need to reduce the mechanical timing by 6 degrees.
The arrow indicates the slot and pin that determine the ignition's total mechanical advanc
You didn’t mention the kind of distributor you have in your engine. Stock Mopar distributors place the mechanical-advance mechanism underneath the pickup plate. This requires you to remove the vacuum-advance pod and the points or the electronic pickup plate to access the advance mechanism. This is a pain, especially if you have to do more than one change to get the curve correct. Essentially, all the mechanical-advance systems work the same way. The usual method involves a pair of eccentric weights that are forced outward with increasing engine speed. A pair of springs is used to determine the rate at which these weights move outward. As the weights move outward with rpm, they are tied to a single plate that uses a pin inside a slot. The diameter of the pin and the length of the slot determine the amount of mechanical advance. So in your case, the easiest way to limit timing would be to reduce the length of the slot. Most old-school tuning shops used a torch and braze to effect this change. To establish the length, you’ll need to shorten the slot; you might try measuring the length of the pin travel and then dividing that length by the amount of mechanical advance (27 degrees). This figure will be a rough estimate of the degrees of advance for the given distance. For example, if the pin travel is 1.00 inch in length and the total mechanical advance is 27 degrees, then the pin is moving roughly 0.037 inch for every 1 degree of mechanical advance. So, if you wanted to limit the mechanical advance to 20 degrees, you would need to shorten the slot by 0.259 inch or roughly a quarter of an inch.
If you have an MSD distributor, you are in luck. The advantage to the MSD distributor is that it places the mechanical-advance mechanism directly underneath the rotor, as in a GM-style distributor, where it is easy to access. Even better, rather than adjusting the length of the slot, MSD supplies different bushings that fit over the pin. The smallest bushing is red and allows 28 degrees of advance, the silver is 25 degrees, the blue limits to 21 degrees, and the largest, black bushing allows only 18 degrees. For your application, the blue would probably be best, since it would allow you to set the initial timing at 14 degrees before top dead center and still deliver 35 degrees of total timing. If you don’t have a dial-back timing light or a degreed balancer, reading total timing can be problematic. MSD offers timing tapes that glue to the balancer to indicate total timing (PN 8985; $4.95, Summit Racing). The tapes come on a single sheet with several tapes specific to the diameter of the balancer. This should solve your timing problem and make the engine much more fun to drive.
Autotronic Controls Corp. (MSD); El Paso, TX; 915/857-5200; MSDPerformance.com
Watts My Line?
Jay Mandernach; Cucamonga, CA: I’ve heard there is a way of automatically shutting off the alternator at WOT. How do you do it? I’m running a big-block in a first-gen Camaro and need about two-tenths to hit the 10s. Do you think this change would get me there?
Jeff Smith: For a short question, the shorter answer is: No. But because abbreviated answers rarely help, let’s run through some numbers to take the mystery out of what we’re talking about. My handy power-conversion chart tells me that 1 hp equals 750 watts. We measure watts by multiplying voltage by amperage. So let’s assume our car requires 50 amps to run down the track. That’s a little higher than normal, but play along with us. Fifty amps at 14 volts pulsing out of our alternator equals 700 watts, which comes pretty close to 1 hp worth of electrical power. Assuming it takes another horsepower to drive the alternator both from losses due to heat and beltdrive, we’re at a grand total of 2 hp. That’s not going to be nearly enough to cut two- or three-tenths of a second from your elapsed time, especially as a typical drag Camaro probably requires less than 30 amps to make a pass down the track, assuming a gnarly CD ignition system and a big electric fuel pump. The classic rule of thumb is that 10 hp is worth 0.10—and 1 mph is not far off in your case. We simulated a big-block Camaro running 10.77 at 129.8 mph and added 10 hp to the simulation, and the result was 10.71 at 130.5 mph, so 10 hp is worth about 0.06 second and 0.7 mph. It appears you’re going to need more than 20 hp to knock 0.2 second off your current times.
What is less obvious is what happens when the alternator is disabled. A typical battery is fully charged at 12.6 volts. The alternator maintains voltage at a much more efficient 14.3 volts. This is the voltage used to push amperage to all your electrical components. If you disable the alternator, the voltage drops quickly to around 11.8 volts. This reduced voltage doesn’t really affect the ignition, since most CD systems will fire the plugs down to as low as 10.5 volts. But your electric fuel pump output will be drastically reduced at lower voltages, and that could be a problem. When we asked Brady Basner of Powermaster about this subject, he agreed, adding that lower voltage can also contribute to run-to-run inconsistencies that are the opposite of what you want for bracket racing. Our recommendation is to keep that alternator charging and look somewhere else to pick up those couple of tenths. You’ve probably already put your Camaro on a diet, but remember that 100 pounds is a tenth of a second. A buddy of mine said he was able to cut 20 pounds off of his race car by replacing all the steel sheetmetal bolts with aluminum fasteners. That’s a great idea. Also, rather than focusing on peak horsepower improvements, consider enhancing the torque in the middle. This will drastically improve acceleration all the way down the track. Nobody said it would be easy! Especially when you’re trying to break into the 10s.