Distributor Gear Size

One of the variables that almost always involved in the ignition is the "slack in the system. How much play is in the timing chain or belt? The amount of backlash in between the cam and distributor gears also adds a certain amount of slack or play in the timing. Some backlash is necessary to keep the gears from grinding themselves apart, but just how much is optimal?

To find out, Dorton tested a variety of different sizes of distributor gears. Check out the chart titled Distributor Gear comparison on this page. This is unusual and a step many engine builders aren't willing to take the time to make. But Dorton says in the right situations, adjusting the amount of backlash in the gear mesh by swapping out an oversized cam gear can be helpful to both an engine's health and performance.

Although Dorton tested with a variety of gear sizes in a variety of different situations, we're posting the results from just two: a standard distributor gear and one that is 0.020-inch oversized (the largest Dorton tried). When testing with an oversized distributor gear, you do need to make sure you still have some free play or backlash between the gears to ensure they don't grind themselves up. Also, check for backlash every 90 degrees to make sure you don't miss a high spot that could wind up damaging the distributor and possibly the entire engine.

Our test subject is the standard Pro Race distributor. The variance didn't change much, which seems to be a function of the distributor or ignition box, but the timing didn't drop as much which makes sense considering we took away some free play in the gears. This seemed to be a consistent result no matter which distributor we tested.

Analog vs. Digital

After testing several distributors and swapping out lots of brass gears, we took a look at the ignition boxes for a change. See the results on this page. The baseline is Crane's HI-6N high-output multi spark box that's approved for NASCAR use. The box is a proven winner, and lots of top race teams swear by it. But that's an old-school analog box, so we're testing it against the next generation, Crane's HI-6R fully digital race ignition.

Honestly, we're not sure what to make of this test. We didn't catch it before we pulled the engine off the dyno, so we didn't have an opportunity to back it up with some more testing (yet). But the digital box seemed to be high throughout the pull. Dorton set the timing at 36 degrees at 5,000 rpm, but during the pull we show the ignition timing for the number one cylinder at 37.3 degrees BTDC at 5,000 rpm. The mostly likely answer is the distributor moved a bit while tightening the hold-down.

The reason why we haven't thrown out this test is the digital box shows remarkable consistency throughout the rpm range despite the timing being set too high. On all the other tests we had a variance from the high to low marks of at least 4.5 degrees. But for the HI-6R box the variance was just 2.5 degrees. And while the percentage change from the target of 36 degrees is high at 5.39 percent, if you compare the 37.941 degree average timing compared to the real timing of 37.3 degrees we saw in the number one cylinder during the test, that makes the percentage change just 1.71 percent. This one definitely deserves to be looked into further, which you can be we will. Stay tuned.

Distributor Comparison

Timing set at 36 degrees at 5,000 rpm - Dyno Pull from 4,000 to 8,000 rpm
Low High Variance Average % Change
Crane Pro Curve Billet 30.4 37.4 7.0 34.288 4.99
Crane Pro Race 32.7 37.2 4.5 35.850 0.42
Crane Pro Race Optical 34.0 39.1 5.1 36.217 0.60

Distributor Gear Comparison

Timing set at 36 degrees at 5,000 rpm - Dyno Pull from 4,000 to 8,000 rpm
Low High Variance Average % Change
Standard Brass Gear 32.7 37.2 4.5 35.850 0.42
Brass Gear +0.020 Inch Oversize 33.6 38.7 5.1 36.196 0.54

Analog vs Digital Ignition Comparison

Timing set at 36 degrees at 5,000 rpm • Dyno Pull from 4,000 to 8,000 rpm
Low High Variance Average % Change
Race Pro w/ HI-6N (Analog) 32.7 37.2 4.5 35.850 0.42
Race Pro w/ HI-6R (Digital) 36.2 38.7 2.5 37.941 5.39

Crane Pro Curve Billet Distributor Test

Engine Configuration: V-8 - Cylinder Configuration: V-8

Cylinder Ordering: Left Bank - 1 - 3 - 5 - 7 - Right Bank - 2 - 4 - 6 - 8

Firing Order: 1 - 8 - 4 - 3 - 6 - 5 - 7 - 2

Cylinder Rotation: 0.0 - 90.0 - 180.0 - 270.0 - 360.0 - 450.0 - 540.0 - 630.0

Cylinder #1 Timing: 36.0
RPM Cyl #1 Cyl #8 Cyl #4 Cyl #3 Cyl #6 Cyl #5 Cyl #7 Cyl #2
4,000 35 36 35.4 35.2 35.4 35.8 35.4 35.2
4,100 36.1 37.1 36.3 35.6 35.7 36.3 36.1 35.7
4,200 35.6 37.1 36.6 35.6 35.6 36.4 36.1 35.6
4,300 33.5 35.6 35.5 34.7 34.6 35.2 34.9 34.4
4,400 33.4 34.4 34.2 33.8 33.5 33.7 33.7 33.8
4,500 34.1 34.4 33.4 33.1 32.8 32.8 33.2 34
4,600 35 35 33.5 33.3 33.2 33 34 34.6
4,700 34.7 35.4 34.2 34.1 33.8 34.1 34.7 34.7
4,800 34.4 35.5 34.5 34.5 34.4 34.5 34.4 34.5
4,900 34.6 35.4 34.6 34.4 34 34.5 34.2 34.4
5,000 34.8 35.4 34.7 34.1 33.7 34.7 33.9 34.4
5,100 34.7 35.4 34.7 33.7 33.8 34.7 33.6 34.3
5,200 35.3 35.3 34.3 33.5 34.4 34.7 33.6 34.7
5,300 35.4 35.2 34.4 33.6 34.4 34.9 33.5 35
5,400 34.8 35.8 34.4 33.5 35 35 33.4 35.3
5,500 34.7 35.8 34.7 33.7 34.8 34.4 33.7 35.3
5,600 34.7 35.1 34.5 34 34.5 34.1 33.9 35.1
5,700 35.1 34.4 34.4 33.6 34.5 34.2 33.4 34.9
5,800 36 34.1 34.8 33.3 34.9 34 32.9 34.9
5,900 35.5 34 35.1 33 34.6 33.6 33.2 34.9
6,000 35.8 33.5 35.4 33 34.6 33.7 33 35.3
6,100 37 32.9 36 33.3 34.8 34.7 32.3 35.8
6,200 37 32.2 36 33 34.3 35.4 31.4 35.3
6,300 37.3 32.2 36.2 33 34.2 36 31.2 35.5
6,400 37.7 31.8 36.7 32.9 34.3 37.1 31 35.8
6,500 36.3 31.6 37.8 32.1 34.7 37.3 30.6 36.7
6,600 33.9 32.7 37 31.2 35.8 36 30.7 37.4
6,700 32.5 34.9 34.6 32.3 36.6 34.2 31.8 36.9
6,800 31.9 35.3 33.9 33 36.2 33.6 32.2 36.4
6,900 30.7 35.8 33 33.2 36.1 32.7 32 36.6
7,000 31 35.5 33 33.3 35 32.6 32.5 35.3
7,100 33 34.9 33.8 33.6 33.8 33.7 33.7 33.4
7,200 33 35 33.7 33.8 33.8 33.8 34.1 33.5
7,300 32.6 34.8 33.1 33.7 33.5 32.9 34.5 33.4
7,400 32.4 35 32.9 33.3 33.7 32.9 34.2 33.3
7,500 31.7 34.3 33.1 32.6 33.5 32.6 33.6 33.1
7,600 31.1 33.7 33.4 32.2 33.1 32.5 33.4 33.4
7,700 31.1 33.9 33.6 32 33 32.8 33.3 33.5
7,800 31.3 34.2 33.4 31.8 33.2 32.9 33 33.5
7,900 31.5 34.1 33.4 31.9 33.2 33.1 33.1 33.9
8,000 31.6 34.2 34 31.7 33.5 32.2 33.6 33.9

SOURCE
Automotive Specialists
4357 Triple Crown Drive
Concord
NC  28027
704-786-0187
www.automotivespecialists.com
Crane Cams
1830 Holsonback Drive
Daytona Beach
FL  32117
866-388-5120
www.cranecams.com/