What difference does a distributor make on the dyno?

[Editor's Note: This article originally appeared in the September 2017 issue of Classic Motorsports, back when Savannah Race Engineering was still in operation.]

Does running the correct distributor matter? For decades we have seen the ads and heard the reports, but how often do you get to compare one setup against another? And is it true that a distributor can make big gains in power? As we recently found during a dyno shootout session, the answer is a bit more complicated.

Our test subject was a 1600cc Lotus Twin Cam engine freshly rebuilt by Ted Wenz’s Savannah Race Engineering. This shop specializes in Lotus Twin Cams but can handle pretty much anything. Our test engine featured all of the good stuff, like a pair of 45mm side-draft carburetors, Dave Bean No. 114 camshafts, and a 10.5:1 compression ratio. The engine ran on 93-octane pump gas and was happiest with 16 degrees of initial advance, leading to 32 degrees of total advance.

We did our testing at Savannah Race Engineering on their Superflow engine dyno. Wayne Brown, who has been building and tuning engines for Ted Wenz for 23 years, oversaw the dyno testing.

Baseline Setup

Test Subject 1

Test Subject 2

Test Subject 3

What Did We Learn?

All of our testing produced nearly identical peak torque and horsepower figures, so does the distributor really matter? Yes, it can, and we’ll get to that in a minute.

First, why the close dyno figures? Because each distributor was timed to provide 32 degrees of total advance. Distributors have an advance curve, which changes from idle until about 3000 rpm. At that point, the distributor has typically delivered its entire advance. Once that advance is all in, it doesn’t change.

The peak numbers don’t tell the whole story, however. On the other hand, it’s hard to see good numbers below 3000 rpm on a dyno–and you really don’t spend much time below that figure anyway.

So, was this a good test? Well, sort of. A good ignition system doesn’t really make power, but a bad ignition system loses power. All the systems we tested were good, so they made good numbers. Had we tested a worn-out distributor, we would have seen some differences. However, we didn’t want to risk using bad equipment on the brand-new engine that had just been built.

What would be a better test? We’ve been trying to find it for years. The bottom line is that the more useful data is easier to measure by seat-of-the-pants feel than with dynos or stopwatches. A good distributor will have a curve that yields great throttle response and drivability. Finding a repeatable way to measure that in the 1000-3000 rpm range is next to impossible, but we know from personal readings that all of these distributors can do that.

So, what’s the difference between these distributors? The key variable that the dyno allowed us to measure was the amount of variation they had at full advance. As you might expect, the hand-built units by S. Jennings and Advanced Distributors held their timing better than our production units. A distributor that provides inconsistent timing loses a bit of power compared to one that’s more consistent.

What else did we learn? Despite the Lotus being a high-performance street engine, Savannah Race Engineering considered it to be rather tame by their standards. As Wayne explains, on a relatively lower-output, production-based engine like ours, one properly operating distributor isn’t going to significantly outperform another properly operating distributor.

If this Lotus engine had been prepared for competition–meaning with a higher compression ratio, increased redline and more radical camshafts–then that distributor would play a more important part, Wayne continues. A few degrees of variation makes a bigger impact at 8700 rpm than 6500 rpm. That accuracy is going to deliver improved performance at higher engine speeds–and, one could surmise, increased longevity.

That accuracy–a product of tighter tolerances–also comes at a cost of time and/or money. A true competition distributor, like the one from S. Jennings Racing, retails for more than a grand and is built to spec. Many of Advanced Distributors’ rebuilds start at less than $200, and each one is hand-assembled by the owner of the company using parts he built or personally sourced.

For less than $300, you can order a brand-new PerTronix distributor. Many retailers keep them in stock, meaning you can get your hands on one right away instead of waiting for a custom unit.

Another consideration: Do you care about period-correct looks? Reproduction or billet parts don’t look the part, so a properly rebuilt original may be the answer.

And finally, do you want to mess with points, or does the set-it-and-forget-it nature of the PerTronix better fit your lifestyle?

While all of our distributors pretty much produced the same performance figures, each one has a different role in our world.

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Comments

Jerry From LA SuperDork
10/10/18 2:42 p.m.

Accuracy is one important component. A hot reliable spark is the other, especially considering the higher combustion pressures in a modified race engine. Hard to make power at 6,000+ rpm if one cylinder misses three of the 50 sparks per cylinder generated in a four-cycle engine running at 6,000 rpm. It gets worse as the rpm climbs.

s2europa New Reader
10/11/18 5:08 p.m.

I know it's a little late, but it would have been nice to see how the 123 Ignition distributor stacked up against the other ones. I'd be anxious to know if that hefty investment is worth it.

Your engine is running 45 side drafts with 10.5:1 compression ratio. The stock home market European Lotus twin cams were 10.25:1 and ran 40 DCOE or Del Ortos. Are you choking the 45s down? Is this because of the different cams?

Vigo UltimaDork
10/12/18 10:02 a.m.

4 cyls with 90* between each cap terminal and a grand total of 20* of ignition swing (did it have vacuum advance too?) are good candidates for making a (tight) distributor work even at high rpm. Less than 1* accuracy has to be considered pretty damn good! I think actually just being aware of the spark timing variation that's possible at high rpm is the first step to not getting bit by it. A lot of ohc engines running timing belts have the harmonics of the belt and then one or two more gear interfaces adding up slack to the distributor's drive system. If you get into 8+ cylinders engines and boosted engines running high rpm, the reasons for getting away from a distributor grow substantially. Even though the dyno test itself didn't exactly illustrate the pros/cons of accurate ignition timing, just bringing up the issue and talking about it is something worthwhile.