Dome Spring Experiment

It's a far cry from the absurdly light Datsun factory springs!

Check out the air/fuel gauge page, as well as the damper oil page for complete tuning information.

Z Dome Springs:

In looking at one of my British SU books, it lists a few cars equipped with 2 liter motors and the HS4 SU, same as the 38mm SUs from Datsun (more or less). Two of these cars were equipped with the Blue 2.5oz spring. Since the Z dome springs were the same weight as the ones I already had, I now had a set of springs to experiment with. I needed to cut the springs I had, then stretch them out to create a stiffer-than-stock spring, as described below:

19.25 coils, resulting in a ~2.27 oz/in spring, measures 2-3/4". After cutting the spring you will have to stretch it out in order for it to provide the proper load against the piston. Keep the space between coils as equal as possible. My custom springs measure 4-7/8" after stretching with about 3/16" of space between each coil.

IMPORTANT!!!:

Since installation of my modified dome springs, I've been experiencing an erradic idle, i.e. higher than what I set it at (set @ 1000rpm, would idle @ 1500rpm). Of course this led me on a chase of the problem, which I imagined first was in the linkage, then in the throttle plates, neither of which fixed anything. I'm sure the linkage needed oiling, and the throttle plates needed adjusting, but the high idle was bothering me (and I had somehow ignored/forgotten that I'd just changed something and my problem developed). Finally I decided to check the dome springs. This was something I'd made myself, so it was more open to error than, say, the factory springs. If you've tried to cut and stretch your own springs, you've probably found that it is pretty hard to get them EXACTLY the same length, with EXACTLY the same space between each coil. Well, this is very important. My springs weren't perfect, but they were as perfect as I could make them. However, they were not good enough. I installed the British Red 4.5oz springs (see below) and, Huzzah!, my idle was a mirror of it's former 1000rpm self! It's probably easier in the long run to simply buy a set of springs in the proper weight...

Results:

Proper dome springs should be used which will allow the piston to reach maximum lift at peak horsepower/rpm. Contrary to popularly held belief, you don't want a light dome spring, especially in a modified street motor. Those who don't use dome springs or damper oil in their street SUs think they are enhancing throttle response because the piston reaches maximum lift at the lightest touch of the throttle. However, if they installed an air/fuel gauge to see what was going on, they'd see that as soon as they stabbed the throttle the air/fuel ratio would go lean, maybe even dangerously so.

Datsun dome springs are surprisingly light. As a stock L16 would draw far less air than a built L20B, it is possible that the lightweight springs are suitable for the stock motor. However, british car companies equiped all their small SU motored cars with heavier dome springs than what Datsun provides (from 2-4 times heavier). Many 2L models even use the 8oz. springs!

If you have an air/fuel gauge in your car and you are seeing a lean condition at the slightest tap of the throttle, first check out the Damper Oil page as dome springs shouldn't be used to cure this problem. As I said, dome springs are used to regulate when the piston reaches max lift. Damper oil is more responsible for regulating piston movement as it relates to throttle position.

British Dome Spring:

I have a set of the Red 4.5oz dome springs, after discovering that Triumphs with 2L engines used these springs. These springs appear to be made from the same diameter wire that the standard Datsun springs use. To achieve the 4.5oz rating, the british Red springs have a free length of 6-1/4".

After running the 2.2oz springs I made (see above) and discovering that it was the cause of my high idle, I installed my set of Red springs and ran them around my test route (streets and a freeway blast). These springs seemed a little heavy for my motor as it wouldn't spin up as fast (however, more testing and possibly more tuning are necessary), and acceleration at speed seemed a little off what I had experienced with my homemade dome springs. At speed on the freeway, a tap of the throttle would send the air/fuel gauge lean again, which surprised me. I haven't thought about this enough to have a good explanation yet. Heavier damper oil seems to be the answer though.

Like my search for needles, my search for springs will probably take me to the conclusion that I need something in between what's available (i.e. I could need a 3oz spring where only a 2.5 and 4.5 oz spring are available), but we'll cross that bridge when we get to it (or possibly combine two different weight dome springs in each dome...?). To be honest, the Red springs didn't feel all that much stiffer than my homemade specials. More stiffness was barely noticable as I compared an installed Red spring with one of my homemade springs. Inside the dome of the SU with the damper screwed in, lifting each piston felt nearly the same. I've recalculated the spring weight of my homemade pair and they check in at 2.25oz.

Another hour of running around town has given me a slightly different opinion of the Red springs. They don't seem to let the motor spin up as fast before the cam comes on (slight bog at low rpm). But, my motor seems a bit stronger in the upper rpms, once into the cam's powerband of 3500rpm+. My motor may be a case for the mysterious double-spring approach, however I haven't found any information or sources for this racer's trick as of yet.