ZENITH POWER
STOCK and STREET PERFORMANCE
Background:
By the 1950s, carburetors were asked to do better than carb technology really could do, especially on street engines. They're a puddle of gas, open to the atmosphere, asked to deliver varying (proper) amounts of mixture in all sorts of engine operation regimens with limited feedback. But good carbs matched to a good engine combination continue to deliver power, economy and pleasure.
When 356s were built, Porsche selected from what was available. By comparison, GM could "spec" a carb and get it.
Of the lot even now available, Zenith NDIX carbs seem best suited for our engines driven on the street.
This is partly as a result of the variable configurations available in the Zenith package at the time, but also in that the transition circuitry ('drivability', 'response') seems better than any other option. The construction, with a chrome-plated brass throttle shaft in an iron throttle body has also proven very durable. Zenith continued to develop the NDIX long after 356s were made, making it available in even more alternatives.
A set or exceptionally clean 1958 Zeniths, rebuilt and prepped by Precision Matters for a big-bore "S" engine.
Zenith Options:
Some of the variability of these carbs is a result of the continued development up until the early 21st century. Factory throttle bores are available in 32 and 36mm sizes, venturis can be had in 24mm and 28mm in stock sizes and up to 32mm in 'specials'. Jets are also either available or modifiable to any reasonable need. Peripherals (fuel lines, air cleaners, tin, manifolds, linkage, etc) are also '356-stock'; with these carbs there is no need to jury-rig anything.
As we supply them, they support operation from 1500N engines up to 6,000 RPM in big bore engines with long cams. The 'biggest' of them, on a 1720cc, street-cammed engine developed maximum power at 5,500RPM. Peak power was similar to stock S90 numbers, but the S90 required ~6,000RPM to deliver the peak. Speaks for itself.
Here's the extremes; Stock N at the top and "Big 36" on the bottom .
Those who will or need to run at higher speeds (like you racers) can modify the largest of the carbs to run at even higher engine speeds.
Applications are divided at the throttle bore size as the larger bores require the added cost of CNC-modified manifolds.
These are flexible casts of the stock (right) and the 36mm, CNC-modified (left) manifold passages. We were able to eliminate one of the dog-leg bends in the design and modification.
Engine Applicability, 32NDIX, Stock Manifolds:
By now, the most common 356 engine is either a factory S, an N modified to S-specs, a C, or something built on 912 cases and heads, all with an 86mm 'big bore' kit, a street cam and moderate compression. For these engines, we offer our "Off the Shelf" carbs for immediate delivery. These are dyno-jetted for good performance, response, economy and safety for this majority of 356 engines.
If you have a totally stock engine, you should stick with factory recommendations, including 24mm venturis for N-spec engines. These are what the factory found best, and no one has better data. Zeniths are also dated-stamped; yours may be yours and you may want to keep them. If either of these conditions apply, we will supply them as "Custom" rebuilds at a two week delivery.
All of the above are built on 32mm throttle bodies; they will produce max power between 4,500 and 5,000+RPM and fit the stock manifolds, along with all the ancillaries. .
Special jetting for altitude or other requirements is also a "Custom" rebuild as is custom machining to 30mm venturis. Please call.
Pre Mat rebuild of stock C carbs.
Engine Applicability, 36NDIX, CNC-Modified manifolds:
If you have late heads and a 'big bore' kit, there is more power available from Zeniths with 36mm throttle bodies, mounted on CNC-modified manifolds. Our "S-36s" (28mm venturis) are jetted to produce a strong midrange, max power and safe mixtures on these engines. Maximum power on a well-prepped 1720cc, street cam, stock exhaust is in the low 5,000RPM range.
A comparison of 32mm (top) and 36mm (bottom) throttle bodies. A 20% increase in area compliments big-bore engines.
For heavy breathing, our "Big 36s" with 30mm venturis, enlarged plenums and modifications to the carb top provide the maximum power these can produce for street applications. These kits also include the modified manifolds. You'll need a carefully-prepped 'big bore', high compression engine to take advantage of them, and the power will max out at ~5,500 RPM. These will flow at even higher engine speeds for those running longer cams.
A view of a "Standard 36" (top) and a "Big 36" (bottom). 30mm venturis flow enough to benefit by the removal of the cross-bar.
Here you can see the machining done in the plenum, the new vent ports and the safety wire keeping the screw in place.
Both 36mm programs are normally available from stock. "Custom 36s" are available to your spec. Please call.
Velocity Stacks:
Precision Matters Velocity Stacks are designed to fit inside the stock T6 canisters and they deliver gains where we didn't expect to find them. They were designed, prototyped and tested in the expectation of gains at higher engine speeds. It was there, but not much; a pretty consistent peak increase of 0.5HP.
What was interesting were the gains at lower engine speeds; Torque improved by an average of 4#/ft at 3,350 and those gains continued until they added that 0.5HP above 5,000RPM. This was true of all carbs tested; 32s and 36s.
These are 'drop-in' parts but those with T6B filters can keep your filters from sawing slots in the carb tops by some additional installation efforts.
Both B and C velocity stacks are stocked.
This is the B version. Fitted with screws in a B filter canister, it will rest on the carb air horn, and prevent the canister from cutting a groove in the carb top.
Filters:
We have tested only the T-6 'canister' (separate-element) air filters. Tests have included additional inlet area in the canisters, various filter elements and no elements. We have also tested no canister/filter at all.
No gain was found by increasing the inlet area of the canister. This is obvious by comparing the 356 inlet area to the inlet areas of other high performance engines.
No filter element out-performed the stock, paper, filter element, which also provides very good filtration.
On a 1720, street-cam, 9.6:1CR engine, the next-to-best peak power was found with 'Big 36' carbs, stock canister and filter, and drop-in velocity stacks.
A slight gain in peak power with no gain at peak torque was found with 'Big 36' carbs and no filter at all. Purely extrapolation, but the T5 (mesh) filters should approach the 'no filter' numbers, since they are in effect, no filter. I do not recommend this, but others don't mind.
Stock air filter elements are available from the usual suspects; Registry vendors.
This is the test canister, showing the increased intake area. Sometimes theory doesn't deliver.
Jetting:
Precision Matters 32mm Off-the-Shelf carbs and both of the 36mm versions are jetted for a 1720cc, street-cammed engine with late heads, ~9.5:1 CR. The choices are based on dyno testing at The Stable in San Francisco, near sea level. The jet choices were made with an eye to both power and safety. Special jetting is special order.
Gas odors:
If your carburetors are leaking, they do need to be rebuilt. But all gas 'smells' are not necessarily caused by leakage.
With a thanks to David Jones, modern (say post-2000) gasolines are formulated to favor closed-system, fuel-injected engines. They are far better fuels than we had when 356s were new, but they are very volatile; they evaporate. Our carbureted (sort of a gasoline puddle) engines will lose fuel to evaporation and 'smell' without any leakage at all. Bench tests show carbs to pretty much empty the float bowls in one to two days.
As mentioned in our Instructions, there is another source of gas leaks. After some miles, the fiber inlet (Banjo) washers 'set'. And leak. Tightening the inlet Banjo bolts will solve this problem for many, many more miles; they only seem to 'set' once.
This is our test of a possible solution to quick evaporation. Call it a "still" it is nearly 36" of tubing, fits within a stock filter canister and it didn't slow the evaporation by any measurable amount
Stock Venturis and Jetting:
This is a listing of the venturis and jets supplied by the factory for the various Zenith 32NDIX-carbureted engines
| Engine type | Normal | Super | "C" |
|---|---|---|---|
| Venturi | 24mm | 28mm | 28mm |
| Main Jet | 115 | 130 | 130 |
| Air Correction Jet | 230 | 220 | 210 |
| Idle Jet | 50 | 50 | 55 |
| Idle Air Jet | 120 | 140 | 140 |
| Pump Jet | 50 | 40 | 40 |
| Injection Tube | #8 short | #8 short | #8 short |
| Mixture Tube | #1S | #1S | #1S |
| Injection quantity, all: | .3cc per two full strokes |
| Float level, all: | 18.5mm +/- 1.0mm (.728", +/- .04") |
This is the clearest image we have found detailing the various Zenith NDIX parts. It is applicable to both 32mm and 36mm carburetors.