NewB Contest: Spot the error

[unklpat]

Again, I'd like to meet dann, and thank him for his contribution to our hobby, but it seems unlikely. Pat

[Bwk2000]

Thanks Jon. I do appreciate your input and the thread in general.

As ‘unklpat’ correctly pointed out, we should have had the discussion earlier about which cams were likely in that car - That was an oversight on my part. If it was recurved, I agree, manifold vacuum would not work as the total static timing at idle and connected to manifold vacuum (without mechanical advance) would be at least a full 6 degs higher than if it were the stock emission control cams. It is highly unlikely that anyone would be able to get the idle low enough without having to retard the timing - Which I concur, is not a good idea. There is always the possibility of experimenting with different cams to find the optimal combination, but realistically, I don’t believe most would go to that extent. If it were still the stock cams, different story, as it potentially allows for the use of manifold vacuum as I’ve previously discussed. Whether or not you should, or can, is still clearly up for debate - Which I don’t think we’ll be able to come to a consensus here anytime soon - Which is fine … open discussion, open ideas, etc.

Not sure what the rest of the 35 biggest mistake are (I will look it up though), but I’m sure I’ve covered off most of them at one point or another - You learn as you go.

[jhayden]

If it were still the stock cams, different story, as it potentially allows for the use of manifold vacuum as I’ve previously discussed.

Both engine photos show that the safety valve atop the thermostat housing has been removed and replaced by a plug. If still using the stock distributor cams then that becomes "Biggest Mistake" nr. 30 on Dann's list.

[Bwk2000]

If it were still the stock cams, different story, as it potentially allows for the use of manifold vacuum as I’ve previously discussed.

Both engine photos show that the safety valve atop the thermostat housing has been removed and replaced by a plug. If still using the stock distributor cams then that becomes "Biggest Mistake" nr. 30 on Dann's list.

That, I did not notice! More learning for me …

[unklpat]

Cool, let's debate having manifold vacuum with an emissions distributor. We all like to debate, don't we? Pat

[JT68]

Hi Pat and Jon, There is no need to debate since the intent of this thread was clearly to help educate why connecting to manifold vacuum is a newb mistake, It is.-Dann correctly spelled this out decades ago. I remember reading over all that stuff when I was 18 LOL! The Rallye catalog was great! To clarify.....

There is still no benefit to hooking up to manifold vacuum with the emissions distributor cam- in fact it is worse... here is why:


For the guys and gals who are very new to all this black magic, there are two distributor cams that Nissan produced for the 16/2.0 roadsters, the 7.5 cam and the 17.5 cam.

The 7.5 degree distributor cam is the early pre-emissions version and is commonly considered the "recurve" distributor cam. It provides exactly 15 crank degrees of total mechanical advance.

The 17.5 cam came on the emissions equipped cars and provides exactly 35 crank degrees of total mechanical advance.

To provide the engine with 35 degrees of final timing, the 17.5 cam MUST be set to 0 degrees initial timing. That way you get 35 degrees of advance at max, exactly what Nissan intended. Non-emissions equipped roadster engines HATE zero degrees of initial timing. Makes them run rather pathetic, which is why everyone recurves to the non-emission cam. If you set it much past 0 degrees(TDC) you will get pre-ignition/pinging/detonation with a standard engine. bad again. The 17.5 cam is pretty horrible with or without vacuum advance.

If a de-smogged roadster engine is used with the 17.5 cam with manifold vacuum, at idle the very strong manifold vacuum will max out the vacuum advance to about 7-10 degrees or 14-20 at the crank. The engine will idle just fine like that.

The problem occurs when you open the throttle, all that advance goes away,(retards) back to near zero and the engine will want to bog at exactly the same time you are trying to accelerate. Fail. If it is an SU car, add to that the inherent characteristic of SU's to lean out for an instant (no accelerator pump), and you have a pretty poor startoff until the engine catches up to the throttle position.

The 17.5 cam with incorrect vacuum connection is pretty much the worst case setup.


If you use 7.5 cam, things are not quite as bad, but still kinda suck (pun lol)with manifold vacuum.

You'll recall that the non-emissions cam timing gets set to about 16-18 btdc. Since the mechanical advance will provide EXACTLY 15 degrees advance , that gives a total high speed advance of 31-33degrees BTDC.

If you incorrectly connect the VA to manifold vacuum, that same advance will occur at idle, but since the initial timing is16-18, the actual timing with manifold vacuum is way out to 30+ . Again that is a fail! That is way more than needed at idle, idle will be fast, and the same thing will happen when you open the throttle, it drops down to wherever the mechanical advance is, at exactly the same time you are trying to accelerate and the SU's go lean.

Bottom line: Neither scenario will help your 0-60 time.

Naturally this is where the 123 distributor really shines, all the numbers above are programmable, so there are very few limitations.

If you can't afford a 123, stick with the tried and true method for the last 50+? years: get a non-emission/recurved distributor and set the initial timing to 17 or so, with the VA connected to the carbs. your engine will thank you!

Apologies of anyone considers this arrogant or condescending. It isn't, but it is correct.

If you are totally lost just give me a call anytime and I'll try to explain it more clearly.

Hope this helps, since I know that was the original intent of the thread!

[Datsun.David]

Thanks JT for the thorough analysis.

[JT68]

you 're very welcome David, hope it helps everyone

[iloveredmeat]

you 're very welcome David, hope it helps everyone

Still over my head, but I really appreciate the detailed info and respectful conversation.

Thanks all for taking the time to share your knowledge!

Peter

[Gregs672000]

This has always befuddled my mind, so maybe JT can help... you can skip the first paragraph if you don't care about how my system works and just get to my question.

With my Ford EDIS crank-fired system (no distributor) it uses manifold vacuum (measured by a MAP sensor) to adjust spark timing based in load and rpms. With the throttle plates closed there is a very high MANIFOLD vacuum (strong "suction" on the line if you will) which translates to a low load on the engine. As you open the throttle, that manifold vacuum or suction on the line to the sensor decreases which the computer interprets as an increased load, which triggers it to reduce timing by whatever amount is needed to maintain an efficient timing of the spark and that does not induce detonation by sparking too early. With this design, typically the computer sets the "mechanical" advance based on RPM and REDUCES or advances timing based on load. As the load or vacuum increases or decreases the computer will adjust the timing accordingly to a set timing point. So, imagine that you're cruising along in 4th gear at 3500rpms and 32 degrees advance, then you accelerate hard... the vacuum (pressure) in the manifold drops, the computer senses this and reduces the timing from the set point (32 degrees for example) minus what ever amount I tell it reduce timing to based on load and rpm... say 3-4 degrees to 28, until the load drops (back to cruising, but probably at a higher rpm and a bit more timing advance than at 3500rpms as needed for proper burn) and the engine can be most efficient/effective with more timing advance. (Whew!).

Now, it's my understanding that the stock system takes vacuum signal off a port BEFORE the throttle plates, so there would be no vacuum on the line UNTIL the throttle plates are opened to cause suction... or is this incorrect? (BTW, I never had SUs or vac advance stock, so I'm ignorant here). So how does the vac canister on the stock dizzy operate? Is it really more of a timing retard than a vacuum advance?

Thanks!

[spyder]

I added a map sensor to my crank fired ignition to see if the mileage or performanse would improve. I couldn't see any improvement in either. I guess Solexs are goin' to Solex. Seems the factory learned this many years ago!

[jhayden]

Greg,

IIRC, you owned a 620 PU at one time, which would have had a vacuum advance distributor. If you performed a tuneup, you might recall plugging the vacuum line with a golf tee while setting the basic timing.

Here's a good explanation of why part-throttle combustion somewhat counterintuitively requires more advance (especially paragraph 5):

https://www.onallcylinders.com/2015/03/ ... u-need-it/

JT can no doubt explain it better/more specifically in terms of our cars.

Jon

[JT68]

This has always befuddled my mind, so maybe JT can help... you can skip the first paragraph if you don't care about how my system works and just get to my question.

.....

Now, it's my understanding that the stock system takes vacuum signal off a port BEFORE the throttle plates, so there would be no vacuum on the line UNTIL the throttle plates are opened to cause suction... or is this incorrect? (BTW, I never had SUs or vac advance stock, so I'm ignorant here). So how does the vac canister on the stock dizzy operate? Is it really more of a timing retard than a vacuum advance?

Thanks!

Don't get confused by MAP sensor operation, that is totally different - MAP sensors on an emissions equipped car provide input to the ECU which is always looking to minimize emissions and maximize fuel economy. The manufacturers/ECU move the timing all over the place depending on load and what is measured going out the tailpipe every few milliseconds. That's radically different than the simple little vacuum advance used on all the roadsters.


But yes, you are exactly correct: "there would be no vacuum on the line UNTIL the throttle plates are opened to cause suction"

The VA is a very simple diaphragm, when there is vacuum on the line (part throttle) the little diaphragm pulls the breaker plate clockwise. SInce the distributor rotates CCW, the points open or EI triggers earlier when the plate is pulled CW(advanced). As soon as the vacuum goes away, the spring on the VA pushes everything back to "normal" (non-advanced) position. It's really that simple.

[Gregs672000]

But under throttle or load, shouldn't it pull out or reduce timing? That's what I'm saying and don't understand here... if opening the throttle (increased load on the engine) causes suction that moves the plate to INCREASE or advance the timing that's the opposition what you want, right?

The MAP sensor in my system is designed only to give a measure of engine load so the computer knows what "bin" or data point to look up for the amount/degree of timing advance to give. It gives a different degree of timing at 3500 rpms depending on whether it's cruising, accelerating or decelerating as each condition wants different things.

I don't mean to be dense, and I must be missing something because the stock system clearly works, but I just seem to be missing something in its operation or my conceptualization of it.

Spyder, you probably wouldn't gain any mpg without leaning out your carbs at cruise and advancing the timing so as to burn the leaner mix, so just adding a MAP did nothing to change your fuel ratios. With EFI I can identify cruise loads and rpms, lean the A/F target in those places and adjust the timing to improve mpg. Dyno tests have also shown that engine power is much more influenced by ignition timing than air/fuel ratios by a long ways, and that varies a lot by load, compression ratio and octane.

[JT68]

Hi Greg, I think what you are getting at is the PRACTICAL need to reduce the timing under high load conditions...

Remember the roadster engines were designed back in the days when 98+ octane was common and all the cylinder heads were new, so preignition/detonation was not much of an issue (unless the engine was just timed wrong). A simple system that advanced the timing with rpm worked just great (see the FSM for the simple distributor advance curve)

but yes, due to higher CR's and poor octane/crappy fuels, the roadster engines will definitely want to ping under high load with too much timing.
(think 2200rpm in 4th gear). At 2200 rpm the distributor might be delivering 28 or 30 degrees of total timing, which is great in 2nd, but not so great in 4th with 89 octane gas.

In the ideal world of 1968 100+ octane fuel, the fairly primitive distributor advance system was just fine.

These days (and because most cylinder heads are cut some) we all have to run a bit less timing as a compromise to keep roadster engines happy.

In general, the more advance you can run without pinging/detonation, the happier (more torque/more hp) the engine will be. (until the timing is so early detonation occurs)

Of course modern cars use the ECU to monitor tps, maf, map, knock sensors,rpm and actual load to always optimize injector pw and timing for best efficiency and low emissions no matter what the gear, rpm, speed, throttle position etc. So yes, in that case the ecu is constantly advancing and retarding the timing in realtime to optimize everything and definitely retards timing under load to prevent knock. (*roadster distributors can't and don't do that).

Hope this helps! j

*the 123 distributor pressure sensor could theoretically be connected to either manifold or ported vacuum and be programmed to advance or retard as you like- and even accommodate moderate positive manifold pressures (boost), but I haven't really explored the limitations of those capabilities much yet! The programmable curves really do wake up the roadster engines.