Datsun Roadster Forum-311s.org

....There is no plus to it, unless trying to de-tune the engine. This makes the vacuum advance ass backwards... At idle, you will be at max (vacuum) advance.. when you open the throttle and put the engine under load the timing will retard....exactly wrong. oops

This is the type of discussion I like. I'll stay out. Pat

JT68 wrote: Thu Nov 11, 2021 11:59 am ....exactly wrong. oops

Well, not ‘exactly’ JT.
Lol … So, here we are, the age old Ported vs Non Ported debate

Remember, ported distr. vacuum advances were introduced solely as a pollution control measure mostly in the late ‘60s to early ‘70s (mfg. dependant) to lean out idle mixtures by reducing timing advance - That’s it. They had no other purpose. Before that, manifold vacuum was the standard and everything ran fine with static distributor timing and mechanical advance above idle. If you go through the trouble of removing the pollution control equipment, recurving the distributor, plug gaps, etc, why wouldn’t you go back to non-pollution control manifold vacuum? A lot of times it can also help with vehicles experiencing pre-detonation at w.o.t. without having to reduce the factory timing - And it will run cooler at idle. Also worth pointing out that race cars don’t even have vacuum advances; they run at w.o.t. most of the time.

To keep it in the context of this thread, I agree, sometimes it will work, sometimes it may not. But it’s not necessarily wrong.

Again, I'm staying out. I've started too many threads, had too many warnings, to contribute to this one. Pat

unklpat wrote: Thu Nov 11, 2021 1:48 pm Again, I'm staying out. I've started too many threads, had too many warnings, to contribute to this one. Pat

Really?

For a 60's street car, a properly functioning vacuum advance improves part throttle response-that is the intention, not for emissions. (All pre emissions roadsters had vacuum advances too except the solex cars)

The vacuum advance pulls the timing ahead several degrees as the throttle is opened. That concept preceded emissions issues by decades even though later (70's/80's) distributors did wacky stuff with vacuum modulation for emissions purposes.

When you open the throttle, engine vacuum drops noticeably and advancing the timing helps overcome the tendency to bog. Hooking the standard roadster vacuum advance to manifold vacuum is simply incorrect and won't do anything great for the engine.

If you are running an ECU with a MAP sensor that is an entirely different approach from the 60's design roadster unit. With a map sensor, manifold vacuum is the intended sensor input.

With a race car, there is no need for a vacuum advance since "part throttle" engine response is of little concern. That is also entirely different from street cars.


The original post was correct, it's definitely a newb error to connect it to the manifold vacuum unless you want a "vacuum retard".

I gave up on the pics as they are hard to enlarge , only a bit and cant save

There appears to be dissenting opinions on the subject: Concept is also supported on other Datsun models.


Found a link to the full write-up which quoted John Hinckley of GM:

http://chevellestuff.net/tech/articles/ ... nifold.htm

He is also a National Corvette Museum Hall of Fame inductee:

https://www.corvettemuseum.org/john-hinckley/

You can understand my confusion on this topic when someone who was actually involved with the development of ported distributor vacuum advances for one of the largest auto manufacturers in the world is advocating otherwise.

Kai, now you might understand my staying out. Pat

Obviously the corvette engine is an entirely different beast and optimizing the advance curve for a GM v8 is not at all the same as a Nissan 4-cylinder. Clearly the intent was not to advance the timing with the vette engine, So in that case it makes sense. My guess is the gm guy has little or no experience with Nissan 4-cylinders and SU/Solex carbs.

One can quote all kinds of gm/ford/chrysler/corvette lore that has absolutely nothing to do with tuning roadster engines.

The A-series quote is also useless as those were running emissions gear that most roadsters don't have. (I can't tell you which would be better on a fully emission equipped roadster since I've never dyno'ed one of those)

If you want the street driven non-emission u20/r16 to run right, the vacuum port gets connected to the carb.

Do what you like, If you want a "vacuum retard" go ahead and connect it to the manifold, but you would be probably be better off just capping the vacuum and using mechanical advance. Vintage carb'd non-emission Nissan engines respond well to additional timing advance until you reach the point of ping/detonation or it is too early at max advance/rpm. The latter is immediately apparent on the dyno and when tuning the 123 distributors.

The Chevelle article which quotes the gm developer is also completely out of context for a very simple reason: SU carbs do not have accelerator pumps! - which (greatly) enrich the mixture when you open the throttle. That article is basically useless with regard to tuning roadster engines with SU carbs.

Really, take my advice, Nissan designed the distributor for the U/R engines- The engineers understood how it worked, how to port it and how to tune it-their goal was certainly to optimize power and drivability from the 4-cylinder.

Jt 68
What was the 1st emmissions device and when ?

on roadsters? In 68 when they added the airpump. Before that it was just the VC breather routed over to the air filter. (which also puts the crankcase under vacuum) Ingesting the crank fumes might be the first overall??

ted heaton wrote: Wed Nov 10, 2021 9:30 pm

iloveredmeat wrote: Wed Nov 10, 2021 9:06 pm Okay... I'm an unapologetic newbie... and I don't have a 2L...

but I'm going to throw a guess: the pressure regulator is piped wrong?

Close, but no cigar.
The fuel flow and plumbing thru the thermal valve is correct.
The return line to the tank is not correctly routed but not a problem, just very untidy.

Ted

I demand a Participation Trophy.

JT68 wrote: Thu Nov 11, 2021 8:21 pm on roadsters? In 68 when they added the airpump. Before that it was just the VC breather routed over to the air filter. (which also puts the crankcase under vacuum) Ingesting the crank fumes might be the first overall??

Guess I should have said in the US
it was the vacuum advance 64-65 yr

The vacuum advance in a 64-67 roadster isn't an emissions device. Nissan (and no mfg) cared about emissions in 1964 - The early engines had crankcase "draft tubes"!

The vacuum advance is intended to increase timing under light load to improve throttle response.

This is pretty much spot on:

"Under a light load and part throttle conditions, timing can be advanced. This improves throttle response and makes the engine more efficient. It also helps the engine run cooler. The vacuum advance provides this benefit BEFORE the Mechanical Advance provides Total Timing."