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BRAAPs L6 EFI-induction advice and tips.

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Since we are on the subject of efi and afm stuff Braap, what is "your" take or say on replacing the "stock" non-turbo afm with the larger "turbo" afm for an increase in mass air flow? (changing out the circuit board in the turbo afm for the non-turbo circuit board and going from there?) If, of course, this has been discussed elsewhere, give me the link. But I want to know "your" opinion because you have a well spring of information I haven't acquired just yet. Thanks :)

Edited by wagon jon
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Thank you.

I think that is a great idea, it has been discussed before but, I don't recall to what degree though. Not sure how well it would work, larger cross sectional area, softer spring to maintain similar travel for given air flows?.... B)


Removing the AFM from the air stream all together would be ideal, so getting as close to that ideal without affecting drivability is a good thing. I recall someone offering an interface that allows the AFM to be replaced with MAF. That should help on several levels.


Sorry I don't have any thing concrete on that.

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Thought I'd chime in now that I found the paper I wrote this down on.

I performed the exact same measurements as BRAPP on a P72 EFI manifold about a year ago or so.

The average runner diameter (taken at .25", 2.5" and 4.5" from the flange) came out to be 1.247 inches (31.694mm). Just ever so slightly smaller than the other N/A EFI manifolds already measured.

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EFI system basic run down and simple tuning/drivability diagnosing tool;


These two links are invaluable to to Datsun EFI tuning and diagnosing;







1) Water Temp sensor; In altering the resistance of the water temp sensor circuit, this offers a linear AFR change across the entire RPM range under all conditions. More resistance is richer, less is leaner. The only way to get “leaner†mixtures with the OE EFI is to use the resistor/potentiometer in place of the water temp sensor itself, but then you loose the coolant temperature compensation. I’m sure a savvy electrical engineer could build an electronic doodad that reads the water temp sender and then alters that return signal to the ECU both above AND below that of the water temp sender resistance.



I'm assuming the reason you can't get "leaner" mixtures is because you can't get the resistance low enough with the water temperature sensor. Let's say at 100 deg C the sensor reads 200 ohms. When you put a variable resistor (potentiometer, etc) in series the lowest reading you'll get is 200 ohms when the variable resistor is zeroed out. You could add a 200 ohm resistor in parallel with the thermal sensor to get it to drop to 100 ohms at 100 deg C. The problem being of course is that at our target 100 deg C we are looking at 100 ohms but when the temperature is at 20C and the stock output is 2.5K with our 200 ohm resistor in series our combined parallel resistance would be about 185 ohms instead of 2.5K. A possible solution would be to run two thermal sensors in parallel which would take the curve and essentialy cut it in half but it would remain half throughout the range of the sensor. Not perfect but it would be a lot closer as a solution to run a "leaner" mixture in conjunction with the pot in series. Of course you could also use a micro controller of some sort with an ADC to take a measurement with the thermal sensor then output on a DAC to give the desired output voltage (which is essentially what the voltage divider of the thermal sensor does). The micro controler would just give you the ability to change what the ECU sees instead of what the actual sensor would output.

Edited by FricFrac
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Just a little food for thought. In researching intake manifolds and power produced from those that have ran their cars on the Dyno, was able to find quite a few for the Turbo guys, and a handful for the N/A guys. Essentially all of the Turbo guys running the OE EFI intake manifold are making great torque numbers, but the HP to Torque relationship is reminiscent of the mid ‘80’s GM TPI engine or like turbo diesel HP and torque numbers. Huge torque, but it falls off early in the band. Others that are running similar set ups with the only difference being a different or altered intake manifold, their HP numbers fall in line with the torque numbers respectively, i.e. if they are making 500 ft lbs of torque, they are also making in the neighborhood of 500+ HP. The OE intake guys making similar torque figures are typically making mid 300 to low 400 HP. For the N/A EFI crowd, a similar situation holds true. You will find that the fast EFI N/A guys are running some intakes other than the OE intake! (hmm… seems to be a trend here… ) The throttle valve being used, whether it be OE or the monster 90mm Q-45 unit is irrelevant.


Excellent write-up Paul. I learned some stuff and validated info I thought I knew. I have experienced the above engine characteristics first hand with my turbo L28 and and early N42 intake. Peak torque is exceptional since the ports are so small but big HP (500HP+) requires extreme boost or a BIG turbo as compared to a better breathing engine. A more free flowing intake and head would easily make more power with my same turbo. Unfortunately I think I'd have to sacrifice peak torque due to the velocity reduction caused by bigger ports. A benefit would be a flatter torque curve which would make power delivery a dream compared to what I deal with today. Wait for it......wait for it......BAM!!!!


On the other hand, making peak power at 6K or below (due to small ports) is MUCH easier on the L28 with it's long crank vs. folks who spin 7K and above. I've never broke a crank or damper yet. :blink:

Edited by jgkurz
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btw since it was mentioned earlier in the thread, i searched and couldnt find anything comprehensive... im curious what your methods are for tuning the ignition. ive heard people use vacuum advance and lock out the mechanical advance both to great success... curiously i cant find any real all inclusive thread like this about ignitions.

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... ive heard people use vacuum advance and lock out the mechanical advance both to great success... curiously i cant find any real all inclusive thread like this about ignitions.

Using Vacuum advance only with the mechanical advance locked. Hmmm… As I understand the general ignition requirements of petrol engines and how mechanical and vacuum advance function, that is an odd approach, one that I am struggling grasp how it would successfully deliver a street engines ideal ignition advance curve. I guess it could be done, just seems like the harder way to get the job done.


btw since it was mentioned earlier in the thread, i searched and couldnt find anything comprehensive... im curious what your methods are for tuning the ignition. ...

Dizzy ignition tuning.

First off, I strongly recommend the Jacobs Ignition book. It is a wonderful resource, explains in great detail the ignition process/event, how the engine uses it and how to find your engines ideal ignition advance curve, the procedures I used for tuning my L28, hope this bit of info is helpful.




This is my personal layman’s interpretation of the mechanical and vacuum advance systems.

What the engine really wants for ignition advance;

First off, the combustion event is a burn, NOT an explosion. It is very fast, burn, but again, not an explosion. For your aluminum pistons and head gasket sake, just remember explosions bad. All this fuss over ignition timing has to do with one thing. Getting the cylinder pressures to maximum just after the piston passes over Top Dead Center, (TDC). Some of you may be thinking, “why are igniting the fuel mixture BTDC, (Before Top Dead Center)?” This is because of the time it takes for the spark to ignite the fuel surrounding the plug electrode and for that small kernal of flame to propagate and consume al the fuel and air in the cylinder, refereed to as flame front, which takes time. Remember the fuel burns, not explodes. :wink: As such, we lead the ignition timing so that those pressures take place at the proper time to get the most out of the fuel in the cylinders, i.e. more torque. Because the flame front travels at fairly consistent speed, (factors such as mixture, charge temperature, quality of fuel, cylinder pressure at the time of combustion, etc all have small effect on how fast that flame front travels), the faster the engine spins, the more we have to lead the ignition timing. Now for the wrench. This advancing for added RPM only takes place up to about 2500-3500 RPM or so, beyond that point we do not continue to add more ignition timing. I read whey that is many years ago, forgot that bit so my apologies for not including that, I’m sure one of the more knowledgeable ignition gurus will chime in.

At any rate, if ignition takes place too late in the cycle we end up with cylinder pressure not reaching the pressure it is could pushing on the piston to turn the crank that converts our tires into a dense fog, i.e. energy wasted as heat instead of work. If ignition takes place prematurely we end up with the cylinder pressure peaking before TDC, which we often hear as pinging, again, wasted energy.



As you can see, it is delicate balancing act to get the ignition to take place at the correct time over varying RPMs and load conditions. Ideal ignition advance for most all petrol engines follows this basic trend.

For a given engine load, (can be read as a steady manifold vacuum), the ideal ignition advance curve rises with RPM up to a point, then plateaus. That plateau can be anywhere between 1500-5000 RPM in extreme cases, most 2 vale engines that operate to 7000 RPM will typically plateau between 2500-3500 RPM. The mechanical ignition advance meets this requirement very well.

As engine load decreases, say from WOT down to approx 1/3 throttle for cruising down the highway, the amount of ideal ignition advance for optimum combustion will be more, but still follow that same curve as described above, (for those more advanced ignition tuners, yes the ideal curve/slope will vary slightly as load changes but for this discussion we are calling it the same, just trying to convey the trend that is present), the vacuum advance meets this requirement quite well.


Mechanical advance;

As we just covered, the mechanical flyweight ignition advance system does a good job of meeting the engines advance requirements over a broad RPM range for a given load. Starting low at low RPM, rising over a specific RPM range and then plateau across the rest of the rev range. See graph below for a generic WOT (Wide Open Throttle) mechanical advance curve, not to be used for the L6, (curve hits plateau too soon for most, typical will be between 2500-3000 for the plateau). The amount of advance the dizzy allows to take place is dictated by the amount of throw the flyweights have. The start of that slope and the pitch of that slope is determined by the spring tension on the flyweights and/or mass of the flyweights themselves. (Some of you are already thinking ahead and visualizing modifying the spring tension and/or flyweight mass as well limiting the already long flyweight throw). :wink:

Different engines and configurations will have slightly different starting points for the rise, the slop pitch and ending points for its ideal advance curve, but in general all petrol engines have some form of this shape. For example, multi valve engines with spark plugs dead center in the chamber will have similar shape but the overall values will be much lower.




Vacuum Advance:

As we discussed earlier, as load decreases say from WOT down to approx 1/3 throttle for cruising down the highway, the amount of ideal ignition advance for optimum combustion will increase, the vacuum advance meets this requirement quite well. This Vacuum advance is added to the mechanical advance. For example, cruising the freeway at 3000 RPM, ideal 1/3 throttle ignition advance may be 50 degrees. Using the chart above that means the vacuum advance is adding 15 degrees to the already 35 degrees of mechanical advance.


Now lets make a few extreme examples to show the importance of both mechanical and vacuum advance.


If you locked the mechanical advance in your L6 then set the ignition timing so it was ideal when above 3000 RPM, (35 degrees in this case using the above chart), that means that starting and idle ignition advance will also be 35 degrees. This engine will not start! As the starter tries to crank the engine, each combustion event that ignites will try to push the piston back down before it reached TDC, kick back. Ever try to kick start a motorcycle that kicked back or pull start a lawnmower that ripped the starter handle out of your hand? To much ignition advance or the RPM you were trying to start it from. That’s why if you kick the motorcycle faster you were getting the cranking RPM just high enough to outpace the ignition event same for the lawnmower. For the L6, I don’t, you would have to find a way to get the motor to spin up past 2000 RPM or so, then flick on the ignition system to get the engine lit up.


Removing the vacuum advance. Common among racers and that is fine. As we covered, the vacuum advance only adds ignition advance during part and light throttle operation. That advance helps fuel mileage considerably, (if your vacuum advance is working correctly and your Z is getting 25+ MPG on the freeway. i.e. in a good tune, disconnect your vacuum advance and check your mileage on the freeway. It will drop considerably, up to 5 or more MPG). Racers don’t care about part throttle mileage and they rarely are using part throttle during a race so that is whey they delete the vacuum advance. One more benefit with vacuum advance is throttle response. Throttle response will be much cleaner and crisper with vacuum advance vs without out it.


My starting recipe for L6 ignition advance for stock to mild builds is this, then fined tuned for the specific car; (you need a “Dial Back To Zero” timing light for this. If you don’t have one, then you’ll have to accurately mark the degrees on your damper)

Quality gasoline! If you are running that cheapo no name crap gasoline, don’t even bother as you are wasting your money anyhow, none of this will be of any benefit. I’ll save that rant for another thread. :wink:

Mechanical advance curve, (measured/set with vacuum advance unplugged)

Idle ignition timing between 15-20 degrees. Advance starts immediately off idle, maxes at 2250-2500 RPM. Max mechanical timing of 36-40 degrees. Every engine and combination will response differently, quality fuels, elevation, build up on the back of the valve, injector, spark plugs etc. Using the guidelines in the Jacobs book and if your engine is stock or mild, you should end up somewhere near what I described. Softer springs on the flyweights.

Leave the vacuum advance connected to the ported vacuum port. That is the port that at idle has NO vacuum on it, but does once the throttle is cracked, (little sliver or port hole just ahead of the butterfly in the TB. Carbs also usually have a ported vacuum port).


Hope that helps,


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WOW! That was a alot to digest in one sitting. I appreciate you putting all that down so we could learn ourselves a little sumthin. I'll definitely be picking up that book, Amazon and I have a revolving door it seems lately! Thanks to your explanation I can probably grasp alot of the concepts in the book more confidently, as opposed to having to google half the stuff I read about and hope for some dummied down talk or a diagram, haha. Thanks again, Paul, for an ignition rookie like me that was very helpful.

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I was wondering that if the engine is starving at WOT, would it be a good idea to richen the mix only at WOT, in addition to what the ECU is trying to do using the engines sensors?  I noticed the water temp sensor bypass box early in this thread, and thought that  you might install a single resistor and switch (momentary or toggle) so the mix is enrichened.  Kinda like those nitrous boost buttons... Would that work on the L-Jet?


Maybe you could even set up a  relay thing on the TPS so it inserts the richening resistor automatically.

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Instead of using the TPS for WOT tuning, it would bet better to use the transmission kickdown switch, which only clicks at the last little bit of push on the gas pedal.  The TPS closes at about 2/3 or 3/4 of the way to WOT.  the only alternative to that wold be a disk attached to the throttle linkage with notches or holes that would be sensed by either microswitches or optical sensors.

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