Ignition advance

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Gregs672000
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Ignition advance

Post by Gregs672000 »

Now that my Isky cam is back in and working properly (as far as I can tell anyway!), I wanted to play with ignition timing some more. Background: The engine in crank fired via a Megajolt, which allows me to play with timing anywhere I want or need to. The engine is high compression (11.7 to 1), Dellorto carbs with 37mm chokes, cam is 246 degrees duration at 50 thousands, 536 lift and seems to work really well everywhere. Naturally I'm concerned about ping. I have a timing map we set up on the dyno, but that was when the cam timing was way off. Nevertheless, I will operate under the assumption that this map is adequate, but I will post it when I hook up the computer and write it down for comment. My question at this point is, how much vacuum advance does the stock dizzy add? It is my understanding that it is mainly for gas mileage and emissions but I would love to improve those. Manifold vacuum will be used and is already plumbed from all four runners to the MAP sensor built into the megajolt, and adjusting timing is a simple matter of deciding how much timing to add or subtract based on the demands on the engine... I just need a ball park idea of how much to try. This will probably be in cruising conditions under light loads where I add in more advance, then as the pressure drops as the throttles open more or I stomp on it it will go back to the set "safe" values. I do have the ability to pull more timing out below the set value if needed, which may be helpful if it pings under full throttle for example but not under 2/3 throttle using the set value, but I'm not sure that makes sense. I need to understand where the stock system operated and how much it advanced timing via vacuum.

I have read that the U20 likes a lot of advance (Bob Sharp) but that was with the fuels of the day, and under racing conditions where vacuum advance did not play a role. I think JT had mentioned the same thing about wanting lots of advance.

I know there is a wealth of knowledge here on this site, so I thought I'd put it out there to anyone who knows... thanks!
Greg Burrows
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Re: Ignition advance

Post by fj20spl311 »

I just sort of winged it in my latest advance curve based on the advance SU engine and applied to my FJ20 which also like advance.
The FJ distributor has NO advance. When I got the car, all it had was an MSD retard box for starting, then full advance.
It looks like they are either 20 or 16 degrees crank.

vacuum advance.png

You will need to convert the units to yours.
Mine is in Bars so 290 mm Hg is about .390 Bar
Also your vacuum should be lower
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Re: Ignition advance

Post by Daryl Smith »

Greg,
A few points to ponder, from my understanding at this point, ymmv....

1) The vacuum advance helps with both emissions and milage because it increases efficiency of the burn, increasing efficiency of the engine, therefore more power at part throttle AND better throttle response....I wouldn't run any engine without a vacuum advance...

2) At your compression both a colder plug and slight retard from stock timing should be considered.

3) Ditto on the retard with the larger spark gap and spark provided by the Megajolt system. (Also advised if using projected nose plugs - BPR-6 vs BR-6 on a modified engine - stock, not so much)

4) IF, in your build, you closed up the 'squish' area of the combustion chamber to ~.040" (+/- .005" or so), it should also increase the efficiency and speed of the burn....requiring, again, less timing advance....

5) The stock timing is probably a little on the conservative side because of the spark scatter associated with a distributor being run through chains and gears. With the crank fired system timing will be solid and very accurate, allowing for some advance in the timing (if on a stock engine - kind of irrelephant here)....

6) Changing your timing will change your AFR...possibly requiring carb adjustments.... :(

I'm just guessing here, but I would think your starting point for your map should be about 4* - 5* retarded from stock (maybe more?).
On my maps I am running about 15* vacuum advance, once I get to the dyno, part throttle tuning is the only way to get it 'proper'.

Your engine is so far from stock that stock timing would likely do harm to your engine...

Cheers
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Re: Ignition advance

Post by Gregs672000 »

So I went out and more or less copied what Michael posted a while ago here viewtopic.php?f=3&t=29035&hilit=123#p274133.
I was unable to download maps from the megajolt site, some computer issue. There's one on there for a built L20 running SUs with similar compression and cam, but i can't open it (grrrrr). Anyway, this is what I came up with:
RPM advance
1000 18
1500 23
2000 27
2500 30
3000 34
4000 37
5000 38
6000 38
6500 37
7000 36
I pull 2-4 degrees out based on load and rpm, or add a couple on deceleration (doesn't pop as much). No ping down low unless I'm really stupid, can stomp on it at 3000 with no problems. Not sure it's optimal, but it doesn't give me any trouble. I'm going to replace the coil as I noticed some differences in spark color the other day when testing. After that, some carb jetting and I'll be ready for a dyno pull.
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Re: Ignition advance

Post by Daryl Smith »

At WOT, Michaels map (coloured?) goes from 14.1* @ 500 rpm to 32* @ 6500, then drops to 31.6* @ 7000.
At 0% load he's showing only up to ~40* max timing....
Looks like it is at about 18* BTDC at idle (30 - 45% load)

This looks better to me than what you've posted, safety wise...but, if you've tuned on a dyno for MBT....?

If the map for Megajolt is the same as it was years ago, it should be a 10x10 grid?
If that grid is filled then you should have your 'vacuum' advance....

If you put Michael's #'s above in your rpm range stated @ 100 Kpa (WOT) then work down the Kpa range evenly to about 25 Kpa, adding 1 - 1.2* for each step (9 steps), you should have a safe workable map that will get you to the dyno.
36 - 38* seems high for a small bore engine like yours @ WOT??
I've seen/used 45*+ at part throttle, No way at WOT.

FYI - it is possible to advance the timing beyond MBT (mean best torque) without pinging on some engines. Advancing 'til you hear pinging and backing off is not nescessarily going to be the best power for your (anybody's) setup.
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Re: Ignition advance

Post by Daryl Smith »

Greg,
Sent a quick, conservative, 10x10 ignition map that I think would be a good starting point for your car, via email...Don't know how to post here..properly... :roll:
Kpa row is reversed from what Megajolt is...

RPM
800 1000 1300 1700 2100 2500 3000 4500 6000 7000
100 14 14 17 20 24 27 30 32 32 31
90 14 14 18 21 25 28 31 33 33 32
Kpa 80 14.5 14.5 19 22 26 29 32 34 34 33
load 70 15 15 20 23 27 30 33 35 35 34
60 15.5 15.5 21 24 28 31 34 36 36 35
50 16 16 22 25 29 32 35 37 37 36
45 17 17 23 26 30 33 36 38 38 37
40 18 18 24 27 31 34 37 39 39 38
35 18.5 18.5 25 28 32 35 38 40 40 39
25 19 19 26 29 33 36 39 41 41 40
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Re: Ignition advance

Post by spyder »

I found that using a vacuum advance on my crank fired Electromotive ignition resulted in no increased mileage per gallon. I read somewhere that the side draft carbs fill the cylinders differently than the su's.

I am going to advance the cam back to "0" degrees to see how the engine behaves. I had to retard the cam with the stock pistons and head to reduce / eliminate low end ping / knock. My current combustion chamber is modified to clearance the pistons and doesn't ping if I advance the timing.
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Re: Ignition advance

Post by Gregs672000 »

Appreciate the input guys! Keep it coming. I set my base numbers off a KPa of 60, as that was where it would settle out. However, that was in the garage and not driving cause the battery is dead on the old laptop top I'm using. Daryl, I think my numbers are very close to Mikes at the 60 KPa load. He has a few more bins he can adjust timing in, but the change was small if at all.

I heard no knock or ping, and it was better down low for ping. Before I needed to be just a little careful below 2500, now it seems fine.

Thanks guys!
Last edited by Gregs672000 on Thu Jun 07, 2018 10:48 am, edited 1 time in total.
Greg Burrows
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Re: Ignition advance

Post by 2mAn »

Can you post pics of your crankfire setup?
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Re: Ignition advance

Post by Gregs672000 »

Pics of mine or Spyder's?
Drove the car some more today, alot of low speed/rpm driving, all around Tacoma and Pt Defiance (big forested park with a 5 mile drive). Considering this engine has an aluminum flywheel, 3.9 posi, a relatively "big" cam, high compression, 37mm chokes, it is amazing it can poke along just fine, doesn't complain much, no knock or ping, starts off hills with no problem. It normally cruises along with A/F ratios in the mid 14s. I am overfueling on top a bit, seeing some 10.5 and 11 numbers if I really stomp on it (I want 12.5), so a step up on the air jet is the next likely change. The engine seems happy with this map, but the only way to tell will be on the dyno so we can watch what it does with subtle changes. It does do this weird thing sometimes where it decides to idle funny, with a bit of rocking, and the fuel ratio goes down to 13.1 from 14 or so. Doesn't do it all the time. I'm thinking spark, as there is no way an idle screw is suddenly deciding to let more fuel past. Pluging up? Can happen, but it would go lean. More fuel...how? That says spark to me. The fuel isn't being burned. That's how I interpet the A/F meter. Going to order a new coil pack.
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Re: Ignition advance

Post by 2mAn »

Yours! I need to see a LOT of pics of your car. Its nearly sunny up there, time to go out and get us some pics
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Re: Ignition advance

Post by Gregs672000 »

I'm realizing that in the past I have been giving incorrect information when talking about stock vac advance. I always understood that going direct to the intake manifold (say from one of the manifold ports) and using that for your vacuum source was wrong. Since I have never run vac advance on the Roadster (MIkuni/Variant carbs from the beginning, so I have not thought about it a lot) I thought it was because the stock system worked from the atmosphere side of the throttle plate, and saw no vacuum at idle... but that cannot work right. I know stock SU cars draw from a port in the carb, but again I thought that was in FRONT of the throttle plate, not BEHIND. Is it that going to the manifold for your vacuum source in a stock car is too much? Is there something special about the barb/port location on the carb that makes the stock system work? Just curious, makes no difference to me at this point as the megajolt is designed to draw from the manifold...

Simon, I'll see about doing some pics, but I have had trouble posting pics from my Samsung tablet... not sure what the problem is as I have been able to before.

Spyder, as you know your z-196 cam has almost identical lift, but has 264 duration at .50 per Isky vs the 246 of my z 197 (my lift measured per Delta Cam is 536 but that is with our rockers, is 499 on the Isky site). Impossible to really know where my cam is (advanced, retarded, 0) but my goal was 0. It picks up well at 3200 rpms and I'm not knocking. The car drives around town really well, has good top end and doesn't act like a high duration cam (maybe I'm advanced? But that was the reason why I chose the Z 197, a lower duration to keep the power band lower. The high lift does change the effect of that some). Given the longer duration of your cam and with your cam retarded on top of that, where did the power band come on?
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Re: Ignition advance

Post by spyder »

It started to pull hard around 4K rpm. This was my previous engine and it had stock cast 1.5mm over pistons and an unmodified combustion chamber. I retarded the cam to keep the low rpm pinging down. My current engine has forged pistons and a head modified to match them. This is why I am thinking the squish area is different and want to see how advancing the cam back to 0 degrees will effect the engine. Keep in mind I don't really have a low end lugging issue with a 4.89 rear gear. Which is why I am looking to find a more tame rear ratio so I can pass the car off to the next generation.
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