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Engine Timing

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ccowx View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 5:08pm
Less coils but the same wire diameter gives a stiffer spring. Like how cutting coils out of a front coil spring actually makes them stiffer. Is there not instructions with the dissy that give the various spring rates in order of heavy to light?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spiritdude Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 7:59pm
Bandana,
The best way to attack your issue is to set the full advance timing and work back from there. Given your engine/compression/cam specs and that the car is equipped with a manual transmission I would look at eliminating the vacuum advance for two reasons. First, if you don't need it, why not get rid of it. It will remove an added variable and possibly unnecessary aggravation. Second, with the MSD distributor it comes with enough hardware to tune it with just the springs and bushings. You didn't give vehicle details, i.e. weight/purpose, but your engine combination should run well at 39 degrees total advance. Anywhere between 38-40 should give you more performance if you are running premium fuel(93 octane). AMC engines tend to like two things, timing and fuel. Make sure that it all comes in at no higher than 4000rpm. I set mine up to come all in at 3000rpm. It sounds like as long as your initial advance isn't causing any problems starting when the engine is warm, I wouldn't worry about it as much. I run my car with 40 total and about 17 initial. When I set my distributor up I ended up using the lightest springs and I found that they weren't light enough to bring in the advance in early enough. I ended up 'stretching' them lightly to get the desired results. You indicated that you were running a Holley 700 double pump. I would recommend going bigger as the 700cfm is two small. Be patient while tuning, it can take considerable time to get the tuning/performance you are looking for. Also, the Performer intake will hurt the performance of the 401.
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Edited by spiritdude - Jun/14/2017 at 8:02pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bandana Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 8:46pm
Great perspectives, keep them coming. This is my first AMC car (1970 AMX) so I am still learning about the vehicles and their engines. I have heard and had mopar RB engines that ran upwards to 38 degrees total but have not heard of an AMC liking that kind of timing before.I'm just cruising around in this car so I don't need best ET tuning, more best street manors. I'll be trying this tune with and without the vacuum advance and see if there is a difference. I did replace the blue springs with the light silver ones. Hopefully tomorrow I can take the car out and see what I have.

Right now I am running quite rich, even with this new tune so not sure I want to run to a bigger carb just yet. This engine supposedly made 435hp/450tq on a dyno so while its stout is not a real strong build, plus I don't encounter many WOT conditions. I have heard that an air gap is a better intake for this engine but also taller than the Performer - which already has required modifying the ram air rubber seal.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 8:56pm
Rich is not from the size of the carb, but the jetting. I have a 390 and I run a 780 cfm Holley, fwiw. You could go a bit more if you really wanted, but you may give up a bit around town and bottom end for a gain in WOT.

As for the vacuum advance, if connected to ported vacuum it will improve your highway mileage, if connected to manifold vacuum it will tend to smooth out some shifts, plus the mileage. I am currently running no vac advance on my 390, but that is because of the vintage speed parts I am using. If I was simply looking for best driveability, I would go with the vacuum for sure.

As for the ram air seal, it can probably be made to work. I have RA on my 70 with an R4B and Holley.

Chris
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spiritdude Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 9:40pm
Bandana,
Congrats on the 1970 AMX! As far as I am concerned you are driving the best machinery mankind has ever produced. I have only owned one and regret selling it to this day. It is buy far the coolest car I have ever had. Hopefully, I can acquire one soon!
If you are running rich, you need to tune the carb as well. But get your ignition timing where you want it first, then attack the carb. If street-ability is your prime concern, you won't notice the difference with a 750-850 cfm DP if it is properly tuned. I agree with Chris, the small sacrifice in bottom end shouldn't be a problem with a manual transmission. Depending on the rear end ratio, you may want to lose some bottom end torque anyway. Same goes with total ignition timing. Two to four degrees total probably won't affect drivability much. Most of the drivability will be from initial timing and the advance curve. If an air gap is to tall, try and locate and R4B intake, it will work far better than the performer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SKeown Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 10:02pm

 I eliminated the vacuum advance, used the biggest bushings and weakest springs. Then set the max at 36* and forgot about it, it's all in at about 2500 rpm. Very simple and doesn't fight the starter. That's with a big cam, 10.8 CR and 93 gas. Works fine with either stick or automatic tranny.

 SKeown
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Jun/14/2017 at 11:39pm
Are you completely fed up with advice yet? I would not blame you!

One good point that Spiritdude brought up is that you might consider going conservative on timing initial advance and total. Having it a few degrees back from the most it will take will tend to take a bit of dig out of the lower rpms and give a bit more power top end. It will also be easier on your engine on a hot day or if you get a bad batch of fuel. No one likes detonation! You likely are already traction limited off the line anyway and top end is where you can use the power.

We are starting to enter the realm of fine tuning, but as has been pointed out you will probably be tweaking this for a while, even if you have it running fairly well right off.

Chris
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 3:15am
Every engine is different and every operating condition is different and no single piece of advice will ever be current under all conditions.

The goal is just to find the timing that produces maximum torque at all possible engine speeds, then retard the timing back slightly to give yourself some wiggle room for preventing abnormal combustion (detonation, preignition, other nasty stuff that melts pistons).

So basically if it runs good and runs to your satisfaction, then you've reached your goal.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bandana Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 4:40pm
First drive today. Started good, had to adj the carb a bit but seemed to run ok. Was cruising into town and noticed some pinging at low rpm in 3rd gear. Unplugged the vacuum advance while in town and didn't notice any pinging on the way home. Only one opportunity to get into a bit and seemed to be somewhat responsive but as before after letting off she seems to stumble a bit as if it may have gotten too much fuel? I guess my take-a-way for the day is that it doesn't seem to like the 17 degree initial, with the light gray springs and the vacuum advance for tooling under load. I guess the options are:
1) run without the vacuum advance
2) try going up on spring strength so I get less timing at low rpms.
3) drop down a size on the bushing to get a lower initial timing.

huuummmm!



Edited by Bandana - Jun/15/2017 at 5:36pm
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ccowx View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2017 at 5:56pm
I would try some heavier springs first. Bear in mind that the rate of advance is really mainly going to be about driveability, any "spririted" driving you do with a stick is likely to be above 3500-4000 rpms anyway, so pretty much no matter what you do for advance springs, your WOT power/full advance will be the same.

If that does not work you can also maybe lower the initial as you mention above as well.

Third of all, you could consider adjusting the vac advance for less advance. It is adjustable on your distributor, correct? A stock advance canister adds 16 degrees, perhaps 10 or 12 might be more to the engine's liking? That will still give you something in the high 40's advance for cruising mileage.

Perhaps a combination of slightly slower advance rate plus limit the total vacuum advance might do the trick.

Chris

Edited by ccowx - Jun/15/2017 at 6:15pm
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