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Tune-up Specs |
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Buzzman72 
AMC Addicted 
Joined: Sep/15/2009 Location: Southern IN Status: Offline Points: 2725 |
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 Posted: May/14/2021 at 6:01am |
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I only pointed out that if your engine has a DUI distributor, it is NOT a "stock" distributor. And with all the knock-offs and clones out there, it's hard to tell what your vacuum advance is and what your total advance is...or what advance weights and springs are in it and how fast they come on.
Others have pointed out the same. So saying you have a "stock" DUI actually tells us nothing about what you're starting out with. Knowing your cam specs would help greatly. Where to set initial timing depends on the mechanical components used to build your engine. All the others are giving good advice. Whether your engine requires 5 degrees advance at idle [depending also on where it idles] or 9 or 10 is all determined by the engine itself. Once non-stock components are in it, you have to determine a baseline that won't be found in any book. Cylinder pressure is a function of both compression [combustion chamber volume, quench, head gasket thickness and pistons] and cam timing [some cams are ground straight-up, some have 4 degrees or so of advance ground in]. If your engine isn't stock, stock specs will mean nothing because they won't apply to what your engine wants/needs. I'm sure you know all that. But there is NO "one-size-fits-all" answer once an engine is modified.
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Buzzman72...void where prohibited, your mileage may vary, objects in mirror may be closer than they appear, and alcohol may intensify any side effects.
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AMXFSTBK390
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Joined: Oct/22/2013 Location: SF Bay Area Status: Offline Points: 3489 |
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Posted: May/14/2021 at 11:13am |
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It seems like there is a way to figure out the duration and lift of a cam with a dial indicator that fits in the lifter bore. Measure the travel of #1 lifter exhaust/intake to help determine ideal timing ?
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Questions are powerful tools...what's in your toolbox?
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PHAT69AMX
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Joined: Jul/07/2007 Location: West Virginia Status: Offline Points: 5919 |
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Posted: May/14/2021 at 1:39pm |
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Images from the handy 1970 AMC Factory Technical Service Manual :
  Very 1st thing AMC them self went after in their own corporate performance literature was the timing curve... Excerpt from the rare red white & blue 1970 Rebel Machine Performance Package Booklet : page 10 of 12: rebel machine performance packages When we say "Up With The Rebel Machine" We Mean It ! The very fact that your are a REBEL MACHINE owner indicates you want more than a set of wheels, you demand a GO-MACHINE THAT GOES! Granted the REBEL MACHINE is a good performer, as is right off the showroom floor. But just because you want to put more "muscle" into your MACHINE, we suggest you consider one of the following performance packages. -------------- i. tune-up kit Performance can be improved with out sacrificing any of the built in compromises of stock Rebel Machines, in addition to a basic tune-up we recommend the following: *Change to a fast distributor curve and advance the distributor to a total of 26 Degrees. ( 26 Total Maximum Mechanical, above is a poorly worded sentence imho ). The initial setting should be at approx. 6 Degrees BTDC ( initial static timing advance, & with no mechanical coming in ). The advance should be complete at 2800 ( engine ) RPM. ( 32 degrees total combined 6 Initial + 26 Mechanical ) *Re-jet the secondary circuit in your ( AutoLite 4300 ) carburetor 2% richer. *Although your Rebel Machine is capable of more RPM, experience on the drag strip has shown that optimum performance can be obtained by setting shift points at 5000 RPM. ---------------- ii. tune-up plus Still taking into consideration the need for compromise on stock manifolds, you can achieve even more performance from your Rebel Machine with these further modifications: *Change to a fast distributor curve and advance the distributor to a total of 26 degrees. The initial setting should be at approx. 6 Deg. BTDC. *Re-jet the secondary circuit in your ( AutoLite 4300 ) carburetor 2% richer. *Blueprint cylinder heads. Valves should be reground to a performance seat, combustion chambers should be cc'd and brought to the minimum of 49.10 CC's by milling the head surfaces. Mill .010 of an inch from the head to gain 1.6 CC's. --------------------- iii. street ( all-out ) Once again though such production characteristics as idle, fuel economy, and exhaust sound may be compromised, the all out street machine is still just that, a street machine. Though its idle is rough, and its exhaust level somewhat loud, it is still quite tolerant in traffic, reliable in cold weather, and generally tractable in all-around everyday use. To create this kind of "machine" we recommend the following modifications and changes: *Install tune-up kit ( As described above ). *Blueprint cylinder heads ( As described above ). *Install Service Package Camshaft Kit which includes camshaft, hydraulic valve lifters, and springs. AMC Service Part No. 448-6719 *Install replacement Edelbrock ( R4B ) aluminum intake manifold AMC Service Part No. 448-8409 ( 1970 & Up Manifold Only ) *Install replacement Holley 4-BBL 850 CFM, Dual Pump carburetor ( ? Holley List # ? ) AMC SERVICE PART NO. 448-8658 ( boy... sure would LOVE to KNOW what Holley List # this was...) *Install Doug Thorley Headers AMC Service Part No. Left Side 448-5727 Right Side 448-5726 Purchase headers from hot rod shops, or order direct from Doug Thorley Header Co. *Install Mallory Hi-Output Ignition System AMC Service Part No. 448-7900. The desired ignition curve is pre-set in the distributor from the factory. ------- end of doc
Edited by PHAT69AMX - May/16/2021 at 11:23am |
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IB Sorgn
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Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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Posted: May/14/2021 at 3:52pm |
I'll remember, stock specs mean nothing if your
engine ain't stock, got it. No "one size fits all",
check. Thank you for all the information and advice, especially appreciate the lesson on Cylinder Pressure. |
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Madd X
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IB Sorgn
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Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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Posted: May/14/2021 at 3:59pm |
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Now this is the type of information I was looking for!
Thank you very much PHAT69AMX.
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Madd X
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ccowx
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Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510 |
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Posted: May/14/2021 at 9:30pm |
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Because we are in fact trying to be helpful, let me add to the above. The "Street, all out" build is probably the closest to what you have. If you want something sanctioned by the factory, The Mallory hi output system they mention has 24 degrees of mechanical advance all in at 3000 rpm's. The factory reccomended setting is at 14 degrees BTDC with a total of 38 advance. Given your lower compression that is probably safe. With today's garbage fuel I use more like 12 and 36 total, but I also have full 1970 spec compression.
Chris
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Trader
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Joined: May/15/2018 Location: Ontario Status: Offline Points: 6881 |
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Posted: May/15/2021 at 6:35am |
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That's another good point Chris.
You cannot use factory specifications due to the changes in fuel from the early 70's to today. A lot depends on what is available in you local area and that may also be a crap shoot. Were I live, 91 premium is 10% ethanol. Ethanol is an octane booster but has less BTU. Reducing initial and overall timing, 10 initial and 35 total seem to be were I have to run now. When the Premium had no ethanol, I ran 12 initial and 37 total. We have 93 Premium, 30% ethanol at one local station that my car runs like a toothless dog with a purse dog bark.
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IB Sorgn
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Joined: Dec/17/2012 Location: Kentucky Status: Offline Points: 192 |
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Posted: May/30/2021 at 3:16pm |
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In finalizing this post this was what I changed and what I plan to do in the near future. Want to say thanks for your posts and the ideas that came from them. Using the information provided by this post and my lifetime
of experiences with this old iron I got the AMX running pretty good and meant
my objective of being able to use 89 octane.
Whether all these items had to be done to accomplish the goal I don't
really know, I just started making one adjustment after other until I ran out
of ideas. To inventory changes I; 1) stayed with the same heat range plug except
I opened the gap to .055, 2) leaned the carburetor out by about 4 %, 3) went with
a softer step up spring on the AVS, 4) set initial timing at 10 degrees, the DUI
mechanical advance added an additional 24 degrees for a total of 34 degrees which
was all in at 3000 rpm. That, I found was their recommended curve and advance
for my 390. 4) Reduced throw of the accelerator pump linkage. 5) Will reset the
valve lash when I received my new valve covers and also install a O2 sensor to
help measure A/F ratios. 6) Have decided to go old school and build my own WMI
system to be controlled by vacuum and RPM to eliminate any possibility of higher
load/rpm detonation. |
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Madd X
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Buzzman72 wrote: