HEI advance curve ?

[robby]

I have a 307 with a quadrajet, dual exhaust, later model 305 heads stock automatic and 3:08 rear end in my 70 C-10. I looked at my HEI when I replaced the cap and rotor and saw that the bushings on the centrifugal advance where gone. I bought an advance curve kit with new springs, weights and bushings. It shows 3 curve choices. They come in at about 1,000 rpm, 1,800, and 2,500. I have no idea which to use. This is a daily driver, but I woudn't mind more throttle response. Please help.

[crazy longhorn]

You need the all in point on a daily driver in the 2500-3000 range. you may need to play with springs to tighten the curve up a bit. Good luck......Al

[robby]

so it should come in sooner for a daily driver?

[crazy longhorn]

it should come in later for a daily driver,usually in the 2800-3200 range. depending on weight & gear ratio, some will handle a little faster curve. ........Al

[robby]

does that mean totally in or coming in? Any one else?

[crazy longhorn]

By "all in",it should be fully advanced by 2800-3200 as a rule of thumb for street /drivers. The advance should start @ about 850 -1000 rpms(you dont want it starting @ or below idle speed,or your idle will be "unstable"), but just above idle speed for the beginning of advance. Good luck.....AL

[MikeB]

I disagree. As I recall, the experts like Dr. Jacobs say "all in" at 2000-2200 for a street engine. Accel even suggests 1600 for high-geared low-rpm engines. The 2800-3000 spec is to help reduce emissions. Mileage and performance are best at the maximum advance you can get away with. And you want to get to that max advance as soon as possible without detonation or hard starting. You DEFINITELY want to be there at your cruise RPM! Heck, that's why there's vacuum advance! It goes to max when you are just cruising. When you hit the gas and the vacuum drops, it goes away to prevent detonation under load.

[crazy longhorn]

actually, if you run it in too fast you will have problems.......2800-3200 as a rule of thumb, & some will do well closer to 2500. alot depends on gearing & vehicle weight,converter, ect. My converter is loose enough that it doesnt really make a diff if i run the timing curve looser........as soon as I jab the throttle, Im in the rpm range that i need to be. the ones I have run with no vac advance have had the curve much faster..........Pump fuel, & high comp engines are a different mix at times........AL

[Denny]

Best bet is play with different springs and see what works the best. Like crazy longhorn says the weight of your vehicle, gear ratio and how you drive matters. I have my total advance coming in at 3700 rpm but my truck is heavy and I have alot of big hills where I live. My engine does not like too fast of advance on the hills.

[robby]

sonds like we have some differing opinions. What would it be stock for a 307?

[Denny]

robby I would try the lighter springs first and see if you get any pinging, if you do go the next step heavier till it stops. If it isn't pinging and you have good throttle responce and you can pull hills with out pinging you should be fine.

[robby]

thanks Denny, sounds reasonable.

[Jesse 67 c-10]

I would also like to do the same thing for my engine and I'm wondering if you can mix and match springs, have say a lighter one on one weight and a heavier one on another to make a sort of intermediate step?
Thanks
Jesse

[crazy longhorn]

Yes, mix & match is permitted! One thing to watch is that is the springs are too lite,your advance will be starting at or below idle speed,& will cause an unstable idle.good luck........Al

[mikep]

I run 22 degrees at 650 rpm idle with 38 degrees total in by 2000 rpm. No vacuum advance. I never have detonation even with 9.5 compression on 87 octane.

[MikeB]

Wish we could get a guy from MSD or Jacobs to set us all straight on total mechanical advance! I do know that vacuum advance gives you better fuel economy when cruising, so I'm not sure why you wouldn't use it, except in a race car that's always at or near full throttle.

[crazy longhorn]

My thoughts are that total advance is relative to the engine combo(comp ratio,& quench). Not all will time the same,its a matter of what the eng wants. cam timing has an affect on timing requirements also. My combo is very close to 10-1, & has had the decks cut,but not to minimum specs on the quench. i run 14 initial,34 total . Advance startse @ 900, & is @ 30 degrees by 2700......but the last 4 degrees arent reading till 3200-3300. i have lite aftermarket weights & springs, & any faster on the curve, the idle isnt right cause the timing is starting below idle. i have 8 degrees on the vac can, to give 22 degrees @ idle, & 42 @ 3200-3300 with no load. i have run the curve all in around 2000-2200 on a perf setup with no vac advance..........but I still dont reccomend it on a stock eng with stock cam , & stock deck height in a truck thats pulling a tall gear. I do feel there is an advantage to running a vac advance on a street bound truck,as it gives more timing in the cruise range to help economy, & help the plugs run clean. the vac advance will also let the eng run cooler in the heavy traffic,& back off when pulling hills/heavy loads to keep the eng running with the timing it needs under those conditions. ............AL

[MikeB]

Crazy -- What have you experienced as drawbacks to having the mech advance come in too soon, assuming it doesn't start to come in until above idle? You know, I probably need to disable my mechanical advance and verify that I'm not getting any advance at idle. I can see how that would cause an erratic idle. Timing changing could cause idle speed to change which would cause timing to change...in a endless loop!

[crazy longhorn]

what i have seen is on real loose curves is that the idle is not steady(runs up/down,& wants to die in gear with an auto trans),or if you buzz the throttle in nuetral, the idle wont come back till you drop it in gear. on a stick shift with a loose curve, IAl have had to chugg it down in high coming up to a lite to get the idle to drop, or if you buzz the throttle the advance will open, then if you let up the clutch a little quick (without doin a burnout), the eng will ping sharp.......

[MikeB]

I have none of those problems, but will check to ensure I'm not into my mech advance at idle. Long-term I want to get a MSD distributor with adjustable mech advance, but I already have a lot invested in their HEI module and coil, and they won't sell me their full size HEI without module and coil.

[crazy longhorn]

I have been looking at the MSD stuff also......a lot more adjustable! I have a hot coil& module in the stocker,& as long as it works ill use it.......maybe next season. my thoughts are, if she idles clean,responds good & dont ping..........no problem

[Denny]

I have the MSD 8361 distributor with the 6A unit and love it. It comes with different advance stop bushings that limmit the total advance and a good assortment of springs for different advance curves. You can also run it without vacuum advance if you want to. My engine starts with 1/2 crank. I would highly recommend it to anyone. It is the best all around performance product I have bought for my truck.
I am running the adjustable crane vacuum advance on it.

[mikep]

Heres my probllem with using vacuum advance. I am running 22 degrees initial advance with 16 mechanical starting in at 1100 or so and all in by 2200 for a total of 38 degrees. With my heads and compression that is the most I can reasonably run load or no load. I like using a lot of initial as it gives me great throttle response off idle. With vacuum advance as soon as I open up the throttle the advance drops immediately and kills my throttle response. At cruise I am well over 2200 RPM so my full advance is in already. I suppose I could sneak in another 4 degrees max with an adjustable vacuum can and leave initial at 22 for a total of 42 but I'm not so certain that 42 degrees advance is all that beneficial for the small block chevy chamber.

[Denny]

That's alot of initial advance. I agree with you that the vacuum advance would not work for you. I never get on my truck from a dead stop. I mainly use my truck at highway speeds so the vacuum advance works out great for me. I run about 15 degrees off my vacuum advance with a slow centrifical advance which works out great for mileage and I still have good power for pulling these dang hills.

[crazy longhorn]

Mike p, under part throttle you have a slow burn & the chevy will tolorate 42-46 degrees under certain conditions(depending on comp ratio,ect). 42 total will be too much on initial/mechanical,as it wont retard under full load. Dont get me wrong, I have run a curve like you have,so im not saying that it wont work.......but my travels have showed me that it doesnt work (fast curve+ vac advance). when running a vac advance hooked to full man source, the eng seems to like a little slower curve to keep from over advance. Most guys I have talked with seem to like a total of 32-36 degrees on the street,with 38 degrees as max. the fuel sure isnt what it was back in the day of the 70s muscle cars......Al