Ignition Timing

[Classics Fan]

This is my first foray into distributor recurving. I have a GM HEI on original 350 with Edelbrock Performer intake and RV cam, rebuilt .030 with 9.0:1 pistons and original heads. Ported vacuum advance that comes in with bare tip-in of the throttle. The engine ran well but was sluggish. I replaced the OEM center plate, weights and springs with Mr. Gasket parts. The smallest (black) springs are way too light, coming in at 900 RPM. The next size up (silver) work much better, coming in at 1650 RPM BUT I can only get 10 degrees mechanical advance at 2250 RPM. Initial advance is16 degrees so total all-in is only 26 degrees. The curve diagram from the Mr. Gasket kit seems to show the weight/spring combo only allowing 12 degrees or so. If I advance initial much more than 16, it is hard to turn over. Any thoughts or ideas how to get total timing up to between 34 and 36 degrees? By the way, it runs better than ever as it sits now.
Posted via Mobile Device

[SeventyOne]

Originally Posted by Classics Fan

This is my first foray into distributor recurving. I have a GM HEI on original 350 with Edelbrock Performer intake and RV cam, rebuilt .030 with 9.0:1 pistons and original heads. Ported vacuum advance that comes in with bare tip-in of the throttle. The engine ran well but was sluggish. I replaced the OEM center plate, weights and springs with Mr. Gasket parts. The smallest (black) springs are way too light, coming in at 900 RPM. The next size up (silver) work much better, coming in at 1650 RPM BUT I can only get 10 degrees mechanical advance at 2250 RPM. Initial advance is16 degrees so total all-in is only 26 degrees. The curve diagram from the Mr. Gasket kit seems to show the weight/spring combo only allowing 12 degrees or so. If I advance initial much more than 16, it is hard to turn over. Any thoughts or ideas how to get total timing up to between 34 and 36 degrees? By the way, it runs better than ever as it sits now.
Posted via Mobile Device

Why do you say the Black springs are too light? To me that curve seems to be what you need for a stockish build. You'd have 16 initial timing and ~20 mechanical advance. Depending on what RPM this maxes out at (hopefully you get around 36* at 3000rpm.)

If you get pinging drop your initial timing to 14 and total advance will be 34. You can also mix/match the springs. IE one black and one silver to get somewhere in the middle between the two curves.

Lastly I'd hook your distributor up to full manifold vacuum. It will help immensely with the sluggishness you are experiencing and the engine will run cooler. Not to mention you can lower the initial to help with starting but still get the advanced timing right away to be more in the right torque range.
You may need an adjustable vacuum canister to dial in how much max timing the vacuum can adds at cruise. Run as much timing as you can during cruise without inducing pinging.

One more thing. I ran a Progression Ignition HEI for awhile and it was super awesome to be able to set my own timing tables from my phone (connected with bluetooth) without having to mess with any springs or weights at all. It's pricier that a standard HEI unit but so worth it.

https://progressionignition.com/

[SeventyOne]

Cue argument over Ported vs Manifold vacuum in 3...2...1..

[Classics Fan]

Originally Posted by SeventyOne

Why do you say the Black springs are too light? To me that curve seems to be what you need for a stockish build. You'd have 16 initial timing and ~20 mechanical advance. Depending on what RPM this maxes out at (hopefully you get around 36* at 3000rpm.)

If you get pinging drop your initial timing to 14 and total advance will be 34. You can also mix/match the springs. IE one black and one silver to get somewhere in the middle between the two curves.

Lastly I'd hook your distributor up to full manifold vacuum. It will help immensely with the sluggishness you are experiencing and the engine will run cooler. Not to mention you can lower the initial to help with starting but still get the advanced timing right away to be more in the right torque range.
You may need an adjustable vacuum canister to dial in how much max timing the vacuum can adds at cruise. Run as much timing as you can during cruise without inducing pinging.

One more thing. I ran a Progression Ignition HEI for awhile and it was super awesome to be able to set my own timing tables from my phone (connected with bluetooth) without having to mess with any springs or weights at all. It's pricier that a standard HEI unit but so worth it.

https://progressionignition.com/

Thanks for the reply and info. The black springs were coming in at 900 RPM, below the idle RPM I had set at the time and I could only get 12 degrees mechanical at 2200 RPM, not 20 as shown on the chart. Maybe the black/silver combo will get me more at a little higher RPM since I’ve since reduced idle to 850, but it doesn’t seem like enough.

I’ve read a lot of debate on ported versus manifold vacuum. I know by going to manifold I could reduce initial, but wouldn’t that defeat my quest for more total?

The Progression looks like a nice unit and would no doubt be fun to play with. However I’d rather not spend hundreds on a distributor right now and besides, I’m having too much fun learning and playing with the old-school analog one I already have.

Thanks again.
Posted via Mobile Device

[SeventyOne]

Originally Posted by Classics Fan

Thanks for the reply and info. The black springs were coming in at 900 RPM, below the idle RPM I had set at the time and I could only get 12 degrees mechanical at 2200 RPM, not 20 as shown on the chart. Maybe the black/silver combo will get me more at a little higher RPM since I’ve since reduced idle to 850, but it doesn’t seem like enough.

I’ve read a lot of debate on ported versus manifold vacuum. I know by going to manifold I could reduce initial, but wouldn’t that defeat my quest for more total?

The Progression looks like a nice unit and would no doubt be fun to play with. However I’d rather not spend hundreds on a distributor right now and besides, I’m having too much fun learning and playing with the old-school analog one I already have.

Thanks again.
Posted via Mobile Device

Next question is why is your idle set so high? I've got a street/strip cam in my 383 and it idles at 800 when hot. A tamer build with an RV cam ought to idle fine below that...

And you dont have to reduce the initial w/ manifold vacuum but you had mentioned you were coming into starter issues at 16. It just allows you to run lower initial but with the vacuum advance in play AT idle you will have a peppier engine.

If you cant get the advertised 20* in the chart i'd try another product, personally. That's your true issue. Unless for some reason you have limiter bushing in place that should be smaller to allow for more mech advance...

[Classics Fan]

Originally Posted by SeventyOne

Next question is why is your idle set so high? I've got a street/strip cam in my 383 and it idles at 800 when hot. A tamer build with an RV cam ought to idle fine below that...

And you dont have to reduce the initial w/ manifold vacuum but you had mentioned you were coming into starter issues at 16. It just allows you to run lower initial but with the vacuum advance in play AT idle you will have a peppier engine.

If you cant get the advertised 20* in the chart i'd try another product, personally. That's your true issue. Unless for some reason you have limiter bushing in place that should be smaller to allow for more mech advance...

Yeah, idle was too high for sure. I have it set at 815 now, could go much lower and idle fine in Park but Drive brings it down 215 to 600.

My starter problems begin at 18, no problems so far at 16.

I haven’t seen anything that should limit distributor travel. I am going to try a different product. Morosso seems to make a quality kit. I’ll look deeper. Appreciate your help!
Posted via Mobile Device

[SeventyOne]

Originally Posted by Classics Fan

Yeah, idle was too high for sure. I have it set at 815 now, could go much lower and idle fine in Park but Drive brings it down 215 to 600.

My starter problems begin at 18, no problems so far at 16.

I haven’t seen anything that should limit distributor travel. I am going to try a different product. Morosso seems to make a quality kit. I’ll look deeper. Appreciate your help!
Posted via Mobile Device

Oh gotcha. I was thinking you were in drive at that idle number. That make way more sense. I'd aim for a 600-650 idle in drive. I assume you have a carb as well? We're getting off topic (I like tuning talk haha) but when you tune the idle circuit for best manifold vacuum do it in drive with someone pressing the brakes.