sukhoi
Junior Member
- Joined
- Aug 23, 2024
- Posts
- 5
- Reaction score
- 24
- Location
- CO
- First Name
- Shaw
- Truck Year
- 1973
- Truck Model
- K10
- Engine Size
- 350
Howdy Friends,
If you drive a V8 with a distributor and want to save money on gas, read this!
Okay clickbait aside... Kind of a niche topic, but I'm putting this out there to hopefully help the next person in my situation. Thanks to David Ray of Small Body HEIs, among many others, for sharing their knowledge so freely. I've attached one of the documents from Dave's informational package that's most relevant to this post. There's also a great post by Rich Weyand that's worth checking out if you want to learn more about ignition timing in general: https://www.gmsquarebody.com/threads/ignition-timing-for-first-generation-gm-v-8-engines.14508/
A brief overview of what this post is about...
Any distributor in a street vehicle ought to have a vacuum advance canister. Its purpose is to add additional timing advance whenever engine load is low (idle, cruise, and deceleration). When engine load is low, engine vacuum is relatively higher, and the carburetor delivers a leaner mixture. When the mixture is leaner, it takes longer to ignite. The vacuum can is a neat mechanical way of adding timing advance under these conditions. The result is improved fuel economy and driveability.
If you're not sold on vacuum advance, this video might do it for you:
The challenge is, vacuum varies between different engines. Each stock matched-to-engine distributor had a specific vacuum can, of which there were many multitudes of variants (see Lars Grimsrud’s Distributor Vacuum Advance article, attached). Thus, whatever canister you have on your modified engine probably doesn't advance the timing optimally.
Part of the solution is adjustable vacuum canisters - these allow you to easily customize the rate of the vacuum advance curve with a 3/32" allen key. This is great, but changing the rate doesn't change the endpoints of the curve. Crane Cams used to sell an adjustable canister (99600-1) that came with a limiter plate (99619-1). Oddly, they would have you mount it on what appears to be the wrong side of the canister slot. See the attached PDF from Dave for a visual of what I mean by this. MSD also sells a plate (84281) for this purpose.
My Solution
David Ray has mentioned wanting to make and sell a reproduction of the Crane 99619-1 limiter plate, but I guess it never rose to the top of the pile since you can make a basic DIY stop pretty easily. But it is awfully nice to have it adjustable (especially if you drive to different altitudes and/or tow really heavy things sometimes). So I made what is basically a copy of the Crane limiter plate, sans notches because I don't see what purpose they would serve when the plate is installed on the correct side of the canister slot. I added a small hole for an alignment visual. Pretty simple part, and costs less than $1 each from a laser cutter. Drill and tap your canister for one 8-36 screw, put some loctite on it, and away you go...
Anyway, I'll have at least 10 of these made the next time I do a run of laser parts. Probably .060 stainless. I checked clearances with a paper template on the Moroso 72315 adjustable canister and it seems good, but I'll update this post once I have one installed. Happy to pop them in the mail for anyone who wants one, just send me a message. This is the least I can do in return for all that I've learned here. Forums are the best!
If you drive a V8 with a distributor and want to save money on gas, read this!
Okay clickbait aside... Kind of a niche topic, but I'm putting this out there to hopefully help the next person in my situation. Thanks to David Ray of Small Body HEIs, among many others, for sharing their knowledge so freely. I've attached one of the documents from Dave's informational package that's most relevant to this post. There's also a great post by Rich Weyand that's worth checking out if you want to learn more about ignition timing in general: https://www.gmsquarebody.com/threads/ignition-timing-for-first-generation-gm-v-8-engines.14508/
A brief overview of what this post is about...
Any distributor in a street vehicle ought to have a vacuum advance canister. Its purpose is to add additional timing advance whenever engine load is low (idle, cruise, and deceleration). When engine load is low, engine vacuum is relatively higher, and the carburetor delivers a leaner mixture. When the mixture is leaner, it takes longer to ignite. The vacuum can is a neat mechanical way of adding timing advance under these conditions. The result is improved fuel economy and driveability.
If you're not sold on vacuum advance, this video might do it for you:
xc_hide_links_from_guests_guests_error_hide_media
David Vizard also shares his vacuum advance tuning method in this video, which pairs nicely with the hardware side of the solution presented here.The challenge is, vacuum varies between different engines. Each stock matched-to-engine distributor had a specific vacuum can, of which there were many multitudes of variants (see Lars Grimsrud’s Distributor Vacuum Advance article, attached). Thus, whatever canister you have on your modified engine probably doesn't advance the timing optimally.
Part of the solution is adjustable vacuum canisters - these allow you to easily customize the rate of the vacuum advance curve with a 3/32" allen key. This is great, but changing the rate doesn't change the endpoints of the curve. Crane Cams used to sell an adjustable canister (99600-1) that came with a limiter plate (99619-1). Oddly, they would have you mount it on what appears to be the wrong side of the canister slot. See the attached PDF from Dave for a visual of what I mean by this. MSD also sells a plate (84281) for this purpose.
My Solution
David Ray has mentioned wanting to make and sell a reproduction of the Crane 99619-1 limiter plate, but I guess it never rose to the top of the pile since you can make a basic DIY stop pretty easily. But it is awfully nice to have it adjustable (especially if you drive to different altitudes and/or tow really heavy things sometimes). So I made what is basically a copy of the Crane limiter plate, sans notches because I don't see what purpose they would serve when the plate is installed on the correct side of the canister slot. I added a small hole for an alignment visual. Pretty simple part, and costs less than $1 each from a laser cutter. Drill and tap your canister for one 8-36 screw, put some loctite on it, and away you go...
Anyway, I'll have at least 10 of these made the next time I do a run of laser parts. Probably .060 stainless. I checked clearances with a paper template on the Moroso 72315 adjustable canister and it seems good, but I'll update this post once I have one installed. Happy to pop them in the mail for anyone who wants one, just send me a message. This is the least I can do in return for all that I've learned here. Forums are the best!
