View Full Version : Correct vacuum advance cannaster?
AUTOHOLIC
03-03-2025, 12:37pm
Blueprint makes no recommendation as to what to use for this on there 600 hp 496 engine. Anybody here have any idea what you use?
mudbone64
03-03-2025, 12:43pm
Presuming you will be using manifold vacuum, how much manifold vacuum does the engine make?
Though I may be in the minority in doing so, I generally like to use an adjustable can with some sort of stop.
AUTOHOLIC
03-03-2025, 1:27pm
Presuming you will be using manifold vacuum, how much manifold vacuum does the engine make?
Though I may be in the minority in doing so, I generally like to use an adjustable can with some sort of stop.
The engine hasn’t been installed yet so I don’t know just trying to get a jump on thing so I’ve got all the parts here but yes, manifold vacuum
Need to know: compression ratio, Head chamber type, camshaft profile, rear gear ratio, transmission type, your location, etc. An 11:1 stick car with a big cam and a 4:11 gear needs a completely different advance curve than a 9:1 auto car with a 3.08 gear and a mild cam. Different worlds.
Lots of cans and restrictors available. Lars Grimsruud has a great article on the subject.
These days, the trend is to limit the vacuum somewhat as well as the mechanicals, due to todays crappy gas.
AUTOHOLIC
03-03-2025, 1:55pm
Need to know: compression ratio, Head chamber type, camshaft profile, rear gear ratio, transmission type, your location, etc. An 11:1 stick car with a big cam and a 4:11 gear needs a completely different advance curve than a 9:1 auto car with a 3.08 gear and a mild cam. Different worlds.
Lots of cans and restrictors available. Lars Grimsruud has a great article on the subject.
These days, the trend is to limit the vacuum somewhat as well as the mechanicals, due to todays crappy gas.
Here are the specs on the engine. 4 speed with 3.73 gears. I am at 3500 ft. Above sea level and driving in areas from 1200-5000 ft. On occasion.
https://blueprintengines.com/collections/gm-compatible/products/496-ci-stroker-crate-engine-big-block-gm-longblock-bp4967ct?_pos=5&_fid=723558675&_ss=c
WydGlydJim
03-03-2025, 2:40pm
I never hooked up a vacuum advance on my hotrods.....I was only concerned about WOT and idle.........I wanted the mechanical weights to fly out easily at WOT, so I gave em light springs, and at idle, I want no timing advance........the only thing an advance is good for, is mid range throttle position to increase timing for fuel economy........who gives a rip about that?
:yesnod:
:island14:
DJ_Critterus
03-03-2025, 2:45pm
Need to know: compression ratio, Head chamber type, camshaft profile, rear gear ratio, transmission type, your location, etc. An 11:1 stick car with a big cam and a 4:11 gear needs a completely different advance curve than a 9:1 auto car with a 3.08 gear and a mild cam. Different worlds.
Lots of cans and restrictors available. Lars Grimsruud has a great article on the subject.
These days, the trend is to limit the vacuum somewhat as well as the mechanicals, due to todays crappy gas.
That's new info to me and interesting. Why would you limit the amount of vacuum? Is it for more control of shift points or whatever is being controlled via vacuum?
mudbone64
03-03-2025, 3:08pm
I think GTO meant to say "limit the vacuum advance" rather than somehow limiting the amount of vacuum. I don't think it's possible to limit the amount of vacuum an engine makes. But I digress.
I believe the conventional belief is in order to insure a stable idle the vacuum advance should be limited to 2" below whatever the amount of manifold vacuum the engine creates at idle. For example if the engine creates 14" of vacuum at idle then the vacuum advance can should be full maxed out or limited at 12" of vacuum.
mudbone64
03-03-2025, 3:11pm
I never hooked up a vacuum advance on my hotrods.....I was only concerned about WOT and idle.........I wanted the mechanical weights to fly out easily at WOT, so I gave em light springs, and at idle, I want no timing advance........the only thing an advance is good for, is mid range throttle position to increase timing for fuel economy........who gives a rip about that?
:yesnod:
:island14:
Running as much advance as possible goes a long, long way in resolving overheating issues. I'm not so worried about wide open throttle as I am having the engine run as cool as possible while I'm sitting in line at the car show.
I never hooked up a vacuum advance on my hotrods.....I was only concerned about WOT and idle.........I wanted the mechanical weights to fly out easily at WOT, so I gave em light springs, and at idle, I want no timing advance........the only thing an advance is good for, is mid range throttle position to increase timing for fuel economy........who gives a rip about that?
:yesnod:
:island14:
Vacuum advance at low speed and cruise not only increase economy and all around street performance, it also makes the engine run cooler, which can add in eliminating detonation. I give a rip about that. I'd never run a car on the street without one. In fact, I replaced the original dizzy in my '61 Corvette with a 1965 unit because I wanted vacuum advance.
I think GTO meant to say "limit the vacuum advance" rather than somehow limiting the amount of vacuum. I don't think it's possible to limit the amount of vacuum an engine makes. But I digress.
I believe the conventional belief is in order to insure a stable idle the vacuum advance should be limited to 2" below whatever the amount of manifold vacuum the engine creates at idle. For example if the engine creates 14" of vacuum at idle then the vacuum advance can should be full maxed out or limited at 12" of vacuum.
Exactly. You are limiting the movement of the arm which moves the plate. Different units allow different amounts of movement at different vacuum levels. It really is a rabbit-hole, with today's fuels.
Frankie the Fink
03-03-2025, 3:53pm
mudbone64 has correctly cited the 'rule of thumb' for vacuum advance but it is general guidance and may need tweaked for a 'built' motor.
The guy you want to talk to is Lars:
https://en.wikipedia.org/wiki/Lars_Grimsrud
Assuming he is still around, if you want to reach out to him I'll PM you his email address.
Here are the specs on the engine. 4 speed with 3.73 gears. I am at 3500 ft. Above sea level and driving in areas from 1200-5000 ft. On occasion.
https://blueprintengines.com/collections/gm-compatible/products/496-ci-stroker-crate-engine-big-block-gm-longblock-bp4967ct?_pos=5&_fid=723558675&_ss=c
That's a great value, IMO! If I were you I would contact Lars or read his info (he is on the GTO forum occasionally and is on the 'other' forum, or Google him) since he lives in CO at altitude----or contact Blueprint and give them your numbers. Air is thinner up at 3500 feet, so you'll be running on the rich side unless you re-jet leaner (which you should do to keep the engine in good shape) and as a ballpark I'd want to start about 34 degrees BTDC all in at about 3000-3500 RPM. Your initial with the vac. line disconnected and plugged should be about 8--12 BTDC to start out. The engine will 'tell' you what it wants.
Everything is a tradeoff: a lot of advance will give snappy off-the-line bottom end perfomance, but will crap out on the top end, and vice-versa.
mudbone64 has correctly cited the 'rule of thumb' for vacuum advance but it is general guidance and may need tweaked for a 'built' motor.
The guy you want to talk to is Lars:
https://en.wikipedia.org/wiki/Lars_Grimsrud
Assuming he is still around, if you want to reach out to him I'll PM you his email address.
He's raising his grandkids after his son-in-law murdered his daughter last year.
AUTOHOLIC
03-03-2025, 4:02pm
mudbone64 has correctly cited the 'rule of thumb' for vacuum advance but it is general guidance and may need tweaked for a 'built' motor.
The guy you want to talk to is Lars:
https://en.wikipedia.org/wiki/Lars_Grimsrud
Assuming he is still around, if you want to reach out to him I'll PM you his email address.
Yes, I would like his email address.
Thanks
Frankie the Fink
03-03-2025, 4:40pm
He's raising his grandkids after his son-in-law murdered his daughter last year.
Damn - I forgot about that, horrible, absolutely horrible...
The_Dude
03-03-2025, 4:41pm
He's raising his grandkids after his son-in-law murdered his daughter last year.
:sadangel:
AUTOHOLIC
03-03-2025, 6:47pm
Thanks everyone! I have reached out to Lars
Capt. Shark
03-03-2025, 7:16pm
There is absolutely no one more knowledgeable than Lars. Despite his family tragedy he has continued to share his knowledge and expertise with us mere humans.
* cannister
You're welcome.
There is absolutely no one more knowledgeable than Lars. Despite his family tragedy he has continued to share his knowledge and expertise with us mere humans.
Agreed. He has selflessly helped countless people over the years. I met him on the GTO forum almost 20 years ago. Class Act. :cert:
Capt. Shark
03-03-2025, 7:34pm
Agreed. He has selflessly helped countless people over the years. I met him on the GTO forum almost 20 years ago. Class Act. :cert:
He is, indeed. He has rebuilt my Q-Jet twice over the years and it runs flawlessly. I would love to have participated in one of his "Tuning for Beer" events.
The_Dude
03-03-2025, 7:48pm
Cannaster Supply (https://cannastersupply.com/)
The_Dude
03-04-2025, 11:35am
* cannister
You're welcome.
*canister
WydGlydJim
03-05-2025, 8:03pm
I don't think I have seen this covered in this thread, some vacuum advances are also adjustable on the amount of advance the plate moves....usually an allen head screw inside the vacuum port........:yesnod:
mudbone64
03-05-2025, 11:09pm
The "adjustment" of an adjustable vacuum can only pertains to how much vacuum is required to initiate movement of the vacuum can's arm that connects to the advance plate. If a person wants to limit the amount timing advance the vacuum canister can create, then some sort of stop must be fashioned.
Frankie the Fink
03-06-2025, 7:19am
The Accel adjustable advance can affect the amount and rate of the vacuum applied...
I ran one on my 61 Corvette...
GTOguy
03-06-2025, 12:44pm
The Accel adjustable advance can affect the amount and rate of the vacuum applied...
I ran one on my 61 Corvette...
I've had three of these and each one failed almost immediately with a blown diaphragm. Nowadays, it's experimenting with different pots, bushings, springs, and stops. Lars even sells a limiting plate.
In the old days of good fuel, the stock springs, can, and weights always performed the best with stock-ish engines.
With today's 500-800 monsters, it's gonna take some experimentation.
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