no vacuum advance?
Thread Starter
Racer
Joined: Apr 2007
Posts: 275
Likes: 1
From: Lafayette/marquez La/tx
no vacuum advance?
hey guys long time no see. I have gm's 350/290 hp crate engine. In the specs it says not to run any vacuum advance. I recurved my dist. to be all in by 36 deg. I did not hook up vacuum advance because the distructions said not to. This is contrary to what I was always told. Others who have this engine say they get 20mpg. i get 10mpg. Would vacuum advace help my mpg? Why does GM say not to use the vacuum advance? If i do use it, timed or manifold vacuum, I was thinking manifold pressure. I have a performer manifold, edelbrock 750, headers. Also a 2300-2600 stall. After the holidays i am going to intall some
64cc 2.02 heads ported and polished cast iron (cuz i am poor) and a new cam. I was thinking a 284H-10
Duration at 050 inch Lift:240 int./246 exh. Valve Lift with Factory Rocker Arm Ratio:
0.507 int./0.510 exh.
Lobe Separation (degrees):110
Should I use the vacuum advace with the new upgrades?
What do yall think of this cam?
Thanks Again
Terry
64cc 2.02 heads ported and polished cast iron (cuz i am poor) and a new cam. I was thinking a 284H-10
Duration at 050 inch Lift:240 int./246 exh. Valve Lift with Factory Rocker Arm Ratio:
0.507 int./0.510 exh.
Lobe Separation (degrees):110
Should I use the vacuum advace with the new upgrades?
What do yall think of this cam?
Thanks Again
Terry
Last edited by tgcattle; Dec 20, 2012 at 09:12 PM. Reason: missed something
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
I can't imagine why GM would suggest not running vacuum advance. I suspect you misinterpreted the instructions.
That 350/290 engine used GM's 3896962 camshaft which is an excellent street cam, in spite of the fact its getting a little dated. Its probably not worth changing it. Your money would be better spent elsewhere, especially if you can't afford really good heads.
And yes, hooking up the vacuum advance will improve your mileage substantially!
That 350/290 engine used GM's 3896962 camshaft which is an excellent street cam, in spite of the fact its getting a little dated. Its probably not worth changing it. Your money would be better spent elsewhere, especially if you can't afford really good heads.
And yes, hooking up the vacuum advance will improve your mileage substantially!
Instructor
Joined: May 2011
Posts: 191
Likes: 1
From: Orange Ca
Use manifold vac...check timing at with it connected at 3,000 with a timing light to be sure timing is around 50 degrees give or take...adjust idle speed because the additional timing will most likely increase the idel speed...IMHO
Thread Starter
Racer
Joined: Apr 2007
Posts: 275
Likes: 1
From: Lafayette/marquez La/tx
Distruction clearly state.... Set spark timing at 32deg @ 3000rpm with with vacuum line from dist. removed and plugged. says this will produce 32 deg of total timing at WOT and the vacuum cannister should remain disconnected. This engine is designed to operate using only the interal centrifugal advance to acheive the correct timing curve
Instructor
Joined: May 2011
Posts: 191
Likes: 1
From: Orange Ca
Distruction clearly state.... Set spark timing at 32deg @ 3000rpm with with vacuum line from dist. removed and plugged. says this will produce 32 deg of total timing at WOT and the vacuum cannister should remain disconnected. This engine is designed to operate using only the interal centrifugal advance to acheive the correct timing curve
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Distruction clearly state.... Set spark timing at 32deg @ 3000rpm with with vacuum line from dist. removed and plugged. says this will produce 32 deg of total timing at WOT and the vacuum cannister should remain disconnected. This engine is designed to operate using only the interal centrifugal advance to acheive the correct timing curve
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
It's a small block V8. Is it an HEI distributor? Does it have the timing curve built into it already? If you have a standard distributor with a vacuum can, I say "Bunk!" on the 'instructions'.
To check it out, just put a timing light on it at idle with vacuum can disconnected and plugged. If it reads something in the 12-18 degree BTDC range, your timing system is not going to work well. It needs to be in the mid-20's somewhere....which is what it would do with the vacuum advance active.
Now, if for some reason it reads something in the mid-20 degree range with vacuum can disconnected and plugged...something else is going on that none of us know about.
To check it out, just put a timing light on it at idle with vacuum can disconnected and plugged. If it reads something in the 12-18 degree BTDC range, your timing system is not going to work well. It needs to be in the mid-20's somewhere....which is what it would do with the vacuum advance active.
Now, if for some reason it reads something in the mid-20 degree range with vacuum can disconnected and plugged...something else is going on that none of us know about.
Thread Starter
Racer
Joined: Apr 2007
Posts: 275
Likes: 1
From: Lafayette/marquez La/tx
I have the timing all in at 3000rpm...36deg no vacuum connected. Runs strong, gas mileage sucks though. I don"t understnd why the instructions say that. After the holidays I will hook up vacuum advace and see what happens. I wonder if the other guys who have this motor hooked up the vacuum advance?
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Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
Hook the VA to manifold vacuum. Check it @ steady 3000 RPM. If your @ 54 or under your good. Fuel mileage will increase to over 16 with better cruise. Will likely need to adjust idle speed and mixture. Less advance runs hotter, meets emissions easier, less chance for detonation or engine damage so warrantee claims reduced.
Instructor
Joined: May 2011
Posts: 191
Likes: 1
From: Orange Ca
Hook the VA to manifold vacuum. Check it @ steady 3000 RPM. If your @ 54 or under your good. Fuel mileage will increase to over 16 with better cruise. Will likely need to adjust idle speed and mixture. Less advance runs hotter, meets emissions easier, less chance for detonation or engine damage so warrantee claims reduced.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Performance engines don't need a VC can....Most distributors have 12 degrees built in...double that its 24, and if you put 12 on the crank your total will be 36...I don't know why this story comes up every few months about a VC can....One more time for the record, FI cars did not have them either and a proper tune can give 18-20 miles per gallon at highway speeds and better... and with stiff gears....Lousy gas mileage means the combination is suffering badly from its components and/or choice of parts....
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I believe those modern FI cars have a way of measuring engine load and advancing or retarding timing to compensate for it. It's true, having a functional vacuum advance unit will not increase your engine's performance. It is there to increase fuel mileage and idle quality. Not running any form of vacuum advance on a non-computer controlled engine is strictly for guys who like to brag about what crappy fuel mileage their car gets.
Scott
Scott
Burning Brakes
Joined: Oct 2001
Posts: 1,241
Likes: 4
From: Okinawa Okinawa
This is what the instructions say for my ZZ4.
"Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor
disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI
vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal
centrifugal advance to achieve the correct timing curve."
I have it hooked up and it runs great. With it unplugged, 32 degrees at 2800. Plug it in and I'm at 52. Not sure why it says to keep it unplugged but that is what GM crate engines call for.
"Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor
disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI
vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal
centrifugal advance to achieve the correct timing curve."
I have it hooked up and it runs great. With it unplugged, 32 degrees at 2800. Plug it in and I'm at 52. Not sure why it says to keep it unplugged but that is what GM crate engines call for.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
This is what the instructions say for my ZZ4.
"Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor
disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI
vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal
centrifugal advance to achieve the correct timing curve."
I have it hooked up and it runs great. With it unplugged, 32 degrees at 2800. Plug it in and I'm at 52. Not sure why it says to keep it unplugged but that is what GM crate engines call for.
"Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor
disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI
vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal
centrifugal advance to achieve the correct timing curve."
I have it hooked up and it runs great. With it unplugged, 32 degrees at 2800. Plug it in and I'm at 52. Not sure why it says to keep it unplugged but that is what GM crate engines call for.
Hook the VA to manifold vacuum. Check it @ steady 3000 RPM. If your @ 54 or under your good. Fuel mileage will increase to over 16 with better cruise. Will likely need to adjust idle speed and mixture. Less advance runs hotter, meets emissions easier, less chance for detonation or engine damage so warrantee claims reduced.
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Performance engines don't need a VC can....Nonsense Most distributors have 12 degrees built in...double that its 24, and if you put 12 on the crank your total will be 36...I don't know why this story comes up every few months about a VC can....One more time for the record, FI cars did not have them either and a proper tune can give 18-20 miles per gallon at highway speeds and better... and with stiff gears....Lousy gas mileage means the combination is suffering badly from its components and/or choice of parts....
As I mentioned before, I have yet to see a cylinder head design that enables a combustion burn rate independent of the pressure in that cylinder.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Performance engines don't need a VC can....Most distributors have 12 degrees built in...double that its 24, and if you put 12 on the crank your total will be 36...I don't know why this story comes up every few months about a VC can....One more time for the record, FI cars did not have them either and a proper tune can give 18-20 miles per gallon at highway speeds and better... and with stiff gears....Lousy gas mileage means the combination is suffering badly from its components and/or choice of parts....
Safety Car
Joined: Jan 2009
Posts: 4,000
Likes: 299
From: New York
I think the directions are accurate and the people reading them are messed up .
It is not saying never to use the vac advance it is saying to SET TIMING .. Come on now read it and re read it ... SET TIMING if you got that part NOW realize all our distributors only use centrifugal to achieve total advance and achieve the correct curve. Thats because we all know under wot throttle the vac advance totally drops out of any function affecting timing advance. Matter of fact the instant vacuum drops the advance is pulled off from functioning .
I am wondering if this whole topic was designed to be a play on words to spur a pointless discussion
Lastly the vac adv , its main reason to exist is for steady cruse and idle , where fuel can go un-burned and cause pollution ,the vac adv compensates for this by creating a spark earlier in the pistons travel to TDC making sure the flame front gets a chance to burn the most amount of fuel present. Can this cause better mpg I have never looked at it that way and thus have no idea but I guess ya sure it might help...
It is not saying never to use the vac advance it is saying to SET TIMING .. Come on now read it and re read it ... SET TIMING if you got that part NOW realize all our distributors only use centrifugal to achieve total advance and achieve the correct curve. Thats because we all know under wot throttle the vac advance totally drops out of any function affecting timing advance. Matter of fact the instant vacuum drops the advance is pulled off from functioning .
I am wondering if this whole topic was designed to be a play on words to spur a pointless discussion
Lastly the vac adv , its main reason to exist is for steady cruse and idle , where fuel can go un-burned and cause pollution ,the vac adv compensates for this by creating a spark earlier in the pistons travel to TDC making sure the flame front gets a chance to burn the most amount of fuel present. Can this cause better mpg I have never looked at it that way and thus have no idea but I guess ya sure it might help...
Last edited by diehrd; Dec 21, 2012 at 03:48 PM.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
The reason GM instructs you not to use the vacuum advance is because it supplys too much advance for the engine, it is a 20 degree advance unit. Delco sold a d1370a vac advance that had 10 degrees but it is NLA but they are still avalible thru amazon.com or Standard Hygrade sells one.
I hope this helps Henry @ olescarb
I hope this helps Henry @ olescarb
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
I believe those modern FI cars have a way of measuring engine load and advancing or retarding timing to compensate for it. It's true, having a functional vacuum advance unit will not increase your engine's performance. It is there to increase fuel mileage and idle quality. Not running any form of vacuum advance on a non-computer controlled engine is strictly for guys who like to brag about what crappy fuel mileage their car gets.
Scott
Scott
maybe I could use that little 290 HP thingie to start my 'BIG' ones though....
