I'm changing the vacuum advance can from the stock one to the Duke recommended Napa/Echlin VC 1810 can this morning. I've been through the archives but can't find Dukes posts on "ported" vacuum advance. I'm not sure the 67 L71 is the same as the 66 L72. I don't think it is but I'm not sure. Can someone please clear this up for me. Do I need to build a new minifold vacuum line or can I just install the new vacuum advance can the way it is now. Thanks for the help.
-
-
Re: 67 427/435 L71 Vacuum advance.
It's very easy to determine whether an engine has ported or full vacuum advance.
With the engine idling remove the signal line from the vacuum can. Does the idle speed or character change? Check the signal line for vacuum preferably with a vacuum gage. Does it show the same vacuum compared to teeing the vacuum gage into the choke vacuum break line? Checking the timing with the vacuum can both connected and disconected is another method to determine if an engine has ported or full vacuum advance.
No L-71 owner has ever confirmed the vacuum advance configuration to my knowledge, and all that is required is this simple test. Can you perform it?
If ported vacuum advance is confirmed, a convenient source of full vacuum advance is the choke vacuum break line. You can rig up full time vacuum advance by buying a length of 1/8" vacuum tubing and a tee and make a new harness for the choke vacuum break/vacuum advance.
The VC1810 meets the requirement to keep the vacuum can locked at full advance at idle (with full manifold vacuum signal), but it may be too aggresive and cause part throttle acceleration detonation. A better alternative for SHP BBs may be the VC1765, which provides 16 deg. @ 12" versus 16 deg. @ 8" for the VC 1810.
With full vacuum advance at idle and 10 degrees initial advance, the total idle timing will be 26 degrees assuming the centrifugal has not started, and at an idle speed of 800-900 the manifold vacuum reading should be 14-15 inches with the OE mechanical lifter cam.
Duke -
Re: 67 427/435 L71 Vacuum advance.
Duke, thanks for the response and the help. With the car idleing at 800 RPM, when I remove the line to the vacuum can, the idle drops to almost a stall and idles roughly. The new 1810 can is in and the car detonates on acceleration in all four gears with the timing set at 5 degrees. I have retarded to 3 degrees and added 104 octane boost.....it seems fine now. Anything else I should measure or do. The timimg was set with the car at idle at 800 with vacuum line plugged with a tee....when I reconnected line to vacuum can the idle went up to 1100 or 1200 and I adjusted back to 800. Thanks again for the help.Comment
-
Re: 67 427/435 L71 Vacuum advance.
It doesn't sound like ported vacuum advance, but I'd like to see idle manifold vacuum readings with a vacuum gage teed into both the vacuum can signal line and choke vacuum break line.
Since the VC1765 can has a less aggressive curve it should not produce at much detonatation at part throttle acceleration.
When setting initial timing you should always slowly increase revs and make sure it doesn't start advancing until at least a couple of hundreds revs higher than the speed that you set the timing. Alternatively, you can reduce revs and check that the timing does not retard. The purpose of this is to ensure that the centrifugal has not started. If it has you cannot set the initial timing accurately.
It's always a good idea to use a dial back timing light to characterize the centrifugal curve and compare it to OE spec. You never know on a 35+ year old car if the centrifugal curve has been quickened with lighter springs or heavier weights. Detonation on acceleration can be caused by a too aggressive centrifugal curve.
DukeComment
-
Re: 67 427/435 L71 Vacuum advance.
I have a dial back timing light but limited experience using it at settings other than zero. Can you please explain the procedure I should follow to verify the advance has not started when I reset the idle after reconnecting the vacuum line to the advance can. Thanks for the education.It is much appreciated.Comment
-
Re: 67 427/435 L71 Vacuum advance. Duke
Duke,
My Sears dial back timing light instructions warn that solid core ignition wires may radiat radio interference which can interfere with the operation of the timing light. I have always used Packard 440 solid core wire (and will continue to do so). I your opinion would shielding the inductive pick-up and shield the lead to the light eliminate the interference? Currently I perfer my 40 year old Sears non dial back timing light. I don't have much experience with my newer dial back, it does seem to have some off idle operation quirks. Thanks & Regards,Comment
-
Re: 67 427/435 L71 Vacuum advance.
Checking that the CENTRIFUGAL advance has not started when you set the initial timing just involves slowly bringing up the speed, and watching for the timing mark to advance. If is advances immediately the centrifugal may have started at the speed you set the initial timing, so your initial timing setting is not valid. You don't want the mark to advance until at least a couple of hundred revs higher than the speed you set the initial timing to ensure the centrifugal has not started.
Once the initial timing is set, connect the vacuum can and measure the total timing. Use the dial to bring the mark to zero on the tab and read the dial. This is your total idle advance, and this total minus the initial is the amount contributed by the vacuum can, and it should be the maximum the vacuum can offers if you have full manifold vacuum advance.
Likewise, with the vacuum can disconnected you can take total timing readings at various engines speeds, and at each speed the total minus the initial is the centrifugal at that speed.
DukeComment
-
Re: 67 427/435 L71 Vacuum advance. Duke
It could help. All my cars have RFI wiring, and my Sears dial back works fine. On my Cosworth Vega I placed a very accurate 30 degree advance mark on the balancer, and the light agrees with it as accurately as I can read the dial.
DukeComment
-
Re: 67 427/435 L71 Vacuum advance.
This is turning out to be a great learning experience for me. I hooked up the timing light and took the readings you suggested. Here is what I get:
at idle(800 RPM)with vacuum line connected to vacuum can: 11 degrees
at 1500 RPM: 28 degrees
at 2500 RPM: 55 degrees
at 3000 RPM: 55 degrees
Can you help me to understand what this means in relation to the VC 1810 vacuum can and the distributor set up. It appears to be overheating at idle as well. Thanks for the education.
Rick.Comment
-
Re: 67 427/435 L71 Vacuum advance.
sorry, I should have included that at 2000 RPM I get 48 degrees.Comment
-
Re: 67 427/435 L71 Vacuum advance.
Rerun the test with the vacuum can disconnected.
Do you have a CSM or AMA specs with all the timing map specs for your engine?
Drop the engine revs to just where the engine barely runs and check the timing. Then slowly bring the revs up. If the mark advances immediately your initial timing setting is not valid because it has a quick advance curve.
Now bring the revs up to 1000 and measure in 500 rev increments to 4500 or 500 beyond where it stops advancing. According to the specs in Corvette News, the '67 L-71 centrifugal maxes out at 3800, but it appears someone has installed weak springs for a quicker advance, which could invalidate what you think is an initial timing reading at 800 because the centrifugal may have already started. A BB won't take as quick a curve as a SB due to a higher propensity to detonate because of the larger bore.
I would also like to see idle vacuum readings with a vacuum gage teed into both the choke vacuum break line and vacuum advance signal line.
DukeComment
-
Re: 67 427/435 L71 Vacuum advance.
Thanks Duke. I'll run the test again tomorrow with the can disconnected. My vacuum guage has disappeared...I'll get another tomorrow and post the readings. Am I getting too much mechanical advance from the distributor?Comment
-
Re: 67 427/435 L71 Vacuum advance.
It appears so, but I want to see the number with the vacuum can disconnected.
Do you have OE specs?
DukeComment
-
Re: 67 427/435 L71 Vacuum advance.
Duke:
1) Yes...I have all OE specs.
2) With vacuum line discinnected and plugged with a golf tee here are the readings I get:
At idle(800)RPM: 3 degrees
At 1500 RPM: 8 degrees
At 2000 RPM: 25 degrees
At 2500 RPM: 35 degrees
At 3000 RPM: 34 degrees
With respect to engine vacuum the only place to measure is at the base of the center carb.(line connects to vacuum can) Here are the readings I get:
At idle(800)RPM: 8 inches Hg.
At 1500 RPM: 17 inches.
At 2000 RPM: 20 inches.
At 3000 RPM: 21 inches.
Can you please let me know what you think. Thanks.
Rick.Comment
-
Re: 67 427/435 L71 Vacuum advance.
It appears that your centrifugal is faster than OE. This will contribute to detonation tendency. You can't run such a quick centrifugal unless you're willing to run a high blend of race gas or avgas with pump premium for fuel.
Okay, you have the OE specs for the ignition map, so what are they???? Do a core dump!
The vacuum can source does not appear to be full manifold vacuum at idle. Measure manifold vacuum from the choke vacuum break line.
Here's another useful test. Increase the static timing to 25 degrees, and reset the engine speed to 800-900 and measure manifold vacuum from the choke vacuum break. This is the value used to select a vacuum can.
DukeComment
Comment