Vacuum on '64 L84

[James B.]

I have a 64 roadster; fuelie with 375R unit. On starting it and idling for a while recently I thought that the engine got pretty hot quite quickly (+-200 degrees). On driving the car temp came quickly back down to 180 and stayed there. On checking the vacuum advance I found that the advance held vacuum (proper 236 unit) but the line from the air meter only delivered 2" of vacuum at idle speed of 750 rpm, at 1,000 rpm it was 7.5" and at 1,500 rpm 15". Since this is basically a 365 I am assuming it should give 15" of vacuum min at idle and requires timing advance at idle. The car is a 34,000 mile car and previous owners assured me engine was not rebuilt so the cam should be standard 30-30.
I went through an overheating problem with a '67 L79 coupe I recently bought and ultimately found a bad vacuum advance unit.
A friend suggested the fuelie might use ported vacuum which would provide very low vacuum at idle- I doubt this is right, but will appreciate any help and advice.
John De Gregory sold me parts and gave me lots of helpful advice as I struggled to rebuild the unit a couple of years ago but then the car a not been run for the past couple of years (shame on me). I see in the NCRS fuelie cd it recommends a thicker air meter gasket. I have a new (standard) gasket and could not hear an air leak but that might be the problem. Other than my perception of the car running hotter than it should at idle, the car starts and runs very well. Will appreciate all help and advise, John De Gregory, hope you are on line today... (I've lost your phone number), Regards, Jim Baker

[Duke W.]

Re: Vacuum on '64 L84

For some incomprehensible reason, 375 (and 375R) FI units have ported vacuum advance. Recall that '63 SHP/FI engines were the first of their kind to have OE vacuum advance and Chevrolet did not get it right. For example, with the Duntov cam, typical idle is 12" at 900 and the OE "201" 15.5" VAC used on 340 HP engines did not pull the diaphragm to the limit at idle.

This caused all kinds of idle stabililty/quality problems on my 327/340 until I changed the VAC to the 8" "236" circa 1966.

It didn't make any difference on FI engines as far as idle stability is concerned since they had ported vacuum advance, but ported vacuum advance will increase cooling system load at idle and cause hot running/overheating after extensive idling. The 380 FI unit was configured for full time vacuum advance.

Some very early '64 FI engines were built with both the 375R FI unit and the '63 063 distributor. A 236 VAC on yours indicates the second production 070, which has much better calibration.

The best thing you can do to a 375/375R unit is convert it to full time vacuum advance and ensure that a 236 VAC or equivalent is installed with any of the OE mechanical lifter cams.

The modification will take a little creative work on the air meter to move the vacuum advance port to below the throttle blade in the idle position.

The 30-30 cam with 26-34 degrees of total idle timing (the sum of initial plus full vacuum advance, plus maybe a couple of degrees of centrigual since it starts at about 700) should pull about 10" @ 900, but FI engines will probably be happier with 100-200 more idle revs and the higher vacuum it develops.

Either way, increasing total idle timing from just initial to the above range with full time vacuum advance will considerably decrease idle cooling system load and reduce the tendency to overheat.

For every operating condition there is an ideal spark advance range that will maximize peak combustion temperature, minimize EGT, and yield the highest thermal efficiency, which means the lowest fuel consumption. High overlap cams need lots of total idle timing because the extreme exhaust gas dilution slows flame propagation speed.

If you reduce the total idle timing to below the optimum range, EGT goes up and transfers much more heat to the cooling system.

The best total idle timing for SBCs is the range of around 24-34 degrees with low overlap cams in the bottom half of the range, and high overlap cams in the top half.

Duke

[Duke W.]

Re: Vacuum on '64 L84

For some incomprehensible reason, 375 (and 375R) FI units have ported vacuum advance. Recall that '63 SHP/FI engines were the first of their kind to have OE vacuum advance and Chevrolet did not get it right. For example, with the Duntov cam, typical idle is 12" at 900 and the OE "201" 15.5" VAC used on 340 HP engines did not pull the diaphragm to the limit at idle.

This caused all kinds of idle stabililty/quality problems on my 327/340 until I changed the VAC to the 8" "236" circa 1966.

It didn't make any difference on FI engines as far as idle stability is concerned since they had ported vacuum advance, but ported vacuum advance will increase cooling system load at idle and cause hot running/overheating after extensive idling. The 380 FI unit was configured for full time vacuum advance.

Some very early '64 FI engines were built with both the 375R FI unit and the '63 063 distributor. A 236 VAC on yours indicates the second production 070, which has much better calibration.

The best thing you can do to a 375/375R unit is convert it to full time vacuum advance and ensure that a 236 VAC or equivalent is installed with any of the OE mechanical lifter cams.

The modification will take a little creative work on the air meter to move the vacuum advance port to below the throttle blade in the idle position.

The 30-30 cam with 26-34 degrees of total idle timing (the sum of initial plus full vacuum advance, plus maybe a couple of degrees of centrigual since it starts at about 700) should pull about 10" @ 900, but FI engines will probably be happier with 100-200 more idle revs and the higher vacuum it develops.

Either way, increasing total idle timing from just initial to the above range with full time vacuum advance will considerably decrease idle cooling system load and reduce the tendency to overheat.

For every operating condition there is an ideal spark advance range that will maximize peak combustion temperature, minimize EGT, and yield the highest thermal efficiency, which means the lowest fuel consumption. High overlap cams need lots of total idle timing because the extreme exhaust gas dilution slows flame propagation speed.

If you reduce the total idle timing to below the optimum range, EGT goes up and transfers much more heat to the cooling system.

The best total idle timing for SBCs is the range of around 24-34 degrees with low overlap cams in the bottom half of the range, and high overlap cams in the top half.

Duke

[Dan H.]

Re: Vacuum on '64 L84

Hi Jim, JD phone Nr 724 832 3786. My 64 FI started running hot like yours, the 'repro' 236 advance went belly up, one year old! Put the original 236 on, all in at 8#, runs cool now. When I installed my plenum, I adjusted it by snugging the adapter plate down a little at a time and sat the plenum on it so any 'wobbles' could be minimized by loosening and tightening the adapter, no gasket installed under plenum, until it was as close to no wobble as possible. Your vacuum sounds way low, hopefully it's a gasket failure and not a cracked plenum leg. Happy hunting!
Dan

[Dan H.]

Re: Vacuum on '64 L84

Hi Jim, JD phone Nr 724 832 3786. My 64 FI started running hot like yours, the 'repro' 236 advance went belly up, one year old! Put the original 236 on, all in at 8#, runs cool now. When I installed my plenum, I adjusted it by snugging the adapter plate down a little at a time and sat the plenum on it so any 'wobbles' could be minimized by loosening and tightening the adapter, no gasket installed under plenum, until it was as close to no wobble as possible. Your vacuum sounds way low, hopefully it's a gasket failure and not a cracked plenum leg. Happy hunting!
Dan

[James B.]

Re: Vacuum on '64 L84

Great info- thank you. I have also printed your article on 30-30 lifter adjustment and will correct that too. Thanks again.

[James B.]

Re: Vacuum on '64 L84

Great info- thank you. I have also printed your article on 30-30 lifter adjustment and will correct that too. Thanks again.

[James B.]

Re: Vacuum on '64 L84

Dan, thanks for advice- my plenum and adapter are okay- no cracks and I have been quite careful about snugging them down uniformly. As I said, my most suspect area is the air meter to plenum gasket sealing. I agree that my vacuum is low, even for ported and I know my solids need adjusting so if anything a loose overhead is probably giving me more vacuum than normal. Thanks again, Jim Baker

[James B.]

Re: Vacuum on '64 L84

Dan, thanks for advice- my plenum and adapter are okay- no cracks and I have been quite careful about snugging them down uniformly. As I said, my most suspect area is the air meter to plenum gasket sealing. I agree that my vacuum is low, even for ported and I know my solids need adjusting so if anything a loose overhead is probably giving me more vacuum than normal. Thanks again, Jim Baker

[Duke W.]

Re: Vacuum on '64 L84

Keep in mind that the clearance called out for the 30-30 cam on the Chevrolet drawing is .025" (both sides because this cam uses the same lobe on both sides), which is derived from the "design" rocker ratio of 1.5:1 times the height of the clearance ramp, which is .017" above the base circle.

My .023" recommended clearance is based on the actual measured rocker ratio behavior, which is about 1.37:1 near the base circle (and about 1.44:1 at maximum lobe lift).

The origin of the .030" clearance specification in owner's and service manuals is obsure, but some speculate that it was due to poor FI idle quality at tighter settings.

Increasing clearance reduces effective overlap, which should improve idle quality, but the downside is that running clearance "loose" causes the valves to be jerked open and slammed shut at higher than clearance ramp velocity.

So running the lash at my recommended clearance will reduce valve train loading, but it will also degrade idle quality, so higher idle speed will likely be necessary.

IMO Chevrolet's recommended idle speeds for SHP/FI engines is unrealistically low, and my "rule of thumb" on idle speed is to set it at the "lowest possible speed commensurate with acceptable idle quality". This is a subjective call, and it also depends on the mechanical condition of the engine in addition to whether or not the engine is optimally tuned with the best total idle timing and idle mixture.

Duke

[Duke W.]

Re: Vacuum on '64 L84

Keep in mind that the clearance called out for the 30-30 cam on the Chevrolet drawing is .025" (both sides because this cam uses the same lobe on both sides), which is derived from the "design" rocker ratio of 1.5:1 times the height of the clearance ramp, which is .017" above the base circle.

My .023" recommended clearance is based on the actual measured rocker ratio behavior, which is about 1.37:1 near the base circle (and about 1.44:1 at maximum lobe lift).

The origin of the .030" clearance specification in owner's and service manuals is obsure, but some speculate that it was due to poor FI idle quality at tighter settings.

Increasing clearance reduces effective overlap, which should improve idle quality, but the downside is that running clearance "loose" causes the valves to be jerked open and slammed shut at higher than clearance ramp velocity.

So running the lash at my recommended clearance will reduce valve train loading, but it will also degrade idle quality, so higher idle speed will likely be necessary.

IMO Chevrolet's recommended idle speeds for SHP/FI engines is unrealistically low, and my "rule of thumb" on idle speed is to set it at the "lowest possible speed commensurate with acceptable idle quality". This is a subjective call, and it also depends on the mechanical condition of the engine in addition to whether or not the engine is optimally tuned with the best total idle timing and idle mixture.

Duke