Hay all, Wanted to run something by you to get an opinion on an issue I am experiencing.
I am a huge fan of using a vacuum gauge for tuning and general maintenance. My vacuum is about 15in at 600rpm idle and the gauge indicates "late ignition timing".
My initial timing was set at 12 BTDC. When I advance it more, I can get the vacuum gage to 18.5in before the engine stumbles, dial it back and she idles beautifully and I have about 17 in of vacuum. Now checking my timing again, that would leave it at 22 BTDC!!! Makes no sense. This is in Cali with Cali premium gas.
The original points distributor is dialed in, springs, weights. I don't think that the original harmonic balancer slipped since I checked it versus No1 cyl and it was at the zero mak and lined up with the mark.
Any recommendations? should I just trust the timing light or the vacuum gauge?
Vacuum gauge always tells the true story on what the engine wants. My 468 i had in my 68 before this engine the initial timing was 22-23 degrees with a total of 35 degrees. i have seen timing marks off on these timing covers as much as 4 degrees. I always bolt on the timing cover and balancer and bring the piston to TDC and verify the timing marks before i bolt on the heads. But yea you are good with setting timing with vacuum gauge.
Is the cam stock? A radical cam will show low vacuum at idle. I don't want to start a 'manifold vs ported vacuum advance' pissing contest. I normally use ported vacuum with stock engines. In extreme cam cases manifold vacuum advance does bring the idle vacuum up nicely.
FWIW I too swear by the vacuum gauge for tuning. Another lost art.
The 068? That cam can be kind of lumpy, with low vacuum. I have the 067 in a 455. In an engine that big it is very smooth, high vacuum. Big difference between the two. Pontiac used the same lift on most cams, but the duration was much different. When the duration moves the power band higher up the rpms, the idle and low rpm suffers.
Depends what you consider low vacuum I had the 744 cam in a Pontiac 350 in my lemans, it had 12-13 inches of vacuum. Drove that car up and down 231 every night.
My buddy had the 068 cam in his '68 400 hardtop. I used a Crane HMV-278 in a 400 that was in my '81 Malibu wagon. Both had very low vacuum and needed a vacuum reservoir can to operate the power brake booster. I can deal with a rough idle but I need the brakes to work.
You're missing the point. I don't have low vacuum, 15mg is not low, but 18 is better. I've come to the conclusion to not use the vacuum gauge to set the timing, I'll power time it starting at 18 BTDC and see if she pings going up the hill or throttling it then back it off 1 degree at a time until pinging is gone.
Because I don't know better, I have always (55 years) set my '68 400HO (67 cam) at the factory 9° BTDC during "tune-up". What do you gain with advancing it further.
Note that I now run about 93 octane with corn squeezins and add a little octane booster additive to avoid ping and protect valve seats.
There is a variety of objective evidence out there regarding today's gasoline and the negative impacts on classic car engines. It also suggests not to pay too much attention to "initial timing" as you don't drive you car at idle. The total timing of 32-34 Degrees is what's more important. https://firstgenfirebird.org/faqs/timing-adjustments/
The quality of gasoline here in AZ is abysmal, my previous engine was 9:1 compression and i wasted lots of time and money trying to make it run perfect. I had the best mechanical pump you could buy and had vapor lock issues. switched to an electric pump and return system and still had issues. I completely emptied my system and filled with AV gasoline and no more problems, the engine idled much smoother and ran way better. Since then the only thing that goes in my tank is race gas or AV gas. I don't care how much it cost, just want my engine to run good. The other problem with today's gasoline with the corn juice is if you let the car sit for any length of time it really messes with the carburetor. Also change all the rubber fuel lines in your car. Lead or no lead is really a moot point with a hydraulic tappet cam that only has 100-120 lbs seat pressure. Never seen any adverse effects on any cylinder heads i have worked on.
Everyone has different needs. I always advise guys to do what they can afford, but be prepared to back up and re-group if it doesn't work out. I could care less about originality anymore. I seek out low compression heads with hardened seats for no-lead gas. I also don't use high duration cams because there is no where to use a high rpm engine here in Shitty City. [bigchief knows what I mean] I prefer high lift, short duration cams that make gobs of power way down low. That said, I enjoy very high manifold vacuum like 21" or better. I also like a smooth idle, no pinging, pretty good mileage, peppy performance, buying the cheapest gas I can find, and no overheating. That may sound boring but at this point in my life dependability is key.
Thanks all. I just wondered what further timing advancement adjustments could improve. I suppose if I like the way it runs, I've conquered poor-gas ping, it doesn't overheat, get better than average MPG etc., I should be satisfied. I'm not looking to race it.
Proper tuning has nothing to do with racing a car but overall better drivability. which also includes steering and stoping too. Guys also forget an engine and all its related components that was built in 1967 wears out. Does anybody on this forum have a refrigerator in their kitchen that was built in 1967 ?
Believe it or not, I had a 1950's Tappan 400 stove in my kitchen like forever. The woman I bought my house from in 1975 won it on 'Queen For A day'. She also won a 'Speed Queen' washer and dryer. They gave up after a while but the stove soldiered on because I could get parts. There was an ancient appliance parts place on Atlantic Ave in Richmond Hill that had parts in boxes older than me. The only reason we had to get rid of it in 2004 was the steel bottom rusted to the point of being unsafe. Same reason I got rid of my 1968 hardtop. But the almost 50 year old stove still worked!
I don't race, but I do enjoy tuning for mileage and performance. You can have both. For example, the first thing I do is get rid of points ignition. Sure, they work good when new, but as they wear they degrade, affecting timing, mileage, and power. I grabbed every Pontiac HEI distributor I could get my hands on at swap meets. I tune them for better performance.
Same for Q-jets. The 1960's Q-jets were jetted very rich. By 1980 they were incredibly lean. The happy middle is what I shoot for. I use the 1973 and later models so I can back-fit the electric chokes. I switch metering rods around to get better off-idle take-off. Models with the APT feature can also be played with for some advantages. I have a Firebird 301 that gets 20mpg all day long and runs great.
You have the better gasoline back there. I used to run Sunoco 93 in everything. The junk we get here comes from the west coast. Total garbage that turns brown if you let it sin in a carburetor for any length of time. The problems with the factory HEI distributors the advance slots wear after time and end up with way to much advance. The ignition modules also start dropping off over 4000 RPM.
