Tuning with a vacuum gauge

[Porter]

Back by popular demand, forgot I shared this with Walter Youshock years ago for his 57 Cadillac.

If you live way above sea level like John Washburn I think the factory timing specifications aren't gonna cut it. Or you lose 1 hg. for every 1,000 ft. above sea level. John is a good friend of mine - tuning specialist.

I rebuilt my SBC TBI fuel and ignition system last year, timing mark is a bit cumbersome. After I rebuilt the distributor I dropped it back in per my reference mark. Drove a bit sluggish, advanced the distributor an 1/8" turn CCW and she woke right up. Could bump it a tad more and at some point it will start to ping - ain't broke leave it alone.

Hooked up my vacuum gauge just for laughs to the intake manifold, strongest reading I've ever seen - like 23 hg. My 429 sits at around 21 hg. To quote my esteemed colleague Mike Jones an engine is an air pump,  if you don't have a steady vacuum reading there's some type of problem - internal or an external vacuum leak.

Had mine since 1972, was rummaging at the boneyard years ago for some 65 Impala SW parts, a 65 SS had a vacuum gauge where the clock goes, scoffed that right up

.http://www.secondchancegarage.com/public/186.cfm

Porter

[Ralph Messina CLC 4937]

Porter,

I’ve used that method quite successfully for a long time and we even discussed it off line. I curious about the reading. When I check manifold vacuum my  66’s T port (with other ports closed off) I get a reading that pulsates with cylinder firing. The car runs great with a lot of power and every vacuum circuit was tested when I rebuilt the car. I’ve tested the gauge for PSI accuracy. Are you reading 21 with no vibration in the needle gauge ? I’m wondering if I need a new gauge.

Ralph

[Porter]

Ralph,

I was numb when I saw the reading on my SBC TBI with 165K, never had the vacuum gauge up to 23 hg. GM builds one helluva 350 CID V 8.

My 66 429 pulls a solid 21 hg. with a steady needle. Been trying to find my pictures of that today to no avail.

The beauty of the vacuum gauge is it tells you what the problem is. I had a 71 307 Chevelle years ago and it had a slight fluctuation, likely a bad valve.

Years ago I had a 66 Vette w/ L 79, had the timing advanced 2 degrees - ran premium gas w/ octane booster plus a can of lead. Advance the timing too far and it will ping.

Always a happy medium to be found - timing is not rocket science.

Fine tuning an engine is fun stuff, Mike Jones gets his 70 472, 88 4.5L Eldo and his current daily driver - 87 Fleetwood with 307 totally tuned to perfection with mega high MPG - right off the Richter scale !

Rumor has it you are quite a perfectionist. Your 66 Fleetwood must be a sight to behold. Likely nothing wrong with your vacuum gauge and she drives nice so life is good.

Porter

[P W Allen CLC# 20193]

I just recently used the vacuum gauge method to tune up my 53. I did use a timing light, but only as a reference. By using vacuum, I found that the best vacuum was obtained two or three degrees advanced of spec. I also found that adjusting the two idle mixture screws was much easier and precise using the gauge. I will still use the light and a tach for refrence, but will from now on, use the vacuum gauge to dial everything in.

Paul

[DinoBob]

I met Ralph at the CLC GN in Cherry Hill last year. He is quite the perfectionist all right- to hear him talk about flash-chroming those stainless moldings and restoring his horn ring- wow! I was extremely impressed with his '66 Fleetwood.

[Dave Shepherd]

Great thread, most older engines can benefit from a few more degrees initial to compensate for chain wear and dist wear.  It is almost an issue of " power tuning"

[David Greenburg]

That's the great thing about vacuum tuning; it compensates for wear and/or altitude.  I quit using a timing light a number of years ago when I was in school in Colo. and home was on the east coast; made tuning for those cross country trips a breeze.  That was  with a '67 Rambler, which I believe was the last car to have vacuum wipers, so it also enabled me to tune my wipers.   Maximum vacuum minus 7/8" seems to be the sweet spot for my '59 60S, at least on 91 octane CA gas.  Car and I sure do miss that Sunoco 94 from back east. 

[walt chomosh #23510]

Porter,
  Do I understand that vacumn tuning of ones timing of a SBC can be done on later models such as my 1994TBI?(Roadtrek van) I was under the impression that the computer would control timing after base was set.(max advance,etc.) Another mechanic commented that base timing would only allow one to stray a few degrees in each direction before the computer would"take over",thus limiting advance.....walt...tulsa,ok

[Guidematic]

Great thread, most older engines can benefit from a few more degrees initial to compensate for chain wear and dist wear.  It is almost an issue of " power tuning"

Often a misconception, but may have some merit on another point.

When you time an engine, you are referencing the crankshaft, not the camshaft. So cam timing is not an issue. However, with late cam timing there may be a slight decrease in compression, and slightly advance timing can add a bit of lost power without the peak combustion chamber pressures that would otherwise cause pinging. On and older engine, this becomes a bit of experimentation to see how far the timing can be advanced.

However, as older engines go, they tend to get some carboning up inside the combustion chamber and the intake ports that cause hot spots that promote pre-ignition. Deposits in the intake can cause fuel fall out which can also lead to mpre-ignition and high HC emissions.

Mike

[Guidematic]

Porter,
  Do I understand that vacumn tuning of ones timing of a SBC can be done on later models such as my 1994TBI?(Roadtrek van) I was under the impression that the computer would control timing after base was set.(max advance,etc.) Another mechanic commented that base timing would only allow one to stray a few degrees in each direction before the computer would"take over",thus limiting advance.....walt...tulsa,ok

The ECM can't overcome what the base timing is. On a TBI SBC, the base timiong should be set to 0 DBTDC. The ECM will take over from there, dictating the advance curve. Tjis can vary by more that 30 degrees depending on what the fuel/timing map commands.

Mike

[Porter]

Porter,
  Do I understand that vacumn tuning of ones timing of a SBC can be done on later models such as my 1994TBI?(Roadtrek van) I was under the impression that the computer would control timing after base was set.(max advance,etc.) Another mechanic commented that base timing would only allow one to stray a few degrees in each direction before the computer would"take over",thus limiting advance.....walt...tulsa,ok

Walt,

As we discussed off the forum the title of the thread is "Tuning" not "Timing" with a vacuum gauge.

Seems to be a misconception here.  Mea culpa ? The vacuum gauge is only a fine tuning device and for the most part with a healthy engine the factory timing mark is there for a reason.  For an older engine with some wear or a harmonic balancer that has "slipped" the factory timing mark might not work for you. Two degrees advanced factory spec. will give you a little better performance providing you have sufficient octane in your gas. Advance too far and you will get the dreaded ping. I had a 90 & 93 ETC : permium gas only. Twice for a pit stop they only had regular gas - ping ! Ever get a tank of bad gas and your engine pings ?

Vacuum gauge is great for telling you what is wrong with your engine, if you have low compression for whatever reason you will have a low vacuum reading. If the needle is bouncing around it can give you a good idea what the problem is. You lose 1 Hg for every 1,000 feet above sea level. 18-21 is standard fare for a strong engine at sea level.

I never stated here I used the vacuum gauge to set the timing on my SBC with TBI, only used it to see how "healthy" my engine was with 165K mileage. Strongest reading I've ever seen with a solid needle 23 Hg.

Since I found the timing mark visually "unobtain-em" I said screw that. After the distributor rebuild it easily dropped right back into my reference mark. Test drive was sluggish, an 1/8" turn CCW advanced the timing ( with the engine not running - can't do that with engine running or the computer will just compensate ) and it ran strong -like when I bought it new so leave well enough alone.

Nothing new here, many times in the past a few people came here with old engine problems. I asked what is your intake vacuum reading ?  "16 Hg with a fluctuation" ...... at sea level that's weak compression and one bad valve seal minimum.

Just another tool to use after you run the first test - engine compression. Just wanted to set the record straight.

I hope this is all clear since I started this thread, use your vacuum gauge for a little fine carb air / fuel mixture screw adjustment on a correct healthy engine or to diagnose what is wrong with your engine.

Capish ?

Porter

[Guidematic]

Well said, Porter.

Mike

[Porter]

Well said, Porter.

Mike

Well, just wanted to set the record straight.

There is no magic timing adjustment / setting to compensate for other engine problems: compression , ignition, fuel or otherwise.

If I have an opinion here or advice to share it came from my past experience and hopefully a correct opinion.

Just a shadetree mechanic that's been bending wrenches for 35 years but I can read a shop manual and do auto repair.

My Sears "Penske" vacuum gauge was one of my first diagnostic devices I purchased , has served me well. Must have been one of my mentors recommended that instrument, has far outlived my old tach/dwell and timing light.

Now you can buy a digital MM with a tach / dwell too ? I think you still need a timing light thought.

Where's John Washburn when you need him ?  He has those old SUN diagnostic machines on wheels with oscilloscopes and all kinds of  stuff ........ like an engine EKG reading ........ but he's no saner than me, just has fancier diagnostic equipment, maybe he wears a white coat while using this stuff, match the guys when they come to take him away .....

Porter 

[Ohio57-62Sedan]

Awesome thred Porter...

[Porter]

I have a vacuum gauge / fuel pressure rig , bought that back in 1973 or so.

My free 67 CDV shown here with 45K 429 engine.

Penske was into everything back in those day besides beating everyone at the Indy 500. 

Small wonder he is so successful.

Vacuum gauge tells all. A Ph.D in engine diagnosis.

Porter 

[John Washburn CLC 1067 Sadly deceased.]

Folks,

You got me.

Porter, although I can't express my feelings, here is what is wrong with his response! I wear a blue shop coat with Cadillac Emblems.

Also, the vacuum method is for those old engines without any type of fuel injection, including throttle bodies, and those damn carburetors made by GM that had no adjustments..

If the car is old, smoking, on its last legs, this is still the best way to get it to run to the wrecker or auto auction. One can get, using this metod, the best running car with the gas, altitude, and carb adjustment.

My story -- no white coat. Picture attached of recent tune up clinic for Cad Club at the Washburn Reservation. Guess who the fat boy with the red wine is?

[Ohio57-62Sedan]

Porter I may have to send you my Vac guage to complet you collection.... 

And John your a bunch of old dudes there sitting around the shop drinking wine.. we only drink beer here... young punk's are not so refined...

[John Washburn CLC 1067 Sadly deceased.]

Ha,

Don't worry, as you get older beer will do you in. Soooooooooooo we change to wine, I as did.

Trust me on this, beer will turn on you.

Oh, once you turn 40 never trust a fart. (Opps is this ok?)

[John Washburn CLC 1067 Sadly deceased.]

Ha,

Never trust anyone.

It is never trust a fart, or if this edited it is f a r t.

[Ohio57-62Sedan]

will old farts fall in there too???

And just to keep it on topic 16-21 hg here in ohio...