Thread: Got Time? (Part 1)

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Ok, this seems like the place to answer my question. I unplugged my vaccum advance and set my total timing to 38 at 3000 rpm. This caused initial timing to be 16 degrees. I plugged the vacuum advance back in, on the driver's side port on a Edelbrock 750, and the timing went up to 39 degrees. I have a Mallory Unilite and I believe the vacuum can is adjustable. Does this mean I have 23 degrees of vacuum advance? What should I adjust it to? To check my curve, noting the timing every 500 rpm, should I do it with the vacuum hooked up? This is for a 383 stroker. Wish I knew all the exact specs but I don't. The vacuum at idle is 15.

Originally posted by White Sands
Ok, this seems like the place to answer my question. I unplugged my vaccum advance and set my total timing to 38 at 3000 rpm. This caused initial timing to be 16 degrees. I plugged the vacuum advance back in, on the driver's side port on a Edelbrock 750, and the timing went up to 39 degrees. I have a Mallory Unilite and I believe the vacuum can is adjustable. Does this mean I have 23 degrees of vacuum advance? What should I adjust it to? To check my curve, noting the timing every 500 rpm, should I do it with the vacuum hooked up? This is for a 383 stroker. Wish I knew all the exact specs but I don't. The vacuum at idle is 15.

Referencing the 39 degrees total, it sounds like you're shooting the timing at idle.

Just out of curiosity, how big of a cam are you running and what is your idle speed?

The total timing - assuming it's at idle - of 39 degrees shouldn't be a problem.
The idle fuel/air mixture is lean and it takes a while to burn.

I have very close to the same timing in my 462" Buick engine.
8 degrees initial and 32 degrees all-in on the mechanical.
Most of the Buick speed houses - as well as my own experience - indicate that 32 degrees total is about right.

Depending on the situation (cams, C/R etc.), Buicks will run very well with as little as 28 degrees total all-in.
(All timing figures BTDC.)
Timing at the 550 rpm idle speed with mild cam and manifold vacuum supplied to the vac can the total timing is about 36 degrees.
(When I ran a bigger cam and dual quads the idle speed was right at 600 rpm.)

Looking from here, 38 degrees seems a touch high for a street SBC even if it's a fairly big one.
I believe most Chevy guys like 34-36 degrees total.

Is the engine difficult to get rolling on the starter with 16 degrees initial?

Thank you for your reply. I am not sure about the cam size. It is a little radical but not too bad. It was a custom grind from Lunati to match the head (fully ported Dart). The builder is the one who told me to go 38 degrees total but I am going to experiment with a little less. It actually starts right up at 16 degrees initial. But I have a brand new battery with high cranking amps, good wires, and two ground straps, don't know if that helps but it seems to. My engine was built 5 years ago and has been sitting most of the time due to Military commitments, moves, etc. I recently got back into it and have no idea of all the exact specifications. I am tempted to take it apart and start over to know what everything is.

Also the idle is around 800 in park but hard to nail down due to the cam. It jumps around a little. Is it normal for an engine with a bigger cam to fluctuate the idle speed and have the timing mark at idle move a little. I remember when it was stock the timing mark looked like it wasn't moving. Now it isn't as steady.

hey c9x what do you think of mechanical advance for the street ?

Originally posted by carbuff
hey c9x what do you think of mechanical advance for the street ?

It looks like you're asking, "mechanical advance only."

The answer is, "not much."



I know . . . a lot of guys run their cars that way and claim no problems and I would guess for some of them that could be true.
Most times they find out what it's all about when they hit slow traffic on a 100 + degree F. day in the summer.

Surprising how many "no problem" guys pull over puking coolant on the streets.

Did you read the article?
I thought I made it pretty clear, but perhaps not.

Originally posted by White Sands
Also the idle is around 800 in park but hard to nail down due to the cam. It jumps around a little. Is it normal for an engine with a bigger cam to fluctuate the idle speed and have the timing mark at idle move a little. I remember when it was stock the timing mark looked like it wasn't moving. Now it isn't as steady.

Fluctuation with any non-computer engine is pretty normal.
Bigger the cam and carburetion, the higher the fluctuation levels are - in my experience.
Although the dual quad big cam setup in my Buick engine mentioned above was pretty steady at 600 rpm.
I'm guessing due to the 118 and 112 degree lobe separation in the two big cams I ran it it. Both cams were about the same lift and advertised duration. Duration being 284 & 292 degrees intake and exhaust.

The 118 degree cam idled a little smoother than the 112 degree cam did, but both idled at 600 rpm.
Racing cams as you probably know have 104-108 degree lobe separation angle. The wider separation figures of 110 and up calm down the cam at idle to a great degree.
As does running a particular cam in a large CID engine.
A cam profile that idles well in my 462" Buick may be a lumpy son of a gun in a 265" SBC.


Part of what you're seeing at idle as pertains to fluctuation could also be attributed to a slightly loose chain. One within ok parameters. They all stretch a little bit at first. Even the very high quality chains.


I wouldn't tear down a good running engine just to find out what's inside.

If you're curious about how big the cam is, set up a dial indicator to read lift on either the lifter side or the valve side.
Be sure to take into account the rocker arm ratio if necessary.

If that's not convenient, pulling a cam in most cars isn't too difficult.
Remove the cam, bolt a degree wheel on it, set it in V-blocks and set up a dial indicator.
That'll tell you exactly what you want to know.
Again, don't forget to take rocker arm ratio into account.

Your ID'ing the cam may be made easy once it's removed by finding the cam grinders marks on the back end of the cam.
You can translate from there.

If the marks are unreadable - somewhat common - you'll have to revert to the V-block/degree wheel mentioned above.

Originally posted by C9x
It looks like you're asking, "mechanical advance only."

The answer is, "not much."



I know . . . a lot of guys run their cars that way and claim no problems and I would guess for some of them that could be true.
Most times they find out what it's all about when they hit slow traffic on a 100 + degree F. day in the summer.

Surprising how many "no problem" guys pull over puking coolant on the streets.

Did you read the article?
I thought I made it pretty clear, but perhaps not.

Well i can atest to a 4x4 full size jimmy with a 455 olds a tad over 10:1 comp mild cam that runs mechanical advance which will blow the typical LS1 camero off the which we have done several times and go 4 wheelin at lake mcConaughy in 100 degree heat and the temp in the jimmy will hold at 180, im not trying to start a arguement with you just stating hard proven facts that if everything is tuned proper you wont have no problemos

Originally posted by carbuff
Well i can atest to a 4x4 full size jimmy with a 455 olds a tad over 10:1 comp mild cam that runs mechanical advance which will blow the typical LS1 camero off the which we have done several times and go 4 wheelin at lake mcConaughy in 100 degree heat and the temp in the jimmy will hold at 180, im not trying to start a arguement with you just stating hard proven facts that if everything is tuned proper you wont have no problemos

Like I said, it works for a few, but my experience has been that the great majority of cars running mechanical advance only or vacuum advance sourced to ported vacuum overheat in slow going on a hot day.

As pertains to mechanical/vacuum advance it's not an either/or situation (except for those who chose to run mechanical only).

It's a combination of both.

And I'm still not clear on whether you're running mechanical advance only or you are running mechanical in conjunction with vacuum advance.

Originally posted by C9x
Like I said, it works for a few, but my experience has been that the great majority of cars running mechanical advance only or vacuum advance sourced to ported vacuum overheat in slow going on a hot day.

As pertains to mechanical/vacuum advance it's not an either/or situation (except for those who chose to run mechanical only).

It's a combination of both.

And I'm still not clear on whether you're running mechanical advance only or you are running mechanical in conjunction with vacuum advance.

Sorry if i confused i know you asked earlier also and i forgot to tell you were running strictly mechanical advance, with no vaccumm at all, i was like you at first and hesitate untill i seen first hand cruising 5mph in deep sand at the lake and just sitting idling on 100 + days so my ol 64 is going to be the same way just my 2 cents though maybe someone else has a better idea i dont know but this seems to work for my brother and a buddy of mine.

Originally posted by carbuff
Sorry if i confused i know you asked earlier also and i forgot to tell you were running strictly mechanical advance, with no vaccumm at all, i was like you at first and hesitate untill i seen first hand cruising 5mph in deep sand at the lake and just sitting idling on 100 + days so my ol 64 is going to be the same way just my 2 cents though maybe someone else has a better idea i dont know but this seems to work for my brother and a buddy of mine.

If it works, it works.
Hard to argue with success.

I'm wondering as well if you're running a lot of initial advance and have the mechanical curve cut back?

Wondering too what the engine idles at rpm wise?

Originally posted by C9x
If it works, it works.
Hard to argue with success.

I'm wondering as well if you're running a lot of initial advance and have the mechanical curve cut back?

Wondering too what the engine idles at rpm wise?

Intial timing is 8 degrees before tdc and total timimg is 35, now this is for points, if you have hei go with 12 intial and 35 total and as for idling at 850 - 900 rpm this is what we have found that works best with this motor so stay tuned and ill let you know how it goes on my 514 im building for my 64 galaxie.

Might have missed it - but I'm not sure what ported vacuum is. I have a Ford 302 and it has a single fitting that sits in the back of the engine and that's where the brake booster is tied in, and also the modulator on the C4 tranny.

I've got a vacuum fitting on the passenger side of the Holley, and I'm not really sure what's supposed to be hooked there. When I bought the care is was sitting there, nothging connected to it. Anyone know if this is a source of vacuum, or if I should run a vacuum source to it.

Thanks

Dave

so if my initial timing is 16-20 btdc i should put my ballancer there and drop the dist. in on #1?