Hot Rod Forum banner
1 - 13 of 13 Posts

·
Not Considered a Senior Member
Joined
·
10,722 Posts
Discussion Starter · #1 ·
Since vortec heads are more efficient than the older designs, I need a timing base and curve for my engine, along with which vacuum advance to use.
My combo is a Vortec 350, standard bore, decked .010, Quench distance set at .043, KB 12cc D-cup pistons, reworked vortec heads, stock PM rods, Comp hydraulic roller cam, 206 duration @ .050 .500 lift, Performer vortec intake, SM Quadrajet, HEI, flame thrower coil and module, blue max spiral core plug wires, Hooker super comp headers coated by jet hot.
I haven`t ran it much since I`ve built it, I just need a ball park figure on the timing curve and I can take it from there, should the curve come in fast or slower? I set the base timing at 4 degrees BTDC, then connect the vac advance it goes to around 28 which seems to be too much, I was thinking on getting a adjustable unit. Any help is appreciated.
 

·
Hates: Liver. Loves: Diesel
Joined
·
6,307 Posts
Definitely an adjustable vac canister. With a cam that mild I might consider a ported vacuum source. That response will generate a lot of argument around here, but I prefer it. With a manifold source, you are losing advance off idle and it puts a lot of advance at idle. It works well, but its (in my opinion) better suited to lots of cam where idle quality is of concern. Ported vacuum also adds more advance at light throttle cruise which can help MPG more than a manifold source.

Total timing seems to work well at 30* with vortecs give or take a couple. Initial will be fine over a very wide range, but as long as you're not getting starter kickback or part throttle detonation, the more the better. More timing lets you close the throttle blades more which makes throttle response MUCH better and idle tuning much easier. I have a BBC set for 36* at idle and its fine, but I have more cam and less compression than you.

Your combo sounds great! Very similar to a build I'm doing now.
 

·
Chasing dreams with a ball bat
Joined
·
545 Posts
I would get the mechanical timing sorted out first before adding the vacuum - the vacuum will throw curve balls and make it more difficult. I would try to set the "all in" RPM at about 2800-3000 RPM - quicker and you might run into problems when you add teh vacuum advance.

Try this: Rev the motor up until the timing maxes. out and remember that RPM (a friend is handy for this) Set your max. mechanical timing to be 30-32 BTDC with a dial back timing light or timing tape and then let the motor idle down and read what your initial ends up being. Hopefully, you'll be in the neighborhood or 10-12 or so. I can't recommend an initial without knowing how much mechanical your dist. will add. Don't want to end up at 42 deg. because you put too much initial on it.

On the vacuum advance, Crane Cams makes an adjustable can - and you can change the vacuum rating and also the amount of timing added (with a little sheet metal cam type deal they include). I use them on my street vehicles, and it simplifies the tuning of that system IMO. I ordered mine through O-Reilly Autoparts - they had the same price as Summit, but no shipping charge.

I would think you would want somewhere around 14 -18 degrees of vacuum advance. You can tune that amount with the Crane can.
Hope this helps. Good luck.
 

·
Registered
Joined
·
1,875 Posts
That would be the upper end I'd want to see for base timing with vac advance hooked up. It puts it at the sweet spot though for cruise conditions on the hwy. Low to mid 40's for advance under cruise works very well for throttle response as well as economy on most all applications. Really would be better with 18 degrees at idle with vac advance and 45 at cruise but impossible to accomplish absent EFI or Digital programmable box. Done enough dyno testing and reading of other combinations that those heads don't need or want a lot of total timing under power conditions.
 

·
Registered
Joined
·
1,875 Posts
J, High HC has a number of causes but in your case I suspect it's in the carb tune. Either you are very rich at idle or very lean which can lead to a misfire. Some tuning on a gas analyzer should pinpoint the "issue".

With your cranking compression I can't imagine you'd have an issue with detonation. To confirm you can always have a friend ride along. I can almost assure you that if you run with vac advance and the tune is correct you'll have MUCH better throttle response and performance.
 

·
Not Considered a Senior Member
Joined
·
10,722 Posts
Discussion Starter · #8 ·
I`ve tried to run the vac advance to a ported source, when I did it ran like a dog, no throttle response whats so ever, and real low take off power. Since I was told to use manifold vacuum I did and the throttle response was there and it felt like it picked up 50 horses, I couldn`t believe the difference.
That was some years ago on a older 350, so I don`t know if it would act different on the vortec I have now.
I`m trying to work out this engines other issues as well, if I let it idle any lenth of time, I can rev it and it`ll smoke like no tomorrow, it`s blue oil smoke, before it was plauged with intake leaks, until I got the correct gaskets, I thought the problem was solved but it`s still there, I`ve checked for vacuum leaks, and can`t find any, the intake is sealed good, and it has 170 psi compression. It has little blow by so the rings seemed to have seated, the only other thing I can come up with is valve guides but they were replaced during the rebuild, Unless the machinist screwed me over which is possible, I plan to check the guides next. Anyone have any other ideas where the oil smoke could be coming from?
 

·
Premium Member
Joined
·
3,519 Posts
It would be interesting to see a two variable curve for three graphs:

X= engine RPM
Y= Ported vacuum inches

X= engine RPM
Y= Full vacuum inches

X= Vacuum inches
Y= Degrees of advance delivered by vacuum diaphragm

I don't think there's any "magic" to which source you use. I tend to think cars that run a bit better on full manifold vacuum are a little short on idle advance (or have compression that disallows use of it) and picks it up a bit for the part throttle transition to WOT... come to think of it, my car could benefit from that!

K :thumbup:
 

·
Glad To Be Here
Joined
·
2,240 Posts
DoubleVision said:
Anyone have any other ideas where the oil smoke could be coming from?
Here are two possible causes.

No baffle or poor baffle where the PCV valve plugs into the valve cover. This will allow oil to be sucked into the intake.

Another possibility (but less likely) is oil not draining back from heads quick enough. This will "flood" the heads and cause oil to be forced down the valve guides.
 
1 - 13 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top