Dkarr

I don't think that's my issue because I am running the carb gasket that came with my holley carb. I assume you are talking about exhaust gas flowing over from one bank of the dual plane intake to the other due to intake/exhaust pulse waves during the cycle? But wouldn't running a carb riser or a single plane intake cause the same issue? And wouldn't the larger overlap of a cam like mine cause some of that anyway regardless of a single/dual plane intake or carb gasket? The concept you are proposing seems to make sense just wondering if I am understanding you 100%.

Matt Gruber

Good Q.
Exhaust doesn't necessarily flow over the divider, it would turn black if it did.
The vacuum flows over, and pulls a little exhaust in by the intake valve. Then on the next intake stroke, it gets sucked in. How much comes in, depends on the amount of overlap. Is your dual plane fully divided? A single plane would offer no barrier! An open spacer/riser is almost the same bad effect under 3000 rpm. Is the carb gasket a 4 holer, or an open center?

Dkarr

Yes, my dual plane is fully divided. The carb gasket is not a 4 holer, it's got a divider in the middle (to separate the planes) but nothing separating the primary and secondary throttle blades on each bank.

Matt Gruber

The divider in the middle gasket works unless it is too thin and gets ripped apart by the violent pulses.(which it often does) the center often will disappear(eaten) causing no damage.
The thicker the better.
The Mr. Gasket #97 heat isolator kit has 2 aluminum 4 holers, even using just 1 with the supplied gasket on top would be much much stronger.
Might want to check and see if that gasket is still all there!(since 27* sounds very high)

larrywalk

Don't over think this - your cam's overlap at .050 is only 13 degrees which is relatively small for a performance cam. Even the stock 30-30 cam had twice this. Something is amiss if you need radically more timing than others with similar combinations.

I proposed a very simple test, requiring only removal of one valve cover, in post #15 to discover if you have a cam timing error - in fact, you don't have to even measure it - if you see at TDC-overlap that the exhaust valve is more open than the intake valve (it will be obvious), your cam timing is very retarded and that would cause your discrepancy.

BTW, this check will also include the possibility of a damper slip if it fails so you would need to check TDC afterwards to determine which is the cause.

Matt Gruber

larry has a good idea! could also get clues from cranking compression, and idle vacuum.

68thumper

23 at idle. I have tried it down to 15 and up to 28.

Dkarr

I do appreciate the advice Larry. I'll get this done tomorrow first thing when I'm back in town. I did degree the cam when it went in and it read about 1 degree retarded, given it was so close I left it as it was. Wouldn't this rule out the issue of cam timing? Again, this was my first build so I could have screwed that up... but it seemed pretty straight ahead. The motor runs great with the current timing settings. No hesitation or issues with low end power.

As far as the damper, it could be off somewhat. It's brand new, Powerbond model PB1046SS, so I doubt it spun the marks. I called their tech line and did verify that it does read with a 2 o'clock timing marker and not a 12 o'clock. I've got a piston stop so I can roll the motor and check TDC.

Dkarr

One other thing in regard to the cam... I am running 1.6 rockers. I know this will impact the valve events so just wanted to clarify.

larrywalk

Rocker ratio 1.6 vs 1.5 will add about 2 degrees of duration - of negligible consequence.

WCC Josh

Changing rocker arm ratios only effect lift. Duration is changed by cam lobe profiles.

Dkarr

Okay, so I dug back through old photos of my engine build. I knew I took some shots of the degree process and sure enough, I've got a few that show the intake timing at .015 spot on 60* ABDC. This should rule out cam issues, correct? Here are the specs on the cam again http://www.compcams.com/Company/CC/c...?csid=225&sb=2

ajrothm

DKarr,

If the engine runs well at 20* initial, why worry about it? You guys are WAYYYYY over thinking this stuff...

Give it the initial it wants, modify your mechanical to match the initial and yield the desired total... (Aka 36* at 3000), add your vacuum advance but limit its travel to say 8-10* , thus giving you a cruise advance of 44-46*....or less if it pings. Call it good!

I just did this on Monday on my dads 66' L79. I had the distributor recurved on a Sun machine. On his, it ran best with 15* initial+mechanical at 750 rpms without the vac advance. That combo got me 32-33*@3100 rpms and 36* @4100. I added the vacuum advance but made a limiter plate to restrict it to only 5* of advance.

Now at idle I have 20* (initial+mechanical+vac advance), at 3000 rpms it is right at 39-40* (initial+mechanical+vac advance)... WOT timing is 36* from about 3800 rpms and up.

Car runs perfect, I drove it around all day on Monday...
.030" 327, 9.5-1, ported 461 double humps, Crane 224/230 .475.495 on a 110 lsa. Car drives like an absolute dream...

I'll post up some pics of the limiter plate in a few mins.

Give it what it wants and don't worry about the theories... If it runs great, run it!

ajrothm

I made this:



And I put it here:



That limits the amount of advance I get from the vac advance. Without it I was getting 16* of advance and the car was missing/surging at light cruise..1600-2500 rpms. Now I only get 5* of vac advance and the car runs perfect. In reality, I could have just plugged it and not ran any vac advance at all and the car ran great at 32-33* at light cruise but... I figured another 5* may help the fuel mileage a bit so...

Works good for us. This is a brand new motor so I am still dialing it in but it's pretty much perfect now.

WCC Josh

also make sure the vacuum advance is hooked up to manifold vacuum and not ported vacuum

garygnu

you can buy adjustable vacuum advance canisters and install them in 1 hr.at WOT you loose vacuum and only have mechanical advance to add to the initial timing.stiffer springs can prevent too much timing at partial throttle.get a spring kit to customize you mech advance curve.

Matt Gruber

I often start out with "too much" advance on a new build. Mainly i want to break in the cam so "too much" doesn't matter,it helps. Any cold engine likes way more advance than normal. i've locked the vac advance at full, +15, on a cold day, and it pulls out instantly at 38F, not even 2 seconds to warm up! and no choke! Got that idea from the chrysler lean burn from the 70's. Timing is substituted for fuel! It works, lean burn.
Back on topic, on a hot summer day is when i find the true year round maximium base timing. and it is always way less than a winter max. I doubt there is anything seriously wrong with the OP's car.
after 50 miles i always re-set the valves and a few usually need it. then they stay good for a LONG time.
My 72 has 9.7:1 and 12* base timing and uses 87 octane. Base would be MUCH higher on Hi-test. Race gas would be even higher as higher octane burns slower. I'd guess some cars are using hi-test that really don't need it.

Dkarr

Okay. Got this worked out. Lars was spot on. I trashed the stock vacuum can and went with the "B28" vacumm can from dura last, part number DV1810. It provides 16 degree advance coming in around 8" of vacuum. With this I was able to get my initial down to 18 and still have the car crank nice and easy and run well at partial throttle cruising. With the new can I'm around 34* at idle with vacuum connected on FULL manifold vacuum, not ported.

I read 33-34 degrees at 3000 and I plan on playing with my new distributor advance spring kit to fine tune this even more. Thanks to all those that chimed in.

Crepitus

Great thread! keep us in the loop.

toddalin

You're wrong.

Duration is always measured at some given lift (e.g., 0.05") and that lift will always be achieved earlier and held later using a higher ratio rocker assuming nothing else changes. The change in duration may be small, but it is there.