Timing w/o vacuum advance - Team Camaro Tech
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post #1 of 13 (permalink) Old Jul 26th, 01, 09:47 PM Thread Starter
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Is this a good idea or not? In order to do this should I rely solely on total advance? Reason for me asking is that on take off I need to get the RPM's up to 1500-2000. Also when cruzin, if the RPM's fall below 1800-2000 it fells almost dead. When I give er' more gas and get the R's up to or past 2000 I can really fell it kick. I had talked to one person who had said that this may not be abnormal for my cam. The cam is Crane .480 lift w/228* duration and 284* advertised duration. The motor is a very recently rebuilt 355 with 10:1 piston for 64cc heads. The heads I have are 305 that have been opened to 1.94 intake. with rough cc being 60-65cc. The other thing not helping is the 2.7 something rear end.
Thanks in advance,
Tim
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post #2 of 13 (permalink) Old Jul 27th, 01, 06:40 AM
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That cam might be a little lumpy at idle, but shouldn't give you THAT much grief. Just about any engine in a higher gear is going to feel pretty dead below 1800 rpm - shouldn't be bad in 1st or 2nd, but beyond that it's going to feel like a rock 'til you get it up toward 2000-2500 (at least in comparison with the acceleration you get in the powerband).

Needing to rev to 1500-2000 to get it moving is troublesome though. That cam shouldn't be causing you that kind of trouble, I wouldn't think.

You might want to re-tune your ignition, but I wouldn't completely get rid of the vacuum advance - tuned right it will help light acceleration performance and economy a lot.

I thought most of the 305 heads were smaller than that (56cc, or something??). You sure you're only running 10:1 compression?? Any indication of detonation on the plug porcelan? (dark flecks) Pistons' "compression ratio" is frequently listed for stock bore, and quench height (as much as .075-.085 depending on the pistons compression height) - any change can make a big difference (mine were listed as 9.35:1 w/ 64cc heads, but with .030 over-bore, .035-.040 quench height, and 63cc chambers, I'm closer to 10.3-10.4:1). High compression and too much vacuum advance could be giving you problems, but you should see that more generally than just off-idle, I'd think.

Maybe your idle circuits just tuned badly??? Smell gas?? Running hot at idle??
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post #3 of 13 (permalink) Old Jul 27th, 01, 08:19 AM Thread Starter
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There are no signs of detonation. I have advance (with vacuum) set at 10* initial. I have tried timing by total advance but had no luck, car do not have near the power, Maybe i did not get it up to 3500 while timing.?! I am actually a little suspicious of the carb. It is a Holley 600 vacuum secondaries. Which sat for a couple of years with on of the float bowl gaskets being loose. A friend is selling an Edlebrock performer 700-something. But it has sat for a couple of years. Is there anything to watch for since it has sat for while ( rotton gaskets.etc.) The heads i have were originally 58-60 cc. Then since they had to open them up to 1.94, they shrouded the chamber. so the are a little bigger.
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post #4 of 13 (permalink) Old Jul 27th, 01, 08:20 AM Thread Starter
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I forgot to mention that it does not matter what gear I am in. If the RPM's fall below 1800-2000 I can fell it. But if there is no load on the motor you could not tell. I double checked the plugs to be sure and no dark spots. For the first couple hundred miles, when I looked at the plugs the used to be ghost white. Just this weekend I drove it for about 100 miles and the plugs look as if it is running rich. Also on every porcelin there is a very dark spot like a burn. What does this mean.

[This message has been edited by 71 maro (edited 07-27-2001).]
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post #5 of 13 (permalink) Old Jul 27th, 01, 10:22 AM
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With such a big cam it may help to switch to manifold vacume. This would give you full advance at idle, and your take off from 0 to 2500 rpm would smooth out. Do a search about it on this site.

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post #6 of 13 (permalink) Old Jul 27th, 01, 10:33 AM
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If you've only got 10 degrees of advance with the vacuum hooked up, you probably don't have nearly enough advance (depending on whether it's hooked up to the timed port, or the manifold vacuum port).

Your initial advance should set without the vacuum hooked up. Pull the line off the advance canister and plug the end of the line. Then you should set it so you've got 32 to 38 degrees total advance (depending on where it starts to detonate at), hopefully by about 3000 rpm. So you set that first, and whatever you get for initial, you get. Probably in the 12 to 18 degree range, depending on your distributor.

You probably want the vacuum advance adding 8-12 degrees advance at idle and part throttle, depending on what your engine will handle without detonation. Get a Crane adjustable advance kit, if you haven't already and follow their directions.

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post #7 of 13 (permalink) Old Jul 27th, 01, 10:44 AM
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DjD help him out!

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post #8 of 13 (permalink) Old Jul 27th, 01, 10:54 AM Thread Starter
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Sorry about the way I had posted my timing w/vacuum. It is set at 10* initial w/out vacuum, then I plug in the vacuum. I just did not want to confuse because my subject questions without vacuum.
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post #9 of 13 (permalink) Old Jul 27th, 01, 01:36 PM
 
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Ok it may not be the best way but it works. I am running a 355 with a 292 cam. I had to get a adjust vac on my HEI. I set timing around 10 degrees then plug Manifold vac in I rev up and watch timing then adjust the vac can to get timing at 36 total. I run about 21 at idle with vac hooked up. You have to keep inital low or you will not be able to start it when Hot.
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post #10 of 13 (permalink) Old Jul 27th, 01, 03:07 PM
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71 maro,

Is that cam an Energizer 284 H12? If so it's power range is 3000 - 6000 rpm. That would explain your car falling over itself below 2000 rpm. Short of a cam swap I would suggest a 1" 4hole carb spacer, small tube headers and 3.73:1 gears to allow you to get into the rpm range sooner. What intake are you running? Dual plane of some kind I hope!

As for your carb, keep the holley and get a rebuild kit and a can of carb cleaner. It's not hard to rebuild but is a bit scary the first time. When you have that done spend some time tuning it (floats, accelerator pump, idle mixture etc). It will make a workd of a difference. Like everything else, don't be afraid to do it a few times until you're sure it's right.

For the timing, I think Huck has the right idea. Unless you plan to recurve it, find out where the total is (rpm where the timing doesn't go any higher). You'll need a dial back timing light or a properly marked dampner to do this. Remember to disconnect and plug the vacuum source to the vacuum advance for this. I think your engine should run with 38deg btdc total but like huck's advice it could be anywhere from 32-38deg's so that means trial and error. Once you set the total take a reading at idle to see what your initial is. Subtract the inital from the total and that will tell you how much mechanical advance is built into your dist. This will vary depending on what distributor you have.

To find out how much vacuum advance your canister is pulling hook the vacuum advance to a manifold source (one that pulls at idle) and take a reading at idle again. Subtract the first initial number from this reading and it will tell you how much the canister pulls.

Once you know the numbers ( total and what rpm it reached, initial and canister vacuum) you can decide if it's time to recurve.

I think ideal would be something like 20 degrees of mechanical timing with the inital set around 12-18 deg's giving you from 32-38 total. If you could get the total all in by 3000 that would be good. The ported vrs manifold you'll have to try and decide for yourself. Get an adjustable canister or limit your canister to 8-10 deg's of advance and then try both sources.

With 8 deg's of vacuum advance and given the numbers 12 initial and 32 total, using a manifold source you would have 12degs to start the engine. At idle you would see 20 degs (12initial + 8vac = 20degs) and at WOT it would go up to 32degs total.

Using a ported source you would see 12 initial and 32 total at WOT but under part throttle above 3000 (number where mechanical is all in at) you would see 40 degs btdc.

You can see both give you the same WOT but treet part throttle completely different.
Some food for thought!!!


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post #11 of 13 (permalink) Old Jul 27th, 01, 11:01 PM Thread Starter
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<BLOCKQUOTE>quote:</font><HR>Originally posted by DjD:
Unless you plan to recurve it, find out where the total is (rpm where the timing doesn't go any higher). You'll need a dial back timing light or a properly marked dampner to do this. Remember to disconnect and plug the vacuum source to the vacuum advance for this.<HR></BLOCKQUOTE>

Thanks so much for the help. I have one question about the quote though. Since I am doing total advance why would I unplug the vacuum. If I go 36* w/no vacuum then hook up the vacuum (8-10*?), wont this put me at about 44-46* total.

The cam is the Energizer. As for the manifold, it is an Edlebrock Performer (not RPM). As for the carb, I'll probably buy a rebuild kit this weekend. Also before I tear into it I will hook up a vacuum gauge to find which power valve I need. Know about the 3.73 gears. I have heard several scary stories about doing the work just to find out that I will be over geared. Is this something I need to worry about?
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post #12 of 13 (permalink) Old Jul 28th, 01, 05:56 AM
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The timing is set without vacuum as you are setting the mechanical total. Yes the vacuum advance will boost this at part throttle but at WOT vacuum deminishes and it's the mechanical that comes to play.

Depending on tire size and if you have an overdrive or not you might be making some sacrafice at hwy speeds. With 26" tires, 3.73 gears and a 1:1 final trans gear you'll see 3000rpm @ 62mph and 3500rpm @ 73mph. With 28" tires you can lower the rpm some to 3000rpm @ 67mph and 3500rpm @ 78mph. So taller tires will help some as will how fast you drive. If you can handle 65 or 70 mph on that 1 hr drive to the track or work it shouldn't be a big deal.

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post #13 of 13 (permalink) Old Jul 29th, 01, 05:22 PM Thread Starter
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I guess I brain farted that one. Well I am mostly done with the carb rebuild but before I assembly front bowl and metering I have a few questions. Before the rebuild there was a bog in between shifting on heavy acceleration(maybe do to rear end). I am not sure if I should up the shooter or do rear end first. My other problem is clearence with the 1" carb spacer. I have already bought a cheapo chrome drop down air cleaner. But if my math is correct, after the air cleaner is installed. I am only going to have about 1/4" clearance between the air horn and air-cleaner lid. Is this going to be OK. How much differance is there from a 1" to 1/2"?(other than a half inch). My next question is the carb cam. Currently I have the red one set at #3 on the cam and #2 on the linkage. My secondary spring is the heavy black one.
Thanks again!
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