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Discussion Starter #1
I have a 71 camaro with a rebuilt 350, only a few miles on it. It is .30 over with an original rebuilt z28 carb, edelbrock RPM intake and RPM heads, mallory unilite conversion in a stock dist. and using there coil. And a cam that is like the Z28. It runs rough in higher rpms, but idles ok. I have the timing set at 12 before with the vac advance plugged. What is total timing? If i understand, that is the timing with the advance. I have read many of the old posts, but still confused. One said total timing 34-36 degrees at about 2,800 rpm's and set balancer mark at zero. I also have a restrictive exaust, that i am going to fix. Can a too restrictive exhaust cause problems with a high performance motor? Thanks,Roger
 

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Total time= intial + mechincal.
At wide open throttle there is no vacuum so vacuum advance does not work therefore your total time will be your intial + how much your dist. has built into it which is controled by the weights which are controled by springs to regulate the rate(RPM) at which timing advances. So if your intial is 12*,to get 36* total timing your dist. will have to have 24* built in. Vacuum advance only comes in when you are at cruise if hooked up. Most stock vacuum cans have about 20* in them. With the above setup at 2800 rpm cruise you would have 12*+24*+20*=56*. When you step down on the throttle the vacuum *'s drop out making your total time 36*. There is more to it then this but this will get you in the ballpark. Hope I didn't confuse you more.
 

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Discussion Starter #3
Thanks, was working on it last night. blew out a power valve, so have to order some parts now. I see jegs has a plug to eliminate the power valve. Has anyone done this, good idea for a street car? Thanks, Roger
 

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Not a good idea for any car that ever operates at part throttle or more importantly, varying loads. You would need to re-jet the carb to make up for the fuel lost when you plugged the power valve and it would run rich all the time or lean under part throttle acceleration or changes in road incline/load. Not something I would do but maybe someone else has more experience with that. Playing around with the accelerator pump cams/settings might work but why re-engineer the carb design?

-Mark.
 

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I always thought that an engine shouldn't ever have over 36* of advance timing. I use an HEI with a curve kit and light springs. I have about 14-16* initial timing, with a total of 36*. I don't have the vaccuum advance hooked up, and it seems to run fine. Why is a vaccuum advance necessary ?
 

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If all you do is wide open throttle, you do not need it. If you like to have better gas mileage and drivabillity at less than wide open throttle, it helps.

-Mark.
 

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Thanks for the reply. I don't see how you can have +50* timing advance without having detonation. I had my vaccuum advance hooked up one time with a different engine, and it would rattle (spark knock) everytime I got on it. That's why I quit using it all together. Should I be able to hook up the vaccuum advance without making any other changes to the timing ? Thanks, Ron,
 

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Yes you should be able to do that if it is set up right. You may need a vacuum advance limiting cam plate or a whole new adjustable vacuum can. Easy mod that will pay for itself within a couple of tanks of gas. The 36 degree figure is a ballpark for initial plus all the centrifugal (at WOT). 16 degrees on top of that for the vacuum can at part throttle would not be unusual. The factory cans have different advance ranges but the most advance in a factory can is about 20 degrees. You can read the amount of advance available on top of the can where it screws on to the distributor.

-Mark.
 

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Ron GM makes a vacuum can with 10* in it but I forgot the part #. Dealer should be able to look it up.(also AC Delco on Magnolia ave)What kind of motor are you running?
 

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Discussion Starter #11
I found there was a problem with the uni-lite conversion in my stock dist. I went and bought a MSD pro billet with a 6A ignition box. All the plugs were fouled and sure it was not running rich. Once i get the carb fixed,i hope these changes will give me the power this engine has. Thanks for your advice, a great group of people with experience and knowledge.
 

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Thanks for the information guys. jhow66, I've got '96 model Vortec 350 out of a truck that is all stock except for Performer RPM intake, Carter 750 carb, headers, and HEI with Accel coil.

jhow66 - you may remember me; We met at a car show at the Jacob's Bldg a few years back. That's a super-nice '56 you've got ! What are we going to have to do to get Jerry motivated to start working on his ? Thanks, Ron.
 

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* Deleted - wrong info *
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JohnZ
CRG
'69 Z28 Fathom Green

[This message has been edited by JohnZ (edited 04-27-2002).]
 

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I was just reading the Chevy Smallblock Interchange Manual by David Lewis, Motorbooks International Publishers, 1989.

Page 107 has a reference to vacuum advance cans and it says "the number on the can (10, 12, 15, 18, 20 or 24) represents how many crankshaft degrees the cup will advance the time."

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'79 Z28, 4spd, 3.73 10 bolt, 350 4-bolt .030 over with stud kit, 10:1 Compression, Keith Black Flat Tops, Comp Cams Magnum 294 Solid Cam .525 lift, 63cc Heads, Stainless valves, hardened exhaust seats, guideplates, Summit Needle Bearing Roller Rockers 1.5 ratio, Offy Single Plane Intake, Holley 650 Vac. Secondaries, Stock HEI (for now), Headers 1 7/8" primaries, 3" Collectors into 2 1/2" exhaust with "H" pipe and Turbo Mufflers.
 

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Ron--probley steal his keys to his Honduely 2wheeler and his off road 4wheeler.
 

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Tjgerow -

You're absolutely correct - they're marked in crankshaft degrees; I was thinking about the spec chart in the Delco distibutor manual - it shows it in distributor degrees, and I always have to double it. Sorry


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JohnZ
CRG
'69 Z28 Fathom Green
 

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I hooked up my vaccum advance and I could tell a noticable improvement in acceleration. I found that it is 20* unit. Will I be OK to use this having 15* initial + 21* mechanical ? Are there times when the engine will be operating at 56* advance, or do the vaccuum advance and mechanical advance sort of "trade off" as your engine speed increases. Even though I didn't hear any detonation, I'm worried that this much advance will cause damage to my engine. Thanks, Ron.
 

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Ron the only time you will see a full 56* is when you are cruising under light throttle and your RPM's are above where your mechanical advance is all in. When you step on it the vacuum goes away which drops out your vacuum advance degrees(20*). If it is not pinging you should be good to go. Did you hook it up to ported or manifold vacuum?
 

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Hello Howard. I know that there are two types of vacuum, but I'm not sure which is termed "port" and which is "manifold"; "manifold" applies to vacuum that is there when the engine is idling, correct ? If so, I hooked the vacuum advance there just to see how much it would give it. I have a dial back timing light, and it showed that the engine picked up about 20* of timing (maybe 21* or 22* ?). I think the thing may even be a 24* unit - it has the numbers 694 24 on it.

Then I hooked it up to the other vacuum port which only has vacuum when you accelerate. This is the one I left it on when I drove. I checked both vacuum ports with a gauge to find out which one was which. Since my engine has the stock roller cam, it has a lot; it reads about 25 inches of vacuum on the gauge. If you think all of this sounds OK, I'll leave it hooked up.

I finally got my oil pressure "T" hooked up for my "OIL" light. I still haven't hooked the "TEMP" light up. The old style heads used a 3/4" thread sender, and the vortecs use a 5/8" one. I don't know if I can use a sender from the '96 to '98 trucks or not. I'm not sure if they had warning lights or gauges, and since they have computers, if their sender would even work for me.

Thanks for all of your help. I won't be able to check this site again until 6/1. I'd like to come out some time with Jerry and show you my car and check out that nice truck and '56 again ! Thanks, Ron.
 

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Ron sounds like you have a 24* can. If not pinging you will be alright. Ported sounds ok for your case. on the temp deal does your intake have threaded holes for water acess? If so they a normaly the larger size. ok to put temp sender here. You are welcome to come over any time.
Howard
 
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