Small Block 350 Timing

nat

 
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I want to tune up my sbc 350 as I am sure the timing is off. I know it is supposed to be 10 degreees BTDC at 700 rpm. Is that with the vacuum advance connected or with it disconnected and plugged? The timing mark is on the harmonic balancer not on the pulley on the front of it?

Sorry for the questions, but I am used to playing with F and 2F motors.
 

nat

 
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missipboy said:
i thought the vacuum advance didn't matter since it's only at 700rpms??
My understanding was there is more vacuum at idle, its the reason why my vacuum windshield wiper motor in my 1953 GMC works at idle, but not at speed.
 

HI^C

 
 
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LOL, depends on how the vac advanced is hooked up. but should be checked without
vacum, and plugged.

STOCK 6-8 btdc on most sbc.

electric wipers are nice, convert it, 54 GMC :)
 
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Be careful, 10 BTDC is a good rule of thumb.

There are some numbers under the pass side head, under the alternator if it is in the stock location, or on the block on the drivers side just over the oil filter. Punch those numbers into google or yahoo and that should tell you what it came out of and what kind of cam is installed.

I went to the library and copied the pages out of the '67-'79 chevy pickup and blazer manual for tune up specs. Mine is set to 0 BTDC BTW '71 - '72 350. Runs better than at 10 degrees and I get an extra 1-2 MPG on average.
Good luck.
 

miker

 
 
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fussy_cruiser said:
There are some numbers under the pass side head, under the alternator if it is in the stock location, or on the block on the drivers side just over the oil filter. Punch those numbers into google or yahoo and that should tell you what it came out of and what kind of cam is installed.
Not all SBC alternators are mounted on the passenger side. GM had a wide range of alternator mounting locations. There are no numbers stamped on a SBC block that tell you for sure what cam is currently installed. Chances are, the SBC in your cruiser has been rebuilt at least once or had the cam swapped out at one time or another. Unless you installed the cam yourself, you just don't know. The SBC casting numbers will give you a general idea of the block year and type of car or truck GM used it in but they are generally not exact.

How much initial timing you have set is important but it is equally important to check the TOTAL advance timing -> initial+mechanical+vacuum =TOTAL advance

TOTAL advance should not exceed 36 degrees. If it does, make sure the engine is not pinging under heavy loads otherwise you might find yourself having to rebuild the engine. Trust me, I know what pre-detonation can do to pistons, rings, and rod bearings. Is not pretty
 

dgangle

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I second Mike. It's the total advance you should be concerned with. Leave everything hooked up and measure/adjust. I have always set at 38* total advance @2500 RPM. You can experiement with what works best for you. Less advance will run cooler but I was told never go below 32* or the performance falls off pretty bad.
 

miker

 
 
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It's always a good idea to check the total advance even if you know what you purchased:

I bought a "blueprinted" Summit HEI vacuum advance distributor for my SBC. It came with a spec sheet indicating that it had 11* mechanical advance @ 2200rpm and 12* vacuum advance at 2500rpm. So, I set the initial timing to 12* and expected to measure a total of 35*. I was surprised to find that I had over 50* of total advance! Even with 0* initial timing I was still borderline too much advance. It turned out that the "blueprinted" distributor was not at all as the spec sheet indicated and it really had nearly 30* of vacuum advance at 2500rpm
 

dgangle

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Wow! How did it run and what became of this? I am sure Summit exchanged it, and if so was the replacement correct? Assembled/tested incorrectly?
 

miker

 
 
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I never bothered to contact Summit on this one. I discovered that the vacuum module was not adjustable so I just purchsed an adjustable vacuum module and adjusted it to what I needed.

I'm not bashing Summit. I love Summit! I've bought a lot of stuff from them and have even returned items that did not work correctly. The've always glady and promptly taken items back and quickly refunded money or replaced a defective item. At the time I found this, I didn't want to wait for a replacement so I just spent a little money and bought the part locally and got it working correctly myself. From now on I will always make sure if I buy a vacuum advance distributor that it has an adjustable module.

The engine seemed to run ok with 50* advance but I never had it under a load with that much advance. I did distroy another SBC which had too much advance. It ran good up to the point where it blew a hole in the piston. When you are just trying to go fast you kind of ignore the occasional pinging. I guess it was really trying to tell me something.
 

nat

 
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Thanks for the responses everyone. What will be more accurate, the degree plate mounted above the harmonic balancer, or the degrees marked on the balancer itself. Seeing that timing mark is a pain, I used white paint, but it is impossible to see.
 

miker

 
 
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You have to use a minumum of 2 marks. A mark on the balancer and then you also need a stationary mark on the engine block somewhere as a reference. Beware that it's possible to have the timing marks on the block in the wrong location especially if someone has installed one of those aftermarket chrome timing tabs. If it looks like a factory timing tab and if its the correct tab for the size of balancer, its probably close enough.
 

miker

 
 
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Just re-read your message. I prefer to use an adjustable timing light. Then you only need TDC on the balancer and TDC on the tab.

As for best accuracy between the two you mentioned, I don't know.
 

nat

 
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I do have one of those chrome tabs with degree marks on it, but it came with the crate motor, so I am guessing it is ok. I used a whiteout pen and marked 8 degrees BTDC and then lined it up with the 0 on my chrome tab. It runs killer now. Thanks for everyones input.
 
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