Hey everybody. I'm trying to get my car tuned in for its first test run with new 350 engine. I set my timing last night and noticed it has little mechanical advance when adjusting total timing. I spun engine up (till it stopped advancing) and set distributor to about 30 degrees advance. At that setting it has 20 degrees advance at idle(800-1000 rpm initial timing). Its a vacuum advance dist. and vacuum line was unhooked. Thats only 10 degrees of distributor mechanical advance. Is that normal for that type of dist. or should it have more mechanical advance?

Engines fairly rowdy with 10:1 compression and bigger .480 lift cam.

 

Depends on what type of dist it is.

If it's a stock one, no. If it's something intended for more high-perf sort of use, then maybe.

"Ideal" advance is heavily dependent on CID/weight ratio, gear, converter, etc. (IOW, all the things that add up to engine load and engine RPM at any given load) For the car in your profile, if the converter is stock, if the compression "estimate" is just what the catalog says and didn't include the MEASURED deck clearance in the "calculation" and is therefore actually about one full point less than that, and your cam is just the yerbasic old 230/480/108/104 cam that everybody's been selling since cams were invented, it'll probably want around 18° "static", about 36° "total" with the 18ish starting right off idle (1200 or so) and "all in" by 2500 or so, and ALOT of vac adv on top of that since the motor will be so lightly loaded. If it has Vortec or other "fast burn" heads on it it'll want acoupla ° less than that; maybe knock 3° off of each of those #s.

Most stock dists are set up to give around 26 - 28° of centrifugal adv. That results in EITHER the "total" being WAY too much (pings like a beeothch) if it's set for best idle, OR poor idle and lazy low-speed performance if max-HP RPM adv is set right. If such a one is only giving 10°, it's busticated and needs fixed, since its future behavior is now totally unpredictable. Then once it's fixed it'll at least be reliable and repeatable but it'll be wrong. Time for a spring & weight kit and maybe the fixed cam that the weights work against too.

 

Initial or base timing depends on the cam grind. Total advance can be modified by changing the weights and cam. The springs control how quickly the timing advances. For general street or street/strip cars, you should be at full advance by 3000 for good performance.

If using a computer controlled distributor, you need to disconnect the timing wire before setting the timing. Vacuum advance is used for part throttle operation only.

20 degrees base timing is too much even for my race engine.

Typically, it's recommend 10- to 12-degrees of initial timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift; 14- to 16-degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”; and 18- to 20-degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.

Once the base timing is set, you need to modify the distributor to achieve the total timing and not by adjusting the distributor to get it since that will change your base timing. Depending on how much mechanical advance is built into the distributor, you may need to advance or retard the mechanical to reach the desired total advance. Total advance is generally anywhere from 28-36 degrees. It all depends on where the engine likes it. No two engines are the same so if someone says to set it at 32*, that's only a ballpark number. Only trial and error or dyno tuning will tell you how much total advance your engine likes. Once it's set, and other engine tuning will not affect where the total timing should be. Always start tuning with timing then leave it alone as you play with jetting etc.

Many years ago when I was using an OEM distributor, I'd go through the junk yard pulling different weights, cams and springs to fine tune my distributor. It's amazing at now many old distributors were just worn out. Shaft bushings worn, Weight bushings worn and I even found a couple that the weights were seized. I can just imagine the last owners before the car got junked were disappointed with the cars performance when all it needed was a new distributor.

 

http://www.ebay.com/itm/SBC-BBC-CHEVY-65K-RED-HEI-DISTRIBUTOR-6501-R-/331169190781?hash=item4d1b38d37d&vxp=mtr
Lifetime warranty and only $40 so figured I'd try one out.

 

That goes in the "who knows" category... there's absolutely no telling how something like that was built, whether it's working "right", or any of that.

And as well, who knows whether any standard stock/replacement parts will even interchange with anything in one of those cheeeeeep chineeeeez knockoffs.

Everything you need to know about how risky it is right there, in 9 letters.

 

http://www.summitracing.com/parts/cc...make/chevrolet

ok thanks. It says 230-degrees at 0.050 so should be around 14-16 initial. I just set initial at 16-degrees and spun engine till it stopped advancing. Was around 26-28 total timing.

From both your guys's input sounds like I should have more total when spinning up engine. Like 4 to 6 degrees more which would put me at 32-34 degrees total with initial set at 16. I plan on running 150-shot nitrous so would this distributor work to my advantage because it doesn't advance alot?

I'm thinking now with nitrous in its future is that this would be a perfect set up because it timing is already retarded at 28 from the 32 or more it should be. With 150 shot I need to retard timing 4-6 degrees anyway. What do you think?

 

LOL. thanks for the heads up.

 

Just a few more details about my car.

350 bored .060 (360 cid)

coan 9-inch convertor. stall around 3000 on motor. 3500 with nitrous.(per coan salesman specs)

4.10 gears

3500 lbs with me in it

exact static compression is at 9.87:1

 

What's your MEASURED deck clearance on all 8?

 

Found my notes and I got .020 deck clearance without factoring in oil in bearings

 

On all 8? Using a micrometer / feeler gauges / etc.? (as opposed to, some method of "prediction")

 

Couldn't tell you all that. I'm sure its a measurement not a prediction. On 1 or 8 its hard to say cause engine builder wrote it down. Would a compression test help you with determining timing specs

 

There's no way to equate cranking compression to static compression. The numbers from a "test" are affected more by the cam than by the static comp. Case in point: a 8:1 LG4 with 180psi "test" numbers.

Stock, the deck clearance is .025" minimum, and might be over .010" more than that on any given cylinder, and often varies the full .010" from least to worst. It's rarely much less than .030". So your block started out life with that much. The decks are almost never "square" or equal.

A crank that has been ground adds more error to that. The stroke is almost never the "spec" for the motor, but rather is a few .001"s less. That adds yet more to the clearance.

Then, most pistons - virtually ALL cast, MOST hypereutectics, and the majority of forged as well esp TRW/SpeedPro - add .020" MORE. Reason being, the consequences of doing that, to the builder, (low compression) are FAR more acceptable than the consequences of an error the other way (pistons hit the heads). 383 pistons, not being a stock configuration, are less prone to this, but certainly not immune.

To give you some idea how much difference it makes, and how important MEASURING is, let's look at a typical motor: a .030" over 350, with flat-tops with 6cc valve reliefs, and nominal 64cc heads.

Zero deck clearance (piston top EXACTLY in the same plane as the deck), .016" gasket: 10.9:1

Zero DC, .016" gasket, 66cc (we're going to let the valves sink into the seats a bit from acoupla valve jobs): 10.6:1

Zero DC, .016" gasket, 62cc (we had to plane it a bit to clean it up): 11.2:1

Zero DC, .039" gasket: 10.3:1

.020" DC, .039" gasket, 64cc chamber: 9.8:1

Same as above except .030" DC: 9.6:1

Same, .045" DC: 9.3:1

Pretty eeeeezy to see, if you just look it up out of the catalog, you could be WAY off from whatever the motor was ACTUALLY built at; and since most of this is dependent on subtle variation in OTHER parts besides the pistons, the ONLY way to know FOR SURE what you REALLY have is to MEASURE it after it's built. Or, to MEASURE every single part as it's prepped and verify that it's EXACTLY what it's supposed to be.

Best way to "rough in" the timing is to advance it until it pings, then back it off til it quits. Won't be "right", I don't advise leaving it that way or any such, but it gives you a REAL GOOD idea of what THE ENGINE wants.

Absolute BEST way to set "total" timing is on a dyno. Best timing = max power "under the curve". (might not produce the highest "peak" number)

It helps to think of that point right there as the "real" timing number; then, all of that spring and weight stuff, as more like "low speed retard", rather than "high speed advance". While the dist doesn't physically work like that, that's what you're REALLY trying to accomplish. Then of course you add vac adv on top of the "total", usually 12 - 15° more, because the lean mixture during cruise burns slower and therefore needs to be lit off earlier in the cycle for optimum efficiency.

I'd suggest not worrying about some "number" just yet, until you find out where it runs the best. Let THE ENGINE tell you what it wants. Listen to it. It knows better than you ever will.

Remember, it's so simple, it's eeeeeezy to outsmart yourself over it: "If it RUNS good, it IS good". That's how you TUNE an engine: not by plucking some "number" for something out of thin air because it seems "good" TO YOU or because some g00b on the Internet like me told you so, and applying it to the engine, but rather, by making small incremental changes until peak performance is reached and you start to go down the other side of the curve, so that you know where BEST really is.

 

9.87:1 is what was measured and calculated by the napa machinist who built it. Not me or some catalog I read but thanks for the info. If I pull the heads at some point I'll be sure to measure all 8 for my own knowledge.

Back to question at hand. Will this distributor work to my advantage with nitrous because it doesn't advance alot. Which I could set my initial for good idle and throttle responce and my total would be only be 10-12 degrees more. Kind of like retarding the distributor but keeping by initial where it should be.

 

A 150 shot of NOS could use a couple of degrees of retard but will also work fine with a basic timing setup.

 

Sub'ed simply because I need to learn a lot about timing.

Any good articles out there that any of you experts know of? Or a good book about it? I am far off from being done with my build, but I always want to learn more.

Back on topic...

 

ok thanks alkyiroc. Seems to idle good and has crisp throttle response at 16 initial. Going to road test it here soon hopefully after i button a few other things up on the car. I'll fine tune it from there. Got a msd street fire dist I might try too.

 

Nothing wrong with having a mechanical advance system and as mentioned already, vacuum advance is only required for part throttle operation. This all works fine for a street or street/strip car. Even a few full race cars use a mechanical advance system but it's also more things that could cause consistency issues.

My race car is track only. My ignition is locked out at 37*. My ignition box has an adjustable start retard of up to 20*. I think I retard my ignition about 10-15* to get the engine started. Once over 500 rpm, the retard stops and the ignition is at full advance through the entire rpm range. Engine idles around 1200 rpm.

Running at higher altitude and using methanol fuel, my engine loves the extra timing but I've never had it on a dyno to see exactly where the best timing should be. Even with a full race SBC, the best total timing could be anywhere from 32-45* do don't be affraid to keep advancing the distributor to find the best timing. What works well in one engine may not be the best for another. 1 degree difference can mean a few HP are lost so finding the best timing is always essential for best performance. Find a good set point and keep increasing the timing 1 degree at a time until power starts to drop off. On a dyno is easy. At the dragstrip, MPH is your HP indicator. Just make sure your base timing stays the same or at least advance the distributor until you find how much the engine likes then move the distributor back to your base and adjust the mechanical to get the proper total back.