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Old 08-28-2010, 09:31 AM
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Chevy 350 with 292 cam Total timing

Hello. I have a Question on timing. im new to this Vac adacne timing and total timing stuff, but then again im only 17 lol. but i have a 350 bored 60 over with a summit 292/292 duration .488/.488 lift. and edelbrock intake and 650 holley and mds STREETFIRE HEI distruibter. my question is. I have it at 8 degrees for timing. but now from what i been reading i might need more than that? the motor seems kinda ball less if someone could tell me step by step on how to time this thing. it would be great. somone said my totaly timing should be 34 degrees? is that with the vaccum adavace hooked up? or without. and at what rpm should be running? 3000? i didnt get any specs with the cam from summit. I guess maybe if i bought a comp cam or lunai or somitng i would of. but any help would be great..

thanks
NATE

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Old 08-28-2010, 12:05 PM
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http://www.corvette-restoration.com/.../Timing101.pdf

http://www.corvette-restoration.com/...ine_Timing.pdf

http://www.summitracing.com/parts/SUM-1106/
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Old 08-28-2010, 01:38 PM
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You need waaaay more initial than that. I would guess 18-22 degrees initial, with 34-36 total, depending on your heads.
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Old 08-28-2010, 02:16 PM
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Quote:
Originally Posted by ChevyMopar318250
Hello. I have a Question on timing. im new to this Vac adacne timing and total timing stuff, but then again im only 17 lol. but i have a 350 bored 60 over with a summit 292/292 duration .488/.488 lift. and edelbrock intake and 650 holley and mds STREETFIRE HEI distruibter. my question is. I have it at 8 degrees for timing. but now from what i been reading i might need more than that? the motor seems kinda ball less if someone could tell me step by step on how to time this thing. it would be great. somone said my totaly timing should be 34 degrees? is that with the vaccum adavace hooked up? or without. and at what rpm should be running? 3000? i didnt get any specs with the cam from summit. I guess maybe if i bought a comp cam or lunai or somitng i would of. but any help would be great..

thanks
NATE
Nate, were it that simple.

17, don't worry about it; I have it on good authority it's something you'll out grow.

There are reasons why large cammed engines need high compression and lots of ignition advance. These have to do with chemistry and physics of the fuel, air and structural sizes and shapes on the engine. These are the same constraints everywhere which is why you see so much commonality in tune up specs across engines of different manufacturers.

I guess the place to start is the time available for the burn to happen most effectively. To this end the RPMs of the engine is the great arbiter setting the available time for the sparkplug to initiate the burn and the time it takes for the burn to go to completion. While happening in milli-seconds it is by no means instant and many factors will affect the qualities of the burn.

There is a phenomena that occurs where as the mixture density goes up so does the speed of the burn. By mixture density I mean the number or weight of reactant molecules (fuel and air) trapped in the cylinder. Air and vaporized gasoline are what’s called perfect gasses. In gaseous form these materials expand to fill the available space of the container they're in. A gasoline powered engine is what’s called a partial pressure engine, which is to say it's throttled. The position of the throttle blades establishes how much volume and weight of reacting products can get into the cylinder. At low RPMs and a mostly closed throttle not much, at high RPMs and/or an open throttle a lot.

Since the burn speed is directly related to the mixture density, when the density is low an advance needs to be applied to get the mixture burnt at the optimum point of piston position in the time allowed by the RPMs. This then requires methods of applying a spark advance, usually for RPMs below 3000. Because the burn speed increases with mixture density, it turns out that for the most part above 3000 RPMs the mixture burn speed naturally increases at a rate sufficient to be continuously optimal as the RPMs raise beyond 3 grand.

The camshaft has a major effect on mixture density as does the throttle position of the carb or injector throttle body. For different reasons, but the affect on mixture density is the same. This is to say that as the cam becomes more radical the mixture trapped in the cylinder at lower RPMs becomes less dense thus it needs more compression and ignition advance. As RPMs increase with a radical cam a ram effect begins to assert itself forcing the mixture density trapped in the cylinder to increase. So this presents the problem of developing enough spark advance below 3000 RPM to get the low density mixture burnt in the available time. So this requires starting the burn much earlier than would be desirable in a less cammed engine.

The effect that causes a low density mixture at low RPMs with a radical cam is the result of the overlap period where the exhaust and intake are both open and the late point where the intake valve is closed while the piston is rising. These cam timing events at low speed allow the exhaust pressure to blow the intake charge back out the carb and to let the rising piston do the same. These events are a major part of what's called reversion. Both events lower the density of mixture to be trapped in the cylinder and may pollute that charge with exhaust gases as well. This results in a mixture that is hard to light off and slow to burn when it does. These events give rise to the staccato exhaust sound of an engine with a radical cam in it.

The fixes are to get the compression up and to apply much more ignition advance much sooner in the RPM range. Another great help is a multi-strike ignition as these apply upward of 3 to 5 sparks at low RPMs greatly increasing the chances of getting the mixture to burn, thus avoiding misfires.

An engine has two compression ratios we need to be concerned about. The Static Compression Ratio (SCR) is that established by the dimensions of the cylinder and combustion chamber divided by the dimensions of the combustion chamber. There is also a Dynamic Compression Ratio (DCR) which is the static ratio adjusted for the moving effects of the cam timing and rod length ratio against the SCR. You want to keep the DCR in the range of 8 to 8.5 with today's street fuels. Here is a calculator; http://www.kb-silvolite.com/calc.php?action=comp

The other issue is ignition advance. This is typically a three part equation where there is a static amount of advance and two devices that create a variable amount of advance. These parts being the static or base amount and those variables applied by vacuum or centrifugal. Increasingly the latter pair are being replaced by computer calculated electronic controls as this is much more responsive than mechanical controls to more situations than I'll address here.

The vacuum advance is a way to provide advance to the load on the engine while centrifugal is way to apply advance in concert with RPMs. In simple terms a normal car cruises at high speeds using low RPMs and not much throttle opening. This creates, based on what I've discussed in the previous paragraphs, a low density mixture trapped by the cylinder which needs a lot of burn time to complete. Since the throttle is mostly closed the manifold vacuum is high and this is used against a spring loaded diaphragm to increase the ignition advance for optimum power and fuel burn efficiency at these conditions. As the throttle is opened manifold vacuum decreases along with it the amount of advance. This assumes that engine RPMs will increase as the throttle is opened and the centrifugal system will advance the spark. An obvious hole in the logic of this system is pulling a load or climbing a grade where the throttle is advanced but not the RPMs so the vacuum advance is lost, but not replaced by any centrifugal until the transmission is down shifted. This is a hole that computerized timing can cover as a third component of strain on the transmission output shaft can be measured with a strain gauge and that data inputted to the computer can be used to optimize the ignition timing for the RPMs and load.

Here again as the cam becomes more radical, the manifold vacuum goes down with no change in RPM to compensate, so the base setting is usually increased while a like amount has to be removed from the vacuum system if its used and from the centrifugal to keep the combinations of base and variable from over advancing the ignition which will drive the cylinders into detonation and/or pre-ignition.

Radically cammed engines pose a tuning issue with vacuum advances as units made for grocer getter cars tend not to adjustable, these depend a lot vacuum to operate and a big cam isn't making the necessary vacuum so these units become ineffective for our purposes. Adjustable units are available from the aftermarket and can be tuned to operate with a large cam in an engine that cruises on small throttle opening and low RPMs. If your application doesn't fit this scenario, its just as well to fore-go vacuum advance and either drive the static further ahead or soften the centrifugal so it comes in sooner in the rev range. Finding which is best is a tuning function for your total vehicle configuration and where you live topography and environmental factors. The detonation limit is what controls how much and what combinations will work best.

Bogie
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Old 08-28-2010, 02:25 PM
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I.E. lean mixture burns slower
rich mixture burns faster
lean mixture needs more timing adv
richer mixture(at or close to stoich.)needs less timing adv.
Thus the vacuum advance!!
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Old 08-28-2010, 02:40 PM
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Alright guys. let me know what you think. used a advance timing light. Total timing at reved rpm is 40. at idel. with Vac advance hooked up is 23. and without vac advance hooked up its 8. now what do i do? is that good?
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Old 08-28-2010, 03:15 PM
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an other thing, What does the adjustment on the vacuum canister do? advance the timing.? i think i have it like two turns out but i dont understand what it does? make the advance come in faster? or later? or either? thanks guys u been a big help
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Old 08-28-2010, 03:31 PM
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QUOTE You need waaaay more initial than that. I would guess 18-22 degrees initial, with 34-36 total, depending on your heads.




im talking about timing without the advanced hooked up. its at 8. and with the advance hooked up at idel its 23 sound right?
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Old 08-28-2010, 03:37 PM
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and the reved timing was takin with the mark on the 0 is that correct? with the light set on 40. and it was right on the 0 mark. am i suppto use the 0 or 8? maybe thats why my numbers are high.? i thought i read somewhere i read from 0 but im not sure. well thanks guys again
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Old 08-28-2010, 04:32 PM
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Stop playing with it. The stock mechanical advance curve must be modified.

You want 26deg at idle. you want 36deg at max mechanical advance.
That requires that you shorten/physically limit the mehcanical advance travel from stock 20deg to around 10deg max.

That requires tearing down the distributor.

Then vacuum advance: Vacuum advance is for efficient part throttle cruisng at high road speed under light engine load.

You want about 10-12deg of additional vacuum advance at hiway speeds.
Most stock vacuum advance meed to be limited and adjusted to achieve this.
Vacuum advance is a separate issue form mechanicl advance.

mechanical advacnce is set and observed with the vacuum advance disabled.
( vacuum hose disconnected.)


26deg inital at idle +10deg mechanical advance =36deg max mechanical

+10deg of vacuum advance for cruising.

No amount of farting around with the stock distributor will get it right.

Why do I have to post on this every day?
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Old 08-28-2010, 04:32 PM
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Quote:
Originally Posted by ChevyMopar318250
and the reved timing was takin with the mark on the 0 is that correct? with the light set on 40. and it was right on the 0 mark. am i suppto use the 0 or 8? maybe thats why my numbers are high.? i thought i read somewhere i read from 0 but im not sure. well thanks guys again

Disconnect & plug the Vacuum line to Distributor. Set the Initial timing BTDC/BeforeTopDeadCenter, now check Total = Initial + Mechanical timing using 0*. All in by 2500rpm-3000rpm. You want 34*-38*, you may have to limit the mechanical advance inside the Distributor if you have more than the above #.

Don't worry about the Vac advance timing being higher right now. Adjusting the Vac cannister limits the amount of timing added, some Vac cans limit amount and at what Vac level it starts.

You will need to try different Initial & Total & Vac timing settings.

http://www.corvette-restoration.com/...ars_Timing.pdf
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Old 08-28-2010, 05:38 PM
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This engine with this cam will not work with a stock low stall converter.

Get a 10" 3500 stall and 4.10's and recurve the distributor.
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Old 08-28-2010, 08:02 PM
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yup, need 3500 stall, im not going to tell you what gears to run, 373's and up, recurve the dis, and it will run good at 16 int timing...
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Old 08-28-2010, 08:18 PM
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First let me tell u F bird is right
But if u must do something, do this
With vacumm adv. defeated
Hook up ur timing lite bring eng up to 3500 or so adv dizzy until you see 36 deg.
Back to idle
36 to 38 is all the adv u should run, so now its gonna run like crap at idle,\.
This will prove to u what he is trying to say to u.
Step 2......
Take distributor out re-curve to get 10 deg mech adv.
Step3......
Re-install dizzy bring engine up to 3500 time 36 total with vacumm defeated
You should wind up with 26 deg at idle
But I dont think u r gonna like this with a stock converter
Oh i almost forgot, read up on the above mentioned article and hook up your vac adv.
Hey. Are u sure pointer is right? Do u have timing tape on damper?
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Old 08-28-2010, 11:24 PM
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what do you mean take the distrubter our and recurve it? how do i go about doing that?

and why do you guys say i wont like the stock stall converter? im not sure if its the stock one but it came out of a 1985 k10 that somone put a 350 in and a th350 so i got the tranny and stall.? i just got done a install on that transmission cause my 700r4 crapped out. and now u guys are saying im gunna have to take the tranny back out? what would be better with a 3500 stall? i dont know nothing about stall converts thats why im askin. just wondering what would happen with the wrong stall? truck wont have alot of power? or somting?
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