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350 chevy timing with .486 Lift cam. does it need to be advanced

862 views 6 replies 5 participants last post by  johnsongrass1  
#1 ·
Ive recently been building a 350 for my 76 longbed and i had put new cam bearings, cam, springs, and lifters for the block.

when it came for setting the cam gear and the crank gear i had matched dot to dot, without changing the the crank gear to advanced.

when i started breaking the motor in it ran great, steady idle, and would rev up openly. but when i scanned the motor with my timing gun it said i was almost 180 degrees out. you would think it wouldnt run even run at all with the timing like that, which had me confused.

i had checked my distributor for any shaved fins on it or if it was fuctioning properly but it was working fine. i know for sure its not spark timing so, it would only leave it to the crank and cam.

also the balcencer is not bent nor the key was shaved inside the crank

if anyone knows if i should set the crank gear to advanced and to know how many tooths i should move it to?

im 16 years old and im not an expert for stuff like this, but im trying to learn, but im very confused and stuck at the moment
 
#2 ·
No, it will not run with the timing 180 degrees out. You either have the wrong pointer, or the damper ring has slipped, or your timing light is not connected to number one cylinder.

I would suggest bringing number one cylinder up on TDC and see if your timing mark and pointer align or are at least close. On my race car, I could get pretty close to TDC by looking in number one plug hole with a flash light and slowly baring the engine over. When the piston is at the top you can rock the crank back and forth to find the point that the piston is really, really close to TDC.

If you want to be real accurate, you need to make a piston stop you can screw into the spark plug hole on number one cylinder. Then turn the engine over by hand until the piston hits the stop. Make a mark on your damper. Now turn the engine over by hand in the opposite direction until the piston again hits the stop. Make another mark on the damper. TDC is exactly half way between the two marks. Make sure you remove your piston stop. Also, it is much easier to turn the engine over by hand if you remove all the spark plugs.
 
#4 ·
@jaydensalazar971 welcome. You're way ahead of where I was when I was 16 years old. I loved the hot rod game but didn't understand many important aspects of it until I was much older...High school auto shop taught me quite a few things but some of the silly ideas I had in my head persisted until I was finally able to get my hands on a hot rod and work through things myself.

I'd like to share a few things about timing that I've learned in the past few years that might be (hopefully) helpful to you in your understanding and development as a hot rodder.

Visualize the pistons going up and down in the bores of the block through the cycles: valve opens: piston goes down creating "suction" to suck air & fuel into the cylinder, valve closes: piston compresses air/fuel, at some point the spark plug SPARKS to make BOOM which forcefully pushes the piston down the bore creating exhaust in the process, exhaust valve opens: piston pushes gas out of the exhaust valve.

We're focusing on the SPARK PLUG sparking with timing. Visualizing this: there's an optimal time for the spark plug to spark to produce the most amount of power. Imagine if the piston was coming up the bore and the spark happened! The piston would be working AGAINST the explosion and bad things could happen. Likewise, if the piston is already going down the bore and the explosion occurred, we're not taking full advantage of the compressed air/fuel mixture. In other situations (like if you had the distributor 180 out) it might be trying to spark when there's not really any air/fuel in the cylinder.

Let's now talk briefly about advance. Small block chevy engines typically "like" a total of 36 degrees of advance. When you set the timing of the engine AT IDLE (with the vacuum advance disconnected) this is considered your INITIAL timing. When you set this, you want to keep your RPM's down below 1000 usually at 800 or so. WHY?

Distributors have mechanical or centrifugal advance which is activated when RPM's increase. If you take the cap and rotor off of your distributor you can see that there are springs and weights. What happens is as you put the pedal to the floor, these weights fight against the springs and this will mechanically creep your advance up. How much? If you're running an OEM distributor it's hard to tell. You CAN (and should if you're able) get a timing light that has an OFFSET. This will allow you to determine how much mechanical advance is being introduced into your setup as you increase RPM's.
You CAN also figure this out without a fancy offset light but you need to make up a timing tape and apply this to your harmonic balancer. You can also buy timing tapes, you just need to know the size of your balancer and they usually fly off pretty quickly. Fancy and expensive balancers have markings all the way around them to make this easier.

Fancy and expensive distributors usually come with bushings and springs that allow you to adjust how much mechanical advance is coming into the mix and WHEN:

see the following for reference: https://documents.holley.com/44326b1b5d986a37979dd931867f4eda217792e9.pdf

Why is any of this relevant?
Well it works like this. You can pretty safety run a total of 36 degrees of timing. Working backwards you subtract your initial timing of say 12 degrees = 24 degrees of mechanical that you can "bring in" where you want it. If you were racing, you may find that if you bring in too much advance too soon that your rear tires start spinning and while that's cool...you're not moving and you lose the race. So you want to bring in the timing where it makes sense to do with the overall setup of the car.

I really hope that some of this information is helpful to you as you dork with this.

TLDR: As far as OPTIMAL TIMING for your ENGINE. The only way to know 100% is to put it on a dyno. The second best way would be to have it on a drag strip. Most of us just stick to 36 TOTAL. If you don't know how much mechanical your distributor is bringing in, you will want to figure this out because you do not want to crank up your initial too far and have mechanical exceed 36 -- the engine will live a short life if you do that.
 
#5 ·
The purpose of advancing a big cam is to get the intake valve closed sooner in crankshaft degrees of rotation. This plays directly into the concept of DCR (Dynamic Compression Ratio).

There are two common compression ratios, there is the Static Compression Ratio (SCR) which is simply the measure of the sum of all volumes ; displacement, clearance, gasket and combustion chamber. Then there is the working compression ratio of which one of the measures is the stroke consumed This by crankshaft rotation up to the point of the intake valve closing. This effectively makes the engine appear smaller in regard to actual trapped cylinder pressure than the SCR indicates. This is dynamic in that there are serval and compounding events taking place and has a large impact on power and torque in the RPM range below the torque peak. This is the key to the high speed internal combustion engine, suggested technical authors on this is Sir Harry Ricardo and Charles Fayette Taylor Phd. These books are written for engineers and scientists so unless you have a pretty tight grasp of trigonometry and calculus a lot is boring, but you can in the more written content and the pictures, graphs and photos gleam a lot of the concepts of how an engine actually functions. The important thing to take at this point is an engine operates at a fundamental level by the opening and closing of triangles make between the crank pin, connecting rod and piston. The movement of the piston in the bore is not linear to crank degrees and its velocity constantly varies as well both of these reflect trigonometric functions.

The other big concept affecting performance is intake system mixture velocity. In a throttled engine especially but to some lesser extent a diesel has a similar intake gas velocity changes, but these are really remarkable in gas engines. This is the speed of the mixture varies greatly with RPM. At low RPM the rising piston creates a pressure that overcomes the inertia of the incoming mixture and reverses some fraction of what was inducted on the intake stroke, this ia one component in the concept of mixture reversion. The result here is there is less quantity of mixture that the SCR calculation would lead you to understand that tge DCR calculation compensates for. But there is a big, however. That however is as engine speed increases so does the mixture velocity in the intake porting system to where there is a point where the speed induced energy of the intake charge overwhelms the reverse pumping pressure from the rising piston thus rams more mixture into the cylinder. This is the key to top end power and why long duration cams can be made to work. it also is a big hit of gearing selection to where big cams want big gear ratios to get the engine spun up so the ram effect takes over at lower road speeds. Theres other things to this as well

So back to the point of advancing the cam, of which most cam grinders know this so most performance cams come with a couple threes degrees of advance from the true zero point already to help the scientifically unwary customer out a little.

This also gets us to the point of needing to know the cam details as well as those of the engine and its vehicle installation as all of this stuff is related to each other, the answer whether to advance the cam further and by how much requires knowing the total context. I’d also suggest if you are watching the many hot rod shows on the WEB you need to consider that the engine dyno is running to an SAE (Society of Automotive Engineers) test protocol of which there are several, that which is popular these days uses a loading that holds the engine to a crankshaft acceleration rate of 300 RPM per second. This is not a God’s eye view for all installation cases everywhere, all of the time. What makes nice relatable numbers on the dyno may not come across to your world in exactly the same state of tune.


Bogie