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View Full Version : DISCUSSION , Ignition Advance VS High RPM



POS carb
11-05-2003, 06:00 AM
Ok this topic has come up before and people usually say something like "Advancing your timing will give you better low-end power"

now to me in theory this makes no sense, even as RPMs get higher the air's momentum basically stays the same so to me it would seem for high RPM you wnat to advance the ignition more to cause the burn to occur sooner and therefore by the time the burn has max power the engine is at a good position to apply the reciprocating power...
Just like you get a cam that opens sooner for high-rpm cars

Now am I full of $hite and if so can someone clarify?

What if one was to set up the timimg so that the base timing stays normal but the high-rpm has more advance? I can set this up but I don't want to blow up the motor yet without trying some research.

:werd: :stupid:

shepherd79
11-05-2003, 06:56 AM
ok, here is the layout
if you adjust the distributor you will make spark plug ignite earlier, this will give you better spart onthe top end. it may harm the low end.
if you get adjustible cam gear, you can adjust when you want the spark to be. low end or top end.

i would not recomend adjusting distirbutor unless you are racing.

POS carb
11-05-2003, 09:40 AM
Originally posted by shepherd79
i would not recomend adjusting distirbutor unless you are racing.

every day sees the redline :)

A20A1
11-05-2003, 11:19 AM
Distributor advance always hurt my trap speed by like 5 mph on the 1/4 mile, but I gained in off the line acceleration.

I didn't care for the results since I run in the high rpm's anyways so now I usually keep the distributor advance right on TDC and add a stronger vacuum advance just so I don't stumble too much off the line. I don't mind playing catch-up on the circuit.

BootMachine
11-05-2003, 11:53 AM
ummm....ok....

The air momentum does NOT stay the same as the RPM's go up! The faster the rpm's the faster the air is going in. ALSO...because of the flow speed the air will CONTINUE to enter the cyls even after BDC which is why you have overlap cams that cause your car not to idle...

ok there is way too much to explain here.

Just post away and I'll correct anything thats not right....

BootMachine
11-05-2003, 11:55 AM
Originally posted by A20A1
Distributor advance always hurt my trap speed by like 5 mph, but I gained in off the line acceleration. I usually keep it right on TDC and add a stronger vacuum advance.

the results should be linear assuming you are burning the same octane fuel the entire 1/4 mile!

Get an engine knock sensor! There is no way to properly tune the A20A's timing without one! The stock dist. is too old skool for an acurate result. The best you can do is listen for the knock by ear if you are playing with a timnig other than stock!

BootMachine
11-05-2003, 12:01 PM
Originally posted by shepherd79
ok, here is the layout
if you adjust the distributor you will make spark plug ignite earlier, this will give you better spart onthe top end. it may harm the low end.
if you get adjustible cam gear, you can adjust when you want the spark to be. low end or top end.

i would not recomend adjusting distirbutor unless you are racing.


umm....no

an adjustable cam gear does absolutly nothing for the spark. What you are tryign to do with the cam gear is adjust the CAM timing.

If you can hold the cam open for a little longer AFTER BDC you can get a little more air to burn because of the intake velocity. Kinda like a garden hose. When you turn the hose off the water still flows out for a little while. Same with the air in the cyls. Its going in there so fast it takes time to stop rushing in.

Playing with the dist wont do a thing for your engine unless you have the proper fuel to burn with it. High octane fuel has more overall BOOM but it takes longer for the BOOM to happen SO....you need to fire the spark SOONER in the compression stroke SO THAT...you get total gas expansion (BOOM) when the piston is about ...oh....6-10 degrees past TDC on the down stroke.

Otherwise the BOOM happens when the piston is on the way up and slamms into it.....causing a knocking sound!

OR...the spark happens too late and the fuel/ air mix in the cyl has been squeezed so hard it explodes anyway....causing a knock

The higher the octane rating of the fuel...the larger the OVERALL boom, the slower it burns and harder you can squeeze it before it explodes!

A20A1
11-05-2003, 12:29 PM
My big issue is with reversion at low rpm's causing the idle problems.

I don't think POS ment to say air momentum was static thru the RPM range. It might depend where along the intake you are measuring the air velocity.

Eric, were you thinking of hooking the vacuum advance to the venturi vacuum? :eek:

POS carb
11-05-2003, 12:37 PM
1. sorry I chose my words poorly, what I meant to say was that the air/fuel burns at a fixed rate, not that the momentum stays the same (i wan't awake:))

2. Cam gear does affect timing b/c the distributor runs off the cam

3.

POS carb
11-05-2003, 12:38 PM
lol...
3. I will run it off the EGR vacuum port on the Weber since it is the only port where the vacuum present increases as RPMs rise

A20A1
11-05-2003, 12:49 PM
Oh yeah you have a weber... :D

BootMachine
11-05-2003, 01:00 PM
WEBBERS :super:

Thats hard core man! Good luck with those! I cant stand the carbs. I took one set apart and since they have never been put back together! I have a EFI engine!

AND...the cam gear affecting timing...I totally forgot the dist runs off the cam. GOOD CALL! I stand corrected! HOWEVER. If it were me ...I would do ALL my timing adjustments with the light and dist and just adjust the cam with the gear!

And I DISAGREE with the statement that fuel burns at the same rate in the engine at any rpm (given the same fuel is used in each combustion) I totally disagree.As you have more air and therefore more fuel (unless you install a turbo with no fuel control :huh: ) the mixture becomes more explosive and will burn faster!

I'm thinking of a turbo. When you have that much air and fuel under compresion the explosion happens REALLY fast which is why you have people running programmable ignition controllers with huge amts of retard / advance.

Well...at high rpm you are going to have a lot more air than normal because of the intake velocity so I would say that the higher the rpm the faster the burn will be...however...the overall speed may still be very close to the combustion time at idle!

OR...there is no extra air because the intake valves (on stock valve timing) wil slam shut during the scavenging stage and combustion time will be the same...humm...I've stumped myself....

Lets argue some more!!!!

BootMachine
11-05-2003, 01:02 PM
Originally posted by POS carb
lol...
3. I will run it off the EGR vacuum port on the Weber since it is the only port where the vacuum present increases as RPMs rise

on my EFI engine the vaccuum to the advance port on the dist is constant. Its on or off and does not vary!

Off at idle, anything over 2000 RPM it turns on to full vaccuum activating the advance weights which take care of the rest!

Perhaps to change your advance curve you should use heavier springs / weights or....go full electronic ignition!

BootMachine
11-05-2003, 01:03 PM
Originally posted by A20A1
My big issue is with reversion at low rpm's causing the idle problems.

I don't think POS ment to say air momentum was static thru the RPM range. It might depend where along the intake you are measuring the air velocity.

Eric, were you thinking of hooking the vacuum advance to the venturi vacuum? :eek:

IMO...Fawk the idle....get a highprofile cam and forget it!

POS carb
11-05-2003, 02:27 PM
Originally posted by BootMachine
And I DISAGREE with the statement that fuel burns at the same rate in the engine at any rpm (given the same fuel is used in each combustion) I totally disagree.As you have more air and therefore more fuel (unless you install a turbo with no fuel control :huh: ) the mixture becomes more explosive and will burn faster!

OK I agree that the mixture can have different burn rates but seing as burning is a chemical change that occurs at a rate determined by the laws of nature and affected by factors such as ambient temperature, air pressure, etc
I disagree with the mixture becoming more explosive, actually I believe that unless you are running positive displacement (turbo etc) you actually have LESS air going into your cylinders the faster you go...
How?
As RPM's go up yes the engine does use more air and does use more fuel but this is b/c the cylinders are pumping more often (1L for every revolution; more RPM's = more air/fuel being moved in a given time)
and therefore the valves are opening and closing faster (higher rpms + same cam lobe duration = less time in milliseconds that the air has to overcome inertia and get from the manifold into the cylinders to fill them before they shut b/c the cam is spinning faster)
So in theory the faster it spins the less efficient it gets moving air. This may not be a very noticable drop but I am just clarifying.

I may be talking out of my arse here but it makes sense to me. At least people are participating and not saying crap like "Get a Tornado" :lol

A20A1
11-05-2003, 07:17 PM
Duration
280 Intake
288 Exhaust

Like I said... I have reversion at low rpms. It's a issue, but I don't mind it at all, i preffer the loping idle.

BootMachine
11-06-2003, 11:48 AM
Originally posted by POS carb
OK I agree that the mixture can have different burn rates but seing as burning is a chemical change that occurs at a rate determined by the laws of nature and affected by factors such as ambient temperature, air pressure, etc
I disagree with the mixture becoming more explosive, actually I believe that unless you are running positive displacement (turbo etc) you actually have LESS air going into your cylinders the faster you go...
How?
As RPM's go up yes the engine does use more air and does use more fuel but this is b/c the cylinders are pumping more often (1L for every revolution; more RPM's = more air/fuel being moved in a given time)
and therefore the valves are opening and closing faster (higher rpms + same cam lobe duration = less time in milliseconds that the air has to overcome inertia and get from the manifold into the cylinders to fill them before they shut b/c the cam is spinning faster)
So in theory the faster it spins the less efficient it gets moving air. This may not be a very noticable drop but I am just clarifying.

I may be talking out of my arse here but it makes sense to me. At least people are participating and not saying crap like "Get a Tornado" :lol


Humm...I'd agree with that also!

I guess the only REAL way to tell would be to measure the air flow!


ha ha ha.....for sure someone is gonna have to measure this

BootMachine
11-06-2003, 11:50 AM
Originally posted by A20A1
Duration
280 Intake
288 Exhaust

Like I said... I have reversion at low rpms. It's a issue, but I don't mind it at all, i preffer the loping idle.

I think it sounds cool!

Even if a cam didnt give me more power I'd still get one just for the idle and to keep stabbing at the gas to keep the engine from stalling!

he he he

Magny
11-08-2003, 11:49 PM
I guess I could be a total liar of I said this, but IMO if you retard the timing from TDC a few degrees like 1 or 2, it helps "loosen up" the engine a bit. what I mean is that when the spark hits right before the crank pulls down, you are basically giving it a push. thus decreasing the resistance on the "back-spin" ignition. it may sound confusing at first, but if you think of it as like riding a bike, it makes it easier. the only way to go foward on a bike is pushing in front of TDC on the pedals, so taking that and putting it to the engines perspective................you get the idea. Rant away if you'd like, but my peformane on my car is a whole lot better than what it was stock timing.

oh BTW whoever said that our "carbed" cars don't like cold air, mine does. soon as i slam it in gone. summer is over, the heat is gone and the cold is here. ever since the cold came, i am able to pull off the line faster and get goin down the strip faster. thinkin of makin a tru CAI soon and make it adjustable. so when the heat comes, i can open it fully, and when the cold comes around i can adjust it until it's just enough kick.