ignition timing

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im trying to find out the timing advance for a 74 dodge dart

posted by  swinger

50 no more no less :screwy:

posted by  BanffAutoSpa_ap

Do you mean 5° ?

posted by  Wally

Wally, on forced induction engines, you have to retard the timing a little bit right? Do you do this so the pressure wouldnt be too high? Why is it better to retard timing for forced induction.
Thanks wally

posted by  CarEXPERT

Nice to know your name goes with the Dart and not something else. I'm assuming you have the 318 and not a knocked-over 6er. 27 total. 15 with vacuum line disconnected at 1,900RPM.

posted by  DodgeRida67

Yeah peak cylinder pressure has a big bearing on det. And peak cylinder pressure occurs around peak engine torque. Raising the cylinder pressure by introducing boost increases the chances of det. With increased pressure the flame front moves with inreased velocity and ferocity, so you have to retard the ignition or slow the burn rate (eg Octane, lower CR, etc), otherwise the piston will experience considerable pressure against it's upstroke and chaotic burning of the fuel.

As the engine passes the peak torque region, peak pressure drops and controlled advance can be reintroduced.

I should point out that the timing never actually retards past piston TDC. There will generally always be ignition advance, just that it might be 18°compared to 28° without boost.

posted by  Wally

Wally, can you please help me again... I cant find the answer.
Ok, ignition timing has always got to be before TDC even if you retard like you said...so are the CAMSHAFT timimg always before TDC? If you have a stock engine with no adjustments, are the cams advaced or retard?
Wally, if you RETARD the cam, does that mean the cams open nearer to TDC or when the piston is going down and then it opens.
Like I heard 230 degree duration.. (a piston goes from TDC to BDC at 180degrees) so does that mean the cams open before TDC and close after TDC cause 230 degres is a lot more than 180. THANKS wally

posted by  CarEXPERT

Invalid question. You don't even know what you are asking.



Well, If it hasnt been adjusted then wouldn't that make it neither? :screwy: :banghead:



Think about it. Retarding cam timing (in your terms) opens the valves earlier. Farther before TDC. Stop saying "cam opens." Since when does a cam "open"? :screwy:





Look. You don't have a clue what you are even asking. Stop asking questions unless you know what you are asking. YOU DON'T KNOW WHAT YOU'RE ASKING. :banghead:

posted by  DodgeRida67

...Like wally said ignition timing is ALWAYS before TDC of compression stroke. What im asking is if the valves open before TDC of the exhaust stroke on a stock engine.
So in other words, if you retard the cam timing, that means that you delay the opening, but will it still open BEFORE TDC? When you advance timing, the valves will open earlier before TDC but if you retard does it open later or PAST TDC?? am I making sense to you..?

EDIT:I was talking about the intake valves. If you get adjustable cam gears and retard it positive 5 degrees, is it 5 degrees after TDC? Then that means that when thepiston is movingdown, then the vlaves will open.

posted by  CarEXPERT

You have not your homework have you?

The exhaust valve opens before bottom dead centre (BBDC) of the power stroke to "blowdown" .... i wrote about this in that other post. It generally closes after top dead centre of the exhaust stroke (encroaching the intake stroke), although this may only be 1° or less depending on the design and purpose of the engine.

If you advance you make that event begin sooner.

And if you insist on quoting me use the correct words "generally always", because there are always exceptions to the rule and people on forums who argue the toss for self aggrandisement.

posted by  Wally

Wally that wasnt my question. You know how you said igniton timing is always before TDC, but in Cam timing does the valve always open before TDC?
Remember you said advancing timing will make the valves open earlier before TDC. If you retard it happens later, closer to TDC.
If you retard timing, can the valves open after TDC,?

posted by  CarEXPERT

whao there chief.. u guys really gonna try to second guess the engy??
the reason they have valves open and close like that is to let the engine breathe easier. if i remember correctly the intake and exhaust have an overlap.. both of them in open position at some point in time.

if u change them ur car will either great run high speed or low speed rpm.. then engine set it in the middle so we can idle correctly and go past 5000rpm

posted by  thongsai

Damn shut the hell up, we already know they have valve overlap.

posted by  CarEXPERT

I reckon I should write a book and sell it on Ebay :wink2: There are "Cam Durations" and "Valve Durations", but for the purpose of the excercise let's just generalise:

On a run of the mill car engine the exhaust duration is the number of crankshaft degrees the exhaust valve is open. It begins to open while in the power stroke and closes just after the exhaust stroke. For example it may begin "blowdown" at say 60° BBDC of the power stroke and close 13° ATDC of the exhaust stroke, the duration is therefore 60 + 180 + 13 = 253°.

Now when you use terminology like 60° BBDC and 13°ATDC you are talikng "Cam Events" or "Valve Events" depending on which part you are referring to. When you are discussing "Durations" with a cam maker you would be discussing the number of crank degrees opening and closing of either the cam or the valve. For instance in our example above the opening (from closed to fully open) might be 120° and the closing would be 133°:- 120+133 = 253°.

posted by  Wally

Yes you cn retard cam timing so that the "intake valve" begins to open ATDC.

Don't confuse the ignition spark timing with cam timing.

posted by  Wally

yes, but that means that there will be less valve overlap.. so why do you retard for high rpm power if the more overlap you have the more top end power you have?

posted by  CarEXPERT

On a DOHC there will be less overlap, but not on a SOHC or SOHV engine.

Let's spin this around, why do you think someone would want to retard the exhaust or intake or both at high rpm?

posted by  Wally

Hold up.. I thought on a DOHC there is more overlap and you can make it have more overlap because you can time both coms.

Remeber you said that when its retarded, the fuel/air can come in esier at high rpm because of some exhaust thing..

posted by  CarEXPERT

On a DOHC you can have more or less overlap by changing the position of one in relation to the other. I was keeping in tune with your post, where I assumed you were talking about retarding the intake cam (or advancing the exh cam) = less overlap.

Can you cut 'n paste what I posted earlier so I am not confused with what your definition of retardation is and that I'm not being quoted out of context? If you are talking about retarding the exhaust to avoid blowthrough, then yes sometimes this will be necessary on certain pent roof heads.

posted by  Wally

That is incorrect.

posted by  DodgeRida67

You're confusing me. That is backwards.

posted by  DodgeRida67

The only way to change overlap is to regrind the cam. :screwy:

Like DSMer said, you are eager to learn something you can't understand.

posted by  DodgeRida67

'When you advance the intake valve event (open earlier) you actually increase the time fuel can syphon into the into the cylinders at light or moderate engine load, due to the slight negative pressure created by exhaust gas escaping.

At high revs the syphoning occurs later due to higher static pressures from more residual exhaust, so at high revs an advanced intake valve will not allow as much fuel through before it closes compared to an intake valve opening a little later' -Wally

Dodgerider are you sure, cus if you retard the intake cam and retard the exhaust then wouldnt you have more overlap..

Dodgerider, you can have the spark fire after TDC but who would do it. when it sparks all the fuel and air takes time to burn so if you have it past TDC then pressure will be less.

posted by  CarEXPERT

CarExepert find a writeup on VVT and the advance/retard concepts should be more thoroughly expained.

posted by  Wally

the factory timing specs are for a new engine .if you have rebuilt yours to factory spec,great.but if the engine has k,s on it ,ping detectors might work to get the max power ,adjust til ping is heard under load ,then put it back slightly,or install a knock sensor and electronic ignition,and welcome to the 90s. 110 low lead aircraft fuel at the local airport and your happening this timing thing is interesting.if you delay the explosion of gas and pressurized air
until AFTER TDC,the piston is already on its way back down .you would need a very explosive fuel to create any use for this .if you had a very slow burning fuel,and wanted say -5 to +5 TDC,YOU WOULD HAVE TO BURN HYDROGEN .GAS BURNS MUCH TO SLOW,AND ALOT OF FUEL WOULD GET WASTED.so ,put 3 spark plugs per cylinder.fire the first at -10 tdc. one at -2 and one at +4 .the third plug is fired 4 degrees after liquid nitrogen is injected to cool the top of the piston which will approach 3000 f .the nitrogen also increases your compression by 3 to 1 because it reduces your volume in the cylinder.to avoid drying out the oil on the rings ,you need to use ceramic/titanium rings and inject graphite 7 degrees before topdead center.i have been working on a prototype 1.6 litre dohc engine and i have 500 horsepower coming out of it .the problem with burning hydrogen is that it produces water.you also need three computers to control the three ignition points .the liquid nitrogen,can not be mixed with the atomized fuel,because the nitrogen is -250 f and it freezes the fuel. the graphite is the trickiest part because it has to be injected under very high pressure.i am using the 23000 psi fuel rail from the cummins turbo diesel(bosch) to achieve this ......

posted by  the lobster

Can't be done unless you swop cams or if you have DOHC with adjustable timing gears.

The people that would set the timing at or past TDC are the ones that have engines that require it. For instance a 68-69 chrysler 273 engine, timing should be set at 5 degrees after top dead center (ATDC). Alot of polution-controled vehicles sold in california have ignition timing ATDC.

posted by  DodgeRida67

I recommend reading a good engieering text such as "Internal Combustion Engines and Air Pollution Control" by E. Obert for a good explanation of valve timing and cam profiling. With a mechanical engineering background, I am more confused than enlightened by the valve timing discussion - especially in response to a question concerning the ignition timing for a dodge

posted by  tbaxleyjr

I am interested to know more on the 5° ATDC static timing. I don't profess to know much about american domestic market motors and find it intriguing.

How do they mitigate the rise in the CO levels, lower CHT and tendancy to burn valves? I guess the lower Nox and hydrocarbons was the target?

I found this site, but it quotes 5° advance http://www.sweptline.com/tech/engine2.html

posted by  Wally

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