timing vs compression

i was adjusting my timing today and began to wonder which effected the engine more. timing or compression.

compare these situations

1 - stock engine with stock timing and compression running 87 octane gas
2 - slightly lower compression and advanced timing to the point of pinging on 87 octane gas
3 - engine with raised compression and retarted timing to the point of pinging on 87 octane gas

i guess what i am asking is which will drive the best under most circumstances.
when i say raised compression, i am assuming that you dont raise it enough to have to retard the timing any rediculous ammount.
same with lowering the compression.

anybody have any theories on this?

So.... which is the worse evil, spark knock or pre - ignition due to excess combustion pressure..........
IMO both are equally bad, in that both produce excess and damaging forces on the engine parts (reciprocating assembly ) and both, if severe enough, make the engine work against itself by possibly burning fuel as the piston is still moving upwards.

Realistically, it depends on engine design. Certain combustion chamber shapes tolerate higher compression without detonation (ie hemispherical ) than others (ie OHV wedge,or sidevalve). Therefore, higher compression may provide efficiency benefits in some engines without much ping.

However, regardless of engine design, over - advanced ignition is always bad.

Still, for the choice you offer, #1 is best. I believe in most cases excessive compression or timing are both detrimental to engine durability and efficiency and are not useful tuning methods if taken to excess.

The answer is that the correct combination is what's best. Lets assume that your car is properly tuned for your current combo of compression and 87 octane. The total ignition lead lets say is 38*. For most engines and designs, even if you went to 10.5:1 and 94 octane, the best power will still be found right around 38*. The reason is that the distance from the spark kernel to the piston crown hasn't changed enough to make a big difference. You're still shooting for peak cylinder pressure at the proper crank angle regardless of the rest of the combo.

So, in your examples, the stock combo (#1) is going to be the best. If you add compression and have to retard timing to prevent detonation, you have taken the engine out of optimal tune. You're crutching high compression by retarding timing. Peak cylinder pressures will occur after the sweet spot of crank angle and you'll lose power. Same goes for the other way; if you reduce compression and add timing, you're making too much pressure early and its all over before the sweet spot in the crank's travel.

The best way is to do it right... that's not to say that the stock parameters are the best, but assuming they started in tune, changing them like you mentioned would be taking away from optimal tune one way or another.

i guess i mislabeled the conditions

what i meant was this
1 - engine with 9.5 compression and timing advanced as far as possible but still not pinging on 85
2 - engine with 8.5 compression and timing advanced as far as possible but still not pinging on 85
3 - engine with 10.5 compression and timing avanced as far as possible but still not pinging on 85

obviously, with all other things being the same, the timing will have a greater overall advance on the second case, a greater overall retard on the third case, and a medium advance on the first case.

this is a hypothetical question of course, but if you need another way to think about it, think about it like area/volume ratios.
basically i am asking weather advancing the timing or increasing compression on an engine will yeild better drivability.

sorry if it still doesnt make sense





my thinking is that compression ratios have a larger effect on the feel of an engine than timing, so case 3 will probably feel the most like a "sports car" engine, wheras case 2 will feel most like a "classic V8" engine"

to reply to curtis, i get what you are saying.

if i understand correctly, there is an optimal pressure that an engine will have to start ignition.

say that pressure is 175 psi for example.

my thinking is that it doesnt matter so much weather you have a higher max cylinder pressure ( say 190 for example ) and retard the spark until the pressure is 175 psi, or a lower max pressure ( say 180 for example ) and retard the spark less but still ignite at 175 psi.

if this is the case, then my thinking leads me to believe that the higher compression engine will make more power because the piston will have a higher speed at the point of spark than the lower compression engine will.


continuing on with this logic, the higher compression engine will probably have a lower mean piston speed, and will be a high revving engine.


of course i may be wrong here, so let me know what you think

to reply to curtis, i get what you are saying.

if i understand correctly, there is an optimal pressure that an engine will have to start ignition.

say that pressure is 175 psi for example.

Not quite what I was talking about... I was talking about PEAK cylinder pressures... as in, after ignition. For best operation, the expanding flame front should be at its peak pressure at about 20-23 degrees ATDC. Increasing the compression or changing the octane doesn't appreciably alter flame front speed, so that is why ignition advance is most efficient at the same place regardless of other states of tune.

your clarified question can't really be answered. An engine like a flathead Ford couldn't run on 85 octane at even 6.5:1 without detonating, but I can run 85 in my LT1 with 10.5:1. Way too many variables to predict.

But... making some assumptions, like let's assume its a traditional small block chevy with smogger heads. That would fit your #2 scenario of 8.5:1 compression. Optimal spark lead on most SBCs is probably 38* total. So, if you increase compression to 9.5 and retard ignition accordingly, you are out of tune. Your flame front's peak cylinder pressure is occuring after it can do its best work on the crank. Same goes for the 10.5:1 example. Its best ignition lead is still about 38* but you have to retard it even further.

Now lets say you drop the compression to 7.5:1. Now you can (in fact need to) advance the timing to compensate for the extra distance and lower pressure before ignition so that you can still get your peak pressure after ignition in the right place on the crank.

Compression doesn't change flame front speed.
Octane doesn't change flame front speed.
Flame front speed is the largest factor in when cylinder pressures peak, so since flame front speed is relatively fixed, ignition timing is the biggest key. The moral of the story is, build your engine in whatever compression you need for your cam, set the timing for peak power (probably the same for any given engine family), and then run the octane you need to keep detonation away. Anything else is a crutch and will operate with less power and efficiency than if you were running the right combo of parameters.

If that weren't the case, then there would be no need for high octane gas. We could just all run 10.5:1 on 85 octane by just retarding the ignition, but that's not the case.

cool thanks
i guess i didnt realize that compression had such a small effect on flamefront
compression should affect the speed of the reaction, but i guess that other variables rule it out.

thanks for the info



p.s.
is it still true though that higer compression engines usually have a greater overall ignition retard to take advantage of the fact that the crank will be at a greater angle to the rod/piston, and will therefore make more power?
that is what my theorizing leads me to believe, but again, i may be completely wrong

Or with some clever valve timing, you can mimick the atkinson cycle.
Later intake valve closing to lower the compressoin ratio, while keeping a big expansion ratio.

p.s.
is it still true though that higer compression engines usually have a greater overall ignition retard to take advantage of the fact that the crank will be at a greater angle to the rod/piston, and will therefore make more power?
that is what my theorizing leads me to believe, but again, i may be completely wrong

Not really. The way the geometry works out, the same crank angle is where you want peak pressure regardless of compression, rod/stroke ratio, stroke, most anything. The reason is (since ignition happens BTDC) pressures are rising as the piston is still on its upward travel. If you advance too far, you're wasting flame pressure on fighting crank revs in the wrong direction. If you go too late, the piston is falling away faster than the flame pressure can effectively act on the crank and you lose power. That's why you want peak pressure in the same place on the crank. Its also why crutching tune with anything less than the required octane will hurt power.

The retarded timing (if necessary) on higher compression engines has to do more with the reduced distance from the spark plug to the piston crown. Other factors are involved, like piston domes. If you have achieved your higher compression with domed pistons, you may actually have to advance timing to keep your engine tuned. Domes slow flame fronts and make the peak pressure happen a greater amount of time after ignition. BBCs and Hemis are notorious for this. The typically large open chambers require big domes to get decent compression and it makes them somewhat poor in the detonation tolerance category. I run my 8.5:1 454 with 87 octane and it likes to detonate a little because I have to run 40* total advance for best power. Contrast that with LT1s with their fast-burning chamber design which accelerate flame speed, and you can get by with 87 octane on 10.5:1 because you only need about 30-32* total advance to get you peak pressures in the right place on the crank. BUT... typically, just talking about one engine family; usually a 10.5:1 traditional SBC runs best with the same total advance as an 8.5:1 SBC.

Its also important to note that octane is not cumulative; that is to say, more doesn't really affect things. If you have a combo that needs 91 octane, anything below 91 will require you to retard timing, but running 104 won't really let you advance it much. That's because it doesn't really change flame front speed. The cylinder pressures will peak at the same amount of time after ignition regardless of octane... its just that if you don't have enough octane, it might ignite before the spark plug says it should :) Then you get detonation from insufficient octane.

I would agree with most of what Curtis has already stated. There are a few points of clarification that might be useful.

Compression ratio does not affect flame speed. It does raise compression temperature which persuades the ignition point to occur at a slightly earlier crank angle. Perhaps even before the spark plug fires. Given a sufficiently high compression ratio we don't really need a spark plug. When the autoignition point of the fuel is reached, it will ignite without a spark.

Flame speed is primarily determined by squish velocity. Squish velocity will determine the turbulent flame speed by agitating the unburned mixture and bringing unburned fractions in contact with the wrinkled flame front. Squish velocity is determined by combustion chamber design, piston to head clearance and piston speed. An engine with higher squish velocity can run on lower octane fuel without detonation but may run the risk of pre-ignition if excessive. An engine running at very high speed can use lower octane fuel.

Higher octane fuel might control the onset of detonation but it will not prevent pre-ignition due to excessively advanced ignition timing. Therefore, ignition advance timing is much more important than compression ratio. But, as Curtis clearly said, the goal is to produce maximum combustion pressure at the proper crankshaft angle. Pressure rise prior to TDC can destroy pistons and conrods and bottom ends, late pressure rise will reduce power and overheat the exhaust valve and port and pipes and water jacket.

Dyno testing with lambda sensors and knock sensors are needed to provide the absolutely correct answer for a given engine/fuel combination.