Ulimate 98 in a n/a ecotec V6 ?

[ralcool]

Its a bit early for my head to process an answer.... and went looking for a lengthy article instead. (Good for all of us anyway)

http://www.xcceleration.com/cr-boost%20101.htm

Short answer. More Oxygen & heat with boost. Only More heat with higher compression.

[vlad01]

well yes, as you want peak cyl pressure to happen at 14-16º ATDC, so the faster the burn, the later the timing must be to get the peak pressure in that window to make the highest torque.

Yes all that part I understand, if it burns slower then advancing spark timing to get the peak cyl pressure at the optimum spot ATDC & if it burns faster then retarding the spark timing to get peak cyl pressure at that same optimum spot ATDC.

I suppose what I am asking is why raising the C/R as in the example of a Top Swap L67 onto a n/a ecotec V6, creates a faster burn in the combustion chamber. Where as in a L67 if you add more boost (raising the dynamic comp ratio) it doesn't seem to have that effect ?

Cheers

Not entirely sure, I just did all the calculations and the boost has a bigger effect on final compressed charge temp that going up 1 point in CR not considering any other factor.

Might have something to do with the amount of charge temp increase from uncompressed and compressed state when already in the cylinders and CR is directly proportional to this. The other thing is, how much is the charge cooled by the fuel on the way in lower boost vs higher boost?

[Charlescrown]

Vlad01 is spot on the money when he said that high octane fuel does not burn slower but instead has better knock control. There is a big misconception with octane and what happens. Timing is determined to achieve 2 things. Maximum power and knock control. Some engines suffer power loss because the manufacturer wants the car to run on low octane fuel so they run retarded timing to achieve this which . They may well be producing maximum cylinder pressure as late as 20 degrees after TDC. Higher octane fuel and advanced timing will achieve a nice gain with this car but another which has the max pressure at 12 deg after and does not detonate with 91 will gain nothing even if you advance the timing.

[OZ38]

Vlad01 is spot on the money when he said that high octane fuel does not burn slower but instead has better knock control.

Ok so if high octane fuels & low octane fuels don't burn any slower or faster than one another, how does advancing or retarding spark timing to achieve peak cylinder pressures just after TDC work ?
I was thinking that if high octane burnt slower you would need to advance the spark timing so peak cyl pressure occurred just after TDC. But if they (high & low octane fuels) burn at the same speed why not advance the spark timing on the low octane fuel to achieve peak cylinder pressures just after TDC ?
Now your going to say because low octane fuels don't control or resist knock like higher octanes do. So onto my next question for you now (hope I'm not boring or annoying anyone ).

There is a big misconception with octane and what happens. Timing is determined to achieve 2 things. Maximum power and knock control.

I think I understand how Timing determines Maximum power by trying to get that peak cylinder pressure to occur just after TDC. So how does Timing advancing & retarding control knock ?

Some engines suffer power loss because the manufacturer wants the car to run on low octane fuel so they run retarded timing to achieve this which . They may well be producing maximum cylinder pressure as late as 20 degrees after TDC. Higher octane fuel and advanced timing will achieve a nice gain with this car but another which has the max pressure at 12 deg after and does not detonate with 91 will gain nothing even if you advance the timing.

With the end of the above, why would you not gain anything if you advanced the spark timing a little more to get the max pressure to say 6 degrees after TDC ? Or is that small amount just that too fine to achieve realistically ?

Cheers

Deek

[Jayme]

With the end of the above, why would you not gain anything if you advanced the spark timing a little more to get the max pressure to say 6 degrees after TDC ? Or is that small amount just that too fine to achieve realistically ?

actually the peak pressure goal is somewhere between 15 and 20 deg ATDC.
if you move the peak pressure to occur too early, then you arent pushing the crank with as much leverage during the time when your piston is doing the most work, due to the angle between the big end and the crank center. and the power will drop.

I think I understand how Timing determines Maximum power by trying to get that peak cylinder pressure to occur just after TDC. So how does Timing advancing & retarding control knock ?

knock occurs when the pressure rises above a certain point before the piston has reached TDC. this can happen from having too much advance and fuel having too little octane or by fuel igniting without a spark. by retarding the timing, the pressure rises later and is hopefully below the knock threshold at TDC, while also creating max pressure at the right time ATDC. if your octane rating is too low, you might have to retard timing to prevent knock but also not have acheived the optimum peak pressure point.

I dont know much about the reasons WHY octane rating helps, only the affects it has on your timing, but a few things I have read says that higher octane fuels have a higher temp ignition point, which means it can withstand more pressure and heat before igniting. this helps because it takes a higher cylinder pressure and temperature for the flame front from the spark plug to fully ignite the mixture, thus making it ignite later.

[immortality]

There are some interesting arguments. It's interesting that these days that when we talk about CR we are still talking about the static compression ratio which in reality means very little without knowing the cam and cam timing. What is much more important is the dynamic compression ratio.

The Ecotec and L67 make an interesting comparison because at first glance the only difference between these two engine is the static compression ratio and everything else is equal because it has the same cam, however if I'm not mistaken the L67 uses a different timing chain set and the cam is timed in slightly differently compared to it's N/A cousin the Ecotec altering the cam timing will effect the dynamic CR of the engine even though the static CR has not changed. The article linked further up this thread mentions a safe CR of 12:1 on an aluminium headed engine, I say this is total bullshit because the static CR is totally irrelevant without knowing the cam/timing. The Author than goes on to compare the static CR of a V8 engine and a Hayabusa motor cycle engine which have absolutely nothing in common.

This whole topic really should be combined with discussion about knock as it is all linked, engine knock is a factor of time, pressure and heat. Increasing any one of these factors can increase power created but can also induce knock. One way to counter knock is to use a higher octane fuel which is more knock resistant and the topic of this thread but the other 2 factors are just as important.

I would tend to agree with some here that there is more to the different fuels (91, 95, 98 etc) than just the knock rating. Assuming everything is equal other than the knock rating (of the fuel) putting 98 in an engine that is tuned for 91 is a waste of money. I however am of the belief that there is more to the difference between 91 and 98 other than just the octane rating, this belief comes from nothing scientific but what I have experienced (tried running 98 in a stock ecotec) or what I have been told by experienced engine tuners and what various experienced engine builders have said.

Going back to the L67 for a second, OZ38 mentions running more boost but not reducing the timing further back in this thread, I'm going to assume we are talking about things like doing a simple 10psi boost upgrade on an otherwise stock engine/tune. The reason this works (from my point of view) is that the factory tune runs extremely rich when in PE (power enrichment mode) for what is a relative low boost engine in OEM configuration.

(can you tell my dyslexia is kicking in, if I type what is happening in my mind we will be jumping backwards and forwards between the various topics just like my mind does.....)

To go back to the original example of this thread, doing a top swap on an ecotec bottom end, the higher CR of the ecotec bottom end will increase cylinder pressures which will mean the charge will burn faster reaching peak cylinder pressures earlier (using the standard L67 ignition timing) than the lower CR L67 bottom end. This means the top swap engine will make more power but will also get into engine knock sooner than the equivalent L67 engine. There are a number of ways to get around this issue of course, use better fuel, reduce the ignition timing or add more fuel. By adding more fuel (running the mixture richer than ideal) you reduce the burn rate of the charge in the cylinder and this seems to be the way some people do things because the assumption is that more timing is always better but as has been pointed out in this thread the reality is that once peak pressure is happening at the sweat spot (about 15° ATDC) there really isn't any point in adding in any more timing as the engine will not produce any more power but is more likely to have knock issues.

I could spend ages typing out my thoughts on this but someone else has done it much better so I'm going to link the article instead (this article is actually about knock and detecting it)

Knock Control Setup -EFI University

First, here's a brief background about engine knock. On the compression stroke, a mixture of fuel and air in the cylinder is compressed and it's temperature rises. (Boyle's Law) Between 40-10° BTDC the ecu will command ignition of this mixture and a controlled burn moving out from the spark plug will start to release heat, increasing pressure even faster. If ignition timing was right, cylinder pressure should peak right around 15° ATDC to get the most mechanical work out of the burned fuel. At that point, the flame has swept across most of the cylinder but only about half of the energy has been released. At a certain temperature for a given fuel, the mixture will autoignite (knock), which is essentially a bomb going off in the cylinder causing an extreme pressure spike. Although pressure may peak around 15° ATDC (or well before or after depending on ignition timing and burn rate), temperature is still increasing because of the burning fuel, and there's still a large amount of energy waiting to be released. It's in the 0-50° ATDC region that knock will occur. Generally, knock at an earlier crank angle will be more intense since there is more fuel available to spontaneously combust. Combustion chamber shape will have an effect on required timing advance. Less timing advance is needed for a small displacement pentroof combustion chamber. More timing advance is needed for a large displacement wedge combustion chamber. This has an effect on propensity to knock due to the amount of time the air and fuel mixture has to heat up. The faster burning design being less knock prone. A term that I've often seen associated with knock is pre-igniton. Pre-ignition is the ignition of the mixture before the spark plug fires and is caused by hot spots in the cylinder. Pre-ignition will almost certainly lead to knock. Sustained, severe knock will break ring lands, blow head gaskets, and melt holes in your pistons. The heaviest hand in reacting to spark knock is ignition timing retard since this variable will have a drastic effect on cylinder pressure and temperature. Increasing fuel will also absorb more heat, so a combination of these are what we implement to combat knock. For more on this subject, check out chapter 9 of Heywood's book, Internal Combustion Engine Fundamentals.

[TdracerTd]

Just something that I don't think has been mentioned. Higher octane fuels burn cooler and require more energy to ignite. So, by running a higher octane fuel your combustion temp for a given set of conditions will be lower, reducing the heat in the heads/combustion chamber, added to that, the fuel requires more heat/energy to actually combust. That's where the knock resistance comes in.

[vlad01]

Just something that I don't think has been mentioned. Higher octane fuels burn cooler and require more energy to ignite. So, by running a higher octane fuel your combustion temp for a given set of conditions will be lower, reducing the heat in the heads/combustion chamber, added to that, the fuel requires more heat/energy to actually combust. That's where the knock resistance comes in.

Yes it does burn cooler for a given CR, but the idea is its more energy dense and requires higher thermal input to react it with oxygen completely. So its required to have a higher dynamic CR to be able to release its higher energy content.

That is the reason it yields better economy in engines that have a high enough CR, if the CR is sufficient enough better power is yielded too. At those scenarios 91 fuel is virtually out of the questions due to its low knock threshold.


I have noticed 98 fuel always burns blacker in the combustion chamber and exhaust in stock V6 with 8.5:1 compared to 91 with the same AFRs, that means the combustion can't be fully utilized on 8.5, not even 9:1.

On ecotec with 9.4, that would be coming into usable range.

But with the quality of 91, I'd be using 98 regardless.

[immortality]

That may be true to a degree, If you use 98 in a low comp motor you can simply advance the timing and run the mixture leaner, creating more heat and pressure earlier which would have a similar effect to having a higher static CR.

Why not use 95?

I run 95 in the missus L67 simply because you can't always get 98 and I'm not a fan of BP, don't get me wrong, they probably have good fuel but I hate the company. At the end of the day I might be sacrificing a bit of power but the car spends most of it's time doing the work commute etc so it's not like it ever gets to use those extra killer wasps anyway.

[vlad01]

95 has some additive that ruins the fuel system in no time.

I did a write up somewhere in my build thread. I replaced several pumps, 2 tanks and dozens of filters that were all trashed on 95.

Through process of elimination it was not related to what servo I used but came down to the 95 I was using. 91 was fine as well as 98.