951_VAB

Wow Chris, nice write up! Thanks! I sure hope I've made a good decision increasing flow thru my new 2.7 na head, which was a bit more than simple PnP. I do certainly feel better educated!
Dan

JimV

Hey Chris, just trying to get a rise outta you, appreciate the info. That does bring up a question. I have a n/a head that I've been staring at for a couple of years. I had big dreams about installing a larger intake valve to justify the intake port volume but the valves look too close to the chamber wall. Do you know if there is a benifet to going larger intake valve on the 2.7? Also on the exhaust side there's not much of a venturi in the seat. From doing a bunch of big block heads this is a cruital area and I would have to believe it would be the same on the porsche head. With a smaller cross section and more radius there is usually a considerable airflow gain. Do you or anyone know if there has been much done on this?

Chris White

Remember that he 2.7 went to a 104mm bore so there was less shrouding on the cylinder wall side of the valves. So it works for the 2.7 but I don’t know about doing that to a 2.5 head. I looked at them long and hard….and bought 3 16v heads…..!!

Chris White

84black944

so much information, i definitely learned something from this Chris thanks for the post.

Tomas L

I almost missed this thread! Very good info indeed.
Concerning the bore/stroke ratio I guess it's along the lines of J Chen's post. when you alter rod/stroke ratio you will alter the time (and the crank angle) during which the piston is close to TDC. This will probably effect combustion speed but it's also very likely that it changes the point (in crank degrees) where you want max pressure.

I'll guess you could divide these influencing factors in direct and indirect regarding there effect on combustion speed and ignition advance.
For instance camshaft duration would, if I'm correct, be an indirect factor since it's effect is to increase VE which directly increases combustion speed.

The effect of engine speed, is it really through VE increase? I would guess that it was more due to increased turbulence at higher speed?

Regarding spark plug placement, center should be best as it minimizes distance to the end of the combustion chamber, but it also has to be at a point where there is sufficient mixture flow to help spread the flame.

Regarding water injection, it's already in Chris list. "Moisture in the cylinder charge will slow the flame speed."

As I said in the beginning, very good info, RL needs more of this. Only problem with this amount at once is that it gets harder to comment on the individual statements but that I can take.

Tomas

ehall

Flame propagation and the tooth fairy



Oh, I thought this was a post portaining to the effects of posting anything on this board>

mwc951

That's funny, funny sh**, although very true!

OUTSTANDING post, thanks to all for the info.

mark944turbo

Where did you get 12-15 degrees? I have always heard 10 so I am curious. Also, to address your arguement about modifications effecting flame speed, I doubt that Porsche even had the technology at the time of production to analyze this. Sure they tested, but as long as we expect to do the same I dont think there exists an arguement against head or intake modifications.

I agree, cool post, but I think there is way too much magic dust to make an arguement, its more of a cool thing to discuss for me.

mark944turbo

"If by some quirk of fate you dramatically increase the mixing and turbulence in the combustion chamber with the untested porting you could increase the flame propagation speed and get into detonation (bye bye engine) or if you reduced the turbulence designed in by Porsche you will create incomplete combustion – a drop in power."

Another question, since when does increased turbulence cause knock? I think it creates a more uniform mixture that will burn faster, but increased flame speed is usually an advantage, that would decrease tendency to knock.

Chris White

[QUOTE=mark944turbo]Where did you get 12-15 degrees? I have always heard 10 so I am curious.

I have seen different papers come to different conclusions. I don’t think there is one exact answer – It can depend on the combustion speed – you are trying to balance the added resistance of igniting the mixture before TDC with the greater duration of cylinder pressure after TDC. So a quick burning mixture (high boost) might be different that a slow burning mixture. Another one of those ‘area under the curve’ things!

As to the Porsche testing – it is very interesting that they went from a quench style chamber/piston to a swirl type when they changed from the 8v to the 16v heads. The quench heads were an older technology that had been ‘played with’ for quite a while by other manufactures with most of the improvements coming by empirical testing.

The argument is not against modifications per se – it is against ‘blind luck’ or bigger must be better mods. Improvements are made by increasing the efficiency of the system not just adding a ‘bigger, better’ part or mod. I would be willing to bet decent money that a first timer with a die grinder will not increase output. Jon Milledge with a die grinder – that’s a little different!!

The post is way to broad range to be considered basis for an argument, it was more to raise a little consciousness and think about the system as a whole. We always here about getting the A/F ratio dead nuts on but rarely is timing mentioned as an issue. Sure - A/F is relatively easy to measure and to modify but timing is power!

Chris White

Chris White

Now it gets interesting!
I will admit in advance that you would have to make a serious change in flame propagating but if you did increase the rate of propagation and maintain the same ignition timing then the mixture would burn faster and the pressure would increase earlier in the compression cycle. If you get to far advanced the spike in the pressure rise can get significant enough to auto ignite the mixture in front of the flame front. Remember that if you compress and heat a gasoline mixture enough it will ignite by itself – this is preigntion or pinging.

Increased flame speed is an advantage if you change the ignition timing to adjust for it. If not it can cause knocking.

Also – remember what the KLR does when it hears knocking – it retards the timing therefore removing the early pressure rise.

Chris White

PS – glad to see people thinking.

mark944turbo

Chris, now that you have clarified I agree with you. But, I still dont think it is realistic to say increasing turbulence therefore flame speed can cause knock if timing is not adjusted. There isnt data to back this up. Theoretically sure, but how much can flame speed really be increased by pure turbulence? Not much when it is already pretty hectic in there during the compression stroke.

TurboTommy

Mark,
I think Chris means this: The reason that good mixing and turbulence, and therefore higher flame speeds, is good for anti-detonation is that this ALLOWS the ignition to start later, yet still have complete combustion by a little after TDC. Increasing flame speeds, by itself, without adjusting timing, would not be taking full advantage.

Chris, good info
I've read this myself before on the net, in that chart form you mentioned.

There are a few misleading pieces, however.
Somebody else, here, mentioned this already: concerning AFR; anything leaner or richer than stoichiometric slows combustion. This is not correct. Highest flame speeds are at around 12.5 AFR with slight variations in either direction.

Octane ratings of fuel have no effect on flame speed. Some say that lower octane burns more violently; but that's only because it's often at the threshold of detonation.

I don't know if this was clear: a colder charge, although denser, will burn slower.

UK952

I used to live near a company (20 years ago) that did some very secret work for formula one teams. They built cylinder heads in glass (very expensive and short lived) so they could film the effect of different mods on the flame propagation.
Tony

J Chen

The long rod vs short rod thing has been going on
for sometime. Since our engines are'nt high rev
engines, perhaps there may be some benefit by running
longer rod. With longer rods, theres the advantage of
getting more torque out of every power stroke. Yes
some will argue that the piston will move down slower
during intake which does not help cylinder filling but the
question is how much slower is it & what is the optimum
lenght. There is also an added benefit of running longer rods.
Less friction between piston & bore & also a reduction of
secondary harmonics. Chris, you're absolutely correct in
stating that everything must work as a system. For me I
feel that the area to address first is here. This of course
refers to an all out assault on getting the max out of the
engine.