timing vs compression

[psalt]

Which model Turbo had combustion chambers in the pistons ?

[Flieger]

So, just so I understand the basics, whenever the ignition point is set, the goal is to get the maximum cylinder pressure to occur at that point a few degrees after TDC for maximum angular impulse. The time it takes the fuel to burn is determined by octane rating and intake charge density. The degrees before TDC the ignition must fire is determined by engine speed because the burn does not speed up much.

[psalt]

great info here - thanks. what kind of hp changes have been seen with ignition timing adjustments on a dyno? i've seen the muscle car shows that show 10-20hp increases with simple tuning of the ignition on typical small blocks.


You can't really compare a SBC to the 911 engine. It is bad to generalize , but one study I have seen across many engine design types, states that one point of compression is only worth around a 4 to 5% increase in output. The main point about the air cooled 911 hemi is that you cannot reach optimum timing with pump fuel, so the answer is distorted by the knock limit.

The peak of NA hemi specific outputs came in the 1950's GP engines that used 14:1 compression and ignition timing over 50 BTDC. The reason they could do this was by using alcohol, benezine and nitromethane mix fuel. Pump fuel brought back the pent roof, 4 valve design from 1912.

[Steve@Rennsport]

Originally Posted by psalt

The main point about the air cooled 911 hemi is that you cannot reach optimum timing with pump fuel, so the answer is distorted by the knock limit.

Precisely, and that value is dictated by variations in outside air temps as well as gasolines.

Given what I've seen with broken rings in many late-model engines, 3.0 & larger, its best not to time it to maximum power unless one is using high-octane race gas.

Even the Motronic 3.6's have their timing limits with knock-sensing ignition systems. They cannot pull back more than 6 degrees of timing so one must be careful about programming the timing maps in these cars to avoid expensive problems when using pump gas.

[BURN-BROS]

Originally Posted by psalt

Like I said, vacuum advance is irrelevent to output, perhaps you misunderstood.

I do not think I did, this is what you said;

Originally Posted by psalt

Efficiency means BSFC, and if you study the factory figures, you will find a decrease in consumption for each increase in compression. Curb idle timing does effect NOx (not nitrous oxide) at idle, but it has nothing to do with output or fuel efficiency which occur at an advanced timing. The main thing to understand about 911 timing is that the max advance spec went from as high as 38 BTDC to 25 BTDC max because of the knock limit imposed by the factory specified fuel. A hemi headed engine with a wide VIA and domed pistons does not make peak output at 25 BTDC.

Here was my response, just to be clear

"The 25 degree spec is mechanical advance only. It does not include the vac advance which put the timing higher. I do not have the spec on me but about 10 deg of vac advance IIRC. There were a large variety of vac advance units placed on the 3.0 liter SC."


and

"Not trying to be arguementative, but the vac advance IS relevent as it was for fuel economy during cruise, another hit against increased BSFC solely due to a compression bump. One must comprehend ALL the changes made to the engine. The retard was for the nitrogen oxide @ idle.


MY statement is relevent, the vac advance affects BSFC. You are now arguing output so where is output relevent to your above post? Or did I truely missunderstand?

Originally Posted by psalt

If you look at the factory figures and compare them to the advances curves you will see your error. What I am saying is true all the way out to the 10.3:1 Carrera. Compression was increased and timing kept sub optimal to meet efficiency goals within the limit of the fuel spec.

You cannot understand this without looking at the actual data. You can find it by combining the factory manual, spec book, and the factory sales literature for each production change.

EDIT; No argument there, but it does not apply to gestalt1 RS engine nor any potential performance build with MFI, carbs, or even CIS if one were to lump that system in with performance. It's myopic to think Porsche's methods should be applied to a modified build that does not need to account for the limitations Porsche had to...

[BURN-BROS]

Originally Posted by Flieger

So, just so I understand the basics, whenever the ignition point is set, the goal is to get the maximum cylinder pressure to occur at that point a few degrees after TDC for maximum angular impulse. The time it takes the fuel to burn is determined by octane rating and intake charge density. The degrees before TDC the ignition must fire is determined by engine speed because the burn does not speed up much.

Yes,
optimum max cylinder pressure is around 20 deg ATDC and the majority of stock engines never see max cylinder pressure because of the lack of appropriate fuel and higher than optimum compression. That is why twinplugs give the bump in power. You are able to achieve max cylinder pressure and tolerate increased compression because you quite literally reduce the amount of timing in half.

[kenikh]

Now that we have that out of the way, now we can get to the real fun: why turbos are really variable compression ratio actuators.

[BURN-BROS]

Here is a graph of compression ratio vs HP increase 4%-5% is unrealistic unless the compression jump was substantial.
to be fair I am looking for additional charts to compare this one to.

[BURN-BROS]

Originally Posted by kenikh

Now that we have that out of the way, now we can get to the real fun: why turbos are really variable compression ratio actuators.

LOL, I did not think the subject would be so controvertial! But it's been a fun discussion.

[psalt]

Or did I truely missunderstand?


Yes,

I said "peak output does not occur at 25 BTDC (timing)"

You responded "25 BTDC is mechanical advance (and) does not included vacuum advance which puts the timing higher."


Which is irrelevent to measured peak output.

[BURN-BROS]

Originally Posted by psalt

Or did I truely missunderstand?


Yes,

I said "peak output does not occur at 25 BTDC (timing)"

You responded "25 BTDC is mechanical advance (and) does not included vacuum advance which puts the timing higher."


Which is irrelevent to measured peak output.

O.K. fine, I agree with that then. I was specifically targeting your BSFC statement, sorry for the missunderstanding.

[psalt]

So, just so I understand the basics, whenever the ignition point is set, the goal is to get the maximum cylinder pressure to occur at that point a few degrees after TDC for maximum angular impulse. The time it takes the fuel to burn is determined by octane rating and intake charge density. The degrees before TDC the ignition must fire is determined by engine speed because the burn does not speed up much.


If your goal is to get maximum work done from the fuel (output) the ignition timing should be set to create peak cylinder pressure around 14 degrees ATDC. This is largely a function of mechanical advantage. If your goal is something else, emissions, warm up time, efficiency, there are variations. Yes, the burn rate is somewhat of a constant, so you must advance as rpm increases to accomodate the time. If you start the ignition from two points, the amount of time needed to reach the same peak pressure at 14 ATDC is less, and less advance is needed. In a knock limited design, you will have a corrupted curve because the probability of detonation is highest at the torque peak when VE is highest.

[Steve@Rennsport]

Originally Posted by psalt

Which model Turbo had combustion chambers in the pistons ?

This one:






993 Twin-Turbo.

[psalt]

Hi Steve,

Thanks for the photo, I was visualizing something like a V-12 Jaguar Heron piston, but that look like two raised bumps on opposite edges. The actual reduction in combustion chamber volume in the head must be small (<20% cc). Ludvigsen, who is usually good at pointing out innovation, copied or otherwise, describes them as "pistons had coated skirts and lower domes". Do any aftermarket mfg's copy this design for their pistons?

If you compare the M64/50 and the M64/60, Porsche chose to raise the compression half a point and use 1.5 psi lower boost for a net 13% increase in output. The boost reduction should have dropped the output more, because in this case, the half point of compression could not be responsible for a 20% increase. The answer is that the hp peaks 250 rpm higher which skews the output calculations. The torque increase is a better indicator at +4%. It seems this change was also about improving low speed drivability and efficiency, rather than running higher boost and output. Probably the right choice given the customer spends 85% of the time below 35% load.

[J P Stein]

A bit off topic, but marginally relevant.
A funny thing happened on the way to SCCA SD National Tour event. There is a 93db sound limit. The new owner of my 914 AXer took the car to a chassis dyno in search of quiet without power loss to do some comparisons. One aspect was ITG filters vs a semi-stock 911 air box with no other changes.

The motor is a 2.7L, Solex cams, Webers, 10.5:1 Mahle, single plug on race gas home built. For AX, mid range torque is king and compression gives torque. .....

[Steve@Rennsport]

Originally Posted by psalt

Hi Steve,

Thanks for the photo, I was visualizing something like a V-12 Jaguar Heron piston, but that look like two raised bumps on opposite edges. The actual reduction in combustion chamber volume in the head must be small (<20% cc). Ludvigsen, who is usually good at pointing out innovation, copied or otherwise, describes them as "pistons had coated skirts and lower domes". Do any aftermarket mfg's copy this design for their pistons?

Hi Paul,

AFAIK, none of the aftermarket piston suppliers: JE, CP, Wiseco, Omega, Wosner, etc., replicates the factory designed dome shapes except on a custom basis. Their normal offerings feature the standard double valve reliefs.

If you compare the M64/50 and the M64/60, Porsche chose to raise the compression half a point and use 1.5 psi lower boost for a net 13% increase in output. The boost reduction should have dropped the output more, because in this case, the half point of compression could not be responsible for a 20% increase. The answer is that the hp peaks 250 rpm higher which skews the output calculations. The torque increase is a better indicator at +4%. It seems this change was also about improving low speed drivability and efficiency, rather than running higher boost and output. Probably the right choice given the customer spends 85% of the time below 35% load.

I think you're right. Ever since the days of the 930, Porsche tunes their turbo cars for torque, rather than just peak output.

OBD-2 played a role here as well since hemispherical chambers are naturally lazy and far from optimal for emissions compliance.

[BURN-BROS]

Nice sheet. It’s obviously not a street build. As soon as you have access to proper fuel then the paradigm will change. (Edited due to my inability to aparently read). Selling cars in America happened to be important so a great deal of compromises had to be made in order to do so. Output declined until Porsche reached an optimum starting point in which to develop slightly better engines (the 2.7, 3.0, 3.2, respectively). In their particular cases, slight bumps in compression were added, but not for Paul’s output claim, rather it was emissions and the targeted cruise rpm.
Gestalt1’s 2.7 could use the bump in compression, especially if he ran a sport muffler and retained the stock S cam. However, if his pistons are good, he may find that a more modern cam profile such as the MOD S will provide a very nice performance bump.
I know this argument is head into fully controlled alpha N Engine management. The real conclusion I guess should be keep your compression conservative UNLESS you have the means in which you can fully control all aspects of the operating parameters, but this does not fit with the original poster question as well as the majority of guys that visit this board. You can find a number of engine builders with actual empirical data that leads to their conclusions.

[psalt]

Paul, while being well read has drawn an improper conclusion based on a specific series of engines that output was not the number one goal. Selling cars in America happened to be important so a great deal of compromises had to be made in order to do so. Output declined until Porsche reached an optimum starting point in which to develop slightly better engines (the 2.7, 3.0, 3.2, respectively). In their particular cases, slight bumps in compression were added, but not for Paul’s output claim, rather it was emissions and the targeted cruise rpm.


I am sorry you still misunderstand what I said in my first post. Maybe you could try re reading it.

My position was and is that Porsche first lowered compression and kept optimal timing in response to the low octane fuel mandate. This increased consumption, and made raising the compression and backing off the timing more desireable.

Raising compression increases, not decreases, regulated exhaust emissions, especially NOx.

On the other hand, a one point increase in compression increases fuel efficiency 5 to 6%.

[psalt]

The motor is a 2.7L, Solex cams, Webers, 10.5:1 Mahle, single plug on race gas home built. For AX, mid range torque is king and compression gives torque. .....


It is interesting to me that the argument between torque and hp still still going strong. The surprising thing about torque is that it is proportional to displacement and all N/A engine are limited to around 85 ft/lbs liter, regardless of design. A 500 ci V-8 with a 2 bbl or an 18,000 rpm F1 engine have the same limit. And the current 750 hp F1 engine has around the same torque as your 2.7! I wouldn't get cocky though..... Horsepower is only limited by the speed of sound.

I'm not sure what a stock 911 Weber airbox on a 914 looks like, but something is causing the AFR flip on the graph. Do you know the ignition timing and fuel octane that was used ?

[BURN-BROS]

Originally Posted by psalt

I am sorry you still misunderstand what I said in my first post. Maybe you could try re reading it.

My position was and is that Porsche first lowered compression and kept optimal timing in response to the low octane fuel mandate. This increased consumption, and made raising the compression and backing off the timing more desireable.

Yea, I went back and caught it, sorry I'll edit to reflect it. My issue, is that your statements do not really apply with the original posters question, remember he has a 2.7RS spec motor...The low output smog cars and the changes made to them to comply to the standards of the day do not apply .