timing vs compression
 

which is more important - dyno time with a lower CR motor to get an optimized timing curve or to increase the CR to gain more power but having less ideal timing?

i realize that increasing CR and adding twin plug so the timing can be set for greatest power/torque is ideal but how much is the increased CR adding vs the better timing?

Proper timing will be better. The high compression ratio puts more stress on the motor even with optimal timing; with compromised timing, the engine can melt down with heat related stress and/or negative work being done by combustion on the compression stroke.

thanks fleiger. my thought is that the 2.7rs engine had only 8.5:1 CR, increasing that to 9.5:1 might require backing off the timing and therefore the CR increase would not result in more power because of the timing change to prevent detonation.

9.5:1 is fine for single-plug and is a common upgrade. The issues arise in the 10.5:1 range.

Ah grasshopper, you have finally created a thread for a topic that has needed addressing for a long time.

The ultimate answer is BALANCE. The ultimate goal is to balance 'perfect' ignition timing with the highest CR your motor can support without knock. Since knock tendency changes dependent on load and RPM, CR tends to ultimately be a compromise to the highest static CR a motor can support, within the range of an acceptable ignition curve across the entire RPM band.

The rules of thumb of 9.5:1 for single plugs and 10.5:1 for twin plugs are the tried and true specs for a Porsche with a programmed ignition curve (whether it be vacuum or some other advance mechanism). Load is the biggest bugaboo that leads to necessary conservatism, since loaded motors knock more readily.

This is why knock detection is the key to running higher CR. You can be less conservative on static CR, since the motor can detect transient knock and adjust the ignition curve in the short bursts of knock in on the fly in loaded states. Thus, you end up with a more dynamic ignition curve and can thus run higher static CR with these sensors.

Cam choice and overlap/VE are also important elements, but that is a deeper discussion. :)

Net-net, optimal ignition timing will always pay more dividends than artificially high CR band-aided with crappy ignition timing.

The static compression ratio is not as important as the dynamic compression ratio. If the camshafts are changed and intake valve closing delayed the dynamic or effective compression ratio is reduced. If additional static compression is not added to offset the compression lost to a cam change the power will not be optimized. This subject is long overdue and yes "balance" is required.

I will slide back into my corner now as I expect this will be considered wrong by someone or another on this board.

PFM

Good info here,...:)

Its important to understand that thresholds of detonation are dynamic in air-cooled engines where cylinder head temps mirror outside air temps and engine load. This means that one must be exceedingly careful in setting ignition timing to allow for variations in environmental conditions as well as fuel.

More information on this critical subject; Welcome to Rennsport Systems, Porsche Performance Products for the 21st Century

Regarding your original question, all things being equal, I'd rather have a lower-compression engine with a good ignition advance curve, than a higher-compression one with retarded timing to accomodate the fuel you will use. The former is a lot more "eager" to rev,....:)

Steve,

Is this a good time to ask about ceramic coatings on combustion chambers and piston tops? This should help with some of those dynamic conditions. It seems the air cooled engine should benefit more from ceramic coatings than a water cooled engine.

Stay Tuned

PFM

Sure is,....:)

I think the main beneficiaries of internal thermal barrier coatings are turbocharged engines and very high compression race motors. They do help contain the heat inside the chambers where it does the most good: increasing thermal expansion to increase efficiency.

The side benefit is a reduction in piston (thus oil) temperatures and cylinder head temps, both of which are critical in engines with high cylinder pressures.

Another good idea is Grady's "Rubbermaid Solution". That will really reduce cylinder head temperatures which is the key to maintaining HP when the engine is hot,....:)

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. i guess i'm comparing the cost of dyno time to the cost of raising the CR. seems to me that taking the effort to re-build a performance engine is really wasted without dyno time to really set the timing correctly.

which is more important - dyno time with a lower CR motor to get an optimized timing curve or to increase the CR to gain more power but having less ideal timing?

i realize that increasing CR and adding twin plug so the timing can be set for greatest power/torque is ideal but how much is the increased CR adding vs the better timing?



The correct answer to your question is "it depends". There are so many variables, temperature, octane, mixture, piston shape, bore size, etc for an absolute answer. If by "more important" you mean higher output, output increases around 4 to 5% with each point of compression (within a range, and up to around 14:1). Less optimum timing, within a few degrees of MBT, has a lesser effect on output. Just about all aircooled 911 road engines are knock limited and detuned for the factory fuel spec. Once you add this into the mix, the results are different. Porsche's response to the elimination of 5 star 98-100 octane fuel initially was to back off the compression ratio, as in your 8.5:1 example. After the oil embargo when fuel efficiency became an issue, epecially in advertising to sell new models, they changed direction and increased the compression ratio several times and backed off the timing . The increase efficiency from higher compression was more than worth the reduction in timing.

If you mean efficiency in regards to an emission standpoint then kinda. The majority of the changes made to the engines during the 70's-80's was to pass the idle emission limits instituted by the state of California. The CIS cars were designed to run lean so timing was reduced to protect the engine AND to lower the nitrous oxide levels. The bumps in compression had very little to offer in regards to performance IMHO as I doubt you could "feel" it. My guess was to play with cylinder head temps. So if you do not have to run the engine that lean or adhear to the emission laws, then a ignition map with a more initial timing and a quicker ramp will yield much better results than a half a point of compression.

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.

Sorry, I meant Nitrogen Oxide, good catch. I do agree with consumption decrease with the increase in compression, but it was not the driving force behind the changes. You have to look at ALL the changes implemented, not solely on a compression bump. There were BIG changes in camshafts profiles and exhaust backpressure when this all took place.



Depending on the size of the chamber, a Hemi head can require more than 40 degress of advance

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.

EDIT; The original post is....which is better, full advance or higher comp? The changes made to the CIS cars for emissions/cafe only apply if the cars are in a stock configuration.

Some more input to compression and knock.

Below is a quote from David Redszus, a racing engineer. Clearly he is describing the 3.6 piston. The underline I provided. Makes you want to look a little harder at some of the aftermarket pistons.


True hemi chambers have a real problem with compression ratio and squish velocity. The resulting combustion chamber shape does not lend itself very well to flame front travel and is often quenched on contact with a chamber or piston surface.


The best solution I have seen was a design by Porsche. They used a full dome piston nested closely against the chamber wall to provide a high squish velocity. The actual combustion chamber was actually a bowl cut into the top of the piston. This combination produce extraordinary high flame speeds, and a compact chamber to prevent detonation. I could see where they could run very high turbo boost pressures and still prevent detonation.

Regards,

PFM

The drive for better consumption is documented in the factory literature and prominent in the advertisement of the period. The changes were primarily driven by the requirement that all cars run on unleaded, lower octane fuel. Higher octane unleaded fuel was not available. Once you understand the timeline, the changes in the factory's approach is obvious.

Output is measured at WOT, vacuum advance is irrelevent. Vacuum advance is a device to improve part throttle cruise efficiency. The US 911SC has both vacuum advance and retard. The retard is strickly an emissions device, uses manifold vacuum, and is only really active at idle. Most SC cold start better and run cooler at idle with it disconnected and the idle speed readjusted.

I think the best way to answer the OP question is, once the fuel spec drop occured, the 70's and 80's models were detuned timing wise. If you have higher octane fuel, these models respond to advanced timing. Once the emission strategy advanced to the feedback converter, the factory increased compression to improve both output and efficiency. Even when the fuel spec rose, they chose to keep raising compression rather than timing to post better consumption figures.

Good post,
The principal is the same as the earlier CIS pistons which "Quench" the non sparkplug side of the combustion chamber. This yielded a smaller combustion area, pushing the mixture to the ignition source. Combustion management is much easier. It is incorrect that people say these pistons were designed to promote swirl...at least in the initial design, I guess it's plausible that it could. The piston was again for emissions/efficiency. The piston design changed on the 3.6 because of the incorporation of twin plugs

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. The bump in comp yields less than 1% to the total HP output....Porsche was engineering an engine that could cruise below 3000 rpm and targeted almost every aspect to achieve this.


This promotes my philosophy, You have no lattitude with this engine unless appropriate fuel is available. Where is the fun in that build?

Too many CIS owners try to put Euro pistons in their cars, get no increase in performance and either break rings or burn pistons. The same with most single plug builds for that matter. Most people try to push compression beyond the reasonable range.

In regards to gestalt1's engine;

This is the few times where a compression bump is fine and recommended! The bump is only about a 1.5 to 2.0% hp increase, but a bit more torque throughout the rev range. If you need new pistons, then go for the increased compression.

Here is what I would do if it was my engine.
9.5/1 comp
You could keep the curve as is but there will be some latitude depending on exhaust, so a little dyno time may yield some hidden ponies.
MOD S cams is you require a regrind.

You will love it!

I overlooked this part of your post. I agree

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