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Compression Ratio vs BOOST - Knock Control



I copied a version of the Porsche chart above on the ADVANTAGES OF TWIN PLUGGING thread. The more I studied it for my purposes the more I realized it needed it's own string. (Young guys, bare with me it's old school technical presentation)

I expanded the Porsche chart to higher boost pressure and higher fixed compression ratio. To this I added four starred data points for well know factory engines. There are two factory TURBO engines (middle/left) and two N. A. (lower right).

I added data points from three member's engines representing serious modded street TURBO motors and I added Porsche racing motors (upper/left). These points also show features, C.R., BOOST(or not), intercooled (IC), plugging - single(S/P) or twin (T/P)

Please take the time to understand the chart and the trends it shows. I think it goes a long way toward demystifying relationships and component options as they relate to staying within the confines of stable combustion on the air cooled 911 based motor, N.A or TURBO.

This chart does not predict HORSEPOWER.

Last edited by copbait73; 03-08-2008 at 12:05 PM..
Old 03-08-2008, 10:38 AM
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The chart doesn't include the intercooler used, and the intercooler cfm airflow or outlet air temperature either... thats very important when figuring "Compression Ratio vs BOOST - Knock Control"

Compressing air quickly heats it up in a big way and that increases the chance of detonation.

So if you compress the air to around 14psi in the turbo and then cool it off good in the intercooler before compressing it very quickly some more in the cylinders, that helps alot for knock control or detonation.
Old 03-08-2008, 01:37 PM
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Quote:
Originally Posted by copbait73 View Post


I copied a version of the Porsche chart above on the ADVANTAGES OF TWIN PLUGGING thread. The more I studied it for my purposes the more I realized it needed it's own string. (Young guys, bare with me it's old school technical presentation)

I expanded the Porsche chart to higher boost pressure and higher fixed compression ratio. To this I added four starred data points for well know factory engines. There are two factory TURBO engines (middle/left) and two N. A. (lower right).

I added data points from three member's engines representing serious modded street TURBO motors and I added Porsche racing motors (upper/left). These points also show features, C.R., BOOST(or not), intercooled (IC), plugging - single(S/P) or twin (T/P)

Please take the time to understand the chart and the trends it shows. I think it goes a long way toward demystifying relationships and component options as they relate to staying within the confines of stable combustion on the air cooled 911 based motor, N.A or TURBO.

This chart does not predict HORSEPOWER.
Thanks great mapping idea. That indicates my Kremer car which boost to 1.4 bar must be built to the 6.5 of the early 934 935 as it does not detonate with 108 octane and has almost 10K miles on the original build. It has long been my suspicion that Porsche from the early baby 935 and onwards built low compression high boost motors as boost is adjustable for altitude, humidity longivity, etc and compression is fixed. My car was built in 84 and kremer had been king on tracks for many years by then. I do not know if the race cars were still 6.5 in 1984. Andial was competing closely with Kremer by then and my motor from them uses the 1974 factory and 84 Kremer low compression philosophy.

Last edited by voitureltd; 03-08-2008 at 02:28 PM..
Old 03-08-2008, 01:58 PM
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Originally Posted by JFairman View Post
The chart doesn't include the intercooler used, and the intercooler cfm airflow or outlet air temperature either... thats very important when figuring "Compression Ratio vs BOOST - Knock Control"
All true, getting into that level of detail would make the chart very busy but you do have a very good reading on the impact of adding a charge cooler when you compare the two factory TURBO motors. Apparently the factory's testing of their basic intercooler allowed a .75 C.R. increase.
In their paper they identified this addition upped the power and improved fuel economy 2-3%.
Additionally, if you look at the factory TURBO racers you see a two point C.R. increase with their large, highly efficient intercoolers plus twin plugging.
Old 03-08-2008, 02:17 PM
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Originally Posted by voitureltd View Post
Thanks great mapping idea. That indicates my Kremer car which boost to 1.4 bar must be built to the 6.5 of the early 934 935 as it does not detonate with 108 octane and has almost 10K miles on the original build. It has long been my suspicion that Porsche from the early baby 935 and onwards built low compression high boost motors as boost is adjustable for altitude, humidity longevity, etc and compression is fixed. My car was built in 84 and kremer had been king on tracks for many years by then. I do not know if the race cars were still 6.5 in 1984. Andial was competing closely with Kremer by then and my motor from them uses the 1974 factory and 84 Kremer low compression philosophy.
I placed the point using data from one of your postings. You know, assuming it isn't 6.5:1 and your C.R. is actually 7:1, it still places you right in line with Don and Les' cars. 12PLUS to 1 effective compression. That may be on the edge but you guys must be getting along with it.
Looks like it may be time for one of you three to pull out a couple more enabling technologies like water/methanol injection or knock sensors and set a higher point.
Well, Don you have a higher point out there at 13:1 but I think you told me you don't run that level.
Old 03-08-2008, 02:55 PM
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I'm missing something; the numbers seem high to me. Please explain the math behind this chart. Surely a stock 930 does not run a corrected 12:1 C/R.
When I crunch the numbers 7.0 C/R @ 0.8bar = 10.9:1 effective C/R.
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Old 03-08-2008, 05:06 PM
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Originally Posted by copbait73 View Post
I placed the point using data from one of your postings. You know, assuming it isn't 6.5:1 and your C.R. is actually 7:1, it still places you right in line with Don and Les' cars. 12PLUS to 1 effective compression. That may be on the edge but you guys must be getting along with it.
Looks like it may be time for one of you three to pull out a couple more enabling technologies like water/methanol injection or knock sensors and set a higher point.
Well, Don you have a higher point out there at 13:1 but I think you told me you don't run that level.
Manfred Kremers build spec sheet lists the HP @ 480 so that seems about right in line with the chart.
Old 03-08-2008, 05:41 PM
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Originally Posted by RarlyL8 View Post
I'm missing something; the numbers seem high to me. Please explain the math behind this chart. Surely a stock 930 does not run a corrected 12:1 C/R.
When I crunch the numbers 7.0 C/R @ 0.8bar = 10.9:1 effective C/R.
Yes, you are close enough for the point of the chart. Looking at the curved lines of effective C.R. the Factory 3.3L TURBO falls at 10.7:1 and the 3.0L. at 10:1
Old 03-08-2008, 06:22 PM
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Craig 930 RS just PM'd to report he is running 7.5:1 and .9 Bar with twin plug. This drops right on the 12:1 effective C.R. and continues to support the chart for component selection and recommended boost level.

Thanks Craig.
Old 03-08-2008, 06:31 PM
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Quote:
Originally Posted by copbait73 View Post
I placed the point using data from one of your postings. You know, assuming it isn't 6.5:1 and your C.R. is actually 7:1, it still places you right in line with Don and Les' cars. 12PLUS to 1 effective compression. That may be on the edge but you guys must be getting along with it.
Looks like it may be time for one of you three to pull out a couple more enabling technologies like water/methanol injection or knock sensors and set a higher point.
Well, Don you have a higher point out there at 13:1 but I think you told me you don't run that level.
No, I don't go above .8. I can't tell the difference between 500 and 525 hp driving around, but as your chart shows, it seems the engine knows. However, when I built my car, I built it for drivability and for me, that's torque and throttle response. The torque curves for my motor, whether its 525 or 500 hp, are nearly identical, so I see no need for 1 bar.
Old 03-08-2008, 06:44 PM
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Hmm...I know of 7.5:1 C/R 3.6 twin plugged turbo converted motor getting dynoed to 1003 HP at 2.0 bar w/ race fuel...

This graph seems to be somewhat old....
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Old 03-08-2008, 06:48 PM
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Originally Posted by beepbeep View Post
Hmm...I know of 7.5:1 C/R 3.6 twin plugged turbo converted motor getting dynoed to 1003 HP at 2.0 bar w/ race fuel...

This graph seems to be somewhat old....
Goran, there is no claim to HP on this chart however the only thing that is odd is the engine you just mentioned. Is the race fuel methanol? Please get more details, street?

Last edited by copbait73; 03-08-2008 at 07:18 PM..
Old 03-08-2008, 07:12 PM
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While I find it an interesting chart, it is a little to theoretical. (sorry I got long winded)


One thing the chart does not account for dynamic compression, which will radically alter the results. Now I did a few numbers and here are how some things compare.




Now you can see the differences in compression ratio, piston speed, and intake valve dynamics. All pistons speeds were calculated at 7000rpm except for the Baby 935 which was at 8000.

Now the 3.0 street engine will have a much higher dynamic compression ratio because of the camshaft timing than say the 935, but the 3.6 Turbo will probably have about the same dynamic compression with higher boost levels.

Now comparing boost between street motors and race motors is pretty much useless. Even comparing it between different generation street motors, say 3.0-3.6 is relatively useless. Because changes in camshaft timing, piston speed, compression ratio, intercooler design, intake design, exhaust manifolding turbo design, etc will all alter the measured restriction in the intake path (boost level). Pressure ratios would actually be a better way of comparing street engines, but even that is kinda irrelevant, except to show how improvements were made as Porsche learned more about turbo charging.

I suppose on a theoretical level you could perhaps calculate effective pressure, but it will be different in reality, because, for example the K27 will flow more airflow at the same boost levels than say a K26. So in the case of VoitureLTD he will have a different mean and maximum cylinder pressure than that of a stock 930. His turbo will pump more airflow into the cylinder than a stock turbo.

Also timing curve, piston speed, intake valve opening and closing points,cylinder head and piston design, rpm, etc will alter detonation resistance and cylinder pressure levels. This will also alter the burn speed (although somewhat controlled by fuel used) and combustion efficiency.

Maybe I wandered off topic. If so nevermind, I didn't mean to sidetrack the discussion.
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Last edited by turboteener; 03-08-2008 at 08:34 PM..
Old 03-08-2008, 08:09 PM
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I suppose on a theoretical level you could perhaps calculate effective pressure, but it will be different in reality, because, for example the K27 will flow more airflow at the same boost levels than say a K26. So in the case of VoitureLTD he will have a different mean and maximum cylinder pressure than that of a stock 930. His turbo will pump more airflow into the cylinder than a stock turbo.
Brett raised some really good points about how engine design can affect knock that skew straight octane vs compression ration/heat of compression. That I won't go into.
But, at full boost, turbocharging systems will be designed to flow enough air that excess must be vented out the wastegate (or bypass on newer engines). This means that no matter how much compressor "flow" available, the air density supplied to the cylinder is the same. 1.0 bar of charge results in the same amount of air no matter how much flow the turbocharger can supply.

Now, if your turbocharger went into choke at full throttle load, then the higher flowing unit will definitely provide more pressure. But, as I stated before, no manufacturer would design a turbocharger to hit its choke line.

A larger turbocharger combined with a correctly sized turbine wheel will likely hit a higher flow value at a lower turbine rpm (due to its compression curve), perhaps giving a higher boost pressure at a subsequent lower engine rpm. This is not guaranteed, but typically the K27 turbine and compressor wheels are designed for this. This means you may hit full boost pressure at a lower engine rpm, but if you have a 1 bar wastegate spring, the max pressure you will can get is still 1 bar.

Now, completely off topic but this reminded me of something someone said about turbochargers at high rpm. The statement was to increased boost pressure to account for the higher altitude. This is not true. Temperature being constant, the air density at seal level at 1 bar is equal to the density at whatever elevation. The only difference may be a slightly higher compressor discharge temp, due to higher delta P at max boost combined with lower cooling capacity from the intercooler, due to loss of air density all resulting in a higher intake temperature. At the same pressure, the higher temperature will result in a less dense mixture and lower O2 values.

I definitely wandered off topic. And I am pretty sure I had some really long, grammatically incorrect sentences
Old 03-08-2008, 08:45 PM
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Oh, just to clarify, my comment was not against Brett's comments on dynamic pressures seen within an engine's combustion chamber. I believe those to be correct.

As I beleive he stated, it is really hard to compare engines that are not identical in all engine design components from intake to exhaust.
Old 03-08-2008, 08:57 PM
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Wouldn't the airflow (mass) be lower at elevation though due to the lower atmospheric pressure feeding the turbo? Yes a turbo will generate the same level of boost at elevation as it will at sea level, it may take more rpm(turbo) for the turbo to reach that level. Your increased compressor outlet temps would come from the compressor working harder on less air, right?
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Old 03-08-2008, 10:13 PM
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All those opinions and infos are very interesting......does someone able to post a little bit more about the relation between those specs and hp results...?

Thanks,
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Old 03-09-2008, 04:43 AM
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I don't think there really is a relationship. Every engine will be different in what it wants as far as cylinder pressure rise and the resulting HP. I doubt you could lay out some hard ground rules as far as this boost and this compression ratio equal this HP.

Years ago I ask the question: "Would an engine with low compression and high boost make more power than an engine with high compression and low boost"? Well I got a million different answers and none of them really correct. It all depends on the application. I can take a modern pentroof four valve engine and run 11.5 with 10lbs and get excellent drivability, power and efficiency, but I would imagine that same engine running 9.0 would take more boost to make the same level of HP, thus adding more heat to the combustion cycle. It would be interesting to test it. Now with a 911 engine it will be a vastly different situation. The poor combustion chamber design and the aircooling/oilcooling require another approach. If I can get some time maybe I can build a motor with 10.0 and one with 7.0 and see what the differences would be. It would just be a matter of changing pistons. Leave everything else the same. Obviously it wouldn't be optimized for each condition, but it would be an interesting test.

One thing that would really affect the results of the test would be the usage of the engine. A street motor with limited track use, would be better with the higher compression, but the race motor would probably work better with lower compression since your turbo would remain spooled most of the time and you wouldn't have to worry about poor drivability outside of turbo.

Sorry I ramble.
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Old 03-09-2008, 06:31 AM
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I got it now, thanks.

One common data point you need to add to your chart:

Stock 3.3T 7.0:1 C/R @1.0 bar (using an intercooler)

VERY popular setup. My math says 11.6:1 effective C/R.
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Old 03-09-2008, 07:07 AM
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Quote:
Originally Posted by RarlyL8 View Post
I got it now, thanks.

One common data point you need to add to your chart:

Stock 3.3T 7.0:1 C/R @1.0 bar (using an how )

VERY popular setup. My math says 11.6:1 effective C/R.
Brian:

This is the kind of consideration I would make of this data. It isn't a bunch of theoretical data about various engine configurations. It's data comes from the same basic engine from the maker who's evaluated it's likes and dislikes and has the real world experience you can accept for your use or reject.

It's easy to get "wrapped around the axel" in these discussions. Mainly because opening the population of data too broadly includes applications that defy the chart, then you end up discussing the exceptions rather than the rule, so to speak.

In response to your data point I might add I've always heard you can increase the stock 3.3L boost to 1 bar BUT only after upgrading the intercooler. Is this accepted wisdom correct, or is it conditional? Who really knows, in the end it's your choice.

My point is this allows you some reinforcement for the next component. You can visualize how far you maybe pushing the envelop. DonE made on observation using this that I came to also(but didn't tell Don). DonE gave me two boost levels and HP. The high data point is 525HP vs 500. IMO he rightfully assessed his engine didn't like the additional boost and the apparent risk is not worth an additional 25HP on an already very powerful street engine.
Old 03-09-2008, 07:51 AM
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