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Effective Compression on Boost

We see a lot of people experimenting with different static compression ratios and boost levels.

It is interesting to see what the effective compression ratio(ECR) is with different combination. Bellow is the something I saw in another post that displays this nicely.

By playing with a mix of base compression ratio, boost, cam timing, AFR and intake temperatures we try to do the dance around pre-detonation.

It is interesting that a stock 930 running .8 bar boost on a static CR of 7/1 is an ECR of about 10.7/1 against a 993 CR of 11/3.

Looks like it would be fun to build a 6/1 CR motor and run 1.4 bar of boost for a ECR of about 11/1. It also looks like a motor running.8 bar boost on 8/1 CR might be on the edge and in need of race fuel to run safely.

Can anyone help me better understand how some are running boost in the 20 lbs/inc and even 30's?



Old 04-07-2009, 01:05 PM
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Only guys I see claiming that are ricers, and the crew of Fast and Furious movies.
Good question as I wanna know how too.

The only thing I can think is they are retarding timing and running race gas with modern EFI and ignition controllers. Still don't see it though.
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Old 04-07-2009, 03:47 PM
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My son is running 10psi boost and 10.2:1 compression ratio on 91 octane pump gas with no intercooler in a SVT Focus. That is off this chart.
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Old 04-07-2009, 04:14 PM
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Sorry, I'm no enginerd and don't have good technical reasons, but my 'other life' lies within the Audi realm. My former '95 //S6 ran an OE K24 that I had a custom chipset live-burned (on road) to 22.5 psi, on the OE 2.2 5cyl. 9.3:1 CR. All OE Motoronic ECU / Bosch Plat. plugs / individual coils, and 93 octane.

The next (factory!) step up was to run the RS2 K26 hybrid snail, which most guys ran @ 26psi (all day every day day in day out WOT 7,200 rpm 10's of thousands of miles) which produced typically in the 320hp to (all 4) wheels, and some nutters even spike up to 28-30 range.

I put 80k WOT @ 22.5, sold it @ 165k and was running strong. It's quite nutty to experience the caucauphony of madness at that psi / RPM, definitely titalating. On a cool misty morning, boosting it WOT was like VEHICULAR BONGHITS. Miss that car 8-).

So I guess my answer is ECU, chipset burn, and forged crank / internals.

HTH.
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Old 04-07-2009, 05:51 PM
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In our P-51D, we can run 4.5 bar on a 6.0:1 V12 Merlin, WOT on take off. Currently, we have it max 3 bar to extend the life of the motor. The difference it 1750 hp vs 1400. The air fuel ratio is close to 10:1 and EGTs are around 1650 - 1700. On climb or WOT maneuvers, you better have the radiator exposed to the air or you will burn the motor in a very short amount of time. Minimum octane fuel is 100 low lead - 130 would be better.
Old 04-07-2009, 06:08 PM
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I see this chart come up from time to time. There is one important thing missing, ignition timing.

You can run a high effective compression ratio, even with no intercooler. To do so you will run max power timing around 10 to 15 BTDC. With this timing, most of the combustion energy is released AFTER TDC. All it does is make the exhaust real hot, no power increase. Burns the exhaust valves, even our cool, sodium filled ones. At the other extreme, you can run low effective compression ratio, and lots of timing, but after you get to 28 BTDC or so, you are not making any more power, and you are still far from the detonation limit.

Optimising boost, compression ratio, and timing really comes down to choosing the maximum boost that the engine can structurally stand, and then raising the compression ratio and adjusting the timing until you reach the exhaust temp limit.
Old 04-07-2009, 06:12 PM
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I am very curious about higher boost levels. I suspect combustion chamber design including the spark plug location, heat management at the combustion chamber, timing, and AFR all come in to play.

The data points above on pulling the timing back and what it dose to exhaust temps is interesting.

Also, I suspect fattening the AFR's should make the mix less volatile. I wonder what the point of diminishing returns are with this and how much if effects the sensitivity to pre-detonation.

Can we make more power at 11/1 afr with 1.2 boost and the right timing so the mix quickly enough to not burn things up?

What are the limits?

How do people run 20+ pounds like on the Audi.

Old 04-07-2009, 06:36 PM
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Speedy Squirrel,
On the timing issue my car ( 1977 930 ) on the chart ( Voiture ) I have the old style 12 lead RSR/934 type distributer with twin plug heads with the small spark plugs. Timing is set @ 29% @4000 rpm using 108 octane and have run as high as 1.4 BAR up to 7500 RPM but generally stay @ 1.2 bar and 7000RPM. It is a 3 liter motor that was Andial built with a short bellhousing RUF 5spd . It has no low end HP or torque but starts to pull strong after 4500 rpm at over 1 bar boost. Thats the trade off with this car, of low compression, wild high end cams and lots of upper RPM boost. Defiantly not impressive from the stop light.
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Old 04-07-2009, 06:58 PM
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Goose bumps!

High RPM and boost!

What cam please?
Old 04-07-2009, 07:04 PM
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Just a thought, twin plugging may have a different function on a Turbo than a high compression NA motor.

It helps fire the AF mix with low compression pre boost. On boost the mix is so volatile we only need one plug. Note the 993 TT and the RUF Yellow Bird. Per the factory twin plug on there race turbos the second plug was as much insurance that there would not be a miss fire. I could be wrong but that is what I believe at this point in my learning curve.
Old 04-07-2009, 07:09 PM
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Here is another boost / timming / AFR data point from another thread. I wish it noted the fuel.

Quote:
Originally Posted by zcoker View Post
I agree that EBC's have been around for a long time but when it comes to getting the most of things, one needs to just make it yourself. One of the only reasons that I can get a reliable 700hp out of my 3.3 litre bmw is the boost controller system. It is a fully closed loop 8 port solenoid system which measures exhaust pressure and manifold pressure. I can run this on 32 load points at any rpm, so I can map the boost curve to any load range. Normally a mapped system is constant and one would have to change the maps in order to change the boost curve. Or they rely just on manifold pressure. Toss exhaust gas pressure in the mix mated to a computer and things suddenly become very interesting. What I can do with my setup is alter the mapping via a trim knob in the cockpit to any curve variant that I desire. I can have any boost at any rpm with just a radio dial like effect. I've perfected this setup to the point were the motor runs quite stress free yet sees massive boost. I've seen 38psi with no problems. Here's another run verified by Performance BMW Magazine.

Boost 32psi (sustained), 5492rpm, 117.6F intake temperature, 15.3 degrees timing, 11.0 AFR (wide band), 66% throttle (TPS) gives 723bhp/692 lb ft.

Old 04-07-2009, 07:19 PM
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Quote:
Originally Posted by 911st View Post
Goose bumps!

High RPM and boost!

What cam please?
993 Porsche sport cams.
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Last edited by voitureltd; 04-07-2009 at 07:22 PM..
Old 04-07-2009, 07:19 PM
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Quote:
Originally Posted by 911st View Post
Nice chart but it doesn't tell the whole story. It should be used as approximation / hint.

First of all, it doesn't seem care about VE (volumetric efficiency). Otherwise, the smooth concave lines would be full of "hills". Second, not even effective CE adjusted for VE tells the whole story. Knock sensitivity is further influenced by combustion chamber shape, squish, ignition timing, intake air temperature, head temperature and position of spark plugs.

Thus, this fancy graph doesn't really say much.

You can boost 0.7 bar at middle revs and get it to knock despite rich AFR's and at the same time boost 1.1 bar at high revs w/o knock. As cams run out of breath, you simply cannot push that air into cylinders despite positive pressure before the valves.

That's why it's important to compare apples to apples.

A 4-valve pentroof watercooled/intercooled engine with knock sensors and small piston diameter will tolerate much higher effective C/R than badly intercooled, big-bore 2-valve engine with offset ignition plug, hot heads, no knock sensor and CIS.

What it all boils down to is:

Cylinder combustion pressure vs. octane vs. combustion chamber geometry.
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Old 04-08-2009, 02:09 AM
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This graph is just a tool to help us understand how far out there we might be with most combinations of boost and CR. I suspect few have though of the concept of Effective Compression Ratio. Vender's are offering 8/1 CR pistons sets that help our pre boost torque. That is great but if the buyer is running 1 bar boost, they might not otherwise know how close or if they are going over the edge or that they need to work on making sure there timing, IC, AFRs, and turbo efficiency is such to allow doing so.

It is a good point about differences in motor designs and differences and motor efficiency's (VE). We may want to be cautious in comparing ourselves to boost levels being run with more efficient motor designs. Is this why we can not run 22 lbs of boost?

Most of our air cooled turbos are running basically pretty similar non overlap cams and are of similar architecture which should make them reasonable comparable among themselves. The graph is helpful to better understand what the upper limits might be for different motor combinations we might consider.

Yes, differences between our motors should be considered when targeting a given ECR. A 3.2 turbo conversion motor w no intercooler and mild cams might want to run a lower ECR than a motor with aggressive cams, a very efficient intercooler, big effecent turbo where everthing is dialed in to work.

I suspect the normal aspirated 993 might give us a hint as to where our upper limit as to ECR might be on pump gas if we get everything right. Even then, we need to make sure we get our timing and fueling right. Piloting one's self against the curve can help one to understand how close to the limit one might be.
Old 04-08-2009, 07:17 AM
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Here are a couple of clues I see.

The factory 3.0 turbo w/o an intercooler targeted a ECR of about 10/1. The 3.3 with an intercooler added came in at about 10.75. Assuming Porsche was targeting the same margin of safety, the early IC allowed Porsche to reach about .75 higher ECR.

Take that 3.0, add a good intercooler and run it at 1 bar boost and it should be running an ECR that is the same as the 3.3 at .8 bar boost.

A 3.3 with an efficient turbo, larger efficient IC is felt by most to be ok at 1 bar boost. That puts us at about a 11.5/1 which is just a bit more than a normally aspirated 993.

Some of the big dogs seem to be living in the area past an ECR of 12+/1.

How about building a 3.3 at 6.5/1 CR and reach for the same ECR as the big dogs here. This should allow us to boost to about 1.5 bar if we do it righ. Take a na motor that should make 250hp and boost it 2.5 atmospheres (1.5 bar) correctly and efficiently and that could make for about 625hp. Remember the "Monster CIS motor" thread.
Old 04-08-2009, 07:34 AM
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Quote:
Originally Posted by 911st View Post
Here are a couple of clues I see.

The factory 3.0 turbo w/o an intercooler targeted a ECR of about 10/1. The 3.3 with an intercooler added came in at about 10.75. Assuming Porsche was targeting the same margin of safety, the early IC allowed Porsche to reach about .75 higher ECR.

Take that 3.0, add a good intercooler and run it at 1 bar boost and it should be running an ECR that is the same as the 3.3 at .8 bar boost.

A 3.3 with an efficient turbo, larger efficient IC is felt by most to be ok at 1 bar boost. That puts us at about a 11.5/1 which is just a bit more than a normally aspirated 993.

Some of the big dogs seem to be living in the area past an ECR of 12+/1.

How about building a 3.3 at 6.5/1 CR and reach for the same ECR as the big dogs here. This should allow us to boost to about 1.5 bar if we do it righ. Take a na motor that should make 250hp and boost it 2.5 atmospheres (1.5 bar) correctly and efficiently and that could make for about 625hp. Remember the "Monster CIS motor" thread.
Then the question boils down to getting enuff fuel into it. Porsche used MFI to get the required fueling....as you all know and seen alot of threads on limits to CIS out there.
I am still amazed that these guys are claiming to run 10psi boost on normally aspirated engines....with the cast pistons and high static comp ratio they come with from the factory. How long do these last is my next question. If these are as reliable as they say then why doesn't the OEM make the motor this way from the factory? I see it all the time here in Detroit but without tearing into the engine to see for myself what else was done.....find it hard to swallow.
Most turbocharged cars coming from the OEM still run only 14psi of boost or so and they are designed as turbo engines from the beginning not normally aspirated.
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Old 04-08-2009, 08:19 AM
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I love CIS, have spent a lot of time studying and playing with, and truly believe the limit of fuel delivery potential with 6 injectors is capable of supporting this level of HP. This would require a combination of readjustment of the fuel head system pressure, orifice spring tension (differential pressure), and maybe playing with the internal orifice height. As this throughs the fuel curves out of wack, readjustment and modification of the WUR to bring the fuel curves back to the right inclination and to achieve a lower on boost CP to alow the metering pin / valve to advance further. And or lastly, modification of the metering plate profile to ensure it dose not stall and full metering arm movement is achievable. The Monster CIS thread seems to support this has already been done. See:Cis monster

If this is not enough we could always, open up one or two of the other blanks on the fuel head for a 7th and or 8th CIS injector (plumbed to augment the intercooler) but at 625hp I do not expect this is needed. Advanced CIS Turbo Tuning Discussion Thread. Ultimate?

I suspect, air flow is the more of a potential limitation.
Old 04-08-2009, 08:47 AM
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Hi, I haven’t posted here for a while but I marked up the chart shown above.

First, the origin of this chart was a Porsche technical turbocharging paper published close to 30 years ago. It’s relevance is to Porsche air-cooled boxer engines of 3.0 to 3.3l, which still applies to most all posters on this site.

This should not be extended to modern water-cooled , small bore, four-valved, center/multi spark engines with ultra precise fuel delivery backed-up by knock sensor spark control.

Within any given engine configuration which is fully optimized N.A. or supercharged ultimate power is controlled by available fuel octane and how close one chooses to run near detonation and component mechanical limits.

Last edited by copbait73; 04-08-2009 at 10:14 AM..
Old 04-08-2009, 09:35 AM
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Copbait,

Appreciate your contribution. I think that chart is a significant piece of information that more 930 tweekers should be aware of. Most are flying blind when dialing up there boost or building their motor.

Bruce Anderson talks about this some in his book on modifing Porsche's under his section on suppercharging. It is in the form of a math formula.
Old 04-12-2009, 06:39 PM
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Quote:
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Can anyone help me better understand how some are running boost in the 20 lbs/inc and even 30's?
Some systems (like the 930) quote boost relative to atmospheric, e.g. the amount of positive pressure over atmospheric.

Whilst other systems (like the 951) quote boost as absolute pressure - relative to the vacuum that would otherwise be in the manifold on an N/A car at WOT.

Which is why the factory 951 boost gauge goes up to 2 bar (29.4 PSI)
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