Static Compression Ratio vs Boost?

[copbait73]

Originally Posted by 911TT33

I've heard Juan Ruiz runs 1.4 bar+ on his Carrera motor with 930 P&C's. He's way off the chart

I've run 1.25bar on my turbo Carrera with 930 P&C's too!

Actually Juan is on the chart.

Look at the chart again. The bottom line is your engine compression ratio, 7.0:1 for 3.3L P&C. Move vertically on this line to the intersection of 1.4Bar boost. Now look at the diagonal lines representing effective compression ratio. Ruan is not off the chart, he is running at 13:1 effective compression. Right where at least two other site members were running 3-4 years ago and many others run today.

When he runs on ethanol with it's much higher octane and charge cooling effect he can elevate his boost. This is why race sanctioning bodies have different classes for gassers and alcohol fueled cars and why unlimited racers change to alcohol when approved. It's because he kept static compression low at stock 7.0:1that he can run high boost.

Your 1.25 boost level is also on the chart. Vertical on the 7:1 line, intersect the 1.25 boost line. You are roughly 12.5:1 effective compression. This is boarding on the limit, but the same boost/fixed compression ratio Andial and RUF ran Turbos 30years ago.

If you want the most bang for the buck the last thing you want to do is raise fixed compression on a boosted engine.

[Jim2]

This is completely unrelated to 911TT33 inquiry but I just noticed that you have a 3.6 noted on the bottom right corner of your "dusty" chart (hehe). I've just approached stuffing .7 bar into mine and the effective compression is somewhere off in oblivion. I can only say one thing - methanol!

[copbait73]

Originally Posted by Jim2

This is completely unrelated to 911TT33 inquiry but I just noticed that you have a 3.6 noted on the bottom right corner of your "dusty" chart (hehe). I've just approached stuffing .7 bar into mine and the effective compression is somewhere off in oblivion. I can only say one thing - methanol!

Not a lot of wiggle room. I have a 3.6 I bought 8 years ago planning similar. What chassis and trans is it mated with? Light weight, 5 speeds and you hold off the boost build up for higher revs and there is much less risk. You have the colder north country in you favor.

[Jim2]

You pretty much described it. 993 cab (cab not by choice) completely stripped (think tin snips!) Stock six speed. I used a 1.0 split vane housing on the turbo thinking low boost and I wanted to avoid max boost at torque peak. Too many variables though and it ended up building boost far earlier than I expected but I've managed to avoid the 67 piece ring package. 993s don't hook up like 930s, lots of wheel spin, and occasional axle hop. Engine winds out quickly, feels like it's uncorked compared to my 930, and of course drives like a lightened 993 off boost.

[Tippy]

What Jakob is saying is you cannot live or die by that simple chart.

Camshafts' lobe separation angle can allow compression (in PSI) to "build" or to "bleed". The wider the lobe separation, the more the cylinder pressure will bleed off - then vise versa.

Take for example, A) a 12:1 CR motor and B) a 11:1 motor. By using different lobe separation angles on their respective camshafts, you can essentially have the same PSI in the cylinder.

It is about PSI, not as much about CR. CR is the ballpark.

So in layman's terms, even though you are throwing boost into an engine, the benefit of using a wider lobe separated cam will keep the intake valve open longer on the compression stroke (than a tight lobe separated cam), pushing the air out of the cylinder bleeding the PSI.

Breakdown here too:
http://www.compcams.com/technical/FAQ/LSAproperties.asp

[Raceboy]

There you see an argument that rises from time to time again between guys who have basically no experience in real world, hands-on manner of installing turbos, EFI's and tuning them and the guys who had done it many times. Copbait may be turbo-engineer but he sure does not have real world experience with turbocharged 911 engines, only what others had told him.

That chart is a joke in todays standards. Why? Because people have learned to avoid and overcome many obstacles that air-cooled 911 engine has and do you really think in all honesty that old dizzy had accuracy even close to that of a decent crank fire system? Or fuelling accuracy of the MFI compared to controlled and WBo2 monitored efi?

I have built several boosted beetle type 1 and type 4 engines with 8:1 CR and 2+ bars of boost (ported heads too) and guess what, they never blew up, just after several summer seasons had other issues from over-revving. You'd have to draw a new map for them I guess.

Just so no-one thinks I'm a guy who passes others knowledge, I install and tune engine management systems and done it on 60-70 cars of which 1/3 are Porsches, both air and water cooled. What do you think of 993 with 11.3 CR and 0.8 bars of boost? After 3 years, still runs very strong and not only on the streets.

Let's try to stick to the facts and not speculate, let alone refusing to admit that one is wrong?

Pat Williams boosted STOCK 911 T 2.4 some time ago and got 316 rwhp, also held up just fine and engine failed only after hard overrev, not boost. Yes, it was mag case. EFI too.

[copbait73]

Originally Posted by Raceboy

There you see an argument that rises from time to time again between guys who have basically no experience in real world, hands-on manner of installing turbos, EFI's and tuning them and the guys who had done it many times. Copbait may be turbo-engineer but he sure does not have real world experience with turbocharged 911 engines, only what others had told him.

That chart is a joke in todays standards. Why? Because people have learned to avoid and overcome many obstacles that air-cooled 911 engine has and do you really think in all honesty that old dizzy had accuracy even close to that of a decent crank fire system? Or fuelling accuracy of the MFI compared to controlled and WBo2 monitored efi?

I have built several boosted beetle type 1 and type 4 engines with 8:1 CR and 2+ bars of boost (ported heads too) and guess what, they never blew up, just after several summer seasons had other issues from over-revving. You'd have to draw a new map for them I guess.

Just so no-one thinks I'm a guy who passes others knowledge, I install and tune engine management systems and done it on 60-70 cars of which 1/3 are Porsches, both air and water cooled. What do you think of 993 with 11.3 CR and 0.8 bars of boost? After 3 years, still runs very strong and not only on the streets.

Let's try to stick to the facts and not speculate, let alone refusing to admit that one is wrong?

Pat Williams boosted STOCK 911 T 2.4 some time ago and got 316 rwhp, also held up just fine and engine failed only after hard overrev, not boost. Yes, it was mag case. EFI too.

Wow, I'd be more impressed if you told me what fuels used. BTW, I turbo'd my first car a Beetle in '76, 6.25:1 C.R. 15PSI, W/M. EFI and probably you didn't exist then.

[Raceboy]

FYI, yes, I didn't exist then yet, I was born in 1978.

Regarding fuel, we use regular 98 (93 in US) and for street driving, occasionally used 98 mixed with toluene (20%).

Sorry if I jumped on you, I was mistaken thinking you have little experience with turbocharging. But, I would suggest to try EFI, it really helps to get the most of the car and it does well.

EFI existed in fact in 1976, D-Jetronic (1967) and L-Jetronic (1974) were on the scene by then.
Porsche 914 (1.8 liter iirc) had D-Jetronic too.

[copbait73]

Originally Posted by Raceboy

FYI, yes, I didn't exist then yet, I was born in 1978.

Regarding fuel, we use regular 98 (93 in US) and for street driving, occasionally used 98 mixed with toluene (20%).

Sorry if I jumped on you, I was mistaken thinking you have little experience with turbocharging. But, I would suggest to try EFI, it really helps to get the most of the car and it does well.

EFI existed in fact in 1976, D-Jetronic (1967) and L-Jetronic (1974) were on the scene by then.
Porsche 914 (1.8 liter iirc) had D-Jetronic too.

I stand corrected. You are right, we hottrodders tend to discuss EFI as the current user programmable systems. These early EFI were not.

In my effort to stay on track of compression vs boost members get the wrong impression I am somehow against it. I am certainly not. As a matter of fact I witnessed how modern EFI became the enabling technology to bring the explosion of factory turbocharged cars to the masses - not just the rich who could afford a 930 Turbo. Modern EFI has nearly infinite capability.

[JFairman]

Just wanted to add that good old analog bosch L-jetronic efi can be modified to run richer or leaner easily.

To have dashboard adjustable AFR in the richer direction from stock, wire in a linear taper 10,000 ohm potentiometer in series with the single wire water temperature sending unit. All the sensors including the air flow meter on bosch L-jetronic work on variable resistance.

Turn the 10,000 ohm pot clockwise to increase resistance (depending how you wire it) and the fuel injector dwell time will slowly increase richening up the AFR. Turn it all the way and the ECU will think it's around 30 below out and the ecu will keep the injectors open and the AFR will go very rich. L-jetronic fires the injectors in groups of 3 on a six cylinder motor.

To go leaner than stock tap into the wires in the harness next to the ecu thats usually in the top of the glove box that go to the resitance carbon trace in the barn door air flow meter with another linear taper pot. I don't remember which wires it's been so long since I did it but it's easy to figure out by taking the cover off the air flow meter and following the wires.

Yes you can't reprogram that analog ECU but with these potentiometers you can adjust the AFR from the dashboard and also use rpm switches and relays to switch them in and out according to rpm and throttle position using the throttle position switch on the throttle body..

I know it's off topic.

[patkeefe]

Originally Posted by JakobM

Very wrong IMO. Do you map/tune EFI applications on closed loop knock sensor readings? Try to define knock resistance/control .... Knock sensor is just a sensor input nothing more...many sensor input goes into EFI to tune against detonation.




3D programable load maps that is where you have the single most major difference to knock control (out of many). With EFI your ignition now measure and calculate retard/advance intelligent from MULTIPLE parameters (same goes for fuel), such as:

• Oil/water temperature,
• Inlet charge temperature,
• Exhaust gas temperature,
• Air Fuel Ratio
• Manifold Air Pressure
• Voltage
• Frequency - if you choose to use it (Knock sensor)

Knock sensor is only one parameter out out many way more important.
Newer trust a knock sensor alone. Many don't use it and only with calculated uncertain margins for full stop damage control
A knock sensor measures the FREQUENCY in the block/cylinder/head from combustion. The combustion frequency makes a peak (after normal plug fired combustion) when detonation sets in and varies around 6400 hertz (as I remember for alloy) depending on alloy/cast material and setup! It needs to be calibrated.
I would never trust tuning/mapping on closed loop knock control.
You need same mapping/tuning with or without a knock sensor.
Knock sensor should only be seen as an alarm that you use to set up "damage control" parameters (ign retard, fuel enrichment, boost backoff etc.) Like situations from bad/wrong fuel. Nothing to do with picking your CR.
To say knock sensor alone is the major factor to what makes higher CR possible is totally misunderstanding tuning/mapping IMO.

Knock resistance/control is the combination of programing several important parameters. You need each 3D load point to (which can be as many as you want) automatically calculate the correct output depending on multiple measured input.

Jakob, how would you optimize the OP's setup with the parameters given by the OP? What exactly does a modern EFI/ECU setup provide for power optimization? Optimize with an eye towards reliability and longevity.

Did not all of whatever parameters being used develop from the old school methods of test and tune? And, I assume we will speak of a powerplant which has no capability for valve timing changes while running, such as the Vario-Cam.
I noticed you had a picture of an inline Ford powerplant, which are a bit different beast than the air cooled Porsche. I do realize that the watercooled inlines will develop tremendous power levels, with the associated risk. Where do we draw the line on pressures?
Pat

[Speedy Squirrel]

All aircooled engines, from your lawn mower to the Pratt & Whitney R-4360 Wasp Major aircraft engine, are limited by the temperature reached by the exhaust valves. How far you can go with boost and compression ratio depends on the temperature of the exhaust valve.

The unstable combustion induced when the exhaust valve gets too hot is typically characterized as pre-ignition. In this mode, the entire fuel/air mixture is ignited by the hot exhaust valve. Since this happens on the compression stroke, the loads and temperatures quickly exceed the material limits, resulting in severe damage, often without audible knock.

This is in contrast to water-cooled engines, where unstable combustion is normally caused by autoignition of a much smaller fuel/air mixture, and usually during the expansion stroke. This produces the classic rattling or knocking sound, and can be remedied by intervention of the ignition angle.

Since pre-ignition occurs before the ignition angle, no amount of spark plugs or knock sensors provide any "insurance".

Below is my version of the "Metzger Plot", which can be reproduced by anyone using the following excel macro, and a contour plotting program:

Sub Macro1()
'
For I = 2 To 14 Step 1
Let R = 5 + I / 2
For Boost = 0 To 160 Step 10
Let Reffective = (((101.4 + Boost) * R ^ 1.4) / 101.4) ^ (1 / 1.4)
Let Cells(Boost / 10 + 2, I) = Reffective
Next Boost
Next I
End Sub



On this plot is the 1996 911 GT2 EVO twin K27 turbo, EFI (Bosch), 8.0 CR with 1.25bar boost, SECAN intercooler. Unbelievable? No. This engine has sodium cooled intake and exhaust valves, ceramic exhaust port insulator, and almost 60 degrees of overlap. They are using cooled, boost air to cool down the exhaust valve, which lets them get away with a 14 + effective compression ratio. By the way, this is a single plug motor.

Are you smart enough do all that, and calibrate (tune) it correctly?

Spending 5 seconds at WOT on a chassis dyno and calling it good is probably not going to cut it, unless you plan on spending 5 seconds or less at WOT.

Modern efi can also help a lot. The ability to adjust the ignition angle based on the measured MAT and MAP alone is worth quite a bit. Other capabilities like being able to enrich at WOT after a specified time, and wastegate boost control also make a big difference.

Big enough to be like a water cooled motor? No.

[911nut]

Originally Posted by Speedy Squirrel

. How far you can go with boost and compression ratio depends on the temperature of the exhaust valve.....
.....Since pre-ignition occurs before the ignition angle, no amount of spark plugs or knock sensors provide any "insurance".....
Modern efi can also help a lot....

Wow.
Kim Jung Il passes away and Speedy returns to the bb. Coincidence? Maybe....maybe not.
Yes, the 911 engine is hamstrung by air cooling. Anyone wanting to run epic levels of boost needs to buy a water cooled 911. Water is so much better than air for heat transfer.
Twin plugging, however, will help a little since a flame front moving from two directions will raise the threshold for detonation as the end gas will be combusted by the flame front before it auto-ignites (to a point). EFI will help a little not not "a lot". Maybe with the two together the compression ratio can be raised 1/2 a point but not more than that.
In the end, the limitation is the gasoline. Otherwise, spark ignition engines could run the same compression ratios as diesels.
My compliments on the pretty chart, BTW.

[Raceboy]

Originally Posted by Speedy Squirrel

The unstable combustion induced when the exhaust valve gets too hot is typically characterized as pre-ignition. In this mode, the entire fuel/air mixture is ignited by the hot exhaust valve. Since this happens on the compression stroke, the loads and temperatures quickly exceed the material limits, resulting in severe damage, often without audible knock.

And pre-ignition is exactly where twin-plugs come in handy. Because twin plugs move MBT closer to TDC (detonation treshold will be the same anyway when talkind about crank degrees) and coolest EGT's are when spark is ignited in MBT position.
You can cool down the combustion with more fuel but at some point combustion slows down (due to rich mixture) enough to burn in the exhaust and that what induces pre-ignition. Indeed, pre-ignition is not audible and will burn hole through the top of the piston in no time.

But that's where standalone fuel and spark management is coming in handy: control. You can tune whatever loadsite you like and with parameters and corrections you like and it will work that way constantly.

Originally Posted by 911nut

EFI will help a little not not "a lot". Maybe with the two together the compression ratio can be raised 1/2 a point but not more than that.
In the end, the limitation is the gasoline. Otherwise, spark ignition engines could run the same compression ratios as diesels.

Based on the above, controlled fuel and spark is actually the very key having these kind of things under control. Could you tell me other ways to fully control spark and fuel in different rpms and loads with precision and consistency other than standalone ECU?
Also, Ediie Bello seems to get away with air cooled engine and his car has some pretty impressive power if you ask me. Of course that is not an endurance race-car but OP asked a question about an engine for a street car not a 24hour race-car.