Static Compression Ratio vs Boost?

[911TT33]

Can anyone tell me whether it's possible to run 1.0 - 1.1 bar boost on an EFI motor running 8.5:1 static compression ratio, fully programmable ECU, full bay intercooler, and twin plugs on pump fuel?

[CaptainCalf]

I don't think I'd try it cuz you'd be running at an effective CR of 17.75:1 at 16 psi...

[drmatera]

I don't believe that is correct. Combining static comp and boost is not the whole story. A turbocharged engine may only see 20% higher peak cylinder pressure over it's NA counterpart, the power comes from more pressure during the whole cycle compared to the NA motor

I see no issue running 16psi on a 8.5:1 motor

[JakobM]

Personally I would not go lower than 8.5:1 with such setup

Much more into it than static pressure from boost + CR .... think about cam and duration overlap for a second in regards to static pressure...then think squish, ignition, fuel, etc etc.. IMO Forget static pressure as argument. It is not static pressure but in the end combustion pressure and detonation that needs focus/concern

Your EFI and twin plug makes it possible to control combustion and detonation in a professional manner on these aircooled engines and thereby skip the old school low CR. I don't understand why anyone that convert to EFI would stay with low CR 7.5:1 ...waste of money and no understanding what is going on IMO.

Porsche, Saab and others (pioneers in mass manufacturing turbo cars) started with low 6.5, 7, 7.5:1 in CR before converting to OEM EFI ...

Jakob

[copbait73]

Sure you can do it. Then rebuild your motor when something goes wrong like overheating in traffic or a bad (mis- labeled) load of fuel. You don't have four valves, water cooling, center plug, variable cam and most important knock sensors.

[911nut]

Originally Posted by JakobM

Porsche, Saab and others (pioneers in mass manufacturing turbo cars) started with low 6.5, 7, 7.5:1 in CR before converting to OEM EFI

EFI has no magic effect on knock resistance on it's own. What made higher compression ratios on turbo cars possible is the knock control that is integrated into EFI.

[Tippy]

I'm not an expert, but the fact you have a fully programmable EFI makes it safe to do from my understanding.

Since you have programmable EFI, I'm assuming you have a wideband to monitor AFR's?

What motor?

[911nut]

Originally Posted by Tippy

I'm not an expert, but the fact you have a fully programmable EFI makes it safe to do from my understanding.

Since you have programmable EFI, I'm assuming you have a wideband to monitor AFR's?

What motor?

If a fully programmable EFI cannot detect knock and pull back the timing, your just as screwed as if you are running an old-fashioned vacuum advance distributor.

[Tippy]

So you're saying a fully programmable setup utilizing a wide band is no better than the Motronic from the '80's?

From my limited knowledge, if you use ancient EFI that has simple fuel (and ignition) tables, you will have variation between AFR's during different points of operation.

With modern fully programmable, I was under the impression you can fine tune nearly every aspect of engine operation mitigating risk if you had bad fuel for instance.

[911TT33]

Originally Posted by Tippy

I'm not an expert, but the fact you have a fully programmable EFI makes it safe to do from my understanding.

Since you have programmable EFI, I'm assuming you have a wideband to monitor AFR's?

What motor?

Sorry, this is for a project car... 911SC 3.0 motor running a Carrera EFI intake, programmable EMS, full bay IC and potentially a twin GT28RS turbo configuration.

I think we'll retain the 8.5:1 and use twin Tial 46mm 1.0 bar wastegates. The EMS will have WBo2 input, and also regulate boost pressures above 1.0bar if need be.

Was just curious. We are going into the motor to install new rings and fasteners anyway, so we may slip in a 0.5 or 0.75mm base cylinder shim to reduce static CR to ~8.0:1 if need be. We do like the idea of retaining 8.5:1 for the twin setup to make it ultra responsive pre-boost

Suggestions?

[JakobM]

Originally Posted by 911nut

EFI has no magic effect on knock resistance on it's own. What made higher compression ratios on turbo cars possible is the knock control that is integrated into EFI.

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.

[JakobM]

Originally Posted by copbait73

Sure you can do it. Then rebuild your motor when something goes wrong like overheating in traffic or a bad (mis- labeled) load of fuel. You don't have four valves, water cooling, center plug, variable cam and most important knock sensors.

How old do you think that dusty boost chart is??
It is oooold and from times when igntion control was limited to mechanical vacuum advance up to 3000rpm and then a straight "dead" line through out the rpm.
Nice as history memory.

Use it for a guideline if you have old school mechanical ignition distributor (OEM 930 ignition setup)
IMO dont ever use it as a guide line for now days EFI setups / upgrades.

[JakobM]

Originally Posted by copbait73

Sure you can do it. Then rebuild your motor when something goes wrong like overheating in traffic or a bad (mis- labeled) load of fuel. You don't have four valves, water cooling, center plug, variable cam and most important knock sensors.

He mentioned going EFI and twin plug.

Four valve - numbers of valves has IMO no effect on knock control (squish has)
Center plug - he use two plugs and you get same flame travel distance (depending on bore)
Water cooling - water or air... use temperature sensor on head/cylinder and EGT to map ignition and fuel against overheating/detonation
Variable cam - for knock control ? Twin scroll setup (true build, not single WG) prevent much of the cam overlab issues (backpressure) and makes it possible to stay "on cam" with NA cam together with proper sized turbo.
Knock sensor - tune engine first and then setup as loop "alarm / full stop" parameter (calibrated and tested) to igntion retard and fuel rich compensation.

A knock sensor is a sensor just like all other sensors. EFI reads all input and you tune to map/calculate output.

[JakobM]

Most used KISS and effective amo against detonation when dyno or bench tuning for MBT ... (Minimal Advance for Best Tourge)

[drmatera]

Wow, some people are stuck in the past.

Wasn't the world once thought of as flat? Until technology proved that to be false

Same goes for our precious air cooled Porsches.

Take advantage of modern technology to improve your car. Are we still all running bias ply tires?

[afterburn 549]

Richard Clewit says it can be done

[drmatera]

case closed

[JakobM]

Originally Posted by drmatera

Wow, some people are stuck in the past.

Wasn't the world once thought of as flat? Until technology proved that to be false

Same goes for our precious air cooled Porsches.

Take advantage of modern technology to improve your car. Are we still all running bias ply tires?

Are you referring to the EFI "hear balls" which almost every professional EFI tuners uses when mapping igntition and fuel against MBT ??

Or the general debate from those still keeping 70/80's low CR when doing forced induction + EFI ??

[drmatera]

the latter

[copbait73]

Originally Posted by JakobM

How old do you think that dusty boost chart is??
It is oooold and from times when igntion control was limited to mechanical vacuum advance up to 3000rpm and then a straight "dead" line through out the rpm.
Nice as history memory.

It's Porsche's, published in 1978 when pump gas was much better than now. See the N.A. motors on the bottom line? Motronic, plus the 3.6L has twin plug, CHT and knock sensor. Sorry, push the chart and blow a very expensive motor. I stand by it because it describes the 40 year old air-cooled motor.

Too many guys on this site paid the price and some have bailed this past year.