Maximum dynamic compression ratio w/ single plug?

[Plays with cars]

I'm in the process of building an engine for my 914-6, primary use will be autocrossing but I also want to be able to use it for occasional tours and DE days at the track. I've always thought the combination of 2.4S power in a 914 would be a lot of fun, but the power curve of the 'S' isn't ideal for autocrossing so I headed down the path of an 'E' spec cam.

The motor is now built but I am still working out all of the parts for the MFI system and it has given me time to reflect on the component choices in the engine and I'm second guessing if I'm going to have a motor I can use w/o needing race fuel. What I ended up with was a 2.4 'E' spec motor with 2.2 'S' P&Cs.

The common wisdom seems to be that the 2.2'S' P&Cs work in a 2.4 motor w/ 'S' spec cams and don't require more than premium fuel. But, my concern is that this works because of the overlap in the 'S' cam profile resulting in a 8.9:1 dynamic compression ratio (DCR). With the more docile 'E' cams I'm runnig the DCR calculates out to be 9.5:1. This site Dynamic CR , indicates that the DCRs above 8.9:1 are into racing fuel territory, specifically w/ less efficient combustion chamber designs like the Porsche hemispherical chamber.

I plan to run a MegaJolt electronic ignition system so I will be able to refine the ignition curve, but I'm still not convinced I can meet my goal of not requiring high octane fuel. I'm looking for thoughts concerns from those more experienced on this board.

[Steve@Rennsport]

Mark,

No worries,..you're well within the realm of 92 octane fuel so you can use reasonable ignition advance, too.

Go for it.

[Plays with cars]

Thanks Steve. Good to be able to check this worry off of the list. Also glad to hear that I'll be able to acheive reasonable advance with this combination. That was going to be my next concern; if I had to retard the timing too far I was concerned about the heat that would be transmitted into the piston crown.

So, when do you pass the point of no return for compression on a 2.4 that one would be forced into high octane and/or twin plug situations? I thought I was past the edge, but your response indicates there is still some margin.

[Eagledriver]

What is your static compression ratio? Dynamic compression ratio is virtually meaningless. Dynamic compression ratio would only mean something if computing for an incompressable gas. Of course an incompressable gas would lock the engine so it wouldn't be much use there either. There is a reason that in engine octane and advance discussions static compression ratio's are used. If you are above 9.8 static compression ratio you will probably need to retard your timing or run race gas or a combination of both.

Sorry to be so blunt about this but it keeps coming up. You can get an idea of acceptable CR and advance by looking to the various Porsche factory engines. The 2.2 S used 9,8 pistons (I think). It needs 92 Octane fuel. The 2.4 S used 8.5 pistons and could use 87 octane.

Sounds like a fun project.

-Andy

[Eagledriver]

Thinking more about your combination. I think a longer stroke with the 2.2 S pistons will raise the compression ratio above what it would be on the original 2.2 engine. This puts you well above 9.8 compression ratio and into race fuel territory. This assumes the heads are the same as the 2.2 engine.

-Andy

[Plays with cars]

Yes, you're right. Using the 2.2 pistons w/ the 2.4 stroke results in higher compression; 10.3:1 static compression for this combination.

Because of this though, dynamic compression is important to consider. It is a further refinement of the static compression number and represents the 'fixed' engine variables that determine cylinder operating pressure. Its a combination of static compression, camshaft specs/timing, and engine geometry (specifically stroke and rod length). By including these other variables it is a better representation of engine's efficiency potential and risk for detonation.

You will sometimes see very hot cam advertisements including a recommendation for higher static compression; this is to increase the dynamic compression and improve low speed performance.

I had the opposite concern where I am running high static compression and don't have a hot cam installed. I wanted to make sure I wasn't going to end up w/ too high of dynamic compression and have problems with detontation.

[304065]

Except that Bruce Anderson says that you really don't end up with 10,3 compression with the 2,2 pistons on the 2,4 crank. This is because Mahle cites CR assuming ZERO deck height.

[Eagledriver]

The problem with dynamic compression ratio is that is doesn't take into account the pressure waves of the intake, exhaust tuning and the velocity of the airflow into the cylinder. The peak pressure (which we use to determine safe octane/timing) depends on how much air gets into the cylinder and compressed to TDC for combustion. Dynamic compression may tell you something at idle and low speed but when the engine is operating in it's power band it tells you very little. If Dynamic compression ratio relies on some of the air slipping back out of the intake valve because the valve is still open when the compression stroke starts. Cams with big duration and overlap would allow lots of air to slip out using this theory. The problem is that the air doesn't slip out. Instead it has developed momentum and is forced in even though the piston is rising. It's forced in through a combination of momentum and pressure waves in the intake and exhaust track. Indeed if a cam were to allow the air to slip out at high RPM the engine would not produce the power you want.

The required information for what you want is volumetric effieciency. Even with a very radical cam a well tuned engine can achieve a VE of 1.0 This means that even though the dynamic CR is low the cylinder pressure due to the factors above gets all the way up to what it would be assuming the static CR.

Yes race cams work better with High CR but you also need race gas if you have a well tuned engine for the cam.

Actually a good indication of VE is torque. If you look at the torque of a 2.2 or 2.4 T, E, or S engine you'll see that despite a lower Dynamic compression ratio the S an E have higher torque than the lower performance versions. This indicates that they have higher cylinder pressures not lower as you would assume from dynamic compression numbers.

Hope this makes sence to you.

-Andy

[Plays with cars]

Thank you Andy. Very well put. I was trying to figure out how to incorporate some of those thoughts into my reply but it was turning out much less eloquent than you put it. I understand what you are saying about VE and agree, but because the speed of the flame front increases in the combustion chamber as RPM increases (presumably because of increased turbulence) my experience has been that an engine is more susceptible to detonation at lower operating speeds. While the VE of the E and S do appear to be better than a T, their peak torque occurs at higher RPMs. Dynamic compression seems to be a good measure for recognizing the relative inefficiency of agressive cams at low RPMs where an engine is at greater risk for detonation.

I may be way off, and I would appreciate you feedback if I am, but my line of thinking was to seperate the variables contributing to detonation that are physically built into the engine based on component choices from the ones that are externally changable/adjustable variables and then prioritize them based on their effect and evaluate the likelihood for detonation. Following this line of thinking I prioritized the physical engine variables as:
1 - static compression ratio - at 10.3:1 I suspected I was at risk for detonation
2 - dynamic compression ratio - the basis of my question, was the 'E' cam profile agressive enough to reduce low speed detonation risk.
3 - cam timing - my cams were timed at the extreme end of the spec range on the retarded side effectively reducing DCR.
4 - port sizes - also impacting VE differently as RPM changes. Mine have been opened up to mid-way between T/E and S specs to increase high speed breathing and reduce low speed velocity which should reduce cylinder filling at low speed.
5 - cylinder cooling - not much to be done here. The block has piston squirters to cool the piston crown.
6 - by now the variables remaining affect on detonation seems it would be minimal: port profile optimization, combustion chamber shape, piston crown shape, ..

So, I figured I was at risk with the high static compression but with appropriate choices could reduce the dynamic compression ratio to a reasonable level. Seems like that is in a reasonable range so now I need to start thinking about controlling factors outside of the physical engine.

I mentioned I am planning to run a distributorless electronic ignition system and will be able to program the ignition curve with much better resolution than was available with the distributor. I'm hoping to really acheive some good throttle response while retarding where needed to manage detonation. With the system I am looking to use I am able to program two curves into the ECU and switch between them easily. I'm planning a pump gas curve and a race gas curve. Any thoughts on an advance curve?

Great discussion. Always hoping to understand the theories better and apply them to acheive a superior result.

[Eagledriver]

Mark I think that there is some safety at higher RPM as you suggest. But I believe that the RPM the engine is most likely to detonate at is the peak torque RPM. It sounds like with your combination you might be able to have CR of about 9.8 at normal deck heights. I'm no expert on ignition curves for these engines but you could probably start with the standard 2.2 "s" curve.

-Andy