You may very well be correct Chris. I have not been in the Porsche world very long but, I would never attempt going above 10:1 in an Italian car without knock sensors. The 328 Ferrari is a good example. 328's are fuel injected and have a stock ratio of 9.2:1. Not until Ferrari started using Bosch DME with knock sensors did Ferrari bump the ratio on their motors. The 348 is 10.4:1.

I have the utmost faith in the Porsche community and in this forum. If there are people using higher compression ratio's than 10:1 in street engine's then I will just have to say that it is entirely possibly for anyone who wants to do it as well. That's tough German engineering for ya.

My friend and I are getting ready to start reassembly on a 3.2L top end turned into a 3.4L twin-plug motor next week. The heads are done and just need to be picked-up from Ollie's.

Since the guy wants/needs longevity over every last available horsepower, it will be assembled to "California" spec, i.e. 9.5:1-9.8:1 CR on 91 pump gas.;)

For those of you in California still wanting to run your CIS/DME car over 10:1 SINGLE plug on 91, I wish you guys good luck. It may work short-term but the long-term reliability gods are against you:)

Ralph

So Ralph, its a California problem with the gas then? our midwest 93 gas should be ok then?

Remember, the Mahle CR's are typically overstated by .2-.5/point with factory deck heights. Do your homework and measure the CR......betcha you'll be below 10:1 static.

What octane/RON did Porsche recommend for these engines? I have a sneaky suspicion all these questions were considered when Porsche decided to use the '10.3:1' Mahles with the current fuel avail at the time.;)

As an FYI: I know of 2 local Euro Carreras and both have been fine with 93 octane. Both of these cars are DD's and used during the DE season. One has a custom mapped SW chip.

Follow up:

Just checked with one of the above Euro 3.2 owners. From the original ROW owner’s manual (written in German) Porsche recommended 96 RON (92 octane) for these cars.

I would imagine that you would be perfectly fine with 10.3:1 on your 3.4L and twin-ignition running on 93 octane. If the single-plug Euro 3.2's apparently can get away with it without drama, so will you.:)

In the event that you do run into detonation issues (I would imagine this would only be possible in your instance during hot summers or track use), Steve Wong can dial it out by backing off the timing or you can do the pump gas/race gas blend (which you should probably do for track use as an extra safety margin anyway).

Steve Weiner, what's your opinion:)

Ralph

Just spoke with Peter today at Andial and seemed to think that the 3.2 to 3.4 10:5:1 Mahle pistons would be ok with Sunoco Ultra 94 octane. That's also with a twin plug setup and PMO's.

Ain't cheap though...$4200.00 for P&C's wrist pins and clips. That's not including all the other stuff that needs to get done.

Hey one thing about New Jersey...we have good quality gas and it's relatively cheap;)

So the bottom line is:

You can run higher than 10:1 on pump gas without twin plug unless the ignition control system has knock sensors.

Hi Tom,

You gotta being running somewhere around that in your Monster:D

Yup

The 3.0 in my RSR clone uses 10.5:1 Mahle RSR P&C's. I verified the CR during the build-up. I'm also using a twin plug dizzy and run 92 octane pump gas. So far, so good....But I've not run it hard in hot weather yet.

Hm, I think there is a reason the 964 and later engines with similar compression use dynamic timing retard (knock-sensors in a feedback loop) and twin-plugging. I don't think you'll realize the benefits of the 10.5:1 compression on your engine if you have to run with static retarded timing and/or overly rich to prevent detonation. What you were trying to gain by higher compression you'll loose by running it sub-optimal....

Ingo

The benefits of twin plugging to take advantage of higher CR are well documented here on this board and as well as in many other references on high CR engines. Before the sophisticated engine management systems with knock sensors, this was the only way to prevent detonation.

After all...The early 911 factory race engines all used twin plug.

Ingo,

I have to agree with Tom. One of the reasons that Porsche use the knock sensors in their later model cars is because they have to market their cars to states that have poor gas quality (i.e California). For those of us who live on the east coast, Mobil 93 octane and Sunoco 94 octane is easy to obtain and is relatively cheap. :D

California is a tough place to live if you enjoy "high performance" cars. No offense;) (living in Palos Verdes Estates was nice:cool: )

I will have to double check the sticker on the fuel filler flap, but I think euro cars were recommended to run on something higher than 96RON. I say this because we don't have 96RON gas in Europe (at least not that I am aware of). We have 95RON regular gas and 97/98 super gas.

I have run my euro 3.2 on 95RON on occasion, though I use 98RON Shell Optimax by preference. No problems and extensive track time, though our ambient temps don't get as high as the US in general. I know some euro 3.2 guys who run 95 octane all the time and have never had an issue.

Perhaps we get "magic" gas over here, super resitant to detonation - we certainly pay twice as much for it as you guys in the States :)

Actually today most modern engines use high compression and knock control. Porsche was simply ahead of others in integrating these techniques into their cars. Without this you would not see the high-output engines of today survive on the sh|tty stuff they call premium here in CA :)

Ingo

Not to split hairs, but Porsche was actually behind some other automakers for the implementation of knock-sensors.

For example, Volkswagen implemented a knock-sensor system in 1985 for the 8V "HT" motor fitted in GTI/GLI models. This allowed an increase in static compression from 8.5:1 used in the 83-84 GTI "JH" motor to an advertised 10:1 (actually measured in the 9.5:1 range) for the newer model. A hydraulic lifter cylinder head was also used at the same time (for the first time as well) as the earlier mechanical lifter heads were too noisy and wreaked havoc for the knock-sensor computer.

Another example how far anti-knock control has come is that the new Mk5 GTI 2.0T FSI motor has over 10:1 CR AND is turbocharged. The tiny motor makes 100 hp/liter, pretty impressive indeed.

Ralph

"OK how do the ROW cars that were imported do with the gas here in the US? "

- 88-diamondblue -

Two very knowledgeable guys (Ralph & Steve Weiner) and others who have built 911
engines have explained the high compression problem thoroughly, but yet some fail to
understand. Kinda like not understanding the problematic technicals about using
a performance chip, and how ignition timing, temp, and load affect an engine.
Like other issues, they've been explained over & over again, but some just can't get it!

As implied, most likely without much higher octane, knock sensors, & twin plugs,
the timing will have to be retarded which results in a loss of power. So the overall
performance gain becomes questionable, when using higher CR pistons.

This is a very interesting thread in that you could change the topic from high compression
pistons and change where CR was mentioned & insert performance chips, and basically
the same caveats would apply. Fail to understand one, and most likely one would fail
to understand the other. It appears that on this topic on increased CR most understand
& agree on the issues, but now mention performance chips, well that's a non-issue.

Bottom line: Just like with performance chips, higher compression pistions without
major engine changes reduce the margin of safety in an engine, but many continue
to ignore the facts or fail to understand the similarity.

Loren,

Seems to me you keep saying Porsche is the ultimate in what design can be. Porsche used the 10.3 PC's on the Euro 3.2 engine and with pump gas and no knock sensors. I completely understand the function of what happens with the engine, compression ratio and what can happen in doing an upgrade like this. I am not going to be using the 10.3's because of the feedback I got from Steve W, Ralph, Steve Weiner and Henry Schmidt. You will find out later what I will be doing. But a retaliatory post such as this in an attempt to try to make me look bad just won't work with me or anyone else here. You seem to be the only one that ignores the facts of this thread or any thread on performance chips. I didn't ingnore what was said and found the information that I was looking for. When you know more about what I am doing to my engine then you can go ahead and judge the decisions I have made. And BTW where is that data the I have asked for about your testing with chips???? :rolleyes:

Case study. I have an 87' Euro motor and I can't run it with the crappy 91 octane they sell here without getting some pinging when I put my foot in it. Luckily there is a station with 101 octane less than a mile from my house so every fill-up I put in 2 gallons of 101 and I do fine.

Down the road though I'm planning on going with ITB's and a more aggressive cam. I'll make the jump to twin plug at that time so I won't have to worry about getting stuck away from home without the 101 octane.

"Brother" Ralph, et. al:

Really great thread and I'd offer a few thoughts on this subject.

Without question, one needs to know PRECISELY what the actual, measured compression ratio is in any engine as part of the information suite required to assess octane requirements. :)

The "money" question with these engines is always, "How much comression can I safely use at X compression ratio and pump gas?" and that answer is not a constant. There are several variables that are always changing so there are no definitive answers.

Without active, adaptive knock-sensing, one must build in a margin to proactively prevent ring/piston, and rod bearing damage. We accomplish this by being conservative with CR, ignition timing, minimizing deck heights, and using the best quality fuels we can get.

There are some rules of thumb that have been established with long-term experience, but one shouldn't assume these to be the rule in everyone's engine,.............

Some of them are:

1) Bigger bore engines (95mm and larger) are more octane sensitive than small bore engines. This means that a 2.7-2.8 can use MOST pump premiums at a real 10:1 to 10.25:1 with twin-ignition on 92-94 gas. 3.0's and larger might not tolerate such a compression ratio without some detonation.

2) Cylinder head temps play a HUGE role in determining each motor's octane requirements and that changes constantly with outside air temp, humidity, engine load, and RPM. This is the main drawback to an air-cooled engine; cylinder head temps mimic outside air temps so this is a big variable.

3) Variations in every batch of pump gasolines play havoc with an air-cooled engine's octane appetite. This is why we use 100 octane race gas; its consistent. Further, every region of the US has different gasoline formulations and octanes which is something that our European compatriots do not have to deal with and Porsche did not fully understand until later on (3.6's). One can read up on the differences between RON & MON fuels to appreciate that two different fuel formulations that still average out the same (R+M/2), will behave totally different at part and WOT. Comparing what works with European 95-98 RON fuels to US 92-94 R+M/2 gasolines is not always valid.

4) Engine parameters also affect octane requirements. Excessive deck height promotes detonation as does incorrect spark plug heat range, aggressive ignition mapping, and lean fuel mixtures in these hemispherical, lazy (low swirl) combustion chambers. Everything must be right to PREVENT detonation from low RPM to redline and at all throttle positions.

Hopefully, the reason for Porsche's decision to incorporate active knock-sensing into the last generation of air-cooled engines is clearer,.....one cannot always control the above variables well enough to prevent serious internal engine damage. This is why one must be more conservative when building and operating a high-compression 911 motor without adaptive knock-sensing.

IMHO,....Relying on one's ears to accurately detection detonation (and sub-audible detonation) is a foolhardly exercise that will put a hefty crater into your bank account,...:) I cannot even begin to recount the number of engine found with broken rings (and worse) where the owner never heard anything suspicious. Detonation can be detected by performing a plug cut followed by very close spark plug inspection that frankly, takes a good measure of experience to do accurately. Its smarter to prevent, rather than deal with detonation caused by too much compression and/or too little octane.

In conclusion, my best advice to err on the conservative side with compression ratios when pump gasolines will be the main fuels of choice. In the specific case of this gentlemans's twin-plug, 3.4 running pump at a true 10.3:1, I'd need to have more information about deck height, A/F ratios and timing maps before I could venture an opinion about whether it will detonate in hot weather or not,...:)

My deepest apologies for being so long-winded here,.......I was not trying to avoid an answer, my only intent was to (hopefully) illustrate how complex this subject is and shed some light on all the variables at work. I've just barely touched on this here,....:)

As always, one can drop me an e-mail for any further clarifications if I didn't do that very well.