HP Output of an 8.5:1 CR

[Andras]

Does anyone know what would happen if one were to install Mahle Pistons with an 8.5:1 CR instead of the "normal" stock 9.3:1 CR? Apparently the 1979 Porsche 911SC came with 8.5:1 CR, and yet they seem to be plenty fast when they're with me (1983 911SC) on the track.

1. Would that make for a SLOW car?
2. Can the heads be milled a little to increase the CR?
3. How much could be milled before the valves hit the pistons?

Thanks guys..............Andras

[jmz]

There were some other differences. The port sizes for one are larger no the earlier SC cars.

Do you have a problem with your pistons? ...I suppose if you were going to supercharge or turbocharge your car it might be wise to use the lower C.R. pistons. ...other than that you are going the wrong way.

I have the old pistons from my '81 SC based motor if you need some of the 9:3:1 CR pistons.

[Andras]

Hi jmz,

I was asking, for I have an offer for this set of P&C at $1400, and it seems too good to pass up.

My engine, at teardown, had 4 out of the 5 pistons with broken top rings, thus indicating detonation. And those pistons had very sloppy clearances in the ring lands. Thus I need new pistons.

I have considered getting EBS to match new JE pistons to my Alusil cylinders (after replating with Nikasil), but the "new old stock" Mahles with the 8.5:1 CR are an even better "deal" price-wise ($500 less expensive) than that route.

How can I match your pistons to my cylinders? Are your pistons Mahle Nikasil? Are they in good condition, with the ring lands within tolerance? How much are you asking for them?

You can e.mail me at anagy@zeidlerpartnership.com.

Thanks for your interest..........Andras

[Superman]

Early SCs had different intake systems. Their 8.5:1 CR works well with their fatter intake runners, etc. Best of both worlds would be the early SC intake system combined with higher compression pistons. Worst of both worlds would be lower compression pistons in a later SC.

Sorry to hear that your ring lands are perhaps out of spec. That means your rebuild will be more expensive. $1400 for a set of pistons may be a good deal, but I think $3000 gets you NEW German P&Cs that match. I'd suggest a CR in the 9.8:1 neightborhood since pump gas will work fine. I'd suggest the bigger P&Cs to bump displacement up to 3.2L. The way I see it, those $1400 pistons are a danger, sort of. They are tempting you to cobble together a solution that will be nearly as spendy as NEW P&Cs, and preventing you from jumping on this golden opportunity to build a "short stroke" 3.2.

[Andras]

Yes, I've thought of that, but once you go to 3.2, then I'll have a short-stroke 3.2 with about the same CR (see Wayne's book for discussion).

I can get a new Mahle 3.0 litre, 9.3:1 CR for about $2,000, which might be the best way to go altogether. This would keep my car completely stock (which to me is a plus), and would keep the expenditures reasonable.

The other possiblity is to go with re-plating my cylinders with Nikasil, and having them match new JE pistons. The appearence would still be stock(pictons are "hidden"), and I would feel good abouit re-using my own cylinders. This option costs about $1,900.

So the cjoices are:
1. New Mahles with 8.5:1 CR for $1,400.
2. Replate my cylinders, with new JE pistons for $1,900.
3. New Mahles with 9.3:1 CR for $2,000.
4. New Euro Mahles with 9.8:1 CR for $2,200.


Any thoughts?

[Andras]

I was using 87 octane during "normal" running to and from the track, and 92 octane while at the track. The car racked up most of its 80,000 miles to and from, and at the track. There were times I heard it pinging, but I always let off on the throttle when I hear it. I never heard it at the track.

It's also possible that the carbon build-up, caused by the worn valve guides, cold have contributed to detonation. The machinist is currently cleaning up the heads, prior to resurfacing, and checking the vlaves. He is definitely giving me new valve guides. The worn valve guides could also have contributed to the hot running of the engine, always about 210 in the summer, always above 250 at the track, at 6,000 RPM.

It's one of the reasons I am reluctant to go with higher CR pistons - now I'm skittish about detonation.

[RoninLB]

Originally posted by Andras
The worn valve guides could also have contributed to the hot running of the engine, always about 210 in the summer, always above 250 at the track, at 6,000 RPM.

It's one of the reasons I am reluctant to go with higher CR pistons - now I'm skittish about detonation.

I don't have the hard info, but excessive oil will raise the octane requirements of the engine.. and fwiw I run 8.5RS because of possible fuel problems, it's not getting better.. and I always run 92.. and lower comp doesn't sound as sharp as the higher cr

[Early_S_Man]

Andras,

I suspect that 9.3:1 is too high to run regular gas these days ... unless timing is retarded a 'significant' bit and checked regularly.

There was no change in the power rating of SC engines [which may or may not have been a 'mind game' they were playing] when the compression was raised, but the requirement to use premium is getting painful these days, so I suspect ... as Ron alludes to ... that planing an 8.5:1 engine rebuild that can handle regular gas makes as much sense as it did when the factory dropped all compression ratios for the '72 models in anticipation of the elimination of premium gas from the USA market [which didn't happen until 30 years later]...

[Andras]

Thanks Warren, this makes sense.

I still have to make a decision soon, as the 8.5:1 piston will not be available for long, and the $600 difference bertween it and the 9.3:1 CR seems like a lot to pay for just 0.8 CR.

In your estimation, would the 8.5 CR be that much more loss of power? Can any of this CR be gained back by shaving the heads a bit?

I have passed beyond needing to have the most powereful engine in my peer group, so I could live with some loss. And since I have the SSI and '74 sport muffler, I was getting good power, as attested by the chassis dyno reading of 154 HP at 6,000 RPM, just before we tore the engine down.

Whcih of my choices seems "better"?

[jmz]

I would think that losing almost a full point in the C.R. would SERIOUSLY impact performance.

[echrisconnor]

JMZ,

If your pistons need a home, I have my engine apart and getting ready to put the 8.5s back in it. Let me know if you'll part with them.

I agree with JMZ though, that a full point drop in CR will drop substantially lower your power. I'm eager to rais my CR if possible. I've read somewhere that (this is a very loose standard) a 1 pt bump in compression will give you approximately 10% more/less power. And, you may not just notice it at peak power but torque across the lower band.

Chris

[jluetjen]

In your estimation, would the 8.5 CR be that much more loss of power?

From what I've seen, it won't make a huge change. I'd look no further then the fact that the HP didn't make a significant jump up when Porsche shifted from the 8.5 to the 9.3. Since that was the case, why would it go down by much going in the other direction? 911 motors just don't seem to respond greatly to increased CR. Another for example is the earlier carb'd and MFI'd cars where the torque and HP/liter did not drop significantly when Porsche decreased the CR's in the early 70's. The biggest change in performance seems to be at low rev's when the engine is "off-cam".

Since you've taken the rather mature outlook that you're not trying to have the most powerful motor, but rather are looking to cost and reliability the lower CR pistons might be just the ticket for a long running motor that doesn't require the most expensive fuel. You're right that you can gain some of the CR back by shaving the heads, but then your spending money to get back a portion the performance that you mikely likely won't even be aware was missing. If you decide to do it, you should come back and post the results on this thread so that we get some closure on the result.

[jmz]

John,

As stated above, the early SC vs later SC were different in more ways than CR. The later ones were a bit port restricted which can certainly effect HP in a negative way.

Eric, The pistons are available to sell only if Andras decides he doesn't want them.

[beepbeep]

Hmm, Is only me who thinks "great opportunity to turbocharge?

[Early_S_Man]

Andras,

I tend to agree with jluetjen that there won't be any significant loss in power ...

If you want to be able to run regular gas, you would have to retard the timing to use the 9.3 pistons, and that kills a bit of potential power right there! Unless you plan to use premium gas and find no real utility in regular gas ... the 9.5:1 JE pistons are not much better as an option, either.

I use the RS 3.0 engine development as an example, and it was quite a bit higher on the performance scale. Back then, 8.5:1 pistons were tried, and only 6 hp was gained over the RS 2.7 engine, i.e., 216 hp ... until 9.8:1 pistons were tried, then the hp jumped to 230! That is an additional 0.5 compression over the SC discussed here, so perhaps the difference would be 5 - 7 hp or so, considering the considerably more aggressive cam profile of the Early 'S' cams ... and higher volumetric efficiency of the high-overlap cam profile, and higher rpm power peak!

[jluetjen]

I use the RS 3.0 engine development as an example, and it was quite a bit higher on the performance scale. Back then, 8.5:1 pistons were tried, and only 6 hp was gained over the RS 2.7 engine, i.e., 216 hp ... until 9.8:1 pistons were tried, then the hp jumped to 230! That is an additional 0.5 compression over the SC discussed here, so perhaps the difference would be 5 - 7 hp or so, considering the considerably more aggressive cam profile of the Early 'S' cams ... and higher volumetric efficiency of the high-overlap cam profile, and higher rpm power peak!

Warren, it's funny that you brought up the 3.0RS since I was just looking at the specs for that motor earlier today for a thread on the Engine Rebuilding forum. If the 2.7RS was restricted somewhat by the 2.4S's induction system and 36 mm intake ports -- the 3.0RS was even more restricted with the same induction system -- even with the 3.0's larger valves. I suspect this is why the 3.0 made so few extra ponies over the 2.7.

Increasing the CR has more value if the engine is seriously air restricted since it can help to increase the dynamic CR in the cylinders even as the air is getting choked off. At peak RPM's, another 500 RPM of torque can make a significant difference in the total HP of a motor, but you'll only notice it at peak RPM's. This is why the NASCAR guys run 14:1 or 16:1 (or more?) in restricter plate motors when their unrestricted motors are running a more normal ~13:1. The extra few RPM are very noticible when running wide open around a super speedway.