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need advice on pistons/cylinders

I am backdating a '79 930 to a longhood look, and to avoid needing a spoiler on the enginelid, I am installing a 3.0 non-intercooled 930 engine. The engine was bought from another boardmember and supposedly in good condition and resealed. Some clumsy details on the engine (two types of chain tensioners, timing way off, etcetera) made me doubt the build quality, so I had my local engine builder (40+ yr Porsche mechanic) take the engine apart. That turned out to be a good idea, many things were wrong. Disappointment... However, that's not the point of this thread.
Not surprisingly, there is some wear on the pistons and cylinders. Nothing really serious, they could be re-used. But we're getting to the familiar "now we're at it" point, and considering to do a little upgrade.
Things we'll be doing for sure are:
- install 964 camshafts
- install RarlyL8 headers, central outlet muffler, 934 style wastegate pipe
- install K27S turbocharger

The car will be used for fast street / DE / trackdays, and driveability (i.e. low end torque) is a high priority.

Now my question; should I consider replacing the pistons to raise the compression? If so, how much? 7.5:1 ? What type/brand pistons should I use?
Or take it even one step further and replace the cylinders as well?

Again, the goal is to build a fast and fun car, but not break the bank...

Anybody have any experience with this?
Any thoughts or advice is much appreciated!

Arjen

Old 07-12-2011, 05:28 AM
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thats going to be a sweet sleeper. But why not get creative and mount the intercooler slanted
Old 07-12-2011, 02:18 PM
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Max Sluiter
 
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A number of people have air/air intercoolers under ducktails. Not sure if the same setup would fit under a flat lid or not.
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Old 07-12-2011, 06:01 PM
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Personally I wouldn't drive fast without a spoiler. At the very least go with a period correct ducktail. http://members.rennlist.com/imcarthur/tail%20tales.htm

Also wouldn't build a turbo motor without an I/C.

Your purpose or tastes leading you to want a flat back aside, IMHO esthetics should never be prioritized over function for a Porsche - especially a driver (as opposed to a Concours purpose car).

Ducktail is a win / win with better handling, better power = Porsche the right way.

Last edited by pdqcarrera; 07-12-2011 at 06:22 PM..
Old 07-12-2011, 06:17 PM
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Thanks for all the comments. I realize using a rear spoiler has the advantages of more downforce and room for an IC, and I admit that my preference for a basic decklid is technically not smart. However, for me, this is not just a basic backdate RSR project. What I am trying to build is my idea of my personal favourite 911. For me, that means not only a long hood, but also all SWB details (from doorhandles, to vent wings, green gauges, even swb turn signals, and no tail wing) and full luxury trim. So it not only has power windows, sunroof and full leather trim, but also steel bumpers with trim.
So the rear tail just doesn't fit in my vision of a fast, luxurious 1960's GT car. Sorry.
I have considered an IC under the flat deck, but I would prefer not to do that right now, financially and technically. I'd rather get it running good now without the IC at a lower boost pressure (maybe 0.8 bar) and, if I want, install an IC later on.
I figure, if I raise the compression ratio now to maybe 7.5:1, combined with the lower boost pressure of 0.8, it should result in a good running, powerful engine with some good low-end torque. I am not (yet) looking for maximum hp, first I want the whole car set up right. If I want more hp later on, than I could install the IC under the decklid and raise the boost pressure.
However, I am no expert at this game, and willing to learn.
Does my theory make sense?
And back to the original question; what to do with pistons and/or cylinders at this point?
Thanks for the input!
Arjen

Old 07-13-2011, 03:33 AM
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Arjen, cool project. Not sure you want to delete the IC with an increase in CR running at stock boost. I'm sure you can get away with it but get some bad gas on a hot day might be trouble. I think you should try to retain the IC.
Old 07-13-2011, 09:24 PM
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Not an expert by far, but raising compression with piston choice is no different than raising effective compression by increasing boost, both a big no no for a non intercooled boosted car
that will be seeing some track time. If looking for some cheap insurance, take a look at the meth injection thread started by cole930. I'm slowly working on installing mine so its hidden when lifting the lids.
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Old 07-13-2011, 10:39 PM
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Neat project, I'm old school with turbos. Raising static compression ratio is the last thing you want to do, especially if you are on an engine budget.
No, static compression vs. boost is not the same. A lower static compression ratio with increased boost will stuff more charge in the cylinders thus make more power than a higher compression ratio and lower boost configuration. Leave your C.R. as is, increase the boost and spend money on a water/methanol system. Old school GT's usually made you keep the engine in a power band.
I'd paint two decklids; one ducktail, one clean.
Old 07-14-2011, 03:28 AM
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Quote:
Originally Posted by e170drvr View Post
Arjen, cool project. Not sure you want to delete the IC with an increase in CR running at stock boost. I'm sure you can get away with it but get some bad gas on a hot day might be trouble. I think you should try to retain the IC.
He is not deleting the intercooler, 3.0 liter turbos did not come with them, and neither did the 917.10 or 917.30 as used in Can-Am races since full throttle was not used for more than a few seconds at a time.
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Old 07-14-2011, 09:38 AM
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Quote:
Originally Posted by copbait73 View Post
Neat project, I'm old school with turbos. Raising static compression ratio is the last thing you want to do, especially if you are on an engine budget.
No, static compression vs. boost is not the same. A lower static compression ratio with increased boost will stuff more charge in the cylinders thus make more power than a higher compression ratio and lower boost configuration. Leave your C.R. as is, increase the boost and spend money on a water/methanol system. Old school GT's usually made you keep the engine in a power band.
I'd paint two decklids; one ducktail, one clean.
I have been wrong before, but I do not see how the two are different. You take the intake pressure and then compress it by a certain amount in the cylinder in order to get the final pressure just before compression. Squeezing air is squeezing air.

If you fill the cylinder with 1 atmosphere of pressure and compress it to 1/12 the volume, isn't that 12 atmospheres, same as having 2 atmosphers of charge and compressing it to 1/6 the volume? Or does the temperature rise more in the turbo, meaning that there is less air mass for the same pressure?

Larger expansion ratios (the reverse of compression ratio) are more efficient. Porsche used higher static compression ratios with the water cooled head cars along with reduced boost for better throttle response and better engine longevity with the same power, or else upped the boost for more power and similar longevity.
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Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 07-14-2011, 09:44 AM
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I wonder if this intercooler is still for sale...

TurboKraft Intercooler for sale



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Old 07-14-2011, 10:24 AM
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Seriously cool project Arjen - really dig your path and goals. Please keep us posted w/ pics along your journey!

Vent windows - sigh, I often dream of them! 8-)

Coupla thoughts:

Totally agreed w/ everyone about the IC - you state it's going to be driven hard / tracked, you need an IC. Water/meth inj as an alternative, I'd prefer both tho.

If EP Slick's IC that Patrick posted above has sold, how about considering a fender-well mounted version? Search here for Turboo934 (IIRC), he fab'd one up in his purple monster out in New Mexico. I understand the budget aspect (believe me), but no sense biting off your nose to spite your face right? You're going through quite a lot of great effort to have a fantastic execution, don't leave a glaring hole in the project (and later a few pistons d'oh)...

You also mention seeking low end torque (me too!), but are going w/ 964 cams. Everything I've read here suggests those are top-end cams, whereas SC's are better for enhancing lower end.

My useless $0.00003 Rupees, arbitraged thru the Peso...
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Old 07-14-2011, 11:09 AM
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There are many good debates about compression vs boost

Lets look at the facts

an engine makes x amount of power at sea level ( 14.7 atmospheric pressure )

Theoretically if you double the atmosphere you will double the engines output.

So a 150 hp engine boosted to 14.7psi would make 300 hp / With that basis a 180 hp engine will make 360 hp at 14.7 psi. So if 300 hp is your goal you can then lower the boost pressure to about 10psi and forgo the intercooler.

So by increasing the engines efficiency (ie: higher compression , better flowing heads, better cams) you would need less boost to hit a certain hp target.

Pressure creates heat. So the lower the boost you run the less the turbo heats the air. On a car with no intercooler that's important.

Decide on the amount of power you require. Size your turbo to achieve that power at the lowest pressure ratio and you will have a great running car.

Now, personally when I rebuild my 930's engine it will have 9:1 compression and I will run as much boost as I need to get to my target hp. And I think it goes without saying that I WILL run the appropriate octane fuel for the job.
Old 07-14-2011, 02:58 PM
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Since no one is posting on topic, why not C2 Turbo P/Cs? Then spec your piston to play with your CR.
Old 07-14-2011, 06:57 PM
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That IC is mine just like the car is so not for sale.

Not much bigger/better can be had if choosing wingless lid but ductail helps a lot if going for turbokraft custom IC.

I just got my TRE engine lids/bumpers so hopefully next summer I have my RS3000/IROC wingless look ready. Nice to see bosman likes the same look although he's taking it far beond my vision. I'm just going RS3000 look with optional rear lids (duck, plain, tea, gt2)

About the pistons/cylinders, I would just leave them like they are so you can later get more boost with an IC or WMI.

Last edited by smurfbus; 07-14-2011 at 07:53 PM..
Old 07-14-2011, 07:38 PM
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I believe TurboKraft can make an intercooler that will work for that application.
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Old 07-14-2011, 08:31 PM
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Wow, great how much information always surfaces on this board! Thanks for all the positive comments on the project. I think I'll start another thread on the build in general, I'm just not much of photographer, so you guys will have to do with my lousy snapshots. At this moment, the inside/underside of the car have been painted, and the wiring loom + front/rear suspension + brake system are all back on the car. Next week, the car is going to the painter to get the outside painted (outside is in primer now). Here's a picture of the car a few weeks ago, sorry I don't have a recent picture.

Back to the topic; I tend to think along the lines of drmatera; build the engine now as wanted (and without IC), keep the boost low enough to avoid any damage, and accept whatever amount of hp that brings as the maximum for now. Later on, when the whole car is set up well, and my savings account is out of the danger zone again, add an IC (probably air/water) and increase the boost to get real horsepower. I have been in touch now with Turbokraft, and Chris has already given me some (much apreciated) advice along this line of thinking.
You guys have given me some leads to some interesting threads, so I have plenty of reading to do! Learning a lot here.
I'll keep you all posted on what's going on.
Arjen

Old 07-14-2011, 10:01 PM
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