Markus951

Hey guy's.

I am just looking for information is this the right way to do it! Many of you know that I broke my 2.5 liter engine last year. https://rennlist.com/forums/showthre...ighlight=GT30R

And as winter is cold and dark in this side of planet I am planning new and something different for next season!

As I have most of the parts like programmable fuel management, Turbo, Intercooler, nice 3" exhaust, tial WG, bosh o44 from my last setup going with bigger displacement is not that big of an issue anymore! So I am thinking to create something based on 968 variocam engine! I know this is something what is done really rearly on 951-s but Zoran Letunica has given me a great deal of inspiration!

http://autospeed.drive.com.au/cms/A_1327/article.html

Vids:
http://www.rescueforce.com.au/images/expose-car.avi
http://www.rescueforce.com.au/images/race-pi.wmv

Now to the deal:
I know that this is not the best way to do it but it can be done (proven by Zoran)

As you all know 968 comes with varicocam witch I plan to keep, bacause I am building a streetcar not a circuit racer and that low end torque really conts here. Many cars are running deleted variocam and 2 exhaust cams, and adjustable cam pulley's installed but I will do it different because my Plug and Play fuel management can control even 2 vanos, or variocam, or vetec systems, on both in and exhaust cam if needed. In this case I need to map intake cam only. Porsche stock variocam work's like this..when engine is started intake cam has 15 degrees advance and will be clocked back in 5500 rpm in stock form - thats how I understand it. For turbocharged engine this would mean too much overlap under the boost, so I will use that 15 degrees advanced untill turbo starts to spool and then ECU will clock it back and allowing engine to create power on higher RPM. Stock cam profiles are pretty mild anyway so they should work well..

Now about the other things. As you all know 968 has fully forged bottom end (only in 92 the rods where cast) it has oil squirtels under the pistons etc....sound like a perfect for turbochargine, but it has one small problm. compress ratio is too high for forced induction and pistons are allmost impossible to find.. only if you do the darton mid-sleeve + JP pistons from Chris White wich is quite a pice of work and hard to find a experienced specialist who could do it..

So my solution would be exactly as described in the article.. by installing special custom con.rods to bring down the comp ratio. I do have a connection in Finland where those billet con.rods would be made. They are proven to last at least 200hp/per cilinder so there is plenty of reserve and insurance with them over stock units.. I did some calculations and I got into the solution that 2mm shorter con.rods should do. Could please someone comment the ideas and formula I have for calculating the compress ration:


Those are my calculations
3.0 16 variocam engine with 11,1 comp ratio on stock

Bore 104mm
Bore radius 52mm
Stroke 88mm
Burn chamber volume 67,31

1 cilinder volume = 747 169,28 witch is (bore radius*bore radius)* 3,14 * stroke

Compress ration in stock 968 is 747 169,28/ burn chamber 67,31 = 11,1:1

Now lowering the rod lengh per 1mm will add 8,490 to burn chamber

so
with 88mm stroke we have 67,31 (stock comp ratio 11,1:1)
with 1 mm shorter rod we have 75,80 ( comp ratio 9,86:1)
with 2mm shorter rod we have 84,29 (comp ratio 8,86:1) would be best solution
with 3mm shorter rod we have 92,78 (comp ratio 8,8053:1)

I calculated the burn chamber by knowing the stock comp ratio, bore and stroke. This is not the most accurate method but it will give some idea!

Solution : 2mm shorter con.rods should do the job!
Is this the ritght way to calculate it?

Markus951

Anyone?

Rogue_Ant

I think it would be easier to simply use an extra thick head-gasket...



Rogue

Markus951

But if you need to change the con.rods anyway? It is quite possible that engine like this would put 700 nm down when turbocharged!

Markus

Chris White

Don’t forget to add the head gasket volume (to the combustion chamber) in your calcs – approx 10cc.
Where did you source the combustion chamber volume number? your number looks a lot more like a 8v head volume....a lot more than a 968 or S2 head!

333pg333

Hey Markus I thought of you when I read that article and posted it on Rennlist. I know you were talking of reducing the compression by shortening the 'rods some time ago. I think Zoran or his engineers have proved it works but bear in mind that this was done quite a few years ago and as he doesn't post on R-list (or that we're aware of) we are not sure what has happened to the car. I think he's still running it on club days in Melbourne but maybe you should try emailing him or even Hally (on R-list) as he has raced against him and may know more about the car than most.
I think you will get more responses once the Americans are awake. Speak to Chris
as well I would suggest.
Best of luck,

Patrick

Markus951

Well I calculated the volum according to the compress ratio (what I knew), 11,1:1 and cilinder volume what I knew (of course it has to be measured with petroleum or some other liquid. to be exact..

Formula should be cilinder volume devided to compress ratio equals burn chamber volume.. and that comes from formula how to calculate compress ratio (cilinder volume devided to burn chamber volume equals compress ratio)

Trucho-951

Static CR = ( piston dish vol. + cyl vol. + deck vol. + gasket vol. + head vol.)/ (piston dish vol. + deck vol. + gasket vol. + head vol.)

also don't forget about Dynamic CR, it uses the trapped cyl. volume instead of total cylinder vol. so you will need to know your camshaft specs. to figure out the intake valve closing point and the remaining cyl vol. after intake valve closure.

Chris White

OK, a closer look shows a significant error in your calcs –
To determine the compression ratio you need to use the total volume (not displacement) of the system – that is the displacement plus the combustion chamber volume.

Your displacement calcs are correct but you need to add the chamber volume before figuring the compression ratio.

Using CC’s to make it easier to read –

Displacement 747.17
“Burn Chamber” 67.00
Total volume 814.17

Compression ratio 814.17/67.00

Compression ratio 12.15 – 1 (not correct)


Displacement 747.17
“Burn Chamber” 74.00
Total volume 814.17

Compression ratio 814.17/74.00

Compression ratio 11.10 – 1 (correct)

RajDatta

Markus, I have successfully done this but in a different manner. When I bought my turbo setup, this is what they did.
They used stock sized carrillo rods but they shaved 2mm off the top of 968 pistons. This did not effect the dish section of the piston because it was deeper than 2mm. The intake valve relief is no longer present but the exhaust valve relief is still there because its deeper. The engine at the time used a 2.7 head. Since I now run a factory 968 turbo RS engine, I sold the setup to a friend who has been successfully running it on a 944 S2 16 valve setup. I know of atleast one more individual who went to dishing the pistons but he also installed shorter rods. Hopefully he will chime in.
Depending on what kind of boost you want to run, I think a thicker gasket might still be the easiest and the best solution. You could easily lower your compression 2 full points by getting a cometic 3mm head gasket (calculations were made by the same friend, thanks ). I know people will tell you this will mess up the squish and all but if you look around there are plenty of turbo conversions on other cars successfully running this setup. These are M3 turbos or other bimmers and they run as much as 1.2 bar before they bother doing the internals on their bottom ends. Heck, even RUF on their new supercharged 997 use this method. They only run .5-.6 bar but their compression is also quite a bit higher. They start with approx 11.8 compression and then lower it down to 9.5 or so. It works and RUF offers warranty on these conversions.
I plan to go the same route. Use a 968 head this time for my cabriolet and stick with variocam. My limitations will be the factory MAF on the 968 which can support upto .8 bar. That is what my friend runs on my previous engine. Even at those numbers, its safe to assume the car is quite fast.
Regards.
Raj

Ken D

Exactly right Raj. Ruf have actually been using a thicker headgasket on all their supercharger conversions, going back to the Boxster. Initially I was surprised that they went this route, but it works.

Porsche-O-Phile

There are Honda guys out there running 10:1 and even higher compression rations - plus boost - without problems. You can certainly (with proper tuning) run stock compression plus boost on a 968 but you might limit yourself to race gas or some kind of premium/toluene mix in order to ward off detonation - or adjust the timing maps a little. There's more than one way to skin a cat and there's no absolute divine decree that one must pay $3,000 for pistons in order to attain the nirvana of boostage on a 968. . . I applaud your looking at "non-standard" solutions.

Chris White

Yes, for a ¼ mile or a dyno run.

That statement needs to be qualified a little better. I think ‘certainly’ might be better changed to possibly. I don’t think an 11:1 turbo motor will last long in a track (road circuit not ¼ mi!) application.

I think that thinking outside the box is a good thing also – but the most important thing has not been addressed – what is the end usage of the motor. One of the big differences between the ‘other cars’ and the Porsches is that the Porsches are designed to run all full out put almost indefinitely, the big HP Supras and Hondas can’t do that. 11:1 turbo motors can’t do that unless the boost is very low.

RajDatta

Chris, I respect you a lot and appreciate you sharing your years of knowledge with us. I know we have spoken to the phone in the past. My question is, if the thicker gasket yields a 9:1 compression and we are working with a 16 valve head, with its better pent roof design, I don't think detonation at 9:1 should be such a concern even if you run 1 bar. I personally think 1 bar should be max for street cars anyway running on pump gas, so I might be on the conservative side.
Maybe just because no one has done it before, people have their reservations, but I do believe this setup would remain intact for upto atleast 1 bar. People do this all the time, this is not trying to reinvent the wheel. People also question shorter rods/dished pistons but my setup has run since 1996 when it was initially done by Heimrath.
Hopefully I will be able to back it up in the near future with a conversion of my own.
Regards.
Raj

Guns951