Piston / Cam - clearance / performance Question

jlmccauley · 08-12-2012, 11:50 AM

In the middle of a 3.0 to 3.4 build. Have a set of Mahle 98mm cylinders and Mahle Max Moritz 9.8 CR piston. I think they are CIS not Motronic. I know, not the best choice, but what I got.

Can I use a S cam with these pistons? Better cam choice?

Will this combination work well with 46 PMOs? Drive-ability, power. Car will be street and track. I like a relative wide power band.

Tippy · 08-12-2012, 12:35 PM

Ask in the engine rebuild section; engine combos are commonly addressed there.

Bob Kontak · Bob Kontak's Garage

CIS can handle larger displacement. 964 cam is a common upgrade when keeping CIS.

If you have your heads done they remove a few thousandths off the cylinder mating surface and than will kick your compression ratio up a wee bit. My US 81 went from 9.3 to 9.5 having six thousandths take off.

With carbs your cam choices increase as non CIS grind cams may be used. I doubt S cams are an optimal choice for a wide power band. Other know way more than me.

polizei · 08-12-2012, 02:10 PM

I looked into this, and wedge shaped pistons are suited only for single plug, common plenum setups which support limited camshaft options. The wedge shape of the cis and motronic pistons push the mixture toward the offset single plug.

In your setup with s cams and the mm pistons, you'd need to machine a valve relief into the high side of the piston to clear the cam. This creates an ignition problem, however, as single plugs wouldn't be able to burn the mixture evenly. To use the pistons you have and run s cams, you'd need to machine the pistons such that they are symmetrical and you need to run twin plugs to ensure proper flame propogation. This would allow proper combustion, but likely at the loss of significant static compression because of how much piston material that would need to be removed. Thinking thru the possibilities, these pistons do not seem suited to ur build.

Id recommend selling them and either purchasing mahle or JE hemispherical domed pistons. I don't have any experience running mahle cylinders with JE pistons, but i have read that they don't play nice with each other. LN engineering makes excellent cylinders (including slip fit 100mm) that work well with JE pistons.

jlmccauley · 08-12-2012, 04:24 PM

Thanks for the intel. Looks like I need to slow down and do my homework. Heads have already been drilled for twin plug and I'm not going back with CIS.

polizei · 05:47 PM

Enjoy the journey getting to the right solution for you - it sounds like you are investing some serious coin. Steve Weiner at rennsport has built many similar engines and can provide parts. He's a great guy to chat with about which direction to take.

jlmccauley · 08-12-2012, 05:46 PM

So true about the $$$$$. How do I get in contact with Steve.

polizei · 08-12-2012, 05:48 PM

Quote:

Originally Posted by jlmccauley

So true about the $$$$$. How do I get in contact with Steve.

Contact Us | Rennsport Systems | Porsche Performance Systems and Support

jlmccauley · 08-12-2012, 06:04 PM

Thanks. I'll give him a call Monday early. Always wanted that special Big Motor - my version of one anyway, but the journey definitely has its ups and downs. Presently very down.

polizei · 06:22 AM

Coming out of another rebuild, my advice is to take the time to understand and articulate exactly what your goals are (e.g. HP, personality, usage). Then let someone like Steve lay out options for you. These cars have been around for 30+ years and there are lots of ways to modify them - not all being good options.

I'm building a car very similar to the one you have in mind, but I'm starting off with Montronic engine management from a 3.2 Carrera. I ultimately would like a 3.4L street/track weapon running ITBs and aftermarket engine management. However, I'm stepping into it by getting the long block into a position where it'll work great with either Motronic or ITBs & aftermarket EFI. This allows me to drive the car sooner, spend less now and ultimately determine whether I really do want to go ITBs with aftermarket EFI down the road.

That's just my example of how to enjoy the car sooner, move in the right direction of your goals and not break the bank. Good luck and enjoy!

jlmccauley · 08-13-2012, 06:00 AM

Excellent advice. I've started doing more research this morning and I'll give Steve and others a call for some advice. Going slow and careful. Even found a local set of Mahle 100s 10.5 CR. Think they are the larger exact set you're using. Requires case mod. Going to think it out and get a plan. God help me, I love this stuff. Greatly appreciate your help, joe

polizei · 08-13-2012, 06:37 AM

Quote:

Originally Posted by jlmccauley

Even found a local set of Mahle 100s 10.5 CR. Think they are the larger exact set you're using. Requires case mod.

Yes, I believe they are otherwise the same.

The one thing I would've done differently is go with a set of 100mm Ps&Cs over 98mm. The reason I went with 98mm at the time was that I did not anticipate needing to rebuild the bottom end AND the lead time on Mahle Cylinders & Pistons can be up to 12 months

Plans changed and the bottom end has been rebuilt

It would have been a walk-in-the-park to bore out the case for 100mm cylinders!!

Lesson Learned: Patience, Patience, Buy Cylinders/Pistons after you determine whether the bottom end should be rebuilt and after you measure cylinder head volume (target compression of pistons means nothing without knowing your head volume), and more patience

Considering a lead time of up to 12 months for Mahle Ps&Cs, this approach isn't always practical. Additionally, Mahle doesn't customize pistons, so you really end up ball-parking what you need in terms of compression regardless. This is where all the JE piston guys chirp up

However, for a street engine, I'd take Mahle's proven reliability over a precise compression ratio.