Help with 3.0 PMO build
 

I have a 76 Carrera 3.0. The engine is stock standard.

So far I have installed SSI's, Dansk single out sports muffler and rebuilt the suspension using 22/28 torsion bars, turbo tie rods and superpro bushes.


I am wanting to do a staged PMO engine rebuild.


Stage 1 would be to replace the CIS with PMO's (no cams or pistions at this stage). Would I use 40mm or 46mm? What are the differences and pro's and con's with each?

Stage 2 would be cam grind and pistons etc. On the basis the car is 50% road and 50% track days, what would be the best pistons and cam grind for me.?I am hunting torque more than a peak HP up high engine and I want the car to still idle and crawl through traffic OK.

What else should I consider doing to the engine during stage 1 and stage 2.

I guess I am asking what is the ultimate 3.0 PMO setup with road use still in mind?

Thanks in advance - Paul

Paul,

You might want to save the staged engine mods and do it all in one go. Others with far more knowledge will help more but the cams, pistons and carbs all work together. If you put carbs on a std CIS motor then the max will be 40mm. 46mm is just too big, however you may want 46mm when doing the cams depending what grind you end up choosing. Pistons will probably need changing when selecting the cam.

Its up to you but I would design the engine upfront and do a single build.

40's are your best bet for lower rpm usage. You'll want to select your primary venturis to be 32mm or 34mm I suspect, based on your displacement and target rpm range.

Some good tips here http://forums.pelicanparts.com/911-engine-rebuilding-forum/562252-3-0-sc-40-ida-dyno-tuning.html

http://forums.pelicanparts.com/porsche-911-technical-forum/598453-urgent-need-help-setup-weber-46-911-3-2-engine.html

46's are too big if you don't have the cams to support them. The larger carbs will pull more air and you'll have a hard time fighting a lean mixture condition. Reason you have a hard time fighting the lean condition is because the larger air volume flows at a slower velocity. The slower velocity in turn does not "pull" as much fuel- carb fuel flow is airflow dependent.

That said, the 906 and other 2.0L race engines used 46 Webers. So they can be used on smaller engines. But you have to be radical with your settings to get them to work and that's not conducive to acceptable street usage like you desire.

Pistons and cams all depend on what you're willing to do. If you're staying with single plug ignition, then you are limited to 9.5:1 compression ratio for high octane street gasoline. At that compression, you'd be best served with Mahle Motorsport or JE pistons that are shaped for carbs or MFI usage- your typical domed pistons with valve reliefs. Something similar to these 98mm 9.8:1 Mahle pistons

http://forums.pelicanparts.com/uploa...1334036557.jpg

http://forums.pelicanparts.com/uploa...1334036446.jpg

http://forums.pelicanparts.com/uploa...1334036418.jpg

Cams are a tough call. I don't want to go there, since I know just enough to be dangerous.

Paul,

JMHO,........

Stage 1: 46mm PMO's with the appropriate jetting package for your engine and location. 40mm & 46mm carbs use the same upper castings so one doesn't improve throat velocity at low RPM (like Webers do). Use the correct venturi and you'll be fine. You can also change the distributor advance curve for improved engine response, depending on local octanes.

Stage 2: There are many choices for cams and variables such as vehicle weight, gearing, type of use, and CR play big roles here. Pistons should be replaced unless you use something like a 964 cams for best low & mid-range torque. One can also bump the CR up again, depending on what fuel octanes you will use. Twin-ignition really helps here as well. :)

Your stock engine will work great with carbs. 46's will give you more power above 6000RPM. 40's will give you better tunability/drivability. You will be very happy with either one.

-Andy

Would it be fair to say that a road going car isn't really that suited to a power range above 6000rpm but limited to around 7200-7300 (depending on your level of nerve)?

I would have thought that 40's would be better unless there was a change in cam, which of course brings about a change in pistons and likely a few other things.

If your going to change the pistons you might as well change the cylinders...3.2. I did exactly that:

Stage I: 46 PMO's, SSI's and M&K Dual Muffler, were fine on the street for a 3.0 with CIS pistons.

Stage II: 98mm Pistons, Cams, Top end refresh

Stage III: Rarly8 Headers

What is this?

Running 46 PMOs on a built 2.5SS on the street, no issues at all, granted it has an 8K redline.... Have a 3.0SS in the works and will be going with 50 PMOs. 46 PMOs on a 3 liter with what you originally stated will be just right.

Pluse, if Steve@Rennsport says 46 PMOs, you are good to go.

Jimmy

Custom made slip fit headers from Brian (RARLY8 on this board) picture below.

This thread if you are interested:

http://forums.pelicanparts.com/porsche-911-technical-forum/596812-sound-video-preview-m-k-rarlyl8-headers.html

Here is my build thread, there is a lot of good discussion:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/524936-shanes-3-2-ss-recommendations-advice.html

http://forums.pelicanparts.com/uploa...1298313254.jpg[/QUOTE]