Classic 3.0 motor build?

[dtw]

Originally Posted by kenikh

What is high enough? Steve Weiner preaches that anything 9.5:1 and higher can significantly benefit from twin plugs due to the fact that you can dial in an ideal ignition curve. He feels that Porsche motors on pump gas should stick to no higher than a measured 10:1 CR with twin plugs. His assertion is that anything higher is robbing Peter to pay Paul in terms of the fact that you will have to dial back ignition timing to accomodate the higher CR.

Around here (east coast) I have heard 10.5:1 from some good builders. However, we do have much better gas out here.

[kenikh]

10.5:1, as a rule, will work fine on a twin plugged motor, as you will ber within the range within which you can time the motor to run fine. Nonetheless, your timing depends on other factors that play in - for example, you will be able to dial in better low RPM timing with big cams, due to lost dynamic CR from overlap at low RPM. Rev specific VE is important, as if you are filling your cylinders at 100% efficiency at high revs, your dynamic CR will be very high at high RPM, but high RPMs are more forgiving of aggressive timing.

If you were running RSR cams, 10.5:1 on a 3 liter that revs to 7500 RPM would be just fine for getting that 'perfect' ignition curve. S or E cams may not perform as well. 93 vs. 91 octane isn't that big a difference; the most important thing is to think of a motor as a system and that absolutes don't apply.

[TibetanT]

From the pictures shown here, it doesn't appear that there is much wear on the main bearings. The crank probably looks good too I would imagine. Any idea what the mileage on this engine is currently? Pistons and cylinders look pretty tight as well.

[tom1394racing]

Shaun

Something to think about when building this motor......

You can have the heads modified for the higher CR and twin plug but save the expense of the twin plug ignition for later. If you build the motor with double stacked base gaskets, You can reduce the CR to an acceptable level to allow you to run single plug ignition. simply plug the second spark hole. When you are ready to spring for the twin plug, you can pull the heads, lift the cylinders off the case, clip and remove one base gasket. This will result in an increased the CR when you reassemble the motor and allow you to take advantage of the twin plug ignition.

A very experienced 911 guy suggested this to me when I was building my 3 liter.

[Shaun @ Tru6]

That is a great idea Tom, thank you. because I don't have to have this motor up and running in the next month and winter is upon us, I'll stay with the twin plug set-up and keep saving my pennies. My first hurdle is to get the pistons from the UK and then send:
P/C
Heads
Rods

all to EBS.

Doing so will allow Don to balance and match all components together. I think my crank is still factory balanced just fine and this isn't a high-revving motor anyway so no sense in sending that across country, though I will ask Don if it makes sense to have him do all the balancing and then put in the new rod bearings and ARP bolts all back together. There's got to be some value in having him do that.

Ed, I have no idea how many miles are on the motor but it was clearly rebuilt at some point. All in all it's in good shape so I feel pretty lucky in finding it.

If I can manage a torquey 220 reliable hp going into my 73, I honestly think this would be all the motor I would ever need. I feel a little sorry for my rear 215 A0008's.

[911 tweaks]

Shaun... melt those 215... life is too short :-)