supertec 2/8
 

Henry, based on your vast experience with building these motors what are you expecting at crank and wheel horsepower (and at rpm) for this build? All other things being equal, what are differences between mfi, webers and newer injections and management in horsepower, torque and mileage figures. thanks for your instructive postings on this list, Henry

Given the relatively low compression and Mod'S" cams I would put the performance at around 260 crank hp @ around 6500RPM. The general number for loss from crank to wheel for these cars seems to be around 15%.
The difference between MFI, Webers and more modern systems is a discussion for a thread of it's own.
As for mileage: Considering this engine will run 46 Webers, fuel efficiency might be measured in yardage rather than mileage.

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Wow! Beautiful! Thanks for sharing.

I see such an incredibly useful technique in this picture. Thank you Henry.

This motor is a work of art.

I noticed that you are using 4 bearing cams. How does this affect the choice of valves and heads? What valve springs are you using?

A little history. Production 911 cams were originally designed with three, 47mm journal and this design was used in production engines up through the 2.7liter.
Back in the early 60s, factory race engines were built with four, 47 mm journal cams because 4 journal cams offered more stability.
In 1975, Porsche decided to make cams for all 3.0 and larger engines with four journals. They also wanted a larger base circle to accommodate higher lift cams so they transitioned to 49mm journals.
Being a small company, Porsche elected to make one casting that could function as a three journal 47mm cams or a four journal 49mm depending on the finish machine work.
What you see in the pictures are 2.7 liter, 47 mm cam towers that were modified (drilled and plugged) to accommodate a 4 journal 47mm cam. For years, in order to get 47mm 4 journal cams you had to find factory race cams but since the advent of billet cams (from the likes of Andial, DC, Web and Elgin) most cam blanks are 4 journal and the cam grinder just determines what size journal diameter to grind on it.

2.7 liter cam towers converted for 4 bearing 47mm cams. Note the set screw where the cam tower was drilled and plugged.

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Because this engine is going in an early version of the 68"L" we also needed to covert the towers to six bolt to accommodate the early aluminum valve covers. The extra studs are removed, the holes drilled and custom made plugs are pressed in then the sealing surfaces are machined flat.

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Thanks for the update.

Are the heads 2,4? I think I see a 73 date? Why aren't the combustion chambers chamfered for the larger bore? Who made the pistons? Factory valve springs?

I can't wait to see the finished product. Will you be posting dyno runs?

The heads are 2.7 liter, CIS heads. Originally installed on a 1974 911. The CIS injector notch has been welded to produce a cleaner intake port.
If you look closely you'll see a slight chamfer. The pistons are made by JE and a closer look at them shows a perimeter flat on the piston dome for piston the head clearance. The valve springs are Eibach racing springs.

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We rarely run engines on the dyno. We are engine builders not turners and as such, leave the finer points of tuning up to the customer. Of course we are happy to tune one of our engines whenever asked. We can do it, no matter how must it costs.....lol

I just drooled all over my iPad

Simply beautiful work

Great stuff as always from Henry. What installed height do you choose to set the springs at with the Mod S cams? Reason I ask is because I have a set of the Eibachs for a WebCam 120/104 cam. The 120/104 is a bit more cam than the Mod S (DC 40) and nearly same as DC 60 I believe?

From memory, I think 120/104 has an intake lift of around .475".
If you set the Eibach spring at around 36mm the seat pressure will be around 90lb and the over the nose pressure will be around 275lb. Those are numbers I could live with.

One more question. I noticed that the cylinders were not "mooned". Can you give us your thoughts on this mod.

Thanks,

Much like boat tailing the case, "mooned" cylinders offers negligible if any enhancement to overall performance while compromising the integrity of the part being modified.

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Your memory is correct. Intake lift is .476" and .450" on the exhaust

Thanks much for the 36mm recommendation. Same as the Eibach published number, but theirs would seem to be just generally speaking and not cam related. I'll be sure to check for coil bind and retainer clearance with the stem seal. Probably not going to be an issue for either of those things but worth looking at since in their measuring things anyway

When working with an after market spring all you really need to know is total valve lift and spring pressures. Porsche is notorious for running very low spring pressure to reduce parasitic loss and wear. The point of an after market spring is to reduce valve float at high RPM.
As long as your valve lift is less than Eibach's published pressure @ full lift number and you can live with those numbers, you're good to go.
If you desire higher full lift (over the nose) pressure then calculating spring height becomes an more challenging endeavor.
When we first encountered these springs we were skeptical so we invested time and resources into testing the validity of Eibach's claimed number. At this point we are satisfied that they (Eibach) did their homework.
Of course for all of you "engineer types" that struggle with believing an "old dog" you are more than welcome to do your own homework.