7r case based engine ideas

[gestalt1]

hi all, i just bought a good partial 75 2.7 engine. it did not include pistons, cylinders and heads. i was planning on selling the crank, rods and S cams and getting a 66mm counterweighted crank, rods, 2.7 S heads and 90mm cylinders with 10.2:1 CR JE pistons. i would have the heads twin plugged and go with either a crank-fired ignition or a 12 plug distributer. for cams i was going to go with something around GE-60, 80 or 906 type and use PMO carbs. basically a high rev 2.5ss. i know this combination has been discussed in previous threads but now i'm thinking of going a little more conservative. instead of dropping displacement in favor of high rpm power, maybe i should keep the 70.4mm crank and rods and build a single plug, 9.5:1cr engine with the same cams and PMO carbs. i think i would be at similar HP, but at lower RPM and more low end torque. i had a very similar engine combo in a previous 911 and it was a fun engine. it would be much cheaper to use the parts i have and keep it single plug also.

this engine would be for my 68 911r project and would be used for street and track use. my goal is a street driveable 200-250hp engine using a 901 transmission.

what would you guys do? is there much difference going from a 2.7 normal stroke to a 2.5 short stroke engine? do you think the loss of the .2L be made up for in higher rev limit, twin plug and higher compression? i was also considering using a non-counterweighted crank because these are cheap and since 99% of the time my redline will be 7400rpm it should provide really quick throttle response - which is what i want.

[Steve@Rennsport]

Hi,

Without knowing what ratio of street-to-track use this car will see, its difficult to offer some specific recommendations. Further, gear ratios make a HUGE difference and plays a major role in your choice.

I'd just say that IMHO, torque is more useful for street use and especially important where stock gear ratios are in use. A long-stroke 2.7 can have excellent drivability while a SS 2.5 with more camshafts really requires proper close-ratio gears for best performance.

I would be very careful about camshaft choices with stock gears,......................

Non-counterweighted cranks are quite strong and we have used them successfully but they do induce more stresses into the engine case and careful preparation is required.

[gestalt1]

thanks Steve, i basically drive to the track, race and drive home with occasional drives on nice days. i guess this would count as 50/50 track and street. when i say track i mean track days at road america, blackhawk, autobahn and gingerman and an occasional parking lot AX. since these tracks vary a lot and i do not want to change gears to suite tracks i would think starting with the AFMSX gears would be in the ballpark for most situations.

Your point about street drivability is well taken, not to mention a larger displacement helps make hot cams more liveable on the street.

as for crankshafts, i'm wondering if a 66mm counterweighted crank with 90mm pistons would be a problem. if the counterweighting is adjusted for a certain weight piston, once you change that isn't the counterweighting no longer correct? i'm sure this is not a big deal, after all porsche was able to use a crank with no counterweights. i read somewhere the effect of lightening up the drivetrain components is very effective that even small weight reductions in the crank and flywheel have a dramatic effect. this one reason i'm interested in the non counterweighted crank. i would be curious about what it would take to use a non counterweight crank work in a engine that needs to survive track use and street driving without a rebuild every season. i would be worried that the 7r case and main bearings would be overstressed.

[Steve@Rennsport]

Originally Posted by gestalt1

..., i'm wondering if a 66mm counterweighted crank with 90mm pistons would be a problem. if the counterweighting is adjusted for a certain weight piston, once you change that isn't the counterweighting no longer correct? i'm sure this is not a big deal, after all porsche was able to use a crank with no counterweights. i read somewhere the effect of lightening up the drivetrain components is very effective that even small weight reductions in the crank and flywheel have a dramatic effect. this one reason i'm interested in the non counterweighted crank. i would be curious about what it would take to use a non counterweight crank work in a engine that needs to survive track use and street driving without a rebuild every season. i would be worried that the 7r case and main bearings would be overstressed.

There is no problem using 90 (or 92mm) pistons with a 66mm counterweighted crankshaft. Those run very reliably well past 8K if needed for racing and put less stress into the case while doing so (as long as several mods are done).

The 66mm non-counterweighted cranks were deemed to be OK given the type of engines that used them and their modest peak RPM's. We see that case cracking happens much more often with 70.4mm cranks operated above 7300.

Non-counterweighted cranks do put a lot more stress into the case above 7300 but thats acceptable in a 100-200 hour race engine. You might not be willing to accept such risks in a engine thats expected to last 100K+.

[gestalt1]

thanks Steve - thats just what i needed to know. the thought of having 8-9k rpm available is to hard to turn down. the other nice thing about the smaller displacement is the ports in the stock S heads will work nicely.