maximum rod length - 964 motor

[Rusty911]

I’d be interested to learn about anyone’s experience about using longer rods for a 964 motor by ordering custom pistons from CP or JE with a reduced compression height. Has anyone gone with a longer rod than 130mm without other engine modifications other than custom rod and pistons?

I would be interested to use a longer rod to get a higher rod to stroke ratio so wondering what the c to c length limitation would be which might be indirectly related to another question as to what’s the smallest compression height for the piston that could reliably be used.

Interested in what’s been done with this. Thanks.

[faapgar]

I have built 3.6 with longer rods.I use a 132mm rod on 66mm crank with 98mm pistons for a SS 3.0. Rod stroke ratio is then 2 to 1.William Knight is now building some with almost 6 inch rods for a wider motor.Neil Harveys new rods and pistons give a longer rod with a 19mm piston pin which allows the pin location to be higher in the piston and keeping the barrel heighth and chain angle normal.2 to 1 for a 3.6 is 154mm rod and then you enter a new world to conquer.Peter Dawe has built these and has longer chain ramps,chains and different number of teeth on the idler gears to pull it off.Not cheap.Fred

[Rusty911]

Not looking for 2 to 1 ratio as don’t want all the other modifications that were mentioned. Yes, would go with a smaller pin to increase length but what I really want to know is can the compression height of a piston be reduced further to use a longer rod.

For instance, the CP x forging has a compression height 31.75 mm with a 23mm pin. The c to c length is 127mm and the rod to stroke ratio is 1.66. My question is whether it is possible to increase the ratio to 1.81 (like a 2.4/2.7 engine) by using a 138.5 mm rod and by reducing compression height of piston from 31.75mm to 20mm? If 20mm is too short, then what would be an acceptable height? 29mm. 25mm? You can reduce by 2mm by using smaller pin but beyond that, not sure how much compression height can be reduced. Anyone done this?

[MBruns]

The common length is 5.250 (133.35mm) with std length cyls. you run into problems when pushing the pin up needing oil ring support rails which are no problem, but when moving the ring package up then you get very close to crashing the outer edge of the intake valve pocket, we built several wide 964 based engines and built custom chain rails long cylinders, a lot of work and cost, when I was at JB with the CNC machines it’s an idea to a part ( and a lot of programming)
Mike Bruns

[K24madness]

Reading between the lines I think the OP is asking what have others done with reducing deck/ring/wrist pin with custom pistons and lengthening rods the corresponding amount. It’s an interesting question as to were to compromise the limited space between the two.

I see the GT3 has continued to increase rod length as the engine evolves. I know it’s a great benefit on high RPM race engines but what’s really a good rod ratio target for the street? Too long and the lower RPM can get a bit lazy. Too short and piston life suffers.

[Rusty911]

Well, I think a longer rod to stroke ratio is fine for the street, too. Especially considering the 67 to 71 engines that used a 66mm crank but had variations in the cams depending if it was an S, E or T motor.

I tend to now agree with Mike on the max rod length of about 133 assuming no other modifications other than piston compression height. I just saw an interesting build on Rennlist where someone used 130mm Carrillo rods with an 80.4 crank and custom 102mm mahle pistons for a 3.9 build. The pistons had a compression height of 26.8 and weighed in the low 400s using a 22mm pin. So, that appears to be close to the border without changing ramps, sprockets, etc, etc.

[Neil Harvey]

Your original post was for a greater ratio number. But, why? Longer or shorter rods have a large part in how the engine efficiency changes. You need to think the whole engine and it’s components before you change, add or subtract Tod length.
Complete intake flow numbers, cam choice, CR, exhaust size etc are all affected by this choice.
If your’e thinking about piston side wall thrust and friction only, you may be disappointed by the engines performance if you make the wrong choice.

[Rusty911]

I agree with you Neil that it’s a total package but my question was focused on one parameter as to what could be done without too many modifications. I did get helpful responses to this specific question. Thanks everyone and happy Thanksgiving!

[Neil Harvey]

Minimum piston compression height is dictated by the piston forging. Most forging available by any supplier will be limited by dome volume/valve pockets and CR#. These engines have large chamber volumes so it’s a compromise often on choice. Gets easier with bore increases.
Decide upon the bore you want, factor in the CR then the pocket depths required. This then determines the ring positions and land widths. Then select the ring packages, face materials and ring widths. This will allow you to move the pin CL up as high as possible. Once you have this figured out, calculate your cylinder pressures and choose the pin size to suit. This will then give you your rod length. You then then decide if a longer rod is what you want.
If there are no forging available, you can always choose a billet piston. Welcome to the world of custom engines!!!