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Ideal Bore to Stroke Ratio for SS?

Bore to Stroke

Hey All -

Iím changing directions on the design of my next engine (was going with a 3.8 long stroke) and wanted to get input on the ideal bore to use for a short stroke built on a 964 case using a 9 bolt 66mm crank, like the supercrank. All components used will be lightweight including valves, rockers, flywheel, rods, etc.

I read that F1 cars have bore to stroke ratios that exceed 2.1 to 1 but they are limited to a 98mm bore size and have other limitations as well; plus those engines seem too different to make reliable comparisons.. But to give another comparison, my BMW race bike has a bore to stroke ratio of 1.61 to 1 which would be the equivalent of using a 106mm bore on a 66mm crank which seems too large. Seems that many other sports cars like Ferrariís have B to S ratios of 1.3 to 1 but that is a long stroke motor.

The largest bore Iíve seen on a 66mm crank was the 3.1 liter built by Supertec using 100mm pistons. From reading that thread, it seems that a 102mm bore would also reliably work. My question is what is the sweet spot for maximizing reliable HP on a 66mm crank? Will 105mm cylinders work and be reliable to 9,200? Or would they be too heavy or have other issues? If 105 is too big, is there a bore between 102 and 105 that would wotk best for torque and reliability?

It seems that a few builders have limited maximum bore size on a long stroke air cooled motor to 102.7 but the short stroke is different enough that possibly a larger bore than 102.7 may reliably work?

Interested in your thoughts on this.
Old 07-12-2018, 08:13 PM
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rod to stroke ratio

Hi,66 x 98mm is the sweet spot for longevity.Use a rod length of 132mm to achieve 2 to 1 ratio.Use CP piston and move the pin location up.19mm piston pin is fine.I am building one now with a 22.5 lb.9 bolt 66mm crank on a 3.2 case.Ti rods and valves of course.Use early 3.6 heads that have not been machined for the updated head gasket and just narrow the stud spacing or use a 3.6 case to start.The new billet heads from Pauter are the hot set up if you can afford them.I have made 373 hp.and 249 ft lbs.of torque from 66 x 95 at 9300.Downside from the over 8000 is you need at least twin coolers up front.Ciao Fred
Old 07-13-2018, 05:14 AM
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Originally Posted by faapgar View Post
Hi,66 x 98mm is the sweet spot for longevity.Use a rod length of 132mm to achieve 2 to 1 ratio.Use CP piston and move the pin location up.19mm piston pin is fine.I am building one now with a 22.5 lb.9 bolt 66mm crank on a 3.2 case.Ti rods and valves of course.Use early 3.6 heads that have not been machined for the updated head gasket and just narrow the stud spacing or use a 3.6 case to start.The new billet heads from Pauter are the hot set up if you can afford them.I have made 373 hp.and 249 ft lbs.of torque from 66 x 95 at 9300.Downside from the over 8000 is you need at least twin coolers up front.Ciao Fred
Damn Fred. Thatís one cool build.
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Old 07-13-2018, 06:26 AM
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You should be thinking not in terms of bore and stroke length ratio but rather the rod length to stroke ratio. Longer rods are better because the offset angle as the crank turns is reduced. This means less angular momentum at high rpm.

Pay close attention to what Fred says about moving the piston pin location up. This allows for custom longer rod to achieve the goal.
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Old 07-13-2018, 10:26 AM
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Originally Posted by jpnovak View Post
You should be thinking not in terms of bore and stroke length ratio but rather the rod length to stroke ratio. Longer rods are better because the offset angle as the crank turns is reduced. This means less angular momentum at high rpm.

Pay close attention to what Fred says about moving the piston pin location up. This allows for custom longer rod to achieve the goal.
Other benefits of longer rod are slower piston speeds and longer TDC dwell time.
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Old 07-13-2018, 11:06 AM
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Other benefits of longer rod are slower piston speeds and longer TDC dwell time.
A nice benefit for power, but timing has to be more conservative too.
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Old 07-13-2018, 11:08 AM
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Originally Posted by MST0118 View Post
Bore to Stroke

Hey All -

Iím changing directions on the design of my next engine (was going with a 3.8 long stroke) and wanted to get input on the ideal bore to use for a short stroke built on a 964 case using a 9 bolt 66mm crank, like the supercrank. All components used will be lightweight including valves, rockers, flywheel, rods, etc.

I read that F1 cars have bore to stroke ratios that exceed 2.1 to 1 but they are limited to a 98mm bore size and have other limitations as well; plus those engines seem too different to make reliable comparisons.. But to give another comparison, my BMW race bike has a bore to stroke ratio of 1.61 to 1 which would be the equivalent of using a 106mm bore on a 66mm crank which seems too large. Seems that many other sports cars like Ferrariís have B to S ratios of 1.3 to 1 but that is a long stroke motor.

The largest bore Iíve seen on a 66mm crank was the 3.1 liter built by Supertec using 100mm pistons. From reading that thread, it seems that a 102mm bore would also reliably work. My question is what is the sweet spot for maximizing reliable HP on a 66mm crank? Will 105mm cylinders work and be reliable to 9,200? Or would they be too heavy or have other issues? If 105 is too big, is there a bore between 102 and 105 that would wotk best for torque and reliability?

It seems that a few builders have limited maximum bore size on a long stroke air cooled motor to 102.7 but the short stroke is different enough that possibly a larger bore than 102.7 may reliably work?

Interested in your thoughts on this.

I rarely go over 98mm on the 3.0 liter based builds as they are pretty reliable, have a decent amount real estate in regards to sealing surface.

That said, to answer the questions...It depends. Changing one dimension can have a positive or negative effect on the other parts. A good example is Valve sizes. I see builders increase the valve size, but the bore is too small. As a result the piston ring lands have to be placed farther down the piston so that valve pockets do not encroach on the top ring. The result is a horrible piston and an unhappy motor at higher RPM. Examples...Factory 2.8RSR and The 3.8RSR's
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Old 07-13-2018, 12:59 PM
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design

Aaron,you are so correct.I repaired about 6 2.8 RSR Engines.The issue is once you get to 100mm the barrels are thin.LN Nickies are stronger than Mahle.I had a 66 x 95 I ran for 25 years on a 76 turbo case for 70,000 miles.When I converted it to Gt-4 I only did a slight refresh as the ring gaps had only grown to 8 thousandths from 6 When you cut the valve pocket any deeper on a 2.8 it is very close to the inside of the top ringland on the intake.Most people do not realize the 2.8 had a 49mm intake like the SC.where the 2.7 had a smaller valve.When you put 92mm on a 2.7 head you end up with 12.5-13.0 to 1 compression.Ciao
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Thanks for the advice. Yes, I am aware of the ideal rod length to stroke ratio and was planning to use a set of Pankl rods so would stretch center to center to 132 by using a smaller 19mm pin. I just wasn’t sure how large of a bore that could be used.

Fred has built several at 95 and 98 and Henry has built some too, one that had 100 bore (9000 rpm) and he thought 102 would also work. Got me thinking that bigger bore (102 plus) might be a better choice but don’t know if others have done this. There has been quite a bit of discussion on rod length to stroke ratio here but could not find much on bore to stroke ratio.

I have a core 1995 case, cylinders and heads so intended to use those parts into the SS build with special pistons. Or maybe go with nickies if the 1995 stock cylinders won’t work well. Was trying to understand if larger bore would increase torque for the SS or help engine rev quicker and/or if there are other issues relating to combustion or piston weight, etc. the F1 cars and super bikes definitely use a higher bore to stroke ratio than what I see Porsche use.

Last edited by MST0118; 07-13-2018 at 10:29 PM.. Reason: Addl info
Old 07-13-2018, 09:35 PM
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bore to stroke

I had 66 x 100 in my Imsa Gtu car in the 80,s.Great motor.You have a good basis to build 66 x 105.What will you use for intake system?I used MFI with Slides then but could not get enough fuel.Tried the BMW Kugelfischer pump as well but motor was on the lean side between 8-9000.Today is different as we are blessed with EFI and all the parameters that it can cover.Your other limiting factor is an exhaust system that actually works.I always built my own.Have not built any in over 25 years and just started pricing the pieces to build more but way more expensive now.I see one on the market that looks like it has potential but have not gotten a look at the type of collector it has which is the key.
Old 07-14-2018, 04:38 AM
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Fred, I really like MFI which is what I have on my current 2.8 long stroke but it was a pain to tune. For that reason, I would like to use EFI for this motor. I’m thinking using those new butterfly tb system that are made in UK and are almost as good as slide valve but if you know a good source for reasonably priced slide valve let me know.

I have a fresh 2.8 motor to use so this is a longer term project and want to take my time to build it right and macimize power. I’d be intersted in your thoughts on building a custom header system and would be great to get your input on that when I get closer.

Yeah 105 x 66 was what I was hoping to build. 3.45 liter. Thanks!

Last edited by MST0118; 07-14-2018 at 06:40 AM.. Reason: Gram
Old 07-14-2018, 06:38 AM
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