96x71 or 96x78 motor - That's the question Options

[KenH]

In playing with my el-cheapo dyno program it looks like you can get the same HP with either a 71mm or 78mm stroke. The 78mm however gives a bit more torque.

Over all the CAM seems to give the biggest improvement.

The question is - is it worth the hassle to go to 78mm stroke (T1 rods, special pistons, clearence issues, cost of rods, crank, pistons) for 10'lb of torque??

Looks like 96mm x 71mm with good heads, proper CAM, optimun fuel delivery, FI or Carb, will make a pretty good motor.

Comments??

Ken

[BMXerror]

"There is a replacement for displacement. It's called good engineering."

Displacement isn't everything, but you are rather limited on how much engineering you can do when you're using someone else's design. My quote isn't completely true either, but it's what I say to people who only think bigger is better. That's why Porsches modern 5.7 liter engine has almost twice the output of Chevy's modern 5.7 liter engines. 78mm WILL give you more torque, but unless you're going with longer cylinders and/or severely offset wrist pins, your rod angle is gonna go to hell. This will cost you in reliability to bottom end as well as the extra wear on your rings. It will also cost you the ability to rev, as I think someone mentioned, AND some tourqe perportionally to the length of your stroke.
As Twystd1 said, it all depends on what you're using it for. If you are concerned about cost (as many of us are), it sounds like you're not going to want the rebuild the thing very often. In that case, I would go with the 71mm stroke. However, if I was building a drag motor, especially if it were forced induction, then I would go with the extra torque and displacement at the expense of some reliability. MY OPINION.
Mark D.

[Mueller]

"There is a replacement for displacement. It's called good engineering."

Displacement isn't everything, but you are rather limited on how much engineering you can do when you're using someone else's design. My quote isn't completely true either, but it's what I say to people who only think bigger is better. That's why Porsches modern 5.7 liter engine has almost twice the output of Chevy's modern 5.7 liter engines. 78mm WILL give you more torque, but unless you're going with longer cylinders and/or severely offset wrist pins, your rod angle is gonna go to hell. This will cost you in reliability to bottom end as well as the extra wear on your rings. It will also cost you the ability to rev, as I think someone mentioned, AND some tourqe perportionally to the length of your stroke.
As Twystd1 said, it all depends on what you're using it for. If you are concerned about cost (as many of us are), it sounds like you're not going to want the rebuild the thing very often. In that case, I would go with the 71mm stroke. However, if I was building a drag motor, especially if it were forced induction, then I would go with the extra torque and displacement at the expense of some reliability. MY OPINION.
Mark D.

did you see the graphs above?

have you read any postings of Raby's about his motors or these combos??

I gather not...the Type IV really seems to wake up with the additional rod length without any real negative effects. As for the Porsche 5.7 putting out 2X that of the Chevy 5.7??? What are you smoking??