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FE Technical Forum / Poor mans 4.5 inch stroke?
« on: August 14, 2016, 11:31:17 AM »
I'm in the process of gathering parts to build a large cubic inch FE motor based on a BBM block.
This is not going to be a drag race motor by any measure, it's going into a 72 2 wheel drive F250
and the motor will never see much more than ~5500rpm.
Right now the motor is still in the planning stages, I have a block and I've made contact with Blair Patric about a set of his pro port heads. Outside of that nothing else has been nailed down.
My Father and I are enjoying putting this engine together on paper, while both of us agree that the only substitute for cubic inches is more cubic inches, he likes to get a little more adventurous than I do, he's not the one buying all the parts after all
.
My initial plan is to just order a 4.375 rotating assembly from Barry R. And call it a day. With a ~4.42 bore ( leave a little room for a rebuild or 2 ) I will have a 537ci motor.
My father would like me to take that crank and offset grind it to the Honda sized rod journals to get as much stroke as can be had before rods start hitting the cam.
Several warning flags pop out at me thinking about this.
First, I know Jay has used a 4.5 inch stroke in his HR motor with a little grinding on the rods, and the smaller overall rod end should help. But outside of bottling it together and measuring it, anything past what Jay has already done (4.5 inches) scares me a bit.
Secondly, and more importantly, offset grinding the crank that far with such a small journal is not going to leave much in the way of overlap( I calculate ~.180ish ), and it might well hit an oil passage or lightening hole.
I tried to debunk it strictly on overlap, so I looked up long stroke cranks offered for other motors.
Scat offers a BBC 4340 steal forging at 4.75 inches of stroke, given the 2.75 man and 2.2 rod journals this crank would seem to have less than .1 inch of overlap. I think this is not an apples to apples comparison, .1 inches of overlap on a 2.2 inch journal is not the same overall overlap area as a .1 inch overlap on a 1.88 inch journal. But since the proposed 4.5 stroke combo has .080 more overlap it still seems plausible.
Thirdly, even if all this is do-able, the cost of the rods becomes an issue.
Still scares me though, and I'm not sure I'm willing to take a gamble on destroying a $3600 block by trying to get another 15 cubes out of a cheep forged crank.
I decided to offer up a compromise combination, as my father keeps pushing the idea.
If I could offset grind the crank to a 2.086 journal
And use off the shelf 5.4L ford rods, I could get close to or attain a 4.5 stroke for relatively cheap.
While I think it would be great to get all the cubes I can, I'm just not sure it's worth the risk.
So I thought I would offer it up here and get some opinions from the experts.
I know the rod stroke ratio is not going to be great on such a combo, but this is a whole different debate. My father thinks the shorter the rod the better for this application, I'm not sure that's the case.
Thanks, Joe
This is not going to be a drag race motor by any measure, it's going into a 72 2 wheel drive F250
and the motor will never see much more than ~5500rpm.
Right now the motor is still in the planning stages, I have a block and I've made contact with Blair Patric about a set of his pro port heads. Outside of that nothing else has been nailed down.
My Father and I are enjoying putting this engine together on paper, while both of us agree that the only substitute for cubic inches is more cubic inches, he likes to get a little more adventurous than I do, he's not the one buying all the parts after all
.My initial plan is to just order a 4.375 rotating assembly from Barry R. And call it a day. With a ~4.42 bore ( leave a little room for a rebuild or 2 ) I will have a 537ci motor.
My father would like me to take that crank and offset grind it to the Honda sized rod journals to get as much stroke as can be had before rods start hitting the cam.
Several warning flags pop out at me thinking about this.
First, I know Jay has used a 4.5 inch stroke in his HR motor with a little grinding on the rods, and the smaller overall rod end should help. But outside of bottling it together and measuring it, anything past what Jay has already done (4.5 inches) scares me a bit.
Secondly, and more importantly, offset grinding the crank that far with such a small journal is not going to leave much in the way of overlap( I calculate ~.180ish ), and it might well hit an oil passage or lightening hole.
I tried to debunk it strictly on overlap, so I looked up long stroke cranks offered for other motors.
Scat offers a BBC 4340 steal forging at 4.75 inches of stroke, given the 2.75 man and 2.2 rod journals this crank would seem to have less than .1 inch of overlap. I think this is not an apples to apples comparison, .1 inches of overlap on a 2.2 inch journal is not the same overall overlap area as a .1 inch overlap on a 1.88 inch journal. But since the proposed 4.5 stroke combo has .080 more overlap it still seems plausible.
Thirdly, even if all this is do-able, the cost of the rods becomes an issue.
Still scares me though, and I'm not sure I'm willing to take a gamble on destroying a $3600 block by trying to get another 15 cubes out of a cheep forged crank.
I decided to offer up a compromise combination, as my father keeps pushing the idea.
If I could offset grind the crank to a 2.086 journal
And use off the shelf 5.4L ford rods, I could get close to or attain a 4.5 stroke for relatively cheap.
While I think it would be great to get all the cubes I can, I'm just not sure it's worth the risk.
So I thought I would offer it up here and get some opinions from the experts.
I know the rod stroke ratio is not going to be great on such a combo, but this is a whole different debate. My father thinks the shorter the rod the better for this application, I'm not sure that's the case.
Thanks, Joe