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Why put the PSRU on the Rear Face of Block

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pfarber

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I'm driving the prop from the PTO end.
Is there a significant reason why most engines put the PSRU on the rear/transmission end? The only thing that I can think of is maybe crank thrust bearings and the harmonic balancer.

The front of V engines seems to be more easily adapted to a PSRU, once you remove the accessory belts and switch to an electric water pump. Rigging up a pulley for the alternator off the rear of the crank is dead simple.
 

TFF

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The rear of the engine is where it is designed in to drive. Bunch of holes in that side designed to bolt drivetrain on. Front only sticks out for harmonic balance and to drive accessories. The part that sticks out the front is a lot smaller diameter than the part in the back.
 

Pops

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The rear of the engine is where it is designed in to drive. Bunch of holes in that side designed to bolt drivetrain on. Front only sticks out for harmonic balance and to drive accessories. The part that sticks out the front is a lot smaller diameter than the part in the back.
Same on the VW type I.
 

wsimpso1

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Is there a significant reason why most engines put the PSRU on the rear/transmission end? The only thing that I can think of is maybe crank thrust bearings and the harmonic balancer.

The front of V engines seems to be more easily adapted to a PSRU, once you remove the accessory belts and switch to an electric water pump. Rigging up a pulley for the alternator off the rear of the crank is dead simple.
Rear Face of Block (automotive engines) is designed for reacting powertrain torque. The rear flange of crank is designed to carry full engine torque and firing pulse torque to the starter ring gear, flywheel, and downstream components. Front end of the block and the front end of the crankshaft are called accessory drive - usually designed to carry accessory torques only, which are much smaller than full engine torque. If you tried to draw propellor power from the front end, most automotive engines would likely break stuff.
 
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pfarber

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Rear Face of Block (automotive engines) is designed for reacting powertrain torque. The rear flange of crank is designed to carry full engine torque and firing pulse torque to the starter ring gear, flywheel, and downstream components. Front end of the block and the front end of the crankshaft are called accessory drive - usually designed to carry accessory torques only, which are much smaller than full engine torque. If you tried to draw propellor power from the front end, most automotive engines would likely break stuff.
You can always go billet or forged.... a custom crank is not a super expensive item. You can mill the nose of the crank to whatever you like. I don't believe even a cast iron crank is gonna have much of an issue with full power out the nose. There is a LOT of metal up there.

My main idea is reduce the dead weight of the bellhousing. The front of an engine gives you may more metal to bite into than the rear, especially since the PSRU adds height to the main load causing part.. the prop.
 

TFF

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First off, I doubt it’s affordable to have a special crank made. One that has no auto basis. The snout of an LS is just less than 1.5”. The main bearing is just over 2.5”. What would it cost to get that snout to main bearing size? For them to stop and load a special program, I say $10,000 for the cheaper quality. If they could make a months worth, they could drop it to not much more than a regular car crank. it’s about volume and at low volume, you will be playing with the racers that own that segment of the market and have to get on their good side just for a slot.

The other end already has 6 bolt holes ready for a custom adapter at max output.

In the end a brand new Lycoming will be less expensive. Not saying it can’t be done, of course it can. If you can’t cast, forge, and machine your own stuff, it’s a $50,000-100,000 dream. It’s not a bad dream, but a costly one.
 

slociviccoupe

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Scat wanted 3700$ for a 1/2 vw crank with chevy rod and main journals with big block thrust bearing and flange in the rear. A custom billet crank isnt cheap and that was only 2 cylinders.
 

wsimpso1

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You can always go billet or forged.... a custom crank is not a super expensive item. You can mill the nose of the crank to whatever you like. I don't believe even a cast iron crank is gonna have much of an issue with full power out the nose. There is a LOT of metal up there.

My main idea is reduce the dead weight of the bellhousing. The front of an engine gives you may more metal to bite into than the rear, especially since the PSRU adds height to the main load causing part.. the prop.
You asked, we answered. If you want to do the engineering to make the front end of the crankshaft and block both strong enough and stiff enough to stand the torques and vibratory inputs, knock yourself out. I know my way around this stuff a little, and I would not dream of messing with it.

If you leave the starter ring gear and flywheel on the back end, the new front end will not need to be as sturdy as the back, but it will still need to be pretty substantial. Then you will have close to original weight at the back PLUS added weight forward. As to mounting the PSRU to the FFOB, Investigate carefully. You will find that the various bosses are usually on different planes and are too small for mounting the PSRU securely enough to keep resonance at bay. The guys who do engine block drafting at the auto companies call it "the biggest bracket in the vehicle", and they do not make those FEAD mounts any more numerous nor massive than they must to securely hang the accessories. Block mods are a huge job.

Billski
 

pfarber

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You asked, we answered. If you want to do the engineering to make the front end of the crankshaft and block both strong enough and stiff enough to stand the torques and vibratory inputs, knock yourself out. I know my way around this stuff a little, and I would not dream of messing with it.

If you leave the starter ring gear and flywheel on the back end, the new front end will not need to be as sturdy as the back, but it will still need to be pretty substantial. Then you will have close to original weight at the back PLUS added weight forward. As to mounting the PSRU to the FFOB, Investigate carefully. You will find that the various bosses are usually on different planes and are too small for mounting the PSRU securely enough to keep resonance at bay. The guys who do engine block drafting at the auto companies call it "the biggest bracket in the vehicle", and they do not make those FEAD mounts any more numerous nor massive than they must to securely hang the accessories. Block mods are a huge job.

Billski

You are assuming a stock crankshaft. I am not. A billet crank would not be a huge investment. I don't think its a stretch to change the nose turning operation from 1in to 4in. Its a dimention change, the tool path would be the same maybe add a fillet.

My main thoughts are: is it a reasonable to consider the front of the engine for a PSRU?

The cost of a custom bellhousing vs the cost of a thicker nosed crankshaft.

Take the SPG-4 PSRU at $1895 and the complete PSUR (incl bellhousing) at $2295 $400 for the additional parts? I would snap one up in a second if they did that for a V-6. Even if they could do a 'generic' v-6 adapter and then only leave it up to me to mill out an adapter plate would be miles ahead of what I have to start with now, which is nothing.
 

wsimpso1

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You are assuming a stock crankshaft.
I did not assume anything. I caution you to read the words written, then think before you post. I also caution you that dismissing good commentary in print tends to cut off access to cost-free thinking that otherwise might have been contributed...

Billski
 
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