ideal rod ratios?

[megafreakindeth]

first post by me on this forum. i own an 88 fiero that i got along with a 3.4l camero engine. orriginally i had planned on just dropping it in but, it being a fiero, it needed enough work as is i wasnt able to do it. my plans are now to build the 3.4 up over the winter and install it in the summer and since ive got the time ive been doing plenty of research...

orrginally i was planning on just porting the heads and dropping in the largest custom piston possible to get a nice 10:1 cr. along with all the good valve train mods and an 8lb flywheel i thought i was good to go. then i saw someone talking on a fiero forum about this money pit...

basicly he wanted to destroke a 3.4 block with a 2.8 crank and drop in some large pistons with a 350's 6in connecting rod to make a 3.1. he personally wanted to use northstar pistons but since id already planned on buying pistons i could just specify a diff wrist pin diameter or have the rod custom drilled to fit right. has anyone heard about this? does anyone have the measurements i would have to give to the machine shop to make a 6 in rod fit a 2.8 crank and what wrist pin diameter to use to fit the piston to the rod?

would doing this even work? i could see this producing quite the rod ratio and the increased dwell time at tdc would produce a nice bump in hp. i had thought of long rodding the 3.4 but since its a v6 i figured it might make it more unstable at higher rpms as the pistons would be inclined to fly outwards. if this 3.1 combo is doable i wouldnt mind someone helping me get the parts/knowledge togeather to do it.

[geoffinbc]

Yes it is doable. But using a 2.8L crank makes it easier to use a longer rod since our motors utilize a lower pin height with a shorter stroke. This means if you increase the rod length you can move the piston pin up, and since a 2.8L piston has less stroke there is more room to move th pin up. You are talking about moving the pin up more than a 1/4" when using a 6" rod versus a 5.7" rod.

You need a
.921" pin bore
.854" Width on the big end
Accept 2.0" ID bearing inserts

The pin size can be at your discretion as long as it fits your pistons. You can also choose floating or pressed pins.

[torq455]

any advantage to a full floating pin or not?

[megafreakindeth]

i plan on using a 3.4 block so i can use the most bore. i am already planning on using custom pistons so having them made differently to accept a different rod wont be to hard vs using a stock piston of some sort. my main request here is what information do i need to give to the machine shop for this:

3.4 block
2.8 crank
6in rod from some year 350 or custom rod
largest piston for 10:1cr (whats the largest safest size to fit in a 3.4 block?)

the shop doesnt really like it if i give them the parts and say it will work, so im going to give them the measurements so they can get what they know will work. i think mabye you thought i was going to use the 2.8 block and pistons.

[Fiero11]

I have a 3.4L with 6.0 inch Carrillo rods. Custom Wisco pistons were made. The piston pin hole goes through the oil ring. They made a bridge ring to hold up the oil rings. 327 rods can be used if narrowed at 5.7 inches without the complex pistons. Also, Crower makes rods for the 3.4L. Wisco charged $126 for each piston with rings 18 months ago.

[geoffinbc]

I thought the higher pin height on a 3.1L crank would interupt the oil control ring. So it would be an advantage to use a 2.8L crank. Having a shorter stroke increases your dwell time just like a longer rod. Also less stroke means less piston speed which also contributes to efficiency and less wear.

[megafreakindeth]

exactly, also since it wont be moving as much the tendency of a V engine to waste energy in outward movement decreases.

whats the biggest safest bore the 3.4 block can hold?

[geoffinbc]

060 over. But keep the walls thick it is better for reliability. If you need to clean them up then remove as little as possible. Same with the deck and heads. Remember you only have 4 bolts per cylinder holding the heads. So try and preserve head gasket sealing. The more you thin things out the more they warp.

[megafreakindeth]

would the 2.8 crank and stock 5.7 rods work as well? just in case i cant get a piston to work without having the pin go through the rings? Or would this setup really only work well using 6in rods(vs keeping it a 3.4 if i dont want to get the rods)? im weighing the cost/benifits between the two designs.

[geoffinbc]

Honestly you wont notice an ounce of difference unless you are trying to squeeze ever single last ounce of power out of a given displacement. But since you have the freedom to choose go with a 3.4L it will yield the most power.

If you were restricted by displacement then having a bigger bore, and shorter stroke will yield more horsepower and move torque into the upper RPM range.

[Will'sFiero]

If you want to build the long rod engine, you'll need 6.000" SMALL JOURNAL Chevy rods. They will need to be narrowed to fit the V6 rod journals. Aftermarket rods are usually bushed for full floating pins.

With a deck heigh of 8.818, crank throw of 1.496 and rod length of 6.000, you'll want a compression height of 1.321" to bring the pistons up to zero deck.

exactly, also since it wont be moving as much the tendency of a V engine to waste energy in outward movement decreases.

What are you trying to say by this? This doesn't make sense.

[Joseph Upson]

What are you trying to say by this? This doesn't make sense.

He's talking about the thrust angle, an issue that was prevalent in the small block chevy 400 where in the rod was shortened for the increased stroke instead of moving the piston pin. As a result the small end of the rod comes further down in the cylinder bore resulting in a sharper angle relative to the cylinder bore so that when the rod begins to push the piston back up there is more force against the side of the cylinder bore since the small end of the rod is closer to pointing straight at the cylinder wall. As the piston starts to move up the rod begins to approach the point where it stands straight up and parallel to the cylinder bore so that the sideward thrusting of the piston toward the side of the cylinder bore via the angle of the shorter rod is transfered to a straight up force toward the combustion chamber with little or no sideward thrust at top dead center or bottom center.

Another way of lookint at it is picturing a rod with no piston on it in the cylinder bore down to the bottom dead center point. It will lay against the cylinder wall. Looking at the angle of the rod at that point, if you start to shorten it the angle will become smaller and smaller to the point that the rod looks like it is pointing nearly straight at the side of the cylinder, so when it starts to move up it will apply more pressure to the cylinder wall do to the way the big end moves on the crank until it begins to straighten up as it moves further up the bore. Lots of cylinder wall and piston wear can occur from short rod configurations due to the increased friction

Sorry so long but hopefully it helps.

[Will'sFiero]

I know about side thrust.

What he's trying to say doesn't sound like side thrust because side thrust isn't unique to V engines...

[megafreakindeth]

no replacement for displacement yes. but im approaching my car differently than before, this isnt a balls to the wall hp battle. im trying to make something with excellent performance. good wide powerband, easy to drive, good mileage, good power. i would be fine with 200 to the wheels. id rather have that 200 at a higher rpm range also, then i could take advantage of the getrags(fiero) long gear ratios.

the origional plan was to build the 3.4 and its fine, but then i found out about mixing and matching the parts to make a destroker 3.4. this would produce a quick power producer with a small stroke and large bore at a higher rpm more reliably. it has its advantages, and disadvantages($$$$) and a turbo would probably be cheaper. im still looking into all of this though, and now its probably going to be influenced by my ever weakening pressure plate(190,000 miles so far) and since i gotta drop the cradle to do the trans anyways might be better/easier/cheaper to go FI on a 3.4 i dunno.