engine lightening

[der affe]

i am building a 2.7i motor for my 1991 325i:
my goal was to go as far as i could with stock or stock modified parts.
first thing i am doing is lightening the connecting rods so i though i would do a step by step DIY with pics and results.
*disclaimer* this does not mean you should go attack your engine parts with a bench grinder. if you do that is your own stupidity, don't blame me.

first some side by side shots

stock ETA rod on the left, semi modded on the right






stock in the middle of the roughed in rods. you can get an idea how much is being removed from the top and sides of the rod


after roughing the rods in with a belt sander (which i forgot to to take pics of) i started hand finishing them with a fordom ( a cable driven die grinder)

mostly finished upper con rod. all of the texture has been polished away with a sanding rood as well as all the edges chamfered. the side forging "flash" is also removed along the sides of the beam of the rod as well as the top of the small end. the protrusions on the sides of the small end have been ground and profiled and blended, but not totally removed. all this will make the con rod lighter and "cut" through the air easier inside the crank case as well as shed the oil splash sooner as well with help with windage.

NEXT off to be machined


now i went over to my friend and fellow burn buddy, Greg's house (what are the odds same name friends and both on fire????) to machine the "tangs" flush on the big end of the rods. we were going to use the lathe, but the arbor would not go down small enough for the ID of the rods...so we used the mill which worked out better in the end


the "tang" we are flushing is located between the 2 halves of the rod. by removing this you make the rod lighter, but are not sacrificing any stregnth.


here's Greg machining the extra material away in the mill.


another shot of the mill going to town.

here is some pics of the rods after machining:

you can see the part that was machined away on the unfinished rods

side shot of a mostly finished rod, just a little more hand finishing to do. my drive cable on the fordom broke so i have to wait until monday to pick up a new on to finish.


the final weights:
STOCK ETA:601.4 grams
ROUGHED IN:596.3 grams
SEMI FINISHED TOP:590.3 grams
MACHINED AND JUST NEEDING HAND FINISHING:563.7

now once all the rods are finished i will then batch weigh them so the are are the same overall weight, then take them to be shot peined/stress relieved (this will also bring back some of the texture to the rod's surface), have them resized on the big end, rebushed on the small end, then also balanced end to end. when you balance rods end to end you make sure that the weight of both the small end and big ends match the other rods in the set not just the over all weight. this just makes for a better balance job.

i will post up completly finished pics tomarrow after i finish them along with weights.

NEXT.... balancing and oil grooving pistons.
since my fordom took a crap i decided to drive out the wrist pins and weigh the pistons and wrist pins seperately.

here is some pics and results.


weights with the wrist pins installed


pistons without the wrist pins installed with the weights marked.


pistons and wristpins weighed


the results of about 5 hours work

next weekend i will oil grove them on the lathe and then balance them and the wrist pins so they all match in weight. if there is time, i will also be knife edging and polishing the ETA crankshaft

[T Verdier]

Way to do work! ROD Stewart.

[dirty30]

Is this weakening the rod?

[NC325iC]

as he is only removing the textured surface, forging 'leftovers", and a very small amount of overlap, as well as having them peened and stress relieved (aka light heat treat) they will end up as strong if not a little stronger (the peening and HT)

the end mill machining is likely bringing the parts closer to their intended design, i doubt they wanted there to be any overlapping between the rod ends and the rods, but individual machining would be too expensive on a large scale.

[der affe]

^exactly. i got the weight down quite a bit, bit it was pretty much "extra" weight. i have done this to lots of motors (although this will be my first BMW motor) and the factory rods are forged so they have plenty of stregnth.

that is actually probably closer to what the factory would have intended the rods to look like if time and money permitted it. i was going to use eagle rods, but they do not make them in the shorter ETA legnth, so i went this roue insted

[Stephen]

Wow, awesome. Id love to do a build like that

[dr.gonzo]

Great build, how many grams are you able to remove?

[der affe]

went from 601.4 grams to 563.7 grams witha little more hand finishing to do. i am hoping with balancing and the final hand finishing i will end up 30 grams lighter. that may not sound like alot of weight, but multiply it by 6 and realize that it is rotating mass which is more responcive to weight loss.

add all the rotational parts being lighter (crank, flywheel, rods, to a very small degree pistons) and balanced to zero, you end up with a smoother running, faster reving, long lasting motor. that makes a little extra hp because it is moving around less mass.

[delatlanta1281]

Awesome man! i am impressed.

[dr.gonzo]

This is the first time I have ever heard of someone doing this. Is it because the rods are so overbuilt?

[blueapplesoda]

as long as they are peened after then itll be fine. what about the fat square balancing chunk on the cap? if i remember correctly, back in school we removed some of this to lose the weight as well. Also i think it was my machinist who was explaining that at 2000rpm a 1/4oz weight on the end of a crankshaft can multiply to 15 lbs, and at 7000rpm its like 150lbs.

since i didn't do a stroker i used s50 rods, which were 100g lighter per rod than the stock m20 and eagle rods. Ill tell you the engine revs so much faster and is relatively more "responsive". couple that with an al fly and the engine will respond crazy haha.

anyhow, good work!!

[lennon]

good to see those pistons going to good use. im doing similar stuff with the rods and pistons on my 24v motor (slowest build ever)

[chris00]

In picture number 3 it looks as if the rod is bent? Is it just the angle of the picture? Good work by the way, it looks like its taken a few hours!

[M3 euro ltw]

...
add all the rotational parts being lighter (crank, flywheel, rods, to a very small degree pistons) and balanced to zero, you end up with a smoother running, faster reving, long lasting motor. that makes a little extra hp because it is moving around less mass.

Technically, you should have exact same peak hp as without the lightening, but you'll have less drag on the motor, hence more area under the curve... right? Like a flywheel. Mebbe not.

Beautiful work btw...very, very thought provoking.

[der affe]

as long as they are peened after then itll be fine. what about the fat square balancing chunk on the cap? if i remember correctly, back in school we removed some of this to lose the weight as well. Also i think it was my machinist who was explaining that at 2000rpm a 1/4oz weight on the end of a crankshaft can multiply to 15 lbs, and at 7000rpm its like 150lbs.

since i didn't do a stroker i used s50 rods, which were 100g lighter per rod than the stock m20 and eagle rods. Ill tell you the engine revs so much faster and is relatively more "responsive". couple that with an al fly and the engine will respond crazy haha.

anyhow, good work!!

i really thought about it. but i coud not get a straight answer from anyone as to how important the blocks were or what their exact purpose was.
absorbing harmonics to a point is what i was thinking.
i would welcome any actual answers as to the purpose of the giant freekin block at the base of the rod caps.

i might see if i can mill .100 or so off and see what happens.