4.6L Cobra Crank Work

DavidHarsay · Post by **DavidHarsay** » Thu Apr 28, 2005 11:18 pm

I've got a nice forged cobra crank sitting here and I'd like to shed a lot of weight.

I'm building a 7,000 rpm 2V motor for road racing with ligthtweight forged I-beam rods and pistons... so the rods and pistons should be somewhat lighter. I also have a couple of cast 4.6 cranks here for gee-whiz comparison, and noticed that one of them (2001 Bullitt) had a lot of weight removed from the rod pin casting areas compared to all the other ones. I am planning on knife-edging the leading and trailing edger of the counterweights, cutting down the diameter, and polishing before and after balancing.

I've seen Sean Hyland's book on how they go about cutting it down, but that actually seems like butchery or guesswork more than anything.

I would also like to balance it for the ligther components of course.

Any info source / advice prior to starting with the grinder would be appreciated... such as what pitfalls to watch out for, why I need the balance weights in the middle, as the cast versions do not have them (I imagine more flex from vibes without them at higher revs)... what sort of bob weight formula for balance... what type of balancing and done by whom is the best.

TIA

Dave

crazycuda · Post by **crazycuda** » Fri Apr 29, 2005 4:07 am

I have found that the best approach to crank weight loss starts with knife edging like you said. Then ill check the balance of the whole assembly, There is no point on doing a large amount of weight removal if you have to re-add the weight in mallory metal to rebalance the assembly.

DavidHarsay · Post by **DavidHarsay** » Sat Apr 30, 2005 12:54 am

Thanks, I'll take that advice, and stick to knife-edging the counterweights for weight reduction in this area.

Aside from cutting the counterweights down though... I specifically need some input regarding the center counterweights from those that understand the engineering and R&D stuff to the extent that exactly what difference they make... they are very close together and directly oppose eachother. And the cast versions which seem to live fine at 800HP+ on drag race cars don't have this, and they weigh about 8 lbs less.

The second area I'd like to evaluate is the amount of material removed from the crank pin area on the Bullitt cast crank compared to the other two I have here... in other words, I'd like to remove a considerable amount of weight from the forged crank in this area as well, and I'm fairly certain that it's not a problem... just looking for some input. The weight difference between the cast crank without all this extra "balance weight" removed here and the Bullit cast crank is about 2lbs.

I'm looking to get some endurance racing level longevity from the forged crank... and since it's in an aluminum block, I understand that rigidity compared to the cast crank is an important factor.

It will be left up to pros to balance and finish of course.

Dave

Steen Racing · Post by **Steen Racing** » Sat Apr 30, 2005 9:48 pm

We have done work with these crankshafts. You can easily turn the rod journals down to remove weight. We have cut the counterweights down with no issues. The amount we cut depends on the rod piston combo so mallory is kept to minimum. We current have one in a 2V turning 8500 and have not had any issue as of yet. Car has 10dyno runs and 3 passes. Engine will be inspected after weekends events

DavidHarsay · Post by **DavidHarsay** » Sun May 01, 2005 2:29 am

Thanks for the info. If I may ask, how much did you guys turn the journals down, how much of the counterweight radius was trimmed, and what postons/rods are you running (or at least what weight)? Also, did you keep the center counterweights?

Thanks,
Dave

Steen Racing wrote:We have done work with these crankshafts. You can easily turn the rod journals down to remove weight. We have cut the counterweights down with no issues. The amount we cut depends on the rod piston combo so mallory is kept to minimum. We current have one in a 2V turning 8500 and have not had any issue as of yet. Car has 10dyno runs and 3 passes. Engine will be inspected after weekends events

Steen Racing · Post by **Steen Racing** » Mon May 02, 2005 7:45 pm

We used Custom GRP rods with custom CP pistons. I think total bob weight was about 1400 grams. We did keep the center weights on the crankshaft. We turned the rod journal to 2.00.

DavidHarsay · Post by **DavidHarsay** » Wed May 04, 2005 12:27 am

Thanks... I need to do some research on off the shelf forged piston weights. I'd like to figure out who has the lightest 4032 pistons without breaking the bank.

I was also doing some math...

It seems that if the crank counterweight radius is 3.5", spinning at 7K RPM, the tip is traveling at 145.7 MPH... slamming into oil globules along the way.

Looking at the drag... times eight countershafts... I think some detail work may make a slight difference... who knows, perhaps 10 HP.

I'll take what I can get.

Dave

Post by **SchmidtMotorWorks** » Wed May 04, 2005 12:48 am

Never ever reduce CW radius to reduce weight, the most effective part of the CW is at the largest radius. If you want to reduce weight reduce the fan angle of the CW. Reducing the CW radius will actually result in a heavier crank after it is balanced.

Regarding the center CWs, it depends on where the centerlines of those CWs are relative in clocking to the 1st and last rod journals. In some crankshafts the center counterweights are not only designed to reduce loads on the center main journal, the other reason for them is it is a cheap way to make-up for end counterweights that are not large enough. You can see if this is the case by looking to see if the cnter CWs are more opposite the end rod journals or the middle rod journals. If they are more in-line with the end rod journals they aren't doing much for bearing loading and are adding a lot of weight for the small advantage they provide in balancing.

Also keep in mind, a major factor in the shape of forged cranks CWs is the sahpes that can be economically forged. The ideal CW shapes for a racing crank are not easily forged.

If you will post some photos of the crank, I can give you some further suggestions.

DavidHarsay · Post by **DavidHarsay** » Wed May 04, 2005 2:12 am

Thanks... I probably will keep the center counterweights... it's tempting though since that's about 5 lbs more weight on the crank that the stock cast crank. I'm thinking that aside from balance issues, it will reduce friction at higher rpm's by keeping the center main bearing loads in check a little bit better. Couldn't hurt reliability either since removing it is essentially R&D work that I'm not too comfortable with.

Here's the forged crank on the right next to the cast crank on the left.... sorry about the camera-phone pictures, but I couldn't find the cable for the digital camera.

http://forums.modulardepot.com/attachme ... tid=506925

This is when I first started on Sunday...

http://forums.modulardepot.com/attachme ... tid=506932

This is today:

http://forums.modulardepot.com/attachme ... tid=506929

I started knife edging the counterweights... didn't want to just do the "wedge" or the bullnose airfoil, but rather a hybrid actually if that makes sense.

One of the counterweights has a very deep (1.5") balance hole that I will press a 4340 plug into, and weld... and I'm keeping that moment weight for balancing in mind when doing the work.

Thanks,

Dave

DavidHarsay · Post by **DavidHarsay** » Wed May 04, 2005 10:07 pm

Here's a better picture of the forged crank likely taken by a professional :wi

http://www.seanhylandmotorsport.com/onl ... 6303-2.jpg

Post by **SchmidtMotorWorks** » Thu May 05, 2005 1:07 pm

This crankshaft is a classic example of center CWs that are designed to compensate for under effective end CWs. If you think of the adjacent rod pin as being at zero degrees, any part of the CW that is between -90 and +90 degrees is counter productive to reducing main bearing loads on the center main. You can cut that hi side of the CW down. You may have to add weight to the end CWs unless your parts are lighter than stock.

DavidHarsay · Post by **DavidHarsay** » Thu May 05, 2005 11:16 pm

SchmidtMotorWorks wrote:This crankshaft is a classic example of center CWs that are designed to compensate for under effective end CWs. If you think of the adjacent rod pin as being at zero degrees, any part of the CW that is between -90 and +90 degrees is counter productive to reducing main bearing loads on the center main. You can cut that hi side of the CW down. You may have to add weight to the end CWs unless your parts are lighter than stock.

That makes a sense, I understand what you're saying.... interesting how this 90 degree rule doesn't quite apply to all the other "non-end" CW's, as they are somewhat biased (perhaps 30%) towards the "other adjacent" crank pin as well. This bias doesn't apply to the center ones, as the "other adjacent" crankpin is 180 degrees out ... that may be tough to follow, hope it makes sense.

The rods and pistons will be lighter than stock.

I also plan on pressing in and welding slugs into some of the lightening holes, as they are deep (I can tell some may be cringing with that idea, but I'm doing it annyway).

I'm keeping close tabs on where the weight removal is, and how much to minimize balance work. Also, the 1-5 and the 4-8 crankpins have a much shallower and smaller lightening hole than the Bullitt (equivalent cast) crank. This was done to strengthen the forged crank additionally for suprcharged applications as I understand... this area will be lightened as well, and of course it's 180 out from the end CW's.

Not a lot to be gained from removing weight from the non-end counterweights I'm sure... except I'm trying to figure out what the "fat part" is for on the crank pin ends of the non-end counterweights... all of the cranks I see here have it, and it certainly is on the counter-productive end. I'm not so willing to trim this too much until I understand it better. Here's a picture of what I'm referencing:

http://forums.modulardepot.com/attachme ... tid=508344

Thanks for the help,

Dave

68corvette · Post by **68corvette** » Fri May 06, 2005 12:27 am

I made my sbc400 crank by Vizards "budget buildin" book.

Straight from Eagle, just taped:
http://www.sunpoint.net/~pirkonen/Crank/Crank1.JPG

Counterveights shaped so that they throw oil / air away from rotating rods. also lots of un needed mass is taken away and everything is rounded.. only waiting for final polish:
http://www.sunpoint.net/~pirkonen/Crank/crank2.JPG

Here it is shotpeened and balanced, waiting for final assembly:
http://www.sunpoint.net/~pirkonen/Crank/crank3.JPG

(vizard claims 10-15hp increase @ 7500rpm)

Out of the box Scat rods:
http://www.sunpoint.net/~pirkonen/Crank/rod1.JPG

balanced, polished, critical places rounded:
http://www.sunpoint.net/~pirkonen/Crank/rod2.jpg

after shot peening, ready to mock up:
http://www.sunpoint.net/~pirkonen/Crank/rod3.jpg

Finally assembled:
(i had nothing else to do, so i polished mainbearing caps too =))
http://www.sunpoint.net/~pirkonen/Crank/assembled.JPG

SStrokerAce · Post by **SStrokerAce** » Fri May 06, 2005 10:54 am

I know you are a Ford guy but the book "How to Build & Modify Chevrolet Small Block V-8 Pistons, Rods & Crankshafts" by David Vizzard has a good section on modifing cranks for lightening, windage and strength.

http://www.amazon.com/exec/obidos/tg/de ... s&n=507846

It's 10 pages on crank mods, i can copy them and fax or mail them too you if you want.

Bret