Building a 6.4 Cummins the CORRECT way

9724VF350 said:
If you want to keep it simple, have rods made 194mm long. This will keep piston protrusion the same as stock 6.7 specs. If you wish to raise compression, make the rods longer than 194mm, and machine the top of the piston down the same amount, reducing the cc of the bowl the correct amount to get your desired compression ratio.
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As much as that makes sense, the 194 mm almost sounds too short for stock protrusion. Grant you that's something that I'd just have to measure for, but the reason I say that is the stock rod lengths on the billets I just sold showed 7.559" (191.9986 mm)

I know it doesn't take much difference in length but only 2 mm seems like it wouldn't be enough. But like I said I'd have to measure for it so until then I'll take your word for it!

The original 205-207 mm length mentioned in the first post was something I saw you mention in another thread so that's why it was initially brought up.

As for compression the plan is to try to end up with 16.5-16.7:1. Mainly due to having to cut valve reliefs for the cam and rather than valve reliefs the plan is to just deck the piston to prevent potential hot spots on the edges of the reliefs.
 
Info card from stock length billets

IMG_0828_zps2cf021b4.jpg
 
adamsmarshall said:
As much as that makes sense, the 194 mm almost sounds too short for stock protrusion. Grant you that's something that I'd just have to measure for, but the reason I say that is the stock rod lengths on the billets I just sold showed 7.559" (191.9986 mm)

I know it doesn't take much difference in length but only 2 mm seems like it wouldn't be enough. But like I said I'd have to measure for it so until then I'll take your word for it!

The original 205-207 mm length mentioned in the first post was something I saw you mention in another thread so that's why it was initially brought up.

As for compression the plan is to try to end up with 16.5-16.7:1. Mainly due to having to cut valve reliefs for the cam and rather than valve reliefs the plan is to just deck the piston to prevent potential hot spots on the edges of the reliefs.
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A 5.9 is 120mm stroke, a 6.7 is 124mm. if you modify stroke, you need to modify rod length or piston od deck height by 1/2 that amount, that's where they are cutting the deck by .080/2mm.

When I mentioned a 205-207mm rod, that was going to be with a deck plate.
 
9724VF350 said:
A 5.9 is 120mm stroke, a 6.7 is 124mm. if you modify stroke, you need to modify rod length or piston od deck height by 1/2 that amount, that's where they are cutting the deck by .080/2mm.



When I mentioned a 205-207mm rod, that was going to be with a deck plate.
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Makes a ton of sense. Thank you!
Make
 
I was told recently that R&R has a impressive forged rod. The raw forging is made with a oval pin boss and the pin bore can easily be located to make the length you want.
 
Drag race.

GTX4202 1.12 T4 over Precision PT98 that's been clipped to 91 G-trim 1.00 T6
 
9724VF350 said:
A 5.9 is 120mm stroke, a 6.7 is 124mm. if you modify stroke, you need to modify rod length or piston od deck height by 1/2 that amount, that's where they are cutting the deck by .080/2mm.

When I mentioned a 205-207mm rod, that was going to be with a deck plate.
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Same thing can be done with a different compression hight on the pistons. Rod angle change is very minimal.

Best to do both accually as there are issue's with both as it sits....
 
DIESEL_POWER said:
Same thing can be done with a different compression hight on the pistons. Rod angle change is very minimal.



Best to do both accually as there are issue's with both as it sits....
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As I fully understand the rod angle change being very minimal, would you care to explain the advantage to changes to both rod length and pin height as opposed to one or the other only?
 
Well rod angle change would be zero if you just add compression height. I'd stuff the longest rod possible every time. In this case the difference is pretty marginal. Other than to use the factory piston compression height and avoid the cost of a custom piston or milling the deck.

The crime of the monotherm is they didn't shorten the compression height considerably and stuff the longest rod they could.
 
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adamsmarshall said:
As I fully understand the rod angle change being very minimal, would you care to explain the advantage to changes to both rod length and pin height as opposed to one or the other only?
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My thoughts are based on a "optimal" configuration, nothing against what the factory has, only IMO it can be better, as JSP said, longer rod helps but also the pin hight has its effect of sidewall thrust, and piston rock if its too low, this leads to worse ring seal and extra friction, and if its too high it can affect the ring stack to be "optimized" because the pin will be into the OCR and taking up valuable space and can cause oil controll issue's and heat issue's.

If your going to "do it right" as the thread title suggests. Then All of this needs to be considered....and there is a few more points i could bring up, but i cant build your engine over the interweb very well...LOL

Im just glad your not considering the "hack job" way of decking and or shaving the piston crown...as this has many unseen issue's.

Hope this helps.
 
Joe is right. I would put the longest rod in as possible. Now that we know you are drag racing, and pretty much not driving, that is what I would do. Using a forged piston would allow you to get the longest rod, and have the piston made so your ring stack in not a problem. This is not the cheapest way to do this. So it can really depend what your budget is. We make custom rods, and pistons. I figure this stuff out all the time. If your interested I can give you quote. Just PM me. Just follow what I told you at the beginning, and you should be able to figure out what you need.
 
THe only reason I would run a 6.4 combination is to try and get the factory CR rods to last between 800-1000 rwhp. You're already getting billet rods; buy a 6.7 crank, use Cummins parts and be done with it. Otherwise, you'll spend a few years getting it right and likely be burnt out on it before the project makes a pass.
 
joefarmer said:
THe only reason I would run a 6.4 combination is to try and get the factory CR rods to last between 800-1000 rwhp. You're already getting billet rods; buy a 6.7 crank, use Cummins parts and be done with it. Otherwise, you'll spend a few years getting it right and likely be burnt out on it before the project makes a pass.
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^^^ that's what I ended up doing. IMO, too much snake oil being sold with yhe "perfect" 6.4 setup.
 
Doesn't larger bore shorter stroke work better for drag racing though? If it gives you an edge in competition, granted you can afford it, I'd say its worth it.
 
joefarmer said:
THe only reason I would run a 6.4 combination is to try and get the factory CR rods to last between 800-1000 rwhp. You're already getting billet rods; buy a 6.7 crank, use Cummins parts and be done with it. Otherwise, you'll spend a few years getting it right and likely be burnt out on it before the project makes a pass.
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I agree, on all our recent 6.7 builds we stayed standard bore and used MJ tool fabs rods, wrist pins, girdle, and main studs (shortblock).

I guess i dont see the point of losing displacement if you do need to?
 
the only reason for the 6.7 block is to run the bigger bore , which allows a much bigger intake valve 1.450 vs stock 1.300. this is close to 20 cfm.

I would not waste the money on rods unless your going over 1200 hp , use a good set of 12 v or 24 valve rods with good bolts and properly bushed and clearanced wrist pins.


Diamond, Ross and Wiseco make pistons . I have used all three , including the new billet ones , and I'm a vendor and a master WD for all three. This is not near as hard and everyone makes this out to be. The stroke is 4.72 Vs 4.88 so the compression height of the piston needs to change by adding .080.

I take the bore to 4.250 and use a much better ring package. and the correct compression ratio.

I did a 5.0 liter cummins a few years back for D/DT this was a 4.00 bore and a 4.00 stroke. it makes very simular power , but 400 rpms higher



remember this is building a motor measure twice , order pistons once



here is my 6.7 pistons with a 5.9 crank, its 19.85 to 1 and uses a small ring package that is worth 60 hp in a 2.6

also if your going to put pistons in , go with a lot more cam then that . I would use a 221 221 112

IMAG0416.jpg
 
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