Small block w/ 3.79 crank

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I sold a W7 shortblock that had some crazy good parts but was beat up like a lot of those engines were. r block, Arrow rods, Calias crank, two sets of pistons my buddy had loosely thrown in a brown box in the back of his open truck for a few months. One of those times I needed some cash so I sold it all off.
 
I sold a Molnar 3.79" stroke crankshaft and 6.400" rods to a guy in Minnesota who drag races. I've had several inquires about 6.500" rods, but no one has ever purchased any from me.

3.79" stroke/ 6.500" long rods yields a 1.715 rod stroke ratio and gives you approximately a 1.25" compression height pistons.

3.79" stroke/ 6.400" long rods yields a 1.688 rod stroke ratio and gives you approximately a 1.305" compression height pistons.

Given that better flowing heads need less rod stroke ratio because dwell time needed at TDC using a super long rod is unnecessary.

With all these factors I would advocate using a 6.400" rod for several of these reasons and having an extra .050" of compression height in the piston will allow for more stability

and more room for the ring package. Unless it's a Comp Eliminator/ Heads Up Pro class engine and you need the last 2-3 HP I would build it for more longevity with the 6.400".

Tom
 
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lead69 said:
That is going to be awesome, my dyno partner Tim has a couple 3.79 cranks and at some point he is going to build one. We are always discussing builds to dyno test with and really wanted to find a way to do one with a 3.79 crank. We think the hp would peak at a higher rpm than a 4" and make more power as a result. I would love to throw one in my stick car, I may have to make it happen at some point.
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We're do you get your machine work done ?
I just got a K1 3.79 crank its new and K1 rods
And a used Callies 4.00 crank
Cast iron 810s
Thanks Doug
 
SandyDoug said:
We're do you get your machine work done ?
I just got a K1 3.79 crank its new and K1 rods
And a used Callies 4.00 crank
Cast iron 810s
Thanks Doug
Click to expand...
I have used quite a few, feel free to pm me if you want to talk about them.
 
tvt59 said:
They ran those heads in some NASCAR classes and made about 850 hp. I can do what I want to do with 800 hp. I hope it's works out.
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Respectfully, I've never looked at the W7 or what may have been the last iteration of a w7 before the w8 but, I'd think 800 is possible! Only person I know of that had a good running W7 was Laverne with the Duster but, I'm not privy to many SBM combos. One thing I can share here is even the W8 NEVER made 850hp in a NASCAR application. It wasn't until the r5p7 when teams were able to get 850hp which is likely because the valvetrain was better in design. Conversely, most of us know of one or more W8 builds that make 1000+ today but they aren't NASCAR friendly. Based on your more recent posts it sounds like you're doing a w5 now?...or is this another 'side project' ?
 
@ValiantS @tvt59 @TT5.9mag

I, like @mopardude318 , am putting together a 3.79 piece meal. After seeing the longer rods in question it makes me wonder if I put the cart ahead of the horse on one of my purchases, rods. And, I'm only two components deep into my parts collecting!

I have a virgin NOS 5.9 bare block I bought from John Irving that MP gifted him during the Magnum 380 crate engine days. Later, I looked at the Monar site and got the spec for the rods, minimum 6.125". So I bought the chevy rods that is 6.125, 2.1 journal, .927 piston pin, ....

Molnar's crank:

360-3790FB6FSmall Block Chrysler -2.100" Rod Pins3.7903602.1006.125


My rods:

Brand:
SCAT Engine Components
Manufacturer's Part Number:
2-350-6125-2100A
Connecting Rod Length Center to Center (in.):
6.125 in.
Connecting Rod Material:
Forged 4340 steel
Wrist Pin Style:
Floating
Pin End Bore Diameter:
0.927 in.
Rod Journal Diameter (in.):
2.100
Big End Bore Diameter (in.):
2.2250
Quantity:
Sold as a set of 8.
Cap Retention Style:
Cap screw
Big End Width (in.):
0.940
Clearanced for Stroker:
No
Connecting Rod Bolt Brand:
ARP
Connecting Rod Bolt Diameter:
7/16 in.
Connecting Rod Bolt Material:
ARP2000 alloy
Fastener Tensile Strength (psi):
200,000 psi
Connecting Rod Bolt Head Style:
12-point
Weight Matched Set:
Yes

There's a set of pistons that Richard Nedball is selling that I was thinking would work with my rods. [FOR SALE] - New BRC 4.030" Forged Pistons and pins for 3.79" Stoke Crankshaft

It would be nice to pick these up if they work with my rods. Piston wrist pin is .984 but my rods have a pin diameter of .927. I figure the rod pin hole can be bored .057"?

In retrospect, its better to rat hole your money instead of rat holing parts so you can buy everything at the same time. I would be very pleased if the rods I have will work, especially with the pistons for sale.

Now, as was mentioned 6.5' rods will have a higher rod ratio that 6.4" rods so the shorter rod will be better for wear and longevity? Is it better the shorter the rod is? Would a shorter rod (6.125) and taller piston be better for stability?

I apologize if these are elementary questions.
 
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go-fish said:
@ValiantS @tvt59 @TT5.9mag

I, like @mopardude318 , am putting together a 3.79 piece meal. After seeing the longer rods in question it makes me wonder if I put the cart ahead of the horse on one of my purchases, rods. And, I'm only two components deep into my parts collecting!

I have a virgin NOS 5.9 bare block I bought from John Irving that MP gifted him during the Magnum 380 crate engine days. Later, I looked at the Monar site and got the spec for the rods, minimum 6.125". So I bought the chevy rods that is 6.125, 2.1 journal, .927 piston pin, ....

Molnar's crank:

360-3790FB6FSmall Block Chrysler -2.100" Rod Pins3.7903602.1006.125


My rods:

Brand:
SCAT Engine Components
Manufacturer's Part Number:
2-350-6125-2100A
Connecting Rod Length Center to Center (in.):
6.125 in.
Connecting Rod Material:
Forged 4340 steel
Wrist Pin Style:
Floating
Pin End Bore Diameter:
0.927 in.
Rod Journal Diameter (in.):
2.100
Big End Bore Diameter (in.):
2.2250
Quantity:
Sold as a set of 8.
Cap Retention Style:
Cap screw
Big End Width (in.):
0.940
Clearanced for Stroker:
No
Connecting Rod Bolt Brand:
ARP
Connecting Rod Bolt Diameter:
7/16 in.
Connecting Rod Bolt Material:
ARP2000 alloy
Fastener Tensile Strength (psi):
200,000 psi
Connecting Rod Bolt Head Style:
12-point
Weight Matched Set:
Yes

There's a set of pistons that Richard Nedball is selling that I was thinking would work with my rods. [FOR SALE] - New BRC 4.030" Forged Pistons and pins for 3.79" Stoke Crankshaft

It would be nice to pick these up if they work with my rods. Piston wrist pin is .984 but my rods have a pin diameter of .927. I figure the rod pin hole can be bored .057"?

In retrospect, its better to rat hole your money instead of rat holing parts so you can buy everything at the same time. I would be very pleased if the rods I have will work, especially with the pistons for sale.

Now, as was mentioned 6.5' rods will have a higher rod ratio that 6.4" rods so the shorter rod will be better for wear and longevity? Is it better the shorter the rod is? Would a shorter rod (6.125) and taller piston be better for stability?

I apologize if these are elementary questions.
Click to expand...

Connecting rods are bushed. It's just not cost effective to push out a bushing, bore the hole oversize, re-install a bushing, bore, and hone for .002" difference in rod length.

You are not going gain anything by increasing the rod length from a stock 6.123" to a 6.125". Sell the Scat rods and just get a set of 6.123" x.984" rods

Having more compression height aids to piston stability within the bore and allows the use of a nice ring package. This is specially important to me on a street engine.

Most importantly to me is that one builds a combination that is serviceable should you need replacements. Using off the shelf parts is the way to go for 99% of builds.

Custom piston prices are now $1600 and up and should you need a replacement piston down the road your looking at $300 and at least 16-weeks to get one.

Tom
 
@go-fish this ^^^^ and I’ll add, there may not even be enough material in the small end of the rod to bush (safely) to that pin size.
 
Just what I figured. I’ll hit up the local machinists and see if they want to buy them. It’s a SB Chevy journal which is a big market.

When the timing is right I’ll just get a complete rotating assembly.
 
PROSTOCKTOM said:
Most importantly to me is that one builds a combination that is serviceable should you need replacements. Using off the shelf parts is the way to go for 99% of builds.
Click to expand...

Am I reading it right? A 3.79 crank with a stock length rod uses a stock piston?
 
go-fish said:
Am I reading it right? A 3.79 crank with a stock length rod uses a stock piston?
Click to expand...

Not that I know of but IIRC you can use a 4 inch stroke piston and a 6.250 rod with a 3.79 crank.

The problem is finding an off the shelf piston with the correct dome and geometry and something with thinner than 1/16 rings.
 
go-fish said:
Am I reading it right? A 3.79 crank with a stock length rod uses a stock piston?
Click to expand...

You had previously mentioned about buying that set of BRC pistons that are listed in the for sale section.

Those use a 6.123" x .984" rod with a 3.79" stroke crank. That set of pistons are about half price of what any new set cost today.

Tom
 
@go-fish

The Molnar crank I got from a member here was at a good price. I think it’s the same one you got. (3.79 with 2.1 rod journals, 360 main) I’m using the Chevy 6.125 “power adder” rods because of a 671 blower on top. I’m already aware of the custom piston cost for my application. Little over 2 grand for a set of blower pistons for my build. It is what it is.
 
If go-fish has a 2.100" rod journal 3.79" stroke crank then he can't get a 6.123" rod with a .984" pin, so the BRC pistons are

of no use to you unless you can rework a set of rods, or just buy a set of pistons that have a .927" pin.

Tom
 
Newbomb Turk said:
Not that I know of but IIRC you can use a 4 inch stroke piston and a 6.250 rod with a 3.79 crank.

The problem is finding an off the shelf piston with the correct dome and geometry and something with thinner than 1/16 rings.
Click to expand...

There are lots of ICON pistons available for the 4" stroke crank with a compression height of 1.445" in most variations except for the dome pistons that come with a 1.465", but they are all made with the .984" pins. Using a 1.445" compression height piston would be great with a 6.250" rod IF you could get them in a .984" pin, but you can't. I know JE makes a shelf piston for a 4" stroke with a 6.125" rod length and .927" pins, but they are over $1500 a set.

Tom
 
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Wallace Racing Calculate Piston Pin Height

F9760D67-6441-4FD8-A460-9604A0F596B1.png


Here’s an old post from what I did.

D0B6FAEA-8524-4BA6-BC0B-9ACB357B0958.png


I don’t have a current Diamond Piston catalog to cross reference CH from a chevy for use in a SB Mopar application. I’m assuming they simply change the milling process to relocate the valve reliefs for a small fee….
 
mopardude318 said:
@go-fish

The Molnar crank I got from a member here was at a good price. I think it’s the same one you got. (3.79 with 2.1 rod journals, 360 main) I’m using the Chevy 6.125 “power adder” rods because of a 671 blower on top. I’m already aware of the custom piston cost for my application. Little over 2 grand for a set of blower pistons for my build. It is what it is.
Click to expand...
I don't have the crank yet. I started my 3.79 parts collection with the NOS 5.9 bare block and the Chevy rods. The BRC pistons in the F/S section would work if the wrist pin was .927 but they are .984.
It looks like I could do custom piston like the BRC but just have them made with a .927 wrist pin instead of .984. I can use my 6.125 x .927 rods with the Chevy piston mentioned below.
If your custom pistons used a .927 piston pin you would be using the rod specs that mine are if I'm not mistaken. I was going to buy the crank you purchased but I was going through another big purchase at the time and my car funds had to rebuild.
MiradaMegaCab said:
Wallace Racing Calculate Piston Pin Height

View attachment 1716263568

Here’s an old post from what I did.

View attachment 1716263569

I don’t have a current Diamond Piston catalog to cross reference CH from a chevy for use in a SB Mopar application. I’m assuming they simply change the milling process to relocate the valve reliefs for a small fee….
Click to expand...

@MiradaMegaCab , If you could send me any notes from your build either through this thread or a PM I would appreciate it.
 
I think Romeo furio or pure Vision built one a while back Lake 30 years ago and it was super nice it ended up selling for 70k at Barrett-Jackson I believe Reggie Jackson bought it but it was supposed to have revved up very quickly
 
@go-fish

I did document the build specs such as ring gap, ring type, piston part#, bearing clearances, cam specs, etc, during the build.
The build was prior to my screenshot post of 2011 and last time I looked up the Diamond Pistons Part#, I couldn’t any info as that Part# was NFG.
I’ll double check receipts and records, I’ll post that info up later in the week.
 
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