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Discussion Starter · #1 ·
I have started several threads and taken part in a few here. I'm the hard head who is still going to build a 3.9 V6 in spite of all the put a V8 in advice. While this is my first V6 Dodge engine build I have built plenty engines. I have been playing with the numbers as far as C/R goes. Working backwards the 3.9 is just a 318 missing some stuff. So I began looking at whats available for the 318. Scat makes a very nice set of connecting rods full floating with 7/16 ARP bolts. Yes, they are a little thick on the big end The locking lips may have to be moved to the other side and a spit hole worked into the parting line. That's just a little work and I have two spares to play with. KB makes a set of fully machined flat top pistons that are tall enough to zero deck the engine with out surfacing the block. These pistons have two valve reliefs giving 5 cc volume for the piston. The std pistons in my engine were .060 down the bores. Doing a little math ( which I'm not great at ) I came up with 9.3 to 1 as a std build. I believe stated C/R for the 3.9 id 9 to 1, but I spoke with someone I thought may have some info. He claims the 3.9 is only 8 to 1. He also suggested I not exceed 9 to in my build. He claimed without alum heads I could not do 10 to 1 on the street. Again doing math I figure the KB .020 O/S pistons with 5 cc reliefs is going to be 10.8 to 1 oops a little high for sure. While they don't offer a 318 piston with a dish they do have a 340 piston with a slight dish and valve reliefs giving it a volume of 12 cc. Again with the math, machining a small dish ( again I have two spare pistons to practice on ) with the reliefs giving a volume of 12 cc should drop me in at 9.8 to 1. That's my target if you are better at math or have some info to share along these lines please feel free to share. Steve
 

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Discussion Starter · #2 ·
It seems we are short math wizards here. No matter I'll get it all sorted when all the parts arrive. The more I think about it the spit holes in the rods are timed shots of oil to the cyl walls. Having just torn down a 3.9 fairly low mileage I'd say under 100,000 judging from wear or lack of on all parts. The one thing I did notice was all oil control rings were locked solid in their grooves. The local machine shop mentioned this is a problem with these engines. Opening side clearance would provide full time oil to the cyls. Five of the stock rods had .008 side clearance one was .014 go figure. The new rods will allow me to custom set side clearance for each journal being about .020 wider than V6 rods. Steve
 

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That .020 is because they're 5.2 rods; you'll need to take that off, due to the 3.9 being a split pin crank.

Or not; but that's why they're different.

RwP
 

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Discussion Starter · #4 ·
Well there you are Ralph, wondering where you were. Of course they are 318 that's why there are eight. And the extra thickness is a plus along with being new, forged, 7/16 ARP, bushed and around 100 grams lighter. They are over kill for this engine but reasonable price and the above mentioned pluses. Same thing with the 318 pistons there is some selection and again machine work will be required. On paper I have missed above piston volume by 7cc. A 7cc dish in a piston this large will hardly be noticed. If I end up with zero deck, .039 quench 9.8 to 1 C/R I think I will have as much as can be had along those lines.
 

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Discussion Starter · #5 ·
It surely is a good idea to collect your own specs. I have been waiting for parts and working with posted specs. I spent a little time measuring today and am pleased to say it's looking good. I drop my rods off tomorrow to have them thinned down. Going to delete the spit hole and maintain the .008 side clearance. I CC ed the heads and did the math only to find everything is half true. I was told by what I felt should be a good source that 3.9 Magnums are not 9 to 1 C/R but as much more in the 8 to 1 range. My math and measurements show the engine I have taken apart is a true 8.6 to 1 C/R. This is pleasing as I get the 1 point I was planning ( 9 to 1 to 10 to 1 ) at 9..6 to 1 all I wanted and a better chance regular gas will work. Pre assembly once bore work is done I hope will agree with my math allowing the heads surfaced .010 or less and the block surface .010 to zero deck.With excellent surfaces I could have Cometic make gaskets to .039 compressed. Steve
 

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Discussion Starter · #6 ·
I picked up my of Scat 318 connecting rod set and visited the block at machine shop. The rods have been thinned down to 3.9 V6 and I'll visit another shop today to recheck balance and adjust if needed. All were machined the same amount on each side, balance should be very close. It's racing time of year so the machine shops are busy with that. On top of that many suppliers are back ordering or drop shipping. I'm glad I have spent the last 8 months collecting parts for the project. Looks like flywheel ring gear and the springs, caps and keepers will hold me up a few more weeks. Steve
 

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Discussion Starter · #7 ·
I checked the balance on my rods and the big end lost 5 grams and the small end remained the same. Side clearance is now correct. I'm off to drop the crank for rod journal grinding. Once the block is back the fun really begins. Steve
 

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Discussion Starter · #8 ·
Just been doing some reading about ring end gap with KB pistons. Also so interesting reading about quench in the effort to reduce detonation. A fine line between compression, octane and intake air temp. Get them all correct and make power wrong and stuff breaks. I'm still reading about ring materials and construction. Waiting for the machine shops to call me to pick up my stuff. Steve
 

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Discussion Starter · #9 ·
I had CC ed the heads from my core engine. In spite of reading that they should be 52 CCs I measured them closer to 63.5. My new cyl heads purchased from Rock Auto state 52 CCs in their info. Just looking ( which is how I missed the cracks in the old heads ) I think these are 63 CCs also. Steve
 
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