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Mild Street /6 build suggestions needed
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Author:  koolaid31 [ Sat Jan 04, 2025 9:54 pm ]
Post subject:  Mild Street /6 build suggestions needed
Hi, I'm new to forums so I hope I'm not doing this all wrong.
We are rebuilding my 17yr old son's very tired 1970 Dart Slant 6 and need any advice we can get.
Can someone recommend how much to deck the head and block. How much to bore the cylinders? What street cam to go with, and any other changes we need. This is pretty much our first engine rebuild. He would like it to have a nice loping sound at idle.
The block and head are at the machine shop soaking for a few days, then they'll be magnafluxed and machined.
We would like to significantly improve the amount of power it makes while still running on 89 pump gas.
He previously put on a 4bbl AVS2 and a 4bbl Offie intake... but everything else appears original.
If you've done a 4 bbl rebuild and love it, Please tell me what you did to yours.
Author:  Dart270 [ Sun Jan 05, 2025 5:17 am ]
Post subject:  Re: Mild Street /6 build suggestions needed
Sounds like a fun project. Happy to make some recommendations and have a discussion.

Can you tell us a bit more about your desired performance, MPG, budget, overall driving situations/plans, etc? That will help us to answer your questions in a more informed way. I wonder if someone might be available to do a little head porting or install oversize valves? Does your machine shop have a website, or can you describe their business? What exhaust do you intend to run? Dutra duals are a really nice solution for a "mild street" engine. The factory exh manifold is really restrictive, but can be OK if you are not looking for a big power bump.

Thanks and Slant on,
Lou
Author:  volaredon [ Sun Jan 05, 2025 5:59 am ]
Post subject:  Re: Mild Street /6 build suggestions needed
On the how much to bore it out... The least amount necessary to restore clearances to specs and take out any taper or out of round, that new pistons and rings can be bought for... whether that's 0.020 0.030, 0.040, 0.060. the decking of the block and head, that's gonna take a little more work to determine. At Max TDC how far below "even" with the top of the block do the pistons sit? Alot of "rebuilders grade" pistons sit lower than the original. Which definitely won't help performance. And head gaskets aren't readily available that are as thin as the original. Most are around double the thickness of original. So to even be the same as stock, in my case 0.050 had to be shaved. I had 0.100" taken off between block and head and my compression ended up exactly at the advertised CR in most manuals of the day, when I was hoping for more.
You have to "CC" the head to find the chamber volume to help decide how much to take off there.
When new there was alot of "tolerance stacking" that usually meant they weren't up to "advertised" compression from brand new.
Author:  koolaid31 [ Sun Jan 05, 2025 9:30 am ]
Post subject:  Re: Mild Street /6 build suggestions needed
The plan is to make this a reliable car for my son to drive during college, and hopefully keep well beyond that.
I did leave out that the previous owner installed dual 2.5" headers that neck down to 2" tail pipes (which look fantastic), so he's keeping it.
Budget is pretty thin ....hoping to get it done for $4k ???
I would LOVE to put in oversized valves so please recommend. Once the head is machined, I'll have my son go through and just "clean up" the casting lines, etc..
We do have a legit engine machine shop working on the block, but I doubt he's got a website. It's just a few old guys who have been building engines all their lives.
Once the block work is done, our mechanic friend has volunteered to help us put the engine back together, so my son learns it first hand.
Author:  Rick Covalt [ Sun Jan 05, 2025 10:44 am ]
Post subject:  Re: Mild Street /6 build suggestions needed
I am assuming this is an automatic car? I know everything adds to the cost. But with whatever engine you build the engine will "act" like it has much more power with a lower rear end ratio. Of course that depends what you have in it now and if you are willing to live with a lower ratio. 3.55's are a nice street gear for many people if they do not have an overdrive transmission.
Author:  koolaid31 [ Sun Jan 05, 2025 7:26 pm ]
Post subject:  Re: Mild Street /6 build suggestions needed
yes Rick, it's got an automatic A904 Tranny in it. I don't know the actual gearing in the rear end, but they're clearly not 4:10 as it doesn't scream at 70mph like my '69 boat anchor does. ;)
Once the shop has measured and magnafluxed the block we'll be able to know how much or little we need to bore out the cylinders, and start making decisions then I guess.
Thanks. ;)
Author:  Reed [ Sun Jan 05, 2025 10:09 pm ]
Post subject:  Re: Mild Street /6 build suggestions needed
My recipe for a fun, reliable, street motor.

1. upgrade the cam to the factory 1972-up slant six cam.

2. have the machine shop do the measurements and calculations necessary to raise the compression to a DYNAMIC compration ratio of 8:1

3. Upgrade to a Holley 2300 two barrel 350 CFM carburetor. Have a machine shop modifythe intake manifold to accept the two barrel carburetor AND install a divider between the front three and rear three intake runners, effectively making the manifold a "dual plane" manifold.

4. upgrade to HEI electronic ignition (instructions are found on this site) and spend the time to fine-tune the mechanical and vacuum advance curves

5. Run a single 2 1/2 inch diameter exhaust pipe to a Walker Quiet Flow muffler and 2 1/4 pipe from the muffler to the rear bumper.

6. Install a cable operated kickdown linkage for the transmission.

7. be sure to upgrade the brakes, or at least get the stock brakes operating optimally.

8. Don;t forget to properly adjust the bands in the transmission and the kickdown linkage.
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