Upgrading 289


New Member
Nov 25, 2019
Hey guys so I have a 1966 mustang 289 3 speed and I plan on upgrading the 289 to get around 300hp. My question is what else would I need to replace like the tranny, suspension, brakes, axles, etc? I’m just not sure my guess is that I would need to start replacing stock parts if I’m pushing that much power but that’s why I’m here. Thank you!
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SN Certified Technician
Apr 12, 2002
what else you upgrade depends a lot on your driving style. at the very least though i would upgrade the brakes to front disc with an option of big rear drums, either the 10x2.5" or 11x2.25" set up. i prefer the latter as it has more swept area.

i do recommend suspension upgrades s well eventually though.

transmission, a T5 will do fine as an upgrade.

rear axle, you might consider a lower rear gear, around a 3.50 will do nicely for all around driving. otherwise you can leave the rear end alone unless you break something, which i doubt.


Aug 8, 2018
You will definitely want to install a limited slip differential in the rear end as a single side winder will not put that power to the ground. If suspension has not been upgraded, consider heavier coil springs for the front and 4.5 leaves for the back. Definitely good shocks at every corner. I like the KYB.

Totally agree about upgrading the brake system to at least front disc if not all 4 discs. Mustang Steve sells a great kit to install 2008 GT brakes at every corner. You will have to run 17 inch or larger wheels all around to clear those calipers. Feel free to hit me up. Just finished rebuilding my buddy's 65 doing everything you asked about.



Active Member
Feb 4, 2001
West Rutland, Vermont
For a 300 hp 289 in a '66 chassis the only thing I would consider a "need" would be a dual-circuit master cylinder, for the safety-aspect in case of failure of a line or wheel cylinder. The 3.03 3-speed transmission was the standard manual transmission behind many a FE and 351W/351C, the 8-inch rear is up to the task though you may want to add a LSD, like the Eaton-Detroit TrueTrac, nothing all that wrong with the drum brakes... many 4-wheel drummed 390's were sold.

You should consider the "Arning Drop" to improve your front suspension geometry, a quality "Export Brace" to stiffen the shock tower-to-cowl connection and, generally, replace any worn suspension components with quality (Moog) replacements. Other than that, whatever your heart (and wallet) desires.
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Active Member
Dec 13, 2019
You guys are giving the 8 inch way more credit for being stout without some aftermarket pats. I went through about10 chunks on my 65 Comet in 68-69 before I lucked up on a 9 inch. I blew the pinion completely out top of the housing once. The driveshaft was still hooked up. Tore up my floorboards .
That was with street tires.
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