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Thinking of adding some hp to my 79


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The THM 325 in your 79 is good for about 200 to 225 flywheel HP. There is no performance after market for this transmission because it was used in luxury cars, not performance cars. That being said, a GOOD transmission guy can make the THM 325 live up to 300 HP at the flywheel.



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I figured there wasn't much for this trans but figured it wouldn't hurt to ask, one just never knows.

I will get pics. out as soon as I can still trying to find time to get pics of my 81 T-Type. The pic's won't be very big because I still have dial up internet and times out whenever I send any decent sized pics.

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Hello Jeff:

At least install a shift kit in your THM 325. The TransGo kit I had installed only modifies the valve body & separator plate so the transmission doesn't have to come out. All shifts will be sharper but not harsh. The intermediate band (creates 2nd 'gear' when applied) is a weak point in this trans (THM 325 based upon the THM 200). Hydramatic engineered in long & slow pressure buildup on the intermediate servo for the band. In time, those 'cushy' 1-2 shifts burned up the band.

I would also install a small oil transmission cooler. Keeping the fluid a bit cooler really prolongs the life of the unit. I installed a Hayden cooler in front of the AC condenser at the bottom right. It's in series with the factory radiator but 'after' it.

These simple mods won't really let you put more HP through your THM 325. However, it will increase overall durability & let you feel better about 'spirited acceleration'...!

FYI: In my '84 Toronado, I got about 77,000 miles out of the original intermediate band. During the rebuild, I had the Transgo kit installed. After another 100,000 miles my 325-4L still shifted perfectly. The recent 2nd rebuild (178,000 miles) was needed as the drive link & sprocket bearings were worn.

Transmission 'hard parts' eventually wear out!

Good Luck,


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Jeff, Paul and Jayson,

A good point about the "Trans Go" shift kit. I have used these on my transmission rebuilds since the 70's. A word of clarification here is that Paul stated he installed his kit at the time of an overhaul. This is a good point. Shift kits will not help add any durability to a severely worn out transmission and may even contribute to it's failure. Freshen up the trans, install the shift kit and you should be good to go for a long time.

I don't know if anyone has ever done this, but a THM 425 out of a 66 to 78 Toronado or a 67 to 78 Eldorado would be able to be built to survive 600+ HP. Would probably take more "engineering" than it may be worth, but it would hold up!

Just some ramblings, my favourite car show was canceled tonight because of the RNC!

Take Care,

Tim McCluskey

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Thanks guys and keep the comments coming, I can use all the advice and parts sources I can get.I am a die hard Mercury Cougar guy (please don't ban me) and this is only the second G.M. product I really got attached to The first being an 84 Monte Carlo that was wrecked more times than I can count and had to get rid of it.

If it doesn't rain today I will have pics of both Rivs. to send!

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