THM400 248/148 vs 275/157 results.
 

After crossing off the new bathtub intake from my bucket list I lost .500 et. Its in my 60ft. -1/8mi. Did some mods to intake and got .250 back but want more. Don't really want to spin my 421sd more. Thanks

So do we have to beg and pled for more info about your combo to try and help you out, of give you a few Oz of truth serum?

You making more power and less traction?

No way on traction . Went from consistant 1.54 60ft. to 1.9-2.0. Doesn't get that seat of the pants till past the 1/8 but the shift light will come on just before finish line like before.

Lets see the timeslips may make more sense.

So are you saying you switched from a 248/148 gear set in your Turbo 400 to a 275/157 gear set and your 60ft went from 1.54 to 1.9-2.0?

I suspect he's considering it, and hoping for input? 60 ft went away after an induction change, even though he's closer now it's still not what it once was. :noidea:

Gary why did you drop the bath tub intake?
Was it because there out ,If so I have one still in the box.

GT

I replced my sd 359 2x4 with the pre 64 style bathtub.

Guy's I am looking for results from anyone who has made the change from 248/148 to 275/157 first and second gears. Not really wanting to spin engine at more rpm's at top end.

What I would do next is get a Tack in the carnthat can play back your runs.

This will allow you to see where in the current rpm band your motor is spending its most time in during a pass.
This greatest amount of time rpm wise is where you should concentrate your efforts on improving.

It sounds like your current combo is more dependent on intake tract air speed then pure top end hp.

What I would do next is get a Tack in the carnthat can play back your runs.

This will allow you to see where in the current rpm band your motor is spending its most time in during a pass.
This greatest amount of time rpm wise is where you should concentrate your efforts on improving.

Sorry about this duel post , this site is having issues this morning since I did not do such.

If you have a Go Pro maybe you can mount it to record your tach during a run.
You could get a Draggy App if you are a smartphone dude. Or get a G Tech

https://www.gtechpro.com/G-TECH-Pro%20SS

The results will differ depending on the combination. I ran a 2.75 first gear for many years. The car initially slowed down a tenth or so going to a 2.48 and another tenth going to a 2.20 gear set. My buddy had a bad fast RAIV headed combination and he found better results using a 2.48 and steeper rear gear compared to a 2.75 with slightly less gear. My brothers Firebird ran a bit slower with the 2.75 gear set. I'd think with your deal it will help do what you want, but I wouldn't expect to see much more that a tenth or so improvement. Are you sure the 60 ft can't be helped with some carburetor or plenum work.... That's a big loss you're dealing with.

What carbs are being run?

Stan

Thanks Stan. I did get .250 back from some plenum work but there's probably more to do. That open chamber they used not user friendly with my combo.

I changed the th400 in my firebird to the 2.75 for the same reason (not wanting to increase trap rpm but wanted to lower 60' times). 69 Firebird, 3.90 gear, 389, iron heads, full exhaust and interior. 60's went from mid 1.70's to 1.66/1.68 with no other changes.