Hi folks , I have a stock original 350 on my 68 Skylark Custom w/2 speed ST300 attach to it Actually I have a 3,08 gear on the rear (born with a 2.56) and cannot spin the wheel effectively , would a 3.23 gear alone will add much "go" to my car without killing the 20 miles/gallon it makes actually ? And by the way how would it cost to replace it (aprox) ? Hope my question is clear enough for you to understand my points (english is my 2nd language) Thank you
Going from a 3:08 to a 3:23 will not be worth the hassle. You likely not feel the difference. You need more of a jump or gap in ratios. Are you planning on swapping the 2-speed to a 3-speed,like a TH350?
Getting the mechanical ignition advance in sooner will help more than going from a 3.08 to a 3.23. The 2 speed 1st gear is only 1.76. That means you have a 1.76 X 3.08 = 5.42 starting ratio. The THM350 1st gear is 2.52, that gives you 2.52 X 3.08 = 7.76. Prior ST300 had the switch pitch torque converter which helped more, but that was gone in 1968.
You will not notice a difference by going from a 3.08 to a 3.23 ratio since the difference is only 5%. You will also not see much (if any) by going from a ST300 to a THM350. There have been several heated conversation on this subject. The switch pitch on the ST300 creates a multiplier effect that does almost the same thing as the extra gear in the THM350. Several people have tested this theory and the difference between those transmissions is like splitting hairs. Have you tried putting a curve kit in your distributor? Sometimes that can wake-up an engine.
Oh so right you are Larry. The '68 st300 only has a 1.2 torque multiplication from the same NON-variable pitch torque converter as a TH350 has because in '68 st300 shared the same converter as the TH350 when they changed over in the next couple of years as they phased out the st300. Not to be confused with the '65 to '67 st300(and maybe the '64 as well?) that had the variable pitch converter that could multiply torque by a factor of 2.45:1. The '68 st300 in NO way could accomplish this unless the input shaft was changed so the variable pitch torque converter would fit and the engagement solenoid added as well as the proper wiring added. Unfortunately the OP has the '68 non-variable pitch st300 that has the same converter as the TH350 so an upgrade would actually be to swap in a TH350 to take advantage of the better gearing it offers for the off the line acceleration if in that's what the OP is after. Or to try and find an actual variable pitch st300 to swap in so the OP can have the 2.45:1 torque multiplication of the high stall. Of coarse the electrical would have to be figured out to activate the variable pitch's high stall. This swap would enable the OP to maintain the original 2 speed shifter. I would opt for the TH350 myself because of superior torque converter technology of today that makes the variable pitch nothing but an antiquated novelty. JW can explain better than I why a fixed pitch is better now but something to the effect that a smaller diameter converter can be used for around the same torque multiplication as the high stall of the variable pitch while maintaining very good street manners when just regular mild when a cop is following driving. The same torque multiplication of 2.45:1 AND the better off the line gearing and even the mighty variable pitch st300 is left in the dust from a TH350!!! Derek
Thank you all for your answer , so gear swap seems not the way to go in my case . More info for the mechanical ignition advance ?
