Generally speaking, steeper gear ratios will improve your acceleration and E.T., while reducing your fuel economy and trap speeds. In round numbers, each gear step (from 2.93:1 to 3.29:1 for example) will reduce your quarter mile E.T. by about a tenth of a second and your trap speed by about 1.0 MPH. Your engine speed (RPM) will increase by about 13% at any given vehicle speed and your gas mileage will be reduced by about 5%. Changing from 2.93:1 to 3.69:1 gears will approximately double all the effects listed in this paragraph.
I am thinking about upgrading my 4t65e-HD trans to 3.69 gear ratio. Anyone have any real world experience with this? Car will be a street driven car but will see lots of 1/4 mile passes. The 4t65e-hd is a different beast in a Fiero than in the W-Body cars so I am looking for some opinions on running the gear set in a fiero.
The only person I know who ran the 3.69 ratio (Don Kraus) currently has the fasted 3800SC setup in the 1/4. I am assuming this is the main reason why he has the record as his engine can't be making that much more power than the other modded 3800SC cars.
What's the torque curve like in your car? Can you extend the torque curve out into higher RPMs safely? What RPM are you shifting at now?
Generally, the bit you quoted is pretty much on point for what to expect. If you switch to the steeper gearing, you'll have to shift sooner, which might have you shifting into another gear. If you can also broaden the torque curve to operate safely into a higher RPM range at the same time though, you will see more advantage to it, as you'll be able to stay in the acceleration gears longer.
I ran 2mph quicker with the 3.29 gears over the 3.69 gears on XP cam setup. Tried 3500 stall with 3.69, then 3000 stall with 3.29 and lost a tenth in 60', then 3000 stall with 3.69 no change to 60' and ran 2mph slower. Finally settled on 3500 stall and 3.29 gears as the quickest/fastest combination it ran best 60' and mph. Your results may vary.
It is for an upcoming build. However,my old build with the 4t65e had 3.05 gears and ran just like a stock 4t65e-hd 2.93 gear trans.
The 1-2 shifts were @6Krpm and 49mph. The 2-3 shifts were @5800rpm and 92mph.
The new build plan is to have XP cam / 130# springs so I can shift up to 6400rpm.
Well, with 3.69, you'll be about 10 MPH slower at the 1-2 shift, and you'll be shifting a bit earlier. To shift at the same MPH with 3.69 gears as with the 3.05, you'll need to be able to shift at 7000 RPM.
Well, with 3.69, you'll be about 10 MPH slower at the 1-2 shift, and you'll be shifting a bit earlier. To shift at the same MPH with 3.69 gears as with the 3.05, you'll need to be able to shift at 7000 RPM.
I don't mind on the shift rpm or mph. I just want a faster 60' and ET.
I ran 2mph quicker with the 3.29 gears over the 3.69 gears on XP cam setup. Tried 3500 stall with 3.69, then 3000 stall with 3.29 and lost a tenth in 60', then 3000 stall with 3.69 no change to 60' and ran 2mph slower. Finally settled on 3500 stall and 3.29 gears as the quickest/fastest combination it ran best 60' and mph. Your results may vary.
Sooo, you have a 3.69 gear set you are not using? I assume 7/8" single chain setup.
I ran 2mph quicker with the 3.29 gears over the 3.69 gears on XP cam setup. Tried 3500 stall with 3.69, then 3000 stall with 3.29 and lost a tenth in 60', then 3000 stall with 3.69 no change to 60' and ran 2mph slower.
It was when I was non intercooled. First was 4t60 with 3.73 gears and 3500 stall, always 1.55-1.56 60' never faster than 12.0 at 111-112. Swapped to 4t65, best et was 11.806 at 115 with zzp 3000 stall and stock 3.29 gears, with only a 1.64 60'. Put in 3.69 gears and ran 1.62 60' but only 11.9-12.0 at 112. Swapped to 3500 stall and 3.29 gears. Ran 1.55-1.56 60' and 115 mph, didn't run any quicker than the 11.80 but did run 11.8x. Added intercooler and then ran 11.42 at 115 but did run as high as 117mph with XP cam but never quicker than 11.42
Found the 3.69 gears, one tooth looks like it got damaged in my parts bin, and pretty rusty from sitting for 8 years, I threw them out.
I would suspect some of don's et was from the cam he ran, and I suspect a higher stall converter than even my 3500. His car did bigger wheelies and 60' harder than my car ever did with the m90.
Scott (Darkhorizon) ran in the 10's but he doesn't seem to be posting of late. Perhaps he would share info on his his choice of FDR but IIRC he was running a stock 4T65eHD. BTW, anyone know where Scott is these days?
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
The other issue with 3.69 gears is that the higher gear ratios are more prone to chain stretch. Might not be an issue on a m90 car, but is a definite issue with high hp turbo setups. I originally put in my existing 3.29 gears with new chain with the turbo setup, against Dave's advice, the chain stretched within a month. I then followed Dave's advice and used 2.93 gears, they lasted the rest of the summer before stretching, including the 10.2 run. After a second chain failed the next year, and snapped with the 2.93 gears, I purchased a gm racing 3.11 gear and chain set no issues since. I believe Scott has a 3.29 GM Racing chain set.
The steeper gears have a smaller drive gear and Dave suspects the chain tries to ride up the teeth as the case flexes, causing the chain to twist and stretch. 2.93 gears have the largest diameter drive gear, making it harder for the chain to climb up.
Anything higher numerically than the 1:1 gear set in 3.29 gears also increases engine torque through the gear set, putting more stress on the clutches and hard parts in the transmission. 3.69 gears put 12% more torque into the transmission than 3.29 gears and 26% more torque than the 2.93 gears.
