I just swapped rear ends. I went from a 7-1/4 2.76 open to an 8-3/4 3.55 Sure Grip. Now, I understand that I’m not driving a race car, but I was certainly expecting a more drastic change upon acceleration. Am I mistaken?
Looking at your sig. and not being familiar with your exact build;I just swapped rear ends. I went from a 7-1/4 2.76 open to an 8-3/4 3.55 Sure Grip. Now, I understand that I’m not driving a race car, but I was certainly expecting a more drastic change upon acceleration. Am I mistaken?
I’m not sure what you were expecting. Your cam is very mild with very small restrictive (as cast) ports of the 302 head. The cams 112 LSA should have had added compression from the pistons in order to negate the lower torque output from the wider LSA. If there was a tire diameter increase, this would slow down the car as well. Added stall to the converter would help the launch. But it should not be needed for a cam that size.71 Scamp, 318LA with 302 heads, .454/.468, 256/262 @ .050, 112* LSA, Performer intake, Eddy 1406, TTi long tube step headers, A904 to 8.75 3.55 Sure Grip
This is incredibly helpful! I knew that signature would come in handy someday.Looking at your sig. and not being familiar with your exact build;
I’m not sure what you were expecting. Your cam is very mild with very small restrictive (as cast) ports of the 302 head. The cams 112 LSA should have had added compression from the pistons in order to negate the lower torque output from the wider LSA. If there was a tire diameter increase, this would slow down the car as well. Added stall to the converter would help the launch. But it should not be needed for a cam that size.
Yep, many will make it seem like you'll drop a second and a half by going from 2.76 to 3.55's. Unfortunately, not the case at all. I've played a ton of gear ratio games. Gears are a multiplier to bottom end grunt. Gears help get a car off the line and get into the power band quicker. Gears also help take the drop from one gear to the next. Gears help the initial launch, but remember, when you shift into second gear with 3.55's, the 2.76's are just rapping out 1st gear and at that point, the 2.76's in 1st gear are better geared than the 3.55's in second gear. Same with the 2-3 shift. So, at points during the run, the 2.76 gears are actually at a better ratio. Gears are more important to low torque motors, or motors that have high flow hp set up that start making hp around 4k rpm's. But you don't take a stock 318 4bbl that runs 15.7's and go from 2.76 to 3.91's and run a 13.9. Not how it works. In one car with a mild 360 I went from 4.30's to 3.91's and ran the exact same MPH and ET. Exactly the same. I then went down to 3.23's and a slightly smaller cam and only lost .3 tenths.I will say that I noticed some difference, but it doesn't feel like I'm driving a completely different car, as some folks had suggested to me when I told them I was doing the swap. The car has a 318, 4bbl, mild cam, headers, 904. The only thing I wish I upgraded that I didn't when the engine was out, was the converter.
The speedo is broken, so all I really have to go by is how my *** feels in the seat.
I may take a look at the timing to double check where I'm at, but it seemed happiest around 14 initial and 32-34 total. Should the timing be adjusted with different gearing?
Since your speedometer is broken, put your phone on Google maps. They give your speed, as well as navigation.I will say that I noticed some difference, but it doesn't feel like I'm driving a completely different car, as some folks had suggested to me when I told them I was doing the swap. The car has a 318, 4bbl, mild cam, headers, 904. The only thing I wish I upgraded that I didn't when the engine was out, was the converter.
The speedo is broken, so all I really have to go by is how my *** feels in the seat.
I may take a look at the timing to double check where I'm at, but it seemed happiest around 14 initial and 32-34 total. Should the timing be adjusted with different gearing?
You hit the nail on the head right there., but remember, when you shift into second gear with 3.55's, the 2.76's are just rapping out 1st gear and at that point, the 2.76's in 1st gear are better geared than the 3.55's in second gear. Same with the 2-3 shift. So, at points during the run, the 2.76 gears are actually at a better ratio.
That jump is plus 28.6%, which is huge. But as said; off-the-line you are still fighting a lo-stall TC, 2700 ft elevation, and lower than stock, cylinder pressure; You cannot make a silk purse out of a sow's ear.
put a 2800TC in it, get the tune right, and never look back.
I'd like to see the result of a compression test.
Yur sig says 256/262/112 @.050.
Ima thinking that is just wroong,lol
Is your engine also a 71? Do you know the exact compression ratio of it
IF that is a 9.2 motor, and
If that cam is really 256/262 advertized (not .050),
then; it will feel like a stock 8.0 smog engine below about 3200, until she begins to get up on the cam. With 3.55s and 25.5 tires, that will be about 23 mph.
That 2700 ft elevation is stealing nearly 15 psi off your cylinder pressure and 10% off your sealevel performance. And the stock TC with this combo at this elevation is probably stalling at under 2000rpm. You cannot bandaid those problems with gears.
But Try and remember how it felt in first gear at 3200 rpm with the 2.76s when you smashed the pedal. Now go do the same with the 3.55s. With 28.6% more Torque Multiplication you should see a HUGE increase in the RATE OF ACCELERATION.This is a fair comparison, because it takes the TC out of the equation, and it puts the cam up on the bottom of the power curve. What you are comparing, is the RATE of Acceleration, or how long it takes to go from 3200 rpm to shift-rpm in a particular gear. If you get wheelspin, the test is invalid. Go to the next gear.
No tach either? Stay off the road?
Let's just say I wish my waistline was as thin as my wallet... I like the plan to continue tuning it; I think it needs it anyway, and I could use the practice tuning carburetors.Just keep working the tune of the engine (carb) and distributor. We (and you) will just have to work with what it has. If you start in with changing parts out, then the ball game changes. How thick is your wallet?
How do I know when it stalls? Torque converters (and stall speed) mystify me, no matter how much I try to read up on them.I feel the torque converter is key.
Did it seem to stall higher with the 2.76?