2.76 open to 3.55 Sure Grip...expectations too high?

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Righty Tighty

Righty Tighty

Blame it on the dog
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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?
 
It's all got to work together. A stock 318 2bbl won't feel drastically faster going from a 2.76 to a 3.55, but if you took a stock 340 car and went from 3.55 to 2.76s it would seem way more drastic!

In my '77 D100 with a warmed over 318, I went from a 3.23SG to a 3.91SG. I noticed right away that the truck was peppier and felt better going through the gears. A few weeks later I went back to 3.23s because I do occasionally do some interstate driving, and while I could tell the difference, it wasn't what I would consider drastic.

I think it always depends on the whole package. Big cams and high compression need more gear. Stock "low" performance engines do not.
 
No!

Big jump, almost as much as going from 4.56 to 3.55 in reverse. As I did.

You should smile! I love my 3.55 gears for driving and some good kick! Almost wish I would have gone 3.73 but not 3.91.

3.55 is a good compromise gear if your motor is up to it.

Course I have a 9” convertor in the auto as well.
 
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If you don't notice night and day, it wasn't running right to begin with.
 
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?
 
I swapped cam, went from 2v to 4v carb, and HE268 cam and headers on my 318. After the engine mods I still had the 2:76 7 3/4 in it and it was still a dog until it got up into the power band of the cam. I built an 8 3/4 with 3:55's and the difference is night and day, all I can say is the grunt factor is way more than what it was. I too have not changed the converter, when I do I suspect it will be even more noticeable from a standstill.
 
Righty Tighty said:
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?
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Looking at your sig. and not being familiar with your exact build;
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
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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.
 
rumblefish360 said:
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.
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This is incredibly helpful! I knew that signature would come in handy someday.

I think this is what happens when my goals change from the beginning of the build to driving the car. Originally, I just wanted a reliable, snappy grocery getter. Once it was running and I felt like I had more of a grasp on things, I wanted to see how much more I could do with it.

I’m starting to feel at ease because it’s likely the cam that’s limiting me.
 
@Righty Tighty Glad you find it helpful. I don’t think it is totally the cam. While small, it fits your description above for a snappy daily driver getting groceries.

Not knowing your actual compression ratio, a possible problem, but a small one since it adds efficiency and power everywhere, I made self self would have gone about the build a little differently using that cam.

A better cam would have had a narrower LSA. That would lead to a much more racier idle. I do t know if the heads are stock or not, but stock 302’s are best on stock mileage getter engines. Once ported out for a maximum power return, there really a good head with upgraded valve sizes. However the drawback to that is expense paid vs the power return gain against aluminum heads.

You can build a kick *** all iron. 318. However, the added pluses of aluminum parts on top of a stout short block will hit home runs over just adding good parts on top of a stock short block even if it is in good condition to start with.

It’s a cruddy balancing act.

Don’t blame the cam so fast. While it isn’t what I would use (but have done so ...) it is capable with the right tune behind it. I used a similar Crane Hyd (272/284-@050-218/224??? On a 112, I’ll leave a link...) that did Eventually power the car to the high 14’s. Stock tire traction is a huge issue.....

A friend had the same cam in his drag only Duster that ran 12’s. The car was seriously lightened. But he didn’t go nuts doing so.

My fun tool around project I did t get to Pursue very far. It had to go when things go a bit tight. All well, there’s always another MoPar to play with somewhere.

The goal of this sick around project was just to see how far I could take a junk low compression (‘79) 318 with basic bolt on parts and a cam. It was fun. Enjoy the read and my wasting money to keep busy thread... LMAO!

rumblefish's Duster project
 
That's a pretty big jump.

Most any time I do that, the car now chirps the tires shifting to second, no mater the engine.
 
Righty Tighty said:
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?
Click to expand...
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.
 
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.
 
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Righty Tighty said:
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?
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Since your speedometer is broken, put your phone on Google maps. They give your speed, as well as navigation.
 
Or just do some math and memorize the relationship between certain key speeds and their associated rpms.
Like;
32mph in second gear at steady throttle, with 25.5" tires, will be about 3700 rpm. or
60 in Drive might be 2800
No tach either? Stay off the road?
 
318willrun said:
, 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.
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You hit the nail on the head right there.
but when you said this;,
the 2.76 gears are actually at a better ratio.
I take that to mean at a higher power to the road number.
To OP; read post 12 a few times, until you get it.

Gears are torque multipliers. At any given rpm, adding gear will add torque.
But because the gears change the roadspeeds at which the torque is put down, it will change your rate of acceleration.
Your power did not change. The thing that changes is the amount of time it takes to go from point A to point B, during the run, because ;
1) bigger gears allow the engine to spool up faster, and
2) therefore it spends less TIME in each gear,
3) while putting down more average power per unit of that time.

Too late now; but you shouldda done a few before and after time-trials. Then you wouldda seen it.

If you want more off-the-line punch, with a given engine size, your choices are; a higher tall, and/or higher cylinder pressure; or earlier closing intake valves, which trap more pressure, usually at the cost of top-end power.
 
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Loosen the distributor slightly and move it around until you find the highest idle, then retard it as far as possible while still maintaining the high idle. If it doesn't run great, then change your vacuum advance line to an off idle port and reset the timing the same way. If the vacuum line was already at an off idle vacuum port, then move it to full vacuum, reset the timing like I described, and test drive again. Use which ever vacuum port works best.
 
AJ/FormS said:
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.
Click to expand...

Thanks, AJ. I know you know what you're talking about, but I thought the 302 heads would give slightly better compression than stock due to the closed chambers? Unfortunately I didn't CC the heads when I built the engine, so I don't have an actual CR to go on, but it was the general consensus that I'd be running just above 9:1. It is a reman '73.

What about the cam numbers do you think is wrong? Maybe I thought all the numbers given were at .050.

The transmission guy I got the TC from said it was around 1700-2000 stall, and I was okay with it being stock...until now haha.

I'll try your test tomorrow and report back.
 
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?
 
rumblefish360 said:
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?
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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.

sccachallenger said:
I feel the torque converter is key.
Did it seem to stall higher with the 2.76?
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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.
 
The 1973 318s were advertised at 8.0 Scr, but generally. come in around 7.8 after a reman. The closed-chambers will bump that up, but I think 8.3 is about where yours might be. At 2700ft elevation, that's gonna hurt the bottom end power. The Wallace calculator predicts about 124 psi cranking cylinder pressure.
I had a freshened 1973 smogger , that I used to use as my winter engine, stock long-block, with a small-port intake, a 4bbl,fresh-air,and headers. I put that engine in every winter 5 or 6 years running, and it never had the same trans or gears in it. But when it had a a 904, then it also had a 2800TC. and when it had the 2800, then it usually had 3.23s. And in that combo, she was a ripper. and here's why; Below is a 5.2 Magnum graph, so much more powerful than either of our engines. But look at the shape of the curve. At 1800, it shows about 270 ftlbs. Let's arbitrarily drop that to 70% for your combo; so 189. And your TC is gonna multiply that by say 1.6 at Zero mph, so 302. Now the trans has a first gear multiplier of 2.45, so now 741. With the rear at 2.76, then 2045 into the rear axles.
But with 3.55s then 2630. Checkmark that.
Lets up the TC to 2800
On the graph, that looks like 295 ftlbs. Again at 70%=207ftlbs. But this time I'll estimate 1.7 in the TC@zero mph, and still 2.45 low and back to the 2.76s; I get 2374 ftlbs. So now with the TC alone, you are half way between 2.76 and 3.55, as compared to the 1700 stall. With 3.55s, this combos to 3054! and now 49% greater than where we started.
Ok but I know you're thinking, that don't sound like all that much,right? Well hang on; 189ftlbs at 1700rpm is 61hp. Whereas 207 at 2800 is 110hp, using the estimated numbers, this is an increase of 80% in horsepower.
Ok now I'm not saying the 2800TC will increase your delivered power to 110hp@2800. I'm saying you could be looking at a power increase of 80%@zero mph, over an 1800TC. And that is a really big deal. You understand the difference?
This is the bandaid for the low cylinder pressure.

power-318-gif.gif
 
But this is why the proper (stall in a) torque converter is important.
 
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