I've got a 2400 stall converter in my 700r4. If I am doing a full throttle acceleration run, how much will my torque converter be slipping by the time I get to 5500 engine rpm?

Meaning if the engine is at 5500, will the trans only be at 5000 for example?

This plays into my selection of rear end gear ratio.

Thanks.

 

Stall speed is when both halves of the converter are turning the same speed. Think of one fan blowing air across another fan. The first fan is spinning faster than the second and eventually both are spinning at the same speed. The second fan however will always be going slightly slower.

Once you reach the stall speed, both halves are roughly turning the same speed. With a lockup converter, they do turn the same speed. The difference in speeds between the 2 halves is how much the converter slips.

Anything less than 10% is considered good enough. The converter in my race car slips at around 4% however early in the season I had different gears in the diff and the converter slip was much higher. How much torque put into the converter will also affect how much it slips. A 10" converter or a 12" converter that both stall at 2400 will have different amounts of slip.

To calculate how much the converter is slipping, you need to know tire diameter, finish line rpm, finish line mph and gear ratio in the diff.

From the tire diameter, the amount of revolutions per mile is calculated. The finish line rpm and gear ratio determine the maximum mph the car should run. The difference between the calculated mph and actual mph is the amount of slip is in the converter.

Optimal gearing depends on tire height and rpm range. To select a gear ratio for the diff, you pick a ratio that will have you crossing the finish line in high gear. If you use a shift light and shift at 5500, you should be in high gear, 1:1 (not OD) as you cross the finish line and the shift light should come on at or a little before the finish. The car isn't accelerating very quickly in the last few hundred feet. You're going fast but you're not accelerating or accelerating very slowly. The rpms will not climb very much in that short a distance.

Too deep a gear and you'll run out of rpm before you cross the finish and with the 700R4, will have to shift to OD. Too shallow a gear and you'll cross the finish before the engine peaks out in the powerband.

With a low 5500 limit, 3.73 gears would probably be best with 26" tall tires. The 700R4 can easily overpower the tires with the deep first gear. 4.10 gears will probably be too much unless you went to 28" tall tires.

 

Excellent answer. The piece of info I really needed was the 4-10%.

I assumed the weight of my car (3600 with driver) and the estimated horsepower (350 or so). I got the tire height from the goodyear website for iroc tires. That gave me the theoretical trap speed, which was about 110 mph.

Anyway, all I needed was the slip (which you have provided) to help me select the rear end ratio. 3.73 was going to just barely let me go 110 mph in third at 5500.

My engine is capable of more than 5500. The cam is 1800-6200 range in a 350, but I have a 327, so it is probably good to 6300 or 6400. However, as I understand it from my transgo shift kit instructions, the 700r4 has issues above 5500 rpm (I assume that's ENGINE rpm). So that was my limiting factor. I didn't install the transgo rev kit in the transmission since going all the way inside to the drums was more intimidating than the shift kit stuff I was comfortable with.

Thanks again.

 

Transmission doesn't have problems above 5500 but a TPI will. Not sure about a TBI.

Unfortunately, there's no easy way to determine exactly what gear ratio will work best in a particular car. I have 32" tall tires. Early in the season, I was trying out some 4.57 gears because a couple of different software programs suggested that to be roughly the best ratio. My car performs much better with 4.86 gears. The next step up would be 5.00 but I think that may be too much.

It's possible that if my converter stalled a few hundred prm lower, 5.00 may be a better ratio to use. My converter stalls at 6000. I shift at 7400 and the rpms drop back to 6400 after the shift. I go through the traps at 7400-7500. Using the 4.57 gears dropped my trap rpm to around 7200.

 

Again, good info and I appreciate the help.

But, I notice you have a powerglide. Are you really really really sure that the stock 700r4 doesn't have problems above 5500? I would have said the same thing you did untill recently. (seriously no offense intended here)

According to some of the information that came with my transgo 2&3 shift kit there are issues with losing "third apply" pressure above that level in a 700r4. That's why transgo included an additional "rev kit" with a whole bunch of springs that go deep inside the transmission (near the drums, clutches, etc). I didn't install that "rev kit" because i would have had to pull the whole transmission apart.

As for TBI rpm range, it depends more on the engine than the fact that it's TBI. (The TBI is almost like a carb in that sense.) The holley throttle body I have has adequate airflow (670cfm). And I have changed injectors, pump, and regulator to provide the needed fuel. I'm in the middle of custom tuning. If I have an rpm weakness, it will be in my intake manifold which is the GM performance TBI vortec manifold, intended for up to 5500 (on a 350 I assume).

 

you really can't give it a percentage. What you can do is notice the engine rpm and road speed right as it's starting to upshift into second. Then calculate what your rpm should be at that mph, and you can then see your slippage. In my '95 Z28, with a B&M TorkMaster2400, I was getting 1200 rpm of slippage. But while it was a heavy car with tall gearing and good power, that converter had been knocked off the workbench onto the floor before installation, so it may have suffered internal damage. In my '78 Camaro I had a B&M TorkMaster 2000, and it had just 400 rpm of slip. That was easier to verify because older TH350s didn't have lockup converters. Cruising on the interstate, the difference between no throttle and full throttle was again 400 rpm.
more engine output, more vehicle weight, taller gearing, and higher stall speeds in larger-diameter converters all contribute to more slippage, while weaker engines, lighter cars, deeper gearing, and lower stall speeds in smaller-diameter converters all contribute to less slippage.

 

If you really want to calculate converter slip accurately, you need to data log engine rpm vs driveshaft rpm during a pass. That is bar none, the only way to 100% accurately calculate it. That said, using your 1/4 trap speed vs rpm and having within the 4-10% range suggested by Stephen is the most comon way and is plenty accurate enough for all but the ultra-competitive heads up racing cars.

You do, however, need to go by the actual measured height of your tires, not by what the manufacturer thinks the tires are on your wheels at your pressure, at your vehicle weight, etc. With a soft sidewall, such as a slick, you also have to account for tire growth at speed.

The high-output pump spring is reccomend for any apps turning over 5500rpm with a 4L60. It does not require complete dissassembly of the trans, just pulling out the front pump and dissasmbling it. Dissassembly of the pump does require some sort of band to properly align the pump halves before re-torqueing. - You can install a lline pressure gauge (even just temporary/occasionally) and monitor your line pressure to find where your stock pump/spring falls off(pressure will drop or become irradicate). Depending on the year of the trans, it may indeed have the stock dual springs, which I have had no issues with up to around 6k.

 

The common formula for calculating rpm, the one that uses 336 as the constant, requires your tire diameter unloaded, as in not even on the car. But as my Z28 shows, 10% wouldn't be even close, in that case it was close to 20%. I have more examples, but those 2 should suffice to prove the point.

 

If you don't account for tire growth in your formula, then the roll-out would be way off, therefore the MPH would be off.

Again, it's still off w/o a data log that overlays engine rpm vs driveshaft rpm. - 1/8 and 1/4 mile MPH on a track is a calculation based on the amount of time it takes you to cover the last 60' of the track; it is not actual vehicle mph at the stripe.

 

street radials don't grow, and neither do drag radials, so the formula works. Now, I've seen Top Fuel and Funny Car tires grow, but this formula is for street and autocross, and autocross tires don't grow, either. So: trans ratio times axle ratio times 336 times mph divided by tire dia equals rpm. For example 1.000 times 4.100 times 336 (=1377.6) times 115 (mph) (=158424) divided by 26.00" (slightly worn 235/60R15 drag radial) would be 6093 rpm with a manual trans, (but if your time slip says 115, then you were really doing around 117-118 at the actual final stripe) and with a good converter, you'd be close to 6500 rpm, and my Z28 would've been at 7300.

 

Lots of good info here.

Note that the transgo part for high rpm I was talking about was not the pump ring & spring kit (P/N 700PKH), but rather P/N 7-CS the "Clutch Spring Kit". Both are apparently recommended by transgo for use in transmissions that go over 5500 rpm.

The clutch spring kit came with my 700-2&3 shift kit. I ordered the pump ring & spring kit separately. I installed neither, but I did install the rest of the 700-2&3 shift kit.

I am getting the message that the converter slip needs to be measured. However, I am wanting to pick a rear end ratio that is "about right" without having to go to the track. It's not really a track car.

Not counting the converter that fell off the workbench, it sounds like 10% is the higher rule of thumb I'm hearing. I don't have the tires yet. They will be 245/50R/16 (or whatever the standard IROC size is) street radial tires. When I looked them up they were 25.7" I will use 10%, 25.7", 5500 rpm, and exagerate my trap speed slightly (call it 110 mph when that's probably a stretch) in the gear ratio calculation to make sure I don't over-rev the trans in the 1/4 mile when/if I ever take it to the track.

Thanks guys.

 

When buying slicks, it's common to buy them as matched sets. My 32" tall slicks both have a circumference of 101" with 10 psi in them. A 32" diameter tire is only 100.5" circumference.

The circumference can be easily calculated to how many revolutions per mile (1/4 mile) the tire will turn. A 101" circumference will turn 627.3 revolutions in a mile (tire slip is not part of the calculations). based on that many revolutions per mile, the tire should be turning 147.7 MPH at 7500 rpm with 4.86 gears. Since my last 1/4 mile pass was only 143.1 mph, I have a calculated converter slip of roughly 3%.

The majority of my 1/4 mile run is also above the converter's stall speed so there's little to no slip recorded below the stall speed. I'm also in high gear by the 330' mark so most of the 1/4 mile distance is in high gear.

 

Don't grow much/a noticably measureable amount. You sling a maleable material at high rpm and it shifts according to load/speed.

I do agree that with a radial you only really need to account for at rest measured height, but again nothing is a perfect calcualtion w/o the expensive equipment. You can get well within reason though, especially for a street car.

327 - I would guess that what you're talking about is the 3-4 release spring assembly. In higher rpm apps, centrifical(sp?) force can overcome the release springs in the 3-4 pack causing it to try to engauge, thus burning the clutches some due to partial engaugement(not full apply force). With everything else correct, I have not had any known centrifical(sp? again) apply issues with any apps in the 6k-6500 range. North of 6500, the return springs become neccessary for reliability, especially for an aftermarket 3-4 clutch that is set-up as reccomended(much tighter than stock).

 

Isn't it easiest way to measure slippage at chassis dyno.It will show rearwheel speed and rpm.My previous "vette converter"(#8650919) slippage was at dyno ~5%.Now I have Edge 2800 rpm stall,havent yet measured it slippage.
edit:typo

 

easiest way to get the most accurate w/o data logging, yes. Rubber (tires) do not have a perfect coefficient of friction, and therefore are never truely 100% applied to the tires. Unless they have it tied down wrong, the amount is very negligible though, and wouldn't likely be an issue for the most competitive of heads-up type racer. Many of those cars would over-power most chassis dynos anyway.