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Converter slip VS Gear ratio.

Posted: Mon Aug 07, 2017 10:53 am
by AMXstocker1
how does gear ratio effect torque converter efficiency, it would seem like the lower your gear ratio the more efficient your converter is going to be since there is less load on the converter. and if this is the case could you see less rpm rise than expected with a gear ratio change due to converter efficiency increase? say going from a 4.10 to a 4.56 is about a 11% increase so if i was previously trapping at 6700 i should now be at 7437, unless the converter picks up lets say 2% efficiency from 9% slip to 7% slip does that mean i'll only see a 9% increase in rpm too 7300. i realize thats splitting hairs but it could be more or less or how would someone come to a educated guess about this type of thing. it could effect your choice of ratios if the converter change was big enough i guess is my point.

Re: Converter slip VS Gear ratio.

Posted: Mon Aug 07, 2017 6:16 pm
by Brian P
Yes, it will be more or less like that. The higher the RPM the "tighter" the torque converter will get. The other thing is that it's likely that the engine will be on a part of the torque curve where the torque is dropping off at higher RPM, which will further reduce torque converter slippage.

"How much" ... is hard to figure out.

Re: Converter slip VS Gear ratio.

Posted: Wed Aug 09, 2017 1:44 pm
by F-BIRD'88
For the converter to produce the expected design slip percentage thru the traps the cars trap rpm should be 1500 rpm or more ABOVE the converter's flash stall speed.
So gear the car to allow this based on 0 percent slippage..
rpm /gear ratio x tire diameter / 336 ='s MPH

1500 to 2000 rpm is about right. EG: converter stall speed ( actual, not advertized)
5200 rpm....trap rpm should be about 6700 rpm or more.
More is better. Remember this calc is based on 0 slip....

So yup changing the car's gear ratio from say 4.10 to 4.56 will raise the 1/4 trap rpm
by some 11.3% and if the converter slip percentage decreases a bit (slip efficiency increases) then yup the actual change in trap rpm will be a bit less, as a result.

To do the math correctly you need to measure your rear tires to see the real effective diameter
(Loaded rolling tire radius x 2) A 28" diameter tire is rarely actually 28".
Measure the distance from the ground to the axle centerline.....x2

I predict a few things on your car. (when going from 4.10 to 4.56 gears)
The tires are actually a bit shorter than 28" tall.
(loaded rolling radius x 2) Measure that... I'd guess 27.5" actual....measure that

The car's 1/4 mile trap MPH will increase a little bit.

The converter slip loss percent will decrease a bit.

The converter will suddenly seem to work a lot better.
The car's ET will improve. The MPH will improve just a hair, too.

The chance that going to a tunnel ram W 2 carbs will improve the cars performance will INCREASE as now you are using the higher rpm "big top end " that a tunnel ram allows.
(same as if you did the Dual carb on a adapter would also)
If you are not reving it high enough the Dual carbs do not realize their high rpm advantage.

Going from 4.10 to 4.56 will be dramatic.

Re: Converter slip VS Gear ratio.

Posted: Wed Aug 09, 2017 2:08 pm
by F-BIRD'88
Another thing you can do to increase the apparent converter slip % efficiency
on the car is to DECREASE the car's aerodynamic drag.
Something as simple as a front end chin spoiler can really make a difference.
Car MPH will increase..Converter slip % will decrease.
It's like free horsepower.

The origional OEM Javelin AMX cars had a Aggressive front chin spoiler.
it is there for a purpose. To direct air around the car instead of under the car
to reduce areo drag and increase car down force at speed.
All the Trans AM racer AMX cars had a very aggressive chin spoiler too.

Where is your front Chin Spoiler? All the air is getting trapped under your car,
slowing it down. You can fix that. The faster you go the more it matters.
The more little details you take care of like this the faster the car will go
and the better the converter will perform.

Re: Converter slip VS Gear ratio.

Posted: Thu Aug 10, 2017 3:25 pm
by AMXstocker1
The chin spoiler is in the works and we are fixing a piece of plexi glass to block off the recessed grill.

Re: Converter slip VS Gear ratio.

Posted: Sun Oct 22, 2017 3:28 pm
by Straight8
I went from 4.30 to 4.86 and converter flash was 400rpm less.picked up a little et and gained 500 rpm at the stripe with mph basically the same. 1/8 mile car.