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Best Gear For AUTO Turbo Car?

7K views 27 replies 10 participants last post by  justinschmidt1 
#1 ·
I know everyone seems to love the 3.27s with turbo cars but I think its safe to say an automatic car is a completely animal.

I think one of the biggest benefits with the manual cars is the ability to stay in boost longer before shifting, thats not really an issue with an auto car of course since you just stay in boost the whole time after the initial spool.

Also, the automatic gears are longer gears internally so I dont see any problem creating load there with the 3.73s.

So my question is basically this.

Is there a point to go towards a 3.27 gear with an auto turbo car?

My car is a 4.6 2v with a 67mm turbo .96 a/r and currently has a built 4r70w with a 3600~ converter and a 3.73 gear in the rear end.
 
#3 · (Edited)
Lol.....well that's not always true...if i was running a 1:1 first gear in my trans surely there would be maasive amounts of load with any rear gear in yhat situation.


Anyone else with some input?

I just don't think that an auto needs the 3.27s lol .I would actually like to go down to a 3.27 since its street car actually but I'm just curious
 
#4 ·
Im hoping to be putting the auto back in my car soo. I have a set of 3.27 gears but Im thinking of going on up to 3.55. Most of the guys with 3.27s on the turbo forums are running 3 speeds or sticks. With the wide ration of the 4r I would think 3.55 would be better. But I have never ran either one so Im subscribing to see what everyone has to say. it would be nice to get some real info, not just "thats what everyone runs so its best". On a side note. what converter are you running?
 
#5 ·
I get bashed when I say this, but I think a 3.73 is the way to go if you are going to utilize rpm over 6K.

3.27 build boost better...cool. But more boost and better boost at 3500 RPM doesnt even compare to the power at full boost at 5500-6500.

If someone has a car that is pretty quick....turbo and 3.27....do they want to cross the traps at 4000 rpm or 6500 rpm?

That's my input. Have fun!
 
#7 ·
Yea, ive noticed that everyone seems to bash you for that opinion but I agree with you....im going to be revving to 6500 rpms or so. If I put 3.27s in the car I feel like I would go through the traps at like 4k haha

Id really love to see someone with a true comparison and actual 1/4 mile numbers to prove something.

Like run the car with 3.73s and 3.27s without changing a single thing.
 
#6 ·
I have a pushrod car so my resluts may not be relevent. I ran 3:73 on my turbo car with a T56 and a C4. The 3:73 gear was far too low for my stock block turbo car with either trans. I am now running a c4, 3:27's and a 28" tire. The car is much faster on the street and the track. I only got to make one pass so far and the track was not prepped but I trapped 124mph. I wasnt able to datalog the pass but did see the shift light come on so I know I was past 5500 rpms.
 
#14 ·
  1. This has been discussed 487 times if you do a quick search in this forum you should find many detailed discussions.
  2. "Best gear for auto turbo car" isn't really a valid question. The gear depends on a number of variables and can't be answered generically.
  3. There are some fundamental principles about how a turbo car works that many people miss in this forum and they are fundamentally different then NA, Nitrous and SC cars
  4. Things to consider:
  • Fundamentally speaking you want a turbo car to be "loaded" at all times. Fewer gears and more time in gear generally results in a better result. If you review data logs on a PG vs. a C4 you can clearly see that the PG is making max boost for more of the time going down the track because there is only a single rpm drop and boost drop from the shift. A 3 speed is going to do this twice, and a stick transmission is going to do it 3 + times. I have even seen that the crispness of the shift on the PG results in less boost drop then my PA C4 because it results in a more consistant load on the motor.
  • Top end of the track mph, this is ultimately what you have to consider being able to get to. In a faster car, you want to be towards the top end of your powerband.
  • # of gears, there is a big difference in how cars launch in a 2 speed vs a 3 speed.
  • Converter slip. There is quite a bit of variablility in how much converters slip, it can be anywhere from 5-15% which will have a dramatic effect on your top end mph and required gear. (for stick cars this should be a non issue as there should be little if any slippage in the clutch at the top end of the track).
  • The launch, depending on how a car spools and what transmission is in it you will be impacted differently by the gear.
I used a 3.27 with my C4. It worked perfectly in terms of the steeper 1st gear in the 3 speed, top end of the track at around 7600 rpm and a fairly significant amount of converter slip I was getting with a PI converter.

Once I installed the PG the 1.80 first gear made it much more of a challenge to get out of the hole. Combine that with the fact that the converter I have from UCC is tighter up top and has less slip has resulted in my changing to a 3.55. The car gets out of the hole easier, and up top I go through the traps at around the same rpm because of less slip. The gear change actually reduced my converter slip slightly because a lower numeric gear further loads the converter and will make it slip more. Right now I'm between 160-165 mph, if we can make more power next year I could be forced to put the 3.27 back in to ge the car into the low 170's. Although I'm dealing with a signficant amount of power and mph the concepts apply equally to a slower car.

My $.02 is all of you guys need to stop academic debates on one gear vs. another one and spend more analyzing your specific setups and logically and mathematically determining what gear to use. That would be a better use of the discussion.
 
#18 · (Edited)
Is it too much to ask for people not to post irrelevant comments that do nothing to help the thread?

Maybe because it's easier/ more cost effective to ask of anyone else has done the testing rather than swapping 2 sets of gears ans going to the track multiple times......everyone doesn't live 5 minutes from a track, have endless supply of.money, or the ability to install rear end gears correctly.


Seems like quite a ridiculous comment to make on tour part.

I will be doing my own testing when i complete the car but was just curious about people's results with autos
 
#19 ·
You didnt ask, you are preaching your thoughts on why the 3.27 is not the right gear choice, despite those of us with running, driving, auto, turbo cars, telling you otherwise.

Please go back and re read your first post.

Then, go do your own testing, and report back your findings.

And FYI, its not that hard to learn how to set up a rear end properly.
 
#22 · (Edited)
Well thats because im looking for some real proof in the form of 1/4 mile times to prove a 3.27 is better than say a 3.55 or 3.73....I dont understand how nobody has any hard data but everyone is so fast to say 3.27s are better.

So basically its safe to assume you guys believe my car would be faster with 3.27s in the 1/4 vs 3.73s running a 600 rwhp lockup 4r70w ?

The cars already got 3.73s so I will see how they feel and maybe swap to a 3.27 eventually.

Maybe I will have to look into doing the rear end gears
 
#20 ·
Racing Calculators - TCI® Auto

Do the math yourself. I am putting a 3.55 in my car, even considered a 3.73. With the amount of rpm that my pile will be turning and a lock up converter, I'll probably run out of power before I run out of gear.

Also, when you're doing your math, keep in mind that a 28" tire will not be 28" on the other end of the track.
 
#21 ·
Also, when you're doing your math, keep in mind that a 28" tire will not be 28" on the other end of the track.
A dr will not grow like a bias ply will. Keep that in mind when calculating.
 
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