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Converter size...... help please!!

 
blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/04/13 09:27 AM

Hi all, I'm new here & need some advice please.
I'm having trouble selecting the size of converter for my build/car.

I have a 77 corvette, weekend street cruiser.
388ci SBC, 9.7:1scr, mp122 mini roots blower.

After installing the blower I decided to change the cam, I just had no top end pull it would die off @4500rpm.
The new cam selected is a Comp NX274H 230/244 @ .05 113LSA. Hopefully this will pull to 6000rpm.

I have a 2200 stall off a mild 350. I was hoping to use this; I was told this would act more like a 2600 stall on my bigger blown engine.
When cruising around town @60mph I sit on 3000rpm.

Will I get away with using this converter?!? If not what size would you suggest?  Thanks.  

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redneckjoe69
I mow my lawn and find Chevys
Posts: 2337
Joined: 03/10
Posted: 06/04/13 01:48 PM

see what these guys recommend for ya.    they'll get right back to you.

http://www.ptcrace.com/spec-sheet-pm-23.html  

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skyeking
I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 06/04/13 05:15 PM

Hi there,
 If your heads and associated parts are good then
 6000 rpm is O.K. for max power..
  I would be looking at 2800/3000 stally depending
  on diff ratio. Check your compression ratio as well
 Hard cams and low comp is a dead loss.  Good luck...  
skyeking

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blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/04/13 07:37 PM

Yeah I think my diff has 3.08 gears but not 100% sure.
I would like to go to 3.55's but my car already cruises @3000rpm @60mph!

Because my car sits on 2800-3000rpm when crusing at 50-60mph, I was thinking a converter 2800 or bigger would be a bad choice. Would a 2600 converter fit my cruising speeds & the cam?

I'll check out the likk also, thanks Redneck!  

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skyeking
I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 06/05/13 05:17 PM

Hi there,
 It all comes down to final drive ratio.
 mix and match trial and error is costly
 but you may find someone who has been there
 and done that with a similar project.
  I know from experience that with an unblown
 motor that a 2800 + is a must for a fairly
 heavy car.  Good luck with the project.....  
skyeking

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Dave632
I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 06/05/13 07:00 PM

As you start making more power and torque you actually need less converter.
There is a good chance that the blower would increase your stall speed to 2600 rpm.
I ran a 10" converter in my last 632 that would probably only stall to 3200 on a NA small block, the 632 could stall that at over 5000. It would blow right thru that converter with 2 stages of nitrous on it. What that means is that as soon as I hit the throttle with 2 stages of nitrous on it would spin to about 7500 and hit the rev limiter.  
[[SUPPORT AMERICAN CARS, STOMP A RICE CAR TODAY.]]

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blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/05/13 07:32 PM

Say my engine does stretch the stall out to 2600, Do you think the cam (NX274 230/244 @.05) & my shitty 3.08 gears will handle it?  

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 06/05/13 09:33 PM

What size tires are you running?

3000rpm @ 60mph doesn't sound like 3.08s to me.

What transmission are you running?  
68scott385 68scott385 68scott385

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 06/05/13 09:39 PM

With 3.08 gears, no overdrive, and a 26" diameter tire, 3000rpm is closer to 75mph. All else being equal, taller tire equals more speed and same rpm. Obvious statement but trying to get idea of combination.  
68scott385 68scott385 68scott385

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blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/06/13 03:57 AM

Mate, your bang on!!!

I have 26" tyres, th350 trans.
Your not the first person to say that I dont have 3.08 gears.

Ok, when I lift 1 rear tyre of the ground I can spin it easy, open diff right?!?
When I lift both wheels off the ground they both spin in the same direction.
I can only assume its an LSD, its also got the LSD tag on the diff.
Anyway with both rear wheels off the ground, 1 full turn of the wheel gives me just over 3 turns on the drive shaft.
Even with just 1 wheel off the ground, when I turn the wheel twice I get just over 6 turns on the drive shaft.
This tells me 3.08 gears, correct  me if im wrong.
The stamps on the diff casing also telll me it came from the factory with 3.08's (not that thats anything to go by).

Is it possible that the 3rd gear in the trans is not 1:1???  I'm sure my tacho is correct. Its got me buggered!!!  Any Ideas???  

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blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/06/13 03:58 AM

Any other ways I can check the ratio?  

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 06/06/13 06:57 AM

From your description it sounds like a limited slip differential to me. Here are some valuable formulas that will help you determine exactly what you have.

rpm = (168 x gear ratio x mph) / tire radius

2587 = (168 x 3.08 x 60) / 13

2985 = (168 x 3.08 x 75) / 13

2892 = (168 x 3.73 x 60) / 13

mph = (tire radius / 168) x (engine rpm / gear ratio)

75.37 = (13 / 168) x (3000 / 3.08)

62.23 = (13 / 168) x (3000 / 3.73)

gear ratio = [(tire radius x rpm) / 168] / mph

3.87 = [(13 x 3000) / 168] / 60

Assuming that your transmission is in good working order and not slipping, I think that either your rear gears have been changed or the speedometer gearing is off. If you know the number of teeth on the speedo drive gear and the number of teeth on the speedo driven gear I can plug that into a formula to see if they are right for the gear set.

The most accurate way to determine rear gear ratio, because we've all been fooled by the tire/driveshaft counts, is to divide the number of teeth on the pinion gear into the number of teeth on the ring gear. This method requires some work but it is fail-safe. IMO, 3.08, 3.23, and 3.42 are all too close together to be accurately determined by driveshaft revolutions compared to tire revolutions.  
68scott385 68scott385 68scott385

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 06/06/13 07:12 AM

Here is another formula that will help, at least it helps me, make sure the speedo, tach, and police all agree.

tire diameter = [2 x (section width / 25.4) x ( aspect ratio / 100)] + rim diameter

26.10 = [2 x (235 /25.4) x (60 / 100)] + 15...for 235/60/15

27.99 = [2 x (275 /25.4) x (60 /100)] + 15...for 275/60/15

This formula comes in handy when determining what rpm & mph changes will occur with a change in tire size, if that is something one might be concerned with.

Once, for five years, when the speedo did not work, this was the only way I knew how fast I was going with the tires on the vehicle. In my defense the speedo did not work when I bought the vehicle and the PO said it had 4.11 rear gears and mph was verified by driving next to a friend in a new vehicle on the freeway at the speed limit, on more than one occasion. GM, according to the parts man, did not mate 4.11 rear gears and th350 transmissions.  
68scott385 68scott385 68scott385

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blowme77
I love my Chevy Chevette!
Posts: 7
Joined: 06/13
Posted: 06/06/13 09:31 AM

Thanks scott, you rock!!  

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 06/06/13 09:40 AM

No problem. It comes from years of not having anyone to ask and having to do the research myself to get answers that make sense to me. Math is your friend. Grin  
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