RPM drop vs. rear gears vs. stall speed, please help

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Matt@RFR
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Location: Paradise, Ca, USA

RPM drop vs. rear gears vs. stall speed, please help

#1 Post by Matt@RFR » Sun Sep 26, 2010 5:12 pm

A little background on me; My dad bought a used race car last year, and neither of us have ever had a drag race car before. He used to drive sprint cars, and I used to work at a race engine shop, so we are not new to cars overall, but some of the fine points of drag race cars are coming harder than I hoped they would.

The car:
125" hard tail 23T altered
1550 pounds WITH driver
PTC spragless 10" converter
PTC 1.82 powerglide
4.11 rear gear
32" tires

The engine:
SBC, 4.04" bore x 3.75" stroke (385ci)
12.5:1 on E85
36º locked out timing
Reed solid roller, 258/267 @ .050", .600"/.640", 106º LSA
Stock replacement crank and rods, so absolute MAX RPM is 6800.

The problem:
Please see the attached image below. The run below was:
1.250 60', 5.58@121.75 1/8th, 8.86@147.38 1/4. This is an average run for us, but on better tracks the car does 1.20's in 60', but at very close to the same ET and MPH. We are still in the process of getting the car to launch, but started seeing a surprising trend as the data become more clear (remember, new driver and new car, so it took quite some time to get good solid data). I'm betting the problem is obvious to you guys, but to be clear, after the shift, the engine is dropping waaay too far and stays at or around the stall speed of 5500 RPM for 3.5 seconds. We changed the transmission fluid this weekend, and it definetely smelled like it got hot, but we do not have a transmission temperature gauge in the car, although we are more than willing to put one in, atleast for testing.

This run, dad shifted at 6200 for various reasons... new driver, new crew chief (me) and some MSD equipment not being all that accurate. Next time out I will try different pills to get him to shift closer to 6800 or so, but I highly, highly doubt that will have much impact on our converter issue. The reason for that is, with various online calculators, it looks like even at a shift point of 6800 RPM, the engine wants to drop clear down to roughly 3800. That obviously aint going to cut the mustard with a 5500 RPM stall, but on the other hand, I doubt a 3800-4000 RPM stall will work very well either. Am I completely off base with that assumption?

I'm not sure what to do from here. The 4.11 gear was put in because the car used to be run on 1/8th mile tracks and had a 5.33 in it if I remember correctly. We went with the 4.11 because of our RPM limit of 6800, and a guess of 160 MPH max.

So, would anyone be kind enough to help us solve this?

Thanks a lot,
Matt
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Marty Chance
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Re: RPM drop vs. rear gears vs. stall speed, please help

#2 Post by Marty Chance » Mon Sep 27, 2010 7:37 am

several things are wrong here and they all point to the converter.
1 10" is way too large of fluid coupling for a nat asp small block with 3.750 stroke
2 spragless is NOT what you want for 1/4 mile racing (will always make heat)
3 the flatline you see after the shift is mostly due to converter too tight but also because of spragless

the cure is to get an 8" converter for your combo (not spragless)
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Matt@RFR
Posts: 5
Joined: Sun Sep 26, 2010 4:31 pm
Location: Paradise, Ca, USA

Re: RPM drop vs. rear gears vs. stall speed, please help

#3 Post by Matt@RFR » Mon Sep 27, 2010 9:46 am

Thank you very much Marty. I have some follow up questions if you don't mind:

Why is the 10" too large in our case? (just trying to learn)

You said the converter is too tight, meaning it stalls too low, correct? I assumed the opposite because, I thought, if the stall was lower, the engine wouldn't be sitting on the converter, but rather drop down to whatever it needed to and still be above the stall speed. What am I not understanding?

Thanks for your time,
Matt

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CDR Performance
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Re: RPM drop vs. rear gears vs. stall speed, please help

#4 Post by CDR Performance » Sat Oct 02, 2010 8:35 am

Marty Chance wrote:several things are wrong here and they all point to the converter.
1 10" is way too large of fluid coupling for a nat asp small block with 3.750 stroke
2 spragless is NOT what you want for 1/4 mile racing (will always make heat)
3 the flatline you see after the shift is mostly due to converter too tight but also because of spragless

the cure is to get an 8" converter for your combo (not spragless)
Good info.

The large converter is causing too large of an rpm drop and the motor can not recover.
Then the converter is just flashing at 5500 and waiting for the chassis/drivetrain to catch up. It does about 3.5 seconds later. There is also a loss for multiplication.
The smaller unit is the way to go.
We run a 10" unit behind a 3400# Street strip car with 1400hp.
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Re: RPM drop vs. rear gears vs. stall speed, please help

#5 Post by jmarkaudio » Sat Oct 02, 2010 9:55 pm

Marty gave you good info, you need an 8" that will stall at least 5500, and with that combo a spragless will kill you in the 1/4. One more note, you run E85, what carb do you run? I'd like to see the AFR after you get a converter for it. I mess a bit with carbs, and I am building an 1150 for E85 at the moment. Might be able to give you some tips to help.
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