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