Need your Insight Please!

Thomas A

Registered
Seeing how a lot of guys around here have had a lot of success getting these cars down the track in a short amount of time, I was hoping someone could offer a little insight on why I am having such a hard time doing it myself. I seem to have narrowed my issues down to 2 areas, but am unsure what is causing the one and not sure how to correct the second.

My car has made the power on the dyno and ran the mph at the track to support much lower ET's that I have been able to manage. I'm running a Vortech S-Trim at 8lbs, 4.10's, 2600 stall converter, and the other supporting mods that go along with them. The tune is dead on as far as air/fuel, and the car runs great. It is a conservative tune, so timing is pretty mild, and it shifts by 6400rpm.

The car made 392rwhp on a DynoJet, and has run a handful of passes at 110-112 mph. My ET's have all been right around 13.0 - 13.1 at two different tracks, both ~1000 ft above sea level. I really feel the car should be running a 12.5-12.6 with it's current power and gearing. Here is the slip from my quickest ET run:

60' 2.047
330 5.586
1/8 8.428
mph 87.13
1000 10.916
1/4 12.970
mph 110.72

It seems the first issue is my 60' times. I have never been able to pull a sub 2.0 short time! The track was hooking very well on Sunday and it didn't seem to spin at all off the line. I tried launching from 1000rpm to flash the converter, and launching at 1800-2000 rpm with it stall up against it. It really didn't seem to make much of a difference. All my other short times for the day were 2.1's. What I did notice was it felt like the car bogged pretty bad off the launch. It would fall on its face for a half second, and then take off like it should. The best way to describe it would be like launching a manual trans car on sticky tires at too low of a rpm. Any idea what to look for as to the cause of this. Do you think it is in the tune, or something mechanical?

Second issue is the 1-2 shift. It is blowing the tires off really bad on the 1-2 every time down the track. It is bad enough you can feel the car stop accelerating for half a second until it grabs traction again. I think I went too aggressive on the JMOD. Any ideas for fixing this short of getting another valve body and going to a milder setting with the JMOD?

If I could get my short times down to a 1.8-1.9, and hook up the 1-2 shift, I'm confident that I could drop .4 - .5 off of my ET. Any suggestions or thoughts?

Thomas
 
What kind of tranny you got in there? And I assume you've got limited slip (it's not listed). What kind of blower?
 
The sixty foot is key IMHO.

2600 stall with 4.10s? Seems to me that will not allow you to quickly spool up into the power band as well as a higher stall, say 3500-3800 or even more. If you look at your dyno sheet(if it goes down that low), you should see the torque difference between 2600 and say 3500 or more. The torque multiplication in first needs higher revs.

I am confident you have the power in the upper rpms based on your 1-2 shift description. I would not do anything to the J-mod but instead fix the traction issue. What tires are you running?
 
I'm running 16" Mickey Thompson Drag Radials. It's not like I'm trying to do this on street tires. As I mentioned, they hooked great off the line. No issues there.

Yeah, LSD rear. I wouldn't put 4.10's in an open carrier! It seems to be working fine as well.

I agree a looser converter would make a drastic change. I'll try and get my dyno graph posted up here as well.

Thomas
 
The dyno graph would be helpful for comparison. Your 330 foot time is in the same range as mine when I get a 1.8x sixty foot time. When my sixty foot is closer to the 2.0 range my 330 foot times are 2-3 tenths slower.

I have a set of M/T DRs and quite honestly I am not as pleased with them as I thought I would be. But if you can get a quicker sixty foot you may find the 1-2 hit not as hard on them as they are now?

I would examine the converter issue first, then see if traction on the 1-2 shift is still an issue.
 
I'm not interested in shifting at 7000rpm on a completely stock motor. The car is tuned very conservative for longevity and safety. I'm fine with that. I don't think that is need to get to a 12.5 given my trap speeds.

I'll post the graph here as soon as I get my other computer back up and going. I had to format it today.

I agree the 60' is the key. That is interesting that you are running the same 330 time as I am, but cutting 1.8 short times. Maybe we can figure out why that is. Could be the difference in stall speed?

Thomas
 
That is interesting that you are running the same 330 time as I am, but cutting 1.8 short times.

Here's a sampling of 60' vs 330' made back in September...

1.882 5.533
1.890 5.577
1.894 5.569
1.885 5.582
1.905 5.581
1.883 5.541
1.898 5.554
1.895 5.548
1.886 5.540
1.872 5.535

... compared to some 1.9 sixty foot times in July...

1.954 5.695
1.946 5.697
1.969 5.702
1.962 5.686
1.966 5.701
1.942 5.678
1.951 5.668
1.959 5.694
1.946 5.691
1.936 5.672
1.937 5.651
1.931 5.645
1.953 5.690

Both at the same track with the same setup. July's DA averaged 2400' while September's DA averaged 1000'.

Now if I go back to July at Joliet with about the same DA but in 'street trim' (stock chrome 10 spokes all around and M/T drag radials), I was cutting 2.0 sixty foots...

1.974 5.898
2.019 5.833
2.023 5.831
2.021 5.815
2.036 5.831
2.025 5.869

... and significantly lower 330 foot times.
 
Your torque converter is the problem(bogging off the lunch). Stock mark VIII torque converter will flash 2600 on supercharged mark VIII. In about 3 years I had 4 different torque converters in my car. The first one was the stock so I will not talk about this one but the second one was 3000-3200 stall 9.5" from Art Carr and it bogged off the line with it, the third one was 5000 stall non lockup converter and car 60' like a big block but had no top end power and the driveability was very bad with it and finally my current Converter is about 4200-4400 stall from Dirtydog and I love it. My sixty foot times are very good(mid 1.6 sec) and the car has crazy top end.
I would recommend you to go with 3500-4000 stall torque converter and maybe shift little higher like 6700-6800 so you can use your power band little better. Personaly I would not go back to anything less then 4000 stall for supercharged Mark VIII as those engines mixed with centrifugal blowers are torque less high reving engines and driveability with 3500+ stall torque converter is still very good.With the power you have and 3500+ stall torque converter your 60" times should drop to high 1.7 or low 1.8 and thats good for another .4 maybe .5 tenths in 1/4 mile.
 
I agree with a higher stall converter. It is amazing what they can do for your time slip :) I really liked my PI 3600 stall single disc for the street with my stock block set-up. You felt the lock up, but it was not so harsh that it became annoying at low speeds.

The PI 3600 triple disc in the car now snaps your neck when it locks - especially at low speeds. Aric had to bump up the lock up speed because at lower speeds, it was not pleasant at all. So far, data log trans temps are still good, so over heating the trans does not seem to be an issue for the higher speed lock up.

I do not understand all the mechanics and tuning of the torque converter, but Aric said he does not have as much control with the triple disc clamping force as with the single disc TC. Another way to describe the difference between a single and a triple disc TC - a dimmer switch vs on/off switch.

I would suggest a single disc 3600 stall TC if you plan to keep the stock block, unless you don't mind the harsh lock up - then get the triple disc. You and I had pretty much the same set-up except for the TC last year, and I managed low 12's shifting at 7150 rpm.
 
Well higher stall converter is obvious... I'm just saying with the same exact setup you have now you will be a few tenths quicker revving out to 7000rpm.
 
I actually can't imagine having a supercharged Mark VIII and short shifting at 6400rpm. If you are worried about the long term durability of the motor you shouldn't have put a blower on in the first place. I can't even shortshift with my NA 4.30 geared Mark VIII, it just feels wrong... now in my old 95 LSC I only revved to 6600rpm but it didn't have Beehive valve springs.
 
Just because I modify my car doesn't mean I want to blow it up. The motor sounds very smooth up to about 6400, and not much past that on the dyno, so that is all the further I choose to go with it. I've hit 7K a few times working on the tune, but it's not something I wish to do every time. You must have a lot more money to spend on cars than I do. The few extra tenths at the track are just not worth it for me.

That sounds pretty reasonable about the TC. That is kind of what I was thinking, but wanted to get some other opinions. Thanks for the suggestions guys!

Thomas
 
60ft = better 1/4 mile times

4200-4400 dirtydog.... is what you want. Go get one, I promise you wont be disappointed.
 
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