How much should RPM's drop on gear changes? I may be overpowering my converter

[Steve]

I have a 67 Firebird/LC2 hybrid that I am just beginning to get dialed in. The motor is built exactly like the motor that was in my full weight T that ran 10.50's and the Firebird is lighter at only 3175. I originally swapped a LS1 into the car and then realized everyone was swapping LSx's in everything so I swapped over to the LC2. I used the same converter I had built for the LS1. I spec'ed the converter for a 3400 stall on a 400whp LS1 with a 150 shot of nitrous so I thought it would be in the ballpark for the LC2 and thought I would give it a try before having it restalled. I took the car out tonight for the first time with high boost and I thought I was late on the shifts because it seemed I could not keep the car off of the shift lite. I then went back and checked the PL files and found that I had the shift lite set at 5300 rpms but the motor was making it 5700 by the time it shifted then only dropped back to 4900 after the shift then the shift lite was right back on. I have had a few 10 second turbo Buicks and I have always run the multi disc Vigilante converters locked so there was plenty of RPM drop with them! The car has ported irons, TE-45A(at 25 PSI) and the normal supporting mid to high 10 second parts, turbo 400 and 3.23 gears with a 28" tall drag radial. I am not opposed to having the converter reworked as I thought this might be needed(I thought it was going to be too tight!:redface but I want to make sure I am on the right track before I go thru the hassle and expense. Thanks for the help.

 

[jasjamz]

So its dropping from 5700 to 4900 on the shifts? I get a 800-1000 rpm drop so far. How does it spool? Have you made any track hits to compute converter slip? I have to do more testing but I know mine is not to my liking on the launch off the foot brake. Its a 9.5 n/l Ptc.

 

[Steve]

So its dropping from 5700 to 4900 on the shifts? I get a 800-1000 rpm drop so far. How does it spool? Have you made any track hits to compute converter slip? I have to do more testing but I know mine is not to my liking on the launch off the foot brake. Its a 9.5 n/l Ptc.

I don't have any track tuns yet. This car should be a mid 10 sec car like it is and I have a few mid 10 second turbo Buicks and I knew something was different the first time I ran it thru a couple of gears. It seemed I was late on the every shift and I thought I just sucked at driving at first!:biggrin: I kept backing up the shift lite each run to try to lower the shift point but then I watched the logs and it seems to be reving so fast that by the time the shift lite came on and I hit the next gear it was over where I intended for it shift. I backed up the shift lite a little more on the that run the ****fpoint was OK but when it shifted the shift lite blinked off for a second then was right back on. It has a turbo 400 which has taller gears than a 200r4 and has 3.23 gears with 28" tall tires so I would think it would have even more RPM drop and than a normal turbo Buick with more gear. It spools ok and seems fine everyhere else and seemed OK with 15 psi of boost but when I added the Alky and started cranking the boost it started to seem different. I went to the track last weekend with a buddy in first track trip ever in his TTA(high 10 second car) and listening to his car after the shifts is night and day different to mine. His car's RPM's dropped way down after the shifts like I am used to(which makes sense because he has a zero pump Viglilante like I have always run) where my car seems to be about 5K or above the entire time I am at WOT.

 

[bison]

How many rpm it drops doesnt matter. What matters is if its spending most of its time where it makes the most power. If you re-evaluate you will find the combo you have probably makes the most power between 5200 and 5300rpm so in reality your converter may be a little too tight since its dropping to 4900. If you turn the boost up higher you will further increase the power band and hopefully get the engine closer to the peak power area. Getting the engine to sit as long as possible where it is most efficient will increase the road horsepower a lot and the car will e.t. much better. You may want it shifting at 5500-5600 and having it drop to 5200 then run it out in 3rd since it should have enough power to run well past 5600 with the light car and your combo. The converter i have in my black car flashes to 5500rpm@24psi and thats right where it propels the car like a rocket. The engine spends at least 80% of its time within 200rpm of that. It shifts at 6100. So im only getting a 600rpm drop. On the top its slipping less than 4% at 142mph. If mine dropped to 5000 the car would slow down. Dont be fooled into thinking you need to have all this rpm drop especially when you are running a hyd roller, with a small ci engine. Higher average road horsepower with traction will make the car quicker. Being as light as you are you should have no problem tweaking that combo into the 9's without running over 30psi.

 

[Steve]

How many rpm it drops doesnt matter. What matters is if its spending most of its time where it makes the most power. If you re-evaluate you will find the combo you have probably makes the most power between 5200 and 5300rpm so in reality your converter may be a little too tight since its dropping to 4900. If you turn the boost up higher you will further increase the power band and hopefully get the engine closer to the peak power area. Getting the engine to sit as long as possible where it is most efficient will increase the road horsepower a lot and the car will e.t. much better. You may want it shifting at 5500-5600 and having it drop to 5200 then run it out in 3rd since it should have enough power to run well past 5600 with the light car and your combo. The converter i have in my black car flashes to 5500rpm@24psi and thats right where it propels the car like a rocket. The engine spends at least 80% of its time within 200rpm of that. It shifts at 6100. So im only getting a 600rpm drop. On the top its slipping less than 4% at 142mph. If mine dropped to 5000 the car would slow down. Dont be fooled into thinking you need to have all this rpm drop especially when you are running a hyd roller, with a small ci engine. Higher average road horsepower with traction will make the car quicker. Being as light as you are you should have no problem tweaking that combo into the 9's without running over 30psi.

Thanks for the reply. The car does seem to be pulling well it just seemed totally different than the other cars I have put together with similar combos(except converters) so I was concerned but your explanation makes sense. I will take it to the track and give her a try and see how it does, but I am going to have to improve my driving skills to keep it where it needs to be!:redface: I would love for the car to get into the low 10's with reasonable boost!

 

[jasjamz]

How many rpm it drops doesnt matter. What matters is if its spending most of its time where it makes the most power. If you re-evaluate you will find the combo you have probably makes the most power between 5200 and 5300rpm so in reality your converter may be a little too tight since its dropping to 4900. If you turn the boost up higher you will further increase the power band and hopefully get the engine closer to the peak power area. Getting the engine to sit as long as possible where it is most efficient will increase the road horsepower a lot and the car will e.t. much better. You may want it shifting at 5500-5600 and having it drop to 5200 then run it out in 3rd since it should have enough power to run well past 5600 with the light car and your combo. The converter i have in my black car flashes to 5500rpm@24psi and thats right where it propels the car like a rocket. The engine spends at least 80% of its time within 200rpm of that. It shifts at 6100. So im only getting a 600rpm drop. On the top its slipping less than 4% at 142mph. If mine dropped to 5000 the car would slow down. Dont be fooled into thinking you need to have all this rpm drop especially when you are running a hyd roller, with a small ci engine. Higher average road horsepower with traction will make the car quicker. Being as light as you are you should have no problem tweaking that combo into the 9's without running over 30psi.

So, is mine too tight? I bet it would be great off a transbrake, but I hate it off the footbrake. I get the 800-1000 rpm drop with boost in the 20-22 range. Do you think the rpm will drop less with more boost, and will that cause it to slip more up top?

 

[bison]

So, is mine too tight? I bet it would be great off a transbrake, but I hate it off the footbrake. I get the 800-1000 rpm drop with boost in the 20-22 range. Do you think the rpm will drop less with more boost, and will that cause it to slip more up top?

It will drop less with more boost. What most dont get is that they need the converter loose enough down low to be able to spool and get up on the pipe and they need the converter tight enough (or less in some cases) to flash stall to the correct rpm for the cam/turbo/heads all together. You cant have the converter ok in one of those areas and expect decent performance. The more torque in the higher the stall. The same converter in my black car at 18psi only stalls to about 5000rpm. The engine still made 523whp at that boost but as boost got increased to around 24-25psi it really dropped it right where it wants to be around 5500 and the difference in acceleration is tremendous. Whenever you order a converter its important to order for the power level you will run. If you run more power you actually need a tighter up top converter than you would for less so you dont drive over it. One nice thing is that increasing the boost typically raises the entire power band which is good unless the valves start floating. This way if the converter is spec'd properly it will continue to work almost linearly as the boost is thrown at it.

 

[Nick Micale]

It will drop less with more boost. What most dont get is that they need the converter loose enough down low to be able to spool and get up on the pipe and they need the converter tight enough (or less in some cases) to flash stall to the correct rpm for the cam/turbo/heads all together. You cant have the converter ok in one of those areas and expect decent performance..............

The only way you can achieve the "right" converter stall for a given combination is to re-stall or change the converter if it is not performing correctly.

Based upon your data, the converter is too tight according to your RPM drop.

For many years, we dealt with converters too tight and would loosen them, usually more than once.

We then found it better to start with a looser converter, and tighten it to optimum.

Since we, and others, have developed builds in certain ranges that work well, with that experience we can get a converter very close the first time.

What I think the smart thing to do is to contact and deal with someone that has done a similar build to yours to help you reach your optimum build.

Personally, it took me about 4 converter changes before all worked together. Since then I have duplicated in 3 other builds, and was able to do a few similar builds also.

After dealing with MANY converter companies and "experts", I have not found any better source for TURBO converters and advise than Dusty and Chris who do the PTC converters.

 

[bison]

The only way you can achieve the "right" converter stall for a given combination is to re-stall or change the converter if it is not performing correctly.

Based upon your data, the converter is too tight according to your RPM drop.

For many years, we dealt with converters too tight and would loosen them, usually more than once.

We then found it better to start with a looser converter, and tighten it to optimum.

Since we, and others, have developed builds in certain ranges that work well, with that experience we can get a converter very close the first time.

What I think the smart thing to do is to contact and deal with someone that has done a similar build to yours to help you reach your optimum build.

Personally, it took me about 4 converter changes before all worked together. Since then I have duplicated in 3 other builds, and was able to do a few similar builds also.

After dealing with MANY converter companies and "experts", I have not found any better source for TURBO converters and advise than Dusty and Chris who do the PTC converters.

Agreed. Especially with the 9.5" PTC converters. Id rather have one a little loose myself. Guys think they can run the same converter at 20psi as at 28-30. Not going to work optimally. Another reason you get what you pay for with converters. A cheap converter will not be able to do all the things a converter should do when looking for optimum e.t.'s

 

[Dusty Bradford]

So, is mine too tight? I bet it would be great off a transbrake, but I hate it off the footbrake. I get the 800-1000 rpm drop with boost in the 20-22 range. Do you think the rpm will drop less with more boost, and will that cause it to slip more up top?

If yours is a little sluggish for your liking on a footbrake launch, it can be loosened to help with that. You will loose some rpm drop but it may not hurt. I'd suggest turning the boost up to where you want to run it at and see what the converter is doing.

What do you not like about the footbrake launch?

 

[jasjamz]

I will get some video and logs for you Sunday Dusty. I will be near if not at 30psi and on race fuel no alky. It hooks but needs 15-18 psi launches to get 60 fts in the 1.6xs. If I leave at anything under 12 psi it will not 60 better than 1.8xs. On the street I leave at 5 psi and it rolls out then comes alive after about three car lengths. Turbo is a 7080 HPS, Sold the billet 6765. On the foot brake at 0-1psi its at 2800-3000. box numbers are PTC-9-5-700-27-20-0

 

[Dusty Bradford]

I will get some video and logs for you Sunday Dusty. I will be near if not at 30psi and on race fuel no alky. It hooks but needs 15-18 psi launches to get 60 fts in the 1.6xs. If I leave at anything under 12 psi it will not 60 better than 1.8xs. On the street I leave at 5 psi and it rolls out then comes alive after about three car lengths. Turbo is a 7080 HPS, Sold the billet 6765. On the foot brake at 0-1psi its at 2800-3000. box numbers are PTC-9-5-700-27-20-0

Was this originally planned for the 7080 or the 67?

 

[jasjamz]

the 7080 .85 housing 8.5:1 235 iron head mild hyd roller car is 3900 with me in it. Does it sound like there could be another choice?

 

[WSLN 6]

I think my PTC is a 27-18. If I left at 5psi on the street I would destroy my 325's....

 

[jasjamz]

If I could put enough power down to do that I'd be happy. Don't get me wrong, when it gets into the 20 psi range its making some power but before that. Its like traction control with 275/60 MT/DRs. When I had the 6765 .85 h cover it would be kinda slow to 10psi then flash to 30 with traction in exactly one second. I contemplated a transbrake but that would still leave me with high boost launches to get the car back in the low 1.5s.

 

[Dusty Bradford]

If I could put enough power down to do that I'd be happy. Don't get me wrong, when it gets into the 20 psi range its making some power but before that. Its like traction control with 275/60 MT/DRs. When I had the 6765 .85 h cover it would be kinda slow to 10psi then flash to 30 with traction in exactly one second. I contemplated a transbrake but that would still leave me with high boost launches to get the car back in the low 1.5s.

Sounds like what has happened is you just need the extra rpm to get that turbo up and going. With extra rpm, comes extra boost unless your using an AMS 1000 or something to control launch boost.

The launch boost being higher isn't a bad thing. It can be misleading. With a very loose or inefficient converter you don't have a lot of boost at a given launch rpm. For example 4600 rpm and 10psi may net a 1.50 60'. Now a tighter converter may only net a 1.60 60' with a 10psi launch but the rpm may only be 4000. So to get the car to 60' it may need more rpm which in turn will build more boost.

If you were to loosen the converter to get a lower boost launch you could run into issues with the converter slipping down track.