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Old 08-17-2006, 10:34 PM
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Need Help Rpm Trouble!!!!!!!!

HELLO AGAIN GUYS
OK first off for the guys out there that haven't seen my past post and dont know about my car,,,,, it is a 55 chevy running a 406sbc 515 lift cam, pop up piston's tunnel ram w/dual 450 Holley's also 2.02 valve 76cc heads. backed with a new jegs 350t trans w/3500 stall and also a 390 gear...


Ok here is my problem i shift my car at 5500 rpm but when it goes from 1st to 2nd gear the rpm only goes from 5500rpm to 5100 rpm and from 2nd gear to 3rd gear it goes from 5500 rpm to 5300rpm!!!! im only getting 200 rpm difference when shifting............and the trans isnt slipping...

IT seems to me that the car is not pulling as much as it should,,,,
I think a automatic car when shifting threw the gears should least drop around 1200rpm between 1st to 2nd and at least 1000 to 800 rpms from 2nd to third,,

what im trying to say is shifting out of 1st at 5500 i think it should drop at least to 4500 on the tach then pull back up to 5500rpm then shift out of 2nd at 5500 in to 3rd and the rpms should drop to at least to 4700rpm then go ahead and run out all off 3rd gear u know...

If you guys can give me any feedback on this it would be great, i have been trying to figure out whats wrong with my car only dropping a couple of rpms while on the gas shifting, it just not feeling as strong as i know it can be.... IM guessing it might be the stall or the tunnel ram doing these but im not for sure.. the stall is only 2 months old...... what is your opinion and/or what is the rpm change in your car when you guys are shifting??????? thanks for any kind of help

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Old 08-18-2006, 06:26 AM
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Your converter stall speed could be rated for a 350 - that 406 will have quite a bit more torque, which could push the stall rating up to 4000 maybe 4500 depending. That stall speed is only a guideline for a motor with an "average" torque.

I would guess this is what's giving you your issue. Can you try shifting a little higher to see what happens?? Seems like that cam would want to spin a little higher, although you didn't give any durations for it.

good luck.
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Old 08-18-2006, 10:53 PM
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hello.... it seems to me that 5500rpms is all my motor wants to pull being a 406,, i have had it to 6000 rpm before but it didnt seem to like it much,,, i would hope to think its not the stall but i guess im leaning that way,,, does anyone think that have that tunnel ram on my car could cause this issue???

On an average when drag racing having a 3500 stall and a 373 to a 411 gear when shifting what are your guys rpms dropping to at shift point???? thanks for any help.
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Old 08-19-2006, 01:26 PM
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Intake manifold will NOT cause that rpm drop. You have either a converter problem or a slipping transmission.

How do you KNOW that the trans is not slipping at 5500 rpm?

RPM drop is controlled by the transmission gear ratios, not rear end ratios. If your 1-2 ratio is 70% then the rpm should drop 30%. 5500 rpm times 70% equals 3850.

At 5500 rpm a 3500 stall converter should be 95% efficient on a shift.

Last edited by xntrik; 08-19-2006 at 01:32 PM.
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Old 08-19-2006, 02:18 PM
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Quote:
Originally Posted by xntrik
Intake manifold will NOT cause that rpm drop. You have either a converter problem or a slipping transmission.

How do you KNOW that the trans is not slipping at 5500 rpm?

RPM drop is controlled by the transmission gear ratios, not rear end ratios. If your 1-2 ratio is 70% then the rpm should drop 30%. 5500 rpm times 70% equals 3850.

At 5500 rpm a 3500 stall converter should be 95% efficient on a shift.
HELLO, i really dont think at all that the trans is slipping because the car is still pulling for 1 and i just bought a new trans from jegs and it seems to act just like the one i took out, dont get me wrong that dont mean that the trans from jegs is good but i think i could feel if the trans was slipping that bad u know.. If you dont mind let me ask your opinion on this since it seems you know what your talking about,,, with a 406 sbc standard ratio 350t trans/ 3500 stall with a 390 gear what do you think the rpms will drop when shifting at 5500 from 1st to 2nd and also 2nd to 3rd? you said up above it should drop about 30% which sets the rpms around 3850, my self dont get me wrong im new at this but i think that is a little 2 much drop with a 3500 stall and your foot to the floor? i was thinking shifting at 5500 it should drop to around 4600 or somewhere around there... but of course my car only drops about 200 rpms.... plz let me know what you think on this ok....

Thanks for the info you posted above
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Old 08-19-2006, 09:10 PM
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First, you said you bought a new trans at Jegs and it is doing the same thing as the old one. So you installed a new trans and a new converter and it still is the same problem..... mmmmmmmmmm?

You are correct, it is not pulling as it should since it is just spinning against the trans/converter.


http://www.phoenixtrans.com/html/faq.html

A TH-350 has ratios of 2.52, 1.52, 1.00.

2.52 to 1.52 is 60%

1.52 to 1.00 is 66%

Assuming zero slippage at the converter or transmission, shifting from first to second at ..... 5500 times 60% = 3300 rpm.



That's why in the old days everyone wanted a close ratio 4 speed stick shift so the drop stayed at about 78%.

A 3500 stall converter spins the engine 3500 against a locked driveline (transbrake)..... when that driveline is spinning at 60 or 90 mph the actual converter slip will be less. Of course the engine is screaming at a higher rpm and making more power, but the driveline is not locked, it is spinning.

Your present rpm after the shift is way too high. If you run 7-10% past your horsepower peak the drop should be to about the torque peak rpm. That makes the engine pull smoothly and accelerate the car well.

If you are making hp at say 5200 and you shift at 5600 the drop would be to 60% (3360 rpm) plus the converter slip. An engine making peak hp at 5200 probably makes the torque about 3400.

If you have a lot of cam and are "short shifting" and depending on a converter to make up the difference, all you are doing is wasting your time and energy. Engines and transmissions need to be "made for each other".

IMO most people run too much (too loose) converter trying to launch and actually run slower in the second half and lose trap speed because of excess slip.

If your trans really is OK, then you need to tighten up that converter a lot. Like you said, 4000 would definitely be better.


www.readershotrods.com find your ride here?

Last edited by xntrik; 08-19-2006 at 09:50 PM.
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Old 08-19-2006, 11:21 PM
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Quote:
Originally Posted by xntrik
First, you said you bought a new trans at Jegs and it is doing the same thing as the old one. So you installed a new trans and a new converter and it still is the same problem..... mmmmmmmmmm?

You are correct, it is not pulling as it should since it is just spinning against the trans/converter.


http://www.phoenixtrans.com/html/faq.html

A TH-350 has ratios of 2.52, 1.52, 1.00.

2.52 to 1.52 is 60%

1.52 to 1.00 is 66%

Assuming zero slippage at the converter or transmission, shifting from first to second at ..... 5500 times 60% = 3300 rpm.



That's why in the old days everyone wanted a close ratio 4 speed stick shift so the drop stayed at about 78%.

A 3500 stall converter spins the engine 3500 against a locked driveline (transbrake)..... when that driveline is spinning at 60 or 90 mph the actual converter slip will be less. Of course the engine is screaming at a higher rpm and making more power, but the driveline is not locked, it is spinning.

Your present rpm after the shift is way too high. If you run 7-10% past your horsepower peak the drop should be to about the torque peak rpm. That makes the engine pull smoothly and accelerate the car well.

If you are making hp at say 5200 and you shift at 5600 the drop would be to 60% (3360 rpm) plus the converter slip. An engine making peak hp at 5200 probably makes the torque about 3400.

If you have a lot of cam and are "short shifting" and depending on a converter to make up the difference, all you are doing is wasting your time and energy. Engines and transmissions need to be "made for each other".

IMO most people run too much (too loose) converter trying to launch and actually run slower in the second half and lose trap speed because of excess slip.

If your trans really is OK, then you need to tighten up that converter a lot. Like you said, 4000 would definitely be better.


www.readershotrods.com find your ride here?

thanks again bro for all your info it is really helpful.....just wanted to let ya know that actually when i changed the tranny last week i used my same converter that i had in my other tranny but it only has about 500 miles on it, as far as the stall goes, it seems to me that it is acting pretty well, for instance when i flashed the converter its going about 2700 to 3000 and it seems to be right for a 3500 stall i do believe......i really dont know if that will help you at all or not but with that said i have a real dumb question for ya..... when you say tighten up the converter what exactly do you mean? im guessing go to a smaller converter?

one more quick question for ya ive been kickin this around also the tunnel ram that is on my 406 is rated 4500 to 10500 rpm. i am going to be changing this soon because of course it isnt matched right with my engine, in my opinion, when switching the intake and carb it should help my motor produce a little bit more hp and torque since it would be closer to being matched to the cam and stall and whatnot...........i would like to get your feedback on that as well. like i said before i appreciate your time and effort on leaving me feedback on my questions so far, (im just a young rookie tryin to learn somethin) lol
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Old 08-19-2006, 11:45 PM
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I woud guess that your 3,500 RPM converter is acting like a 5,100 RPM converter at the higher RPM. That manifold won't let the engine produce much power down low, so the engine will stall out low like you observed. The engine finally gets on its power peak and the converter then won't hold it above 5,000 and it flashes very high. Maybe try running 2nd gear up to 4,800 or so and nail it and see if it immediately flashes higher. This is a little easier to feel than trying to figure out what's happening when there's an upshift involved.
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Old 08-20-2006, 12:06 AM
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Quote:
Originally Posted by lust4speed
I woud guess that your 3,500 RPM converter is acting like a 5,100 RPM converter at the higher RPM. That manifold won't let the engine produce much power down low, so the engine will stall out low like you observed. The engine finally gets on its power peak and the converter then won't hold it above 5,000 and it flashes very high. Maybe try running 2nd gear up to 4,800 or so and nail it and see if it immediately flashes higher. This is a little easier to feel than trying to figure out what's happening when there's an upshift involved.

That is a very good point,,,, you know i have did that before. I've taken the car to around 4500 to 4600 rpm and nailed it and it actually pulled really good like it was supposed to it pulled up to 5500 and shifted into third. but once i was in third just like as stated before it jumped to 5500 almost immediately. it seems when i am having all my problems is just when im dead to the floor first to third gear, it gets up to the high rpms in first and more or less other than just a couple hundred it just stays there. but like i said if i nail it at 4500 in second it pulls to 5500 just as it should????? thats whats messing me up about this car its acting like no other car i have ever had or rode in. ill just keep doing some trial and error and try and figure it out when i do ill let you guys know.

thanks alot for the info guys, believe it or not people haven't heard of this particular problem before......
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Old 08-31-2006, 12:42 AM
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hello 1 more time lol

would having bad valve springs keep my car only dropping 200 rpm when i shift???? i think its the stall like alot of you guys but i heard this could do it also??? any truth in this thanks
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