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Discussion Starter #1
Hi everyone !

I bought a 1990 CORVETTE last week, it has the ZF6 with the 3.33 rear gear ratio, an adjustable fpr and a 58 mm BBK throttle body.

Last time I was driving on 2nd gear and pushed the throttle pedal to the floor, the engine reached the 6k rpm very quickly without any breath problem and without rpm regulation.

On my last 85 auto the engine was out of breath at approximatly 4300 rpm dispite of the gear. I know it is due to the TPI intake that is too restrictive.

is there a max rpm regulation on the tpi ?

The 1990 TPI seems very powerful compare to my 85 is there normal ?

(the 85 had the 3.07 rr ratio, adjustable fpr, k&N air fliter with open lid, 180° f thermostat and muffler eliminator)

my apology for my bad english.
 

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both the more aggressive roller cam timing and the rear gear ratio favor the 1990 TPI,the 1990 has better flowing heads and a better exhaust, over the earlier flat tappet cam and 3.07 rear gear of the 1985, the 1985 had a 230 rated peak hp the 1990, had a 245hp rating
the 1990 has at least a 250rpm advantage in where it first accesses its power curve, the stock tpi intake, cam and heads tend to start restricting potential at about 4500-5000rpm
the cam used head flow intake and runner cross sectional area , reag gear, coverter stall and exhaust restriction,all effect the results

http://forum.grumpysperformance.com/viewtopic.php?f=71&t=555

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=333

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=5537

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=148

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=2203

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=5078
 

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Discussion Starter #3
Thank you for your quick answer.

so the all combo of the 1990 is far better than the 85 one.

I read some topics but didn't find the rev limit of the 1990.

Is the TPI map equiped with a rev limiter ? if so what is the limit RPM ?

the exel-file on the grumpysperformance is very usefull ! :thumbsup: thanks
 

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Thank you for your quick answer.

so the all combo of the 1990 is far better than the 85 one.

I read some topics but didn't find the rev limit of the 1990.

Is the TPI map equiped with a rev limiter ? if so what is the limit RPM ?

the exel-file on the grumpysperformance is very usefull ! :thumbsup: thanks
keep in mind that the O.E.M. hydraulic lifters, cam duration, and valve train limit valve control to about 5800-6000rpm and the head flow also limits the engines power potential to below that rpm range
 

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Discussion Starter #6
Hi !

I have done some test with the corvette and a GPS to verify the exact speed. I have calculate that the rear end ratio is 4.27.

That's could be why the car is pushing so hard. and more; the engine speed goes up quite slowly under the 4000rpm and it push very hard after 4000rpm to the 6K line.

At 2000rpm in 6th the car is at 70 mph but it seems like the engine is too low; I cannot accelerate in 6th.

I guess it's possible that the camshaft has been changed. I would like to test the car in a dyno.
 

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Hi !

I have done some test with the corvette and a GPS to verify the exact speed. I have calculate that the rear end ratio is 4.27.

That's could be why the car is pushing so hard. and more; the engine speed goes up quite slowly under the 4000rpm and it push very hard after 4000rpm to the 6K line.

At 2000rpm in 6th the car is at 70 mph but it seems like the engine is too low; I cannot accelerate in 6th.

I guess it's possible that the camshaft has been changed. I would like to test the car in a dyno.

My 86 has a .62 OD with a 3.07 rear and it runs at about 2k in high. a 4.27 rear would have you spinning at almost 3k at 70 depending on the final drive ratio.
 

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Discussion Starter #8
I'm sorry I'll use metric unit but I can convert if you want.

_ engine speed : 2000rpm
_ wheel development (275/40ZR17) : (17x25,4)+2x(275x40/100))xPi=2047,7 mm
_ ZF6 6th gear ratio : 0,50
_ Rear end ratio 3,33 4,27

so:

([(engine speed/gear ratio)/rear end ratio]x60) x (wheel development/10^6)= veh speed

(((2000/0,5)/3,33)x60)x(2047,7/10^6)= 147,6 Km/h
(((2000/0,5)/4,27)x60)x(2047,7/10^6)= 115,1 Km/h

so when my engine is at 2k rpm my car should be at 148 Km/h while it is approximatly at 115 Km/h.
 

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Discussion Starter #10
yeah, that's true ! :agree:

what I wanted to say is that the engine has less torq in lower rpm band than my last 85 but a lot more in the higher band.
 

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And since it´s a 1990, are you sure the rpm gauge is showing the correct rpm?
Many 1990´s are known for problems with incorrect rpm gauge.

Sooo, are you sure you are at 6000 rpm?

Try hooking a scanner to it to see if it´s showing correct rpm.
 

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Discussion Starter #12
And since it´s a 1990, are you sure the rpm gauge is showing the correct rpm?
Many 1990´s are known for problems with incorrect rpm gauge.

Sooo, are you sure you are at 6000 rpm?

Try hooking a scanner to it to see if it´s showing correct rpm.
HI !

I'm sorry I know there is a long time this topic is closed but I have some news about this problem.

I changed the differential rubber bushings to polyuréthane so I checked the gear ratio. It's still the stock ratio of 3.33 (12/40) so it seems that the RPM gauge is incorrect as vette079 said. I was totaly wrong about the engine behaviour :bang . I have to repair the RPM gauge, I think when it will be done the engine performance will correspond with the RPM.

As grumpyvette said, the difference of performance between the 85 and the 90 is due to the timing, head exhaust ... that are better on the 90. I'm really impressed by the corvettes performances ! :surprised

I will fix the RPM gauge when I will have finished to change the rear suspension bushing and toe brace. I'll keep you informed. Many thanks for you help. :thumbsup:
 

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Discussion Starter #14
that's exactly what I was looking for !

many thanks ! :thumbsup:
 

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that's exactly what I was looking for !
The fact your tacho is working ( wrongly ) means most likely the tach filter is not at fault
and you have the common problem as noted with the dash cluster itself.
 

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Discussion Starter #16
that's what I thought. I will fix it asap.

I thought that I was reving the engine at 6000 rpm (or more) while it was under the 5000 range. The L98 is just perfect ! :excited:
 
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