jackpawt883

so ive been driving the car around for about a month now, and ive noticed my speedo is about 7 mph off (based on my buddies gps) and the tach seems accurate at low revs bu when i hammer on it, it jumps from around 2000-2300 depending on how fast im driving to 6,500-7,000 and never once does it act like theres a rev limiter, i know this car has had plenty of work done before i got to it (has a lt4 intake and i never knew until someone pointed it out) is it even possible to rev these motors this quickly? it appears to rev like a rotary, would be my best description

Fatdog2

If your buddy let's say is going 50mph and you are next to him and your speedo says 62mph,then the lower gears are the reason I think.There is a way to adjust this but I don't know how except to reprogram the computer.Different size tires can change the actual spreedo reading to.My02.

jackpawt883

that makes sense now that i think about it, for some reason i only thought wheels did that

Injuneer

If the speedo is always off by 7 MPH, no matter what the speed, it's a problem with the needle being installed incorrectly. If it is due to a miscalibration due to gear change, it would be off by a fixed percentage. At what true speed was yours reading 7 MPH higher than actual?

While a higher numerical gear, used without recalibrating the rear end ratio in the PCM would cause the speedo to read higher than actual (and the odometer to add higher than actual miles) the 7 MPH error would only be the result of swapping from the auto trans 3.23 gears to the 4.10 gears (would read 26.9% higher than actual) if the car was going 27 MPH when you compared it with the GPS. If your car started with the auto trans' 2.73 gears, the error would be even higher (50.5%).

Run the test again, over a range of MPH .... say true 40, 60, 80 MPH.... and then we can tell if its a fixed percentage error, and if its due to the gear change.

As far as the tach suddenly surging to 7,000 RPM, not sure how that could happen. Stock rev limiter is ~5,800 RPM. And, engine RPM would not suddenly surge to 7,000 RPM with a relatively stock engine, and only a 2,200 RPM stall (not too much higher than stock). If someone has removed the rev limiter in the PCM, the logical explanation would be a badly slipping trans, if the vehicle doesn't surge ahead strongly with the RPM increase.

Hopefully it has more than an LT4 intake.... takes a bit of machine work on LT1 heads to avoid the interference of the LT4 intakes taller runners with the LT1 head valve cover rail.

bw_hunter

Instead of using the GPS, try using a watch or the stop watch function on your phone. Get on the highway and set the cruise at the speed you want and measure the time it takes to go 1 mile. Here's a quick reference:

If you cover the mile in: Your actual speed is:
72 seconds 50 mph
60 seconds 60 mph
55 seconds. 65 mph
51 seconds. 70 mph
48 seconds. 75 mph
45 seconds 80 mph

It might seem old school but I've seen some large linear errors in GPS calculated speeds Vs actual speed as measured by LIDAR. Using a mile and your cruise control reduces some of the variability in using your foot and a momentary read on a GPS unit....

Good luck!

jackpawt883

thanks! i'll run that mile test to see, as well as to see what fred said about determining if its a fixed ratio or if its just 7mph off at all times
as for the revving issue, it doesnt feel like its slipping, not to mention i just put a brand new trans in it about 3 months
i went wide open throttle and it seemed to shift nice and firmly but the tach read 6800 at its highest
as to having the lt4 intake what all involves installing it? might give me a better idea of what this motor has in it, i bought the car with the motor built so im still on a mission to find what all i have, the heads look brand spanking new and everything except the heads had a fresh coat of paint on it when i got the car.
my main reason for asking is soon-ish i'll be getting a decent amount of money, so i can buy some of the bigger power adders like a stroker rotating assembly, a blower, and maybe N02 but id like to know what i have before i just start throwing go fast stuff at it

bw_hunter

Since almost everyone powder coats their intake manifolds red, you can check the part number. The LT4 part is 12552137

If it were mine I'd do a few things to try to figure out what's been done. First, I'd do a compression test. Performance upgrades usually include an increase in compression ratio. The cylinder pressures will give you an idea if something has been done. Then, I'd pull off a valve cover. Most performance builds use roller rockers at a higher ratio than stock (1.6 vs 1.5) if you have stamped metal rockers it's not likely that anything has been done to the engine. While you have the valve cover off, throw a dial indicator on an intake and an exhaust valve and measure lift by rotating the engine over. You can get to the valve lift that way. That will tell you if the cam is stock or higher performance.

Since the car is a bit of a mystery, you might not even have a 4.10 ratio in the rear. While you're checking your speedo out with the watch, jot down your rpm at speed. I think Rob has a calculator that will help you figure the gear ratio...his website is shbox.com. Remember that website, there is so much information there it really is the first place to start when you have questions.

Enjoy!

jackpawt883

i actually did confirm it has 4.10 gears in the rear, did it by the old spin the driveshaft and count how many times it took until the wheel turned one full revolution
ive compared the intake to two other lt1's and the intake is different than both, one was from a 93 z28 and another from a 94 formula trans am, both of those were completely stock cars.
i had the valve cover off but i cant remember if it had rollers or not, i'd be curious to see if i have higher compression because from what ive read don't lt4's have closer to 10.8/1 vs the lt1's 10.5/1
im fairly sure it doesnt have a cam in it, but tomorrow at work i'll look under the valve covers and check lift
and robs site has gotten this car on the road and kept it there, and for that i am truly thankful

Injuneer

The 93 LT1 intake is different than 94-97 LT1 intakes, lacking the diagonal fuel rail crossover pipe depression behind the thtrottle body. I have both my stock 94 LT1 intake and an LT4 intake, and other than the red powder coat, there is very little difference, other than the small amount of extra metal at the top of the slightly taller intake ports. Would take a very experienced eye to spot the difference. There isn't even a difference in the internal plenum or runners. The LT4 intake has to be ported just to match the ports on the LT4 heads. GM ran out of money and never finished the development of the LT4 intake.

Injuneer

It can't rev to that high RPM immediately. I have a TH400 with a converter that flash stalls to 5,000 RPM when hit by the 800 lb-ft of flywheel torque from the 300 HP shot of nitrous. It takes several seconds from that point for the engine to reach the 7,000 RPM nitrous cutoff. I could demonstrate with a video of one of the chassis dyno pulls. And that's with 4.11 gears.

jackpawt883

bad news, 10108630, lt1 intake
and no i dont think the motor is actually revving that high that quick, (also that setup sounds mean) but i was just worried i was over revving the motor, but i think what i really need to do is get a tech2 on the car tomorrow and compare the revs and speed shown vs what is read on the scan tool

maybe2fast

who did the scaler for the 4.10 gears in the tune?

bw_hunter

I don't it can be a scaler issue because he states a steady 7mph difference. If it was a scaler it would be a fixed percent with the value being different at every speed. For example, if he's off 7 mph at 30, he'd be 10 mph off at 40...etc.