Are you short shifting? Tach verification Tech Info

[Guest norm[T12SDSUD]]

Ok, one thing I don't think I have shared in the past regarding the times I run and why others can't seem to get as close with similar buildups is this bit of info.

 

My 240 tach is off 900 rpm at 8000 rpm. So, I actually turn my tach up to 8000 rpm before shifting,however 8000 rpm is actually 7100 rpm.

 

It is possible that some of you in here also have an inaccurate tach and are short shifting your engine.

 

Now, I don't recommend you go out and try to turn 8000 rpm on your car without having first verified your tach readings, but it is possible that since my tach is off then your tach may also be off.

 

I verified my tach years ago by using one of those speed trap radar machines that the cops set up beside the road to show you your current speed.

 

I held my car in 2nd gear and then drove by the sign at 7500 rpm , which indicated a speed of 55mph,( btw I wouldn't recommend holding your engine at that high an rpm without being under a load unless it is in great running shape.)

 

You can just as easliy drive by the sign at 5000 rpm and then extrapolate the figures at higher rpms by using the tach percentage correction factor found at 5000 rpm.However, since Tach error percentage can vary at different rpm levels I wanted to get as close to redline as possible.

 

Anyway, once you have an ironclad rpm-mph reading, what you then do is place a piece of masking tape on the tire sidewall all the way to the edge of the tire tread and then get a measuring tape and line it up beside the car and then roll the car forward until the tape mark is once again lined up perfectly up and down with a position on the tape measure.Use the leading edge of the tape as your exact measurement mark.

 

On my car the tire rolls exactly 70.70 inches on one complete revolution.

At the slow speed of 55 mph tire growth shouldn't be an issue ,so I will stay with that 70.70 inch reading for my calculations.

 

Now, next to get the actual rolling tire diameter of the tire divide 70.70 by pi(3.1416). This gives me a rolling tire diameter of 22.50 inches.

 

Now, once you know what your exact speed is through either a radar measuring device or a race track timing light finish line test you can figure your exact Tach reading with this formula.

 

MPH X rear gear ratio X tranny gear ratio X 336/ Tire Diameter= TACH RPM

 

 

So in my case when in second gear I can figure it this way.

 

55mph X 3.9 X 2.077 X 336/ 22.50inches = 6653 ACTUAL ENGINE RPM

 

 

So, since 6653 / 7500 =0.887 , I have a TACH error reading which should be 88.7 % lower than the indicated Tach reading.

 

So, using this error correction factor I see that at an indicated 8000rpm X 0.887 that my ACTUAL Tach reading is 7096 RPM.

 

 

Remember now that to use this formula you have to know your rear end gear ratio and your transmission gear ratios.

 

Check your Tach using this method and you too can find out whether or not you have been short shifting your engine!

 

later,norm

[Tim240z]

Good info Norm. I guess you could go through the speed trap in 4th. Since that's a straight through (1:1) ratio, you could simplify your equation.

I know that at idle, my Autometer Tach is off by 100rpm. I can see what the PCM is seeing (LT1) via the laptop. I haven't checked the upper ranges yet, but I'm pretty certain that all gauges are slightly off at the very bottom of their ranges?

Tim

[Lockjaw]

I just wait until I hit the limiter and shift.

[Guest norm[T12SDSUD]]

Hey, in absence of a racetrack or radar timing device, another way to find the rpm - mph relationship is to get on the interstate and use a stopwatch between the mile markers while holding the tach at an EXACT rpm in either 3rd or 4th gear for the entire mile.

(Preferably you should use a marked measured mile,although if using normal interstate mile markers try it out a couple of times in different spots and then average out your results to minimize mile marker placement errors or stopwatch timing activation errors) You will need light interstate traffic to do this test.

 

Take the amount of seconds it takes you to travel the marked mile and then divide them into 3600 seconds and then you will get your exact mph.

 

If it takes you 50 seconds to cover the marked mile then you are traveling at 72 mph.

 

If it takes you 60 seconds then you are traveling at 60 mph.

 

If it takes you 30 seconds then you better slow down cuz the law is gonna throw your butt in jail for running 120 mph!! LOL

 

make certain to keep the Tach at an EXACT rpm throughout the test.

 

This is a good method for verifying your speedometer accuracy as well as using the tach reading results to get your actual tach reading using the formula in the post above.

[Guest norm[T12SDSUD]]

Hey Tim, at an indicated 3000 rpm the ACTUAL rpm reading is 84% of the indicated reading while at at an indicated 7500 rpm the ACTUAL rpm reading is only 88.7% of the indicated reading. So the error is definitely not linear.

 

I imagine that may also be the case with speedometers. Maybe at 60mph they are only off 5% whereas at 120 mph they could actually only be off 2% or possibly as much as 10%.

 

later,norm

[Lockjaw]

What about just getting a "good" tach?

[Guest norm[T12SDSUD]]

What about just getting a "good" tach?

 

Well who knows if the new tach would be 100% accurate??

 

You are taking the manufacturer's word for it until you test it all out.

 

later,norm

[JMortensen]

I verified mine by putting a 6800 rpm pill in my 6AL and hitting it. My autometer shows 6800 rpm at the rev limiter. Plus it's just one wire from the MSD to the tach.

 

A friend of mine FREAKED OUT when I drove her Z and wound it up to ~7K. Apparently her tach was showing 8500 or something. I was unable to convince her that her tach was wrong, until I asked why her valves hadn't floated if it was really at 8500 rpm (stock springs, mild cam). She still hasn't fixed it.

 

I've never had good luck with ANY of the stock gauges. Tachs are innacurate, speedos jump, temp gauges are CRAP, oil pressure gauges are a just so vague that they're useless, I've been thru several ammeters, and the gas gauge looses 1/2 tank in the first 1/4 tank but I think that is the shape of the tank more than anything else. I've only seen one working clock ever, if you want to count that as a gauge.

 

I agree with Lockjaw, just replace if you want to really know what's going on.

 

Jon

[Guest norm[T12SDSUD]]

No need to replace a good tach if you have one though!

 

I think my method is just an easy way for "everyman" to check their tach's accuracy without going to a shop and paying someone to put it on an oscilliscope or some other method.

 

Later,norm

[Lockjaw]

So I guess I could use the stock boost gauge then, as long as I tested it. I mean I don't need to know if its at 20 psi or 7, as long as its not pinging, right?

 

Nope that VDO one is staying in there.

 

You guys crack me up, you will spend more time trying to figure out how "OFF" the stock tach is. That cracks me up.

[Guest bastaad525]

and the gas gauge looses 1/2 tank in the first 1/4 tank but I think that is the shape of the tank more than anything else.

 

 

ROFL wow I thought that was only mine that did that I feel better now.

 

Another odd thing for me is that I can't even fill my tank the same amount every time I fill up... I always run to about 1/4 tank, sometimes I can only fill like 6 gallons on top of that, sometimes I can fill 9! I think this is an actual thing though, not just the gauge reading off, as I get a lot more miles to the next time I'm down to 1/4 tank, the times that it will let me fill 8 or 9 gallons.

[Guest pintoz]

jmortensen wrote:

 

I've never had good luck with ANY of the stock gauges. Tachs are innacurate, speedos jump, temp gauges are CRAP, oil pressure gauges are a just so vague that they're useless, I've been thru several ammeters, and the gas gauge looses 1/2 tank in the first 1/4 tank but I think that is the shape of the tank more than anything else. I've only seen one working clock ever, if you want to count that as a gauge.

 

I agree with Lockjaw, just replace if you want to really know what's going on

 

What guages do you or others use? I'm in the process of switching motors and wiring harnesses now, so this is the best time for me to switch guages.

[JMortensen]

Right now I have Autometer tach, speedo, water temp (mech), oil pressure (mech), and a stock ammeter/gas in the last hole. I just have the cheapy Autometer stuff. Sport Comp, I think it is. But its 100% improvement over stock.

 

You can see them in my autox vid recently posted in the Announcements forum...

 

Jon

[spork]

A little info s far as boost guages, I've found similar variances when comparing them to a map sensor. My buddies honda was 4psi off. He thought he was running 15psi and was only running 11. He had an autometer guage for anyone wondering. I talked to the local dyno shop and he said approx 75% of the boost gauges he sees go through when he tunes cars are off. Just thought I'd share.

 

James

[Lockjaw]

Alot of the time a boost gauge is off because someone "tweeks" it by running a little to much boost. I went to a VDO 30 psi one, and it doesn't pick up until 2 psi.

 

Mainly I just want it in there to tell me where I ping on pump gas, on race gas, I don't have to worry.

[spork]

your vdo 30psi gauge...does it read vacuum too? I currently have a vdo that goes to 25 but couldn't find one that went to 30 with a vacuum side. My new motor will runaround 25-26psi so I bought an autometer. Let me know and maybe I'll sell the autometer as I prefered the vdo I had