Two questions, 1 Trans, 1 engine

[Gensaiken]

So heres whats up. First question is the following:

 

When I try and agressively accelerate in 5th gear my car will ping like crazy. Even though I'm running premium. Though every other gear it never pings at all even under agressive acceleration.. Only 5th.

 

Some background info on my car: High compression (internals unknown), L20B, Shaved 219 head, Z-Therapy round top SUs, Stock exhaust manafold thermodyne coated inside and out.

 

 

Second question:

 

When I'm running at 65-70mph in 5th gear I'm hovering at 4k RPMS... whyyy?? I have a 720 pickup 5speed in great internal shape. I have a limited slip but.. I'm not sure on ratio.. I think its a 180?.. so Why are my RPMs so high in 5th gear?

 

I am 100% on my speed because I use a GPS to tell me my speed. So I know my spedo isn't off.

[ggzilla]

Pinging is worse at higher loads (e.g. higher gears). Back off your distributor a bit to where it stops pinging.

 

I'm running at 65-70mph in 5th gear I'm hovering at 4k RPMS

That's what my Datsun B110 runs at, with 3.9 gear and small-diameter tires. With your overdrive gear I would expect it to drop down.

My guess: Your speedometer is off. You are probably doing 75-80. Get on I-5, take a watch with second hand, and time the highway mileposts. At an even 60 mph, it should take 60 seconds to go past each milepost. If it takes you less, the speedo is off.

[DISLEXICDIME]

my car with 370's in my lsd and a truck trans it is at around 32 or 3500

[Gensaiken]

Pinging is worse at higher loads (e.g. higher gears). Back off your distributor a bit to where it stops pinging.

 

That's what my Datsun B110 runs at, with 3.9 gear and small-diameter tires. With your overdrive gear I would expect it to drop down.

My guess: Your speedometer is off. You are probably doing 75-80. Get on I-5, take a watch with second hand, and time the highway mileposts. At an even 60 mph, it should take 60 seconds to go past each milepost. If it takes you less, the speedo is off.

 

Speedo is based on a GPS. GPS is correct as I have tested it on a newer car with a correct spedo.

[datzenmike]

Seeing as how it runs well in all other gears and only pings in fifth on hard acceleration, I would leave it and just back off slightly. You could reduce the base timing but it will reduce performance and mileage on the bottom 4 gears too, and this is where you do most of your driving anyway. It may not ping in lower gears because there isn't enough time for cylinder temperatures to rise high enough to cause it. Take the plugs out after they cool after a good hard run in 5th, and look for signs of overheating. If they are too clean, with very white porcelain, the heat range may be too high. A higher compression motor, generally, will require a colder plug.

 

 

If speedo and RPM are correct then you must have a 4.11 or 4.375 diff. What size rear tires do you have Richard?

[SHADY280]

or fatten up your carb at wide open, that works, but milage goes down. you may be too lean if your timing is dead on, or too small a carb. and the only way your rpms are high is tire size or rear diff ratio. nothing to really do with the tranny.

[racerx]

Check you timing.sounds like it's too advance..not good to run engine like that..

[Gensaiken]

Check you timing.sounds like it's too advance..not good to run engine like that..

 

Me and Kelvin dietz adjusted the timing a bit because at idle my timing was slightly off at higher RPMs, I was pinging going up the hills where I lived..

 

 

Seeing as how it runs well in all other gears and only pings in fifth on hard acceleration, I would leave it and just back off slightly. You could reduce the base timing but it will reduce performance and mileage on the bottom 4 gears too, and this is where you do most of your driving anyway. It may not ping in lower gears because there isn't enough time for cylinder temperatures to rise high enough to cause it. Take the plugs out after they cool after a good hard run in 5th, and look for signs of overheating. If they are too clean, with very white porcelain, the heat range may be too high. A higher compression motor, generally, will require a colder plug.

 

 

If speedo and RPM are correct then you must have a 4.11 or 4.375 diff. What size rear tires do you have Richard?

 

My speedo doesn't actually work. I don't have it connected, but speed based on a GPS and my RPMS based on my tach.

 

I have 195/65/R14 all around.

 

Mayeb I shouldn't worry about the 5th gear issue... I usually have to floor it in 5th to make it ping like crazy.. realisticly I should be down shifting. But the high RPM in 5th kind of pisses me off because my MPG is HORRIBLE on trips when it should be much better.

[datzenmike]

What are the RPMs at say...60MPH? Maybe I can get a better value for your rear end. Right now assuming a 0.813 5th gear I get 4.89 or the closest Datsun rear end gear... 4.88!!!

 

I must have a false assumption in there... Let's see 4,000 RPMs at 70 MPH

 

A195/65/R14 tire is 2 feet in diameter

Thats 6.283 feet per turn (circumference)

There are 396600 feet traveled at 70 MPH in one hour

that's 6160 feet per min.

or 980.42 wheel turns/min.

X 4.375 (rear axle) = 4289 RPMs (4th gear)

X 0.833 O/D 5th = 357.3

So 4289 - 357.3 = 3,931 RPM (engine) Rear axle is 4.375 close enough?

[Gensaiken]

What are the RPMs at say...60MPH? Maybe I can get a better value for your rear end. Right now assuming a 0.813 5th gear I get 4.89 or the closest Datsun rear end gear... 4.88!!!

 

I must have a false assumption in there...

 

I don't have an exact RPM for 60mph. I'll have to get some more exact RPMs for you tomorrow.

[Gensaiken]

I got some more exact RPMs. I wasn't able to get to 70 but I have 3,500 rpm at 60mph in 5th gear and 3,000 at 50ish mph in 5th gear.

 

I believe I was also at 3,300ish? in 4th gear at 45mph.

 

I was keeping track of all kinds of random RPMs

 

I believe 3,000 at 20ish mph in 2nd gear

 

3,500 at 35 in 3rd gear.. It's hard to get exact numbers since my speedo has a slight delay. Also my tach was starting to jump a bit (it does that for some reason when it gets really warm..)

 

But those are rough estimates but fairly close.. the 2nd and 3rd gear numbers are pretty rough, kind of forgot exactly what speed and what RPM but those should be close... the 5th gear speed i'm 99% sure of.

[hang_510]

this is a cool calculator...

http://www.angelfire.com/fl/procrastination/rear.html

 

 

w/205-60/13 at 3500rpm and a 3.9 im doing ~70 in 5th.

now if i could get my mileage to ~20mpg, damn heavy shoes :D

 

 

 

 

as always...timing is everything.

back it off 1-2* try again till it stops. good times :w00t:

[bonvo]

sounds to me like its timing or like datzenmike said the plugs are to hot try colder plugs and maybe a different gap as for the high rpm i have a dogleg5 speed out of a first gen 200sx in my 610 and in 5 im at about 3000 at 65mph with 3.90s out back my tire size is p185/75/r14 is your diff possibly out of a 4x4 720? that i would think would have a low gearing like that

[datzenmike]

Richard, I cant get it to work, the numbers are insane.

 

3,300 in 4th at 45 MPH???? Double the speed= double the RPMs That works out to 6,600 at 90 MPH???????? This worked out to a 5.12 ratio!!!!

[datzenmike]

Well worked out for 5th at 60 @ 3,300 and 5th at 50 @ 3,000 and both work out to 5.12 and 5.26 ratio

 

The 4th gear @ 45MPH and 3,300 works out to 5.23

 

I used the 0.813 over drive 5th because it agrees closely with the 4th gear 1 to 1 tranny ratio.

 

I would have to say that 3,300 RPMs in fourth at 45 MPH is wrong. Speedo measurement is off or RPMs are off.

 

I would expect about 2,500 RPMs at that speed using a 3.9 rear axle. What do you think??

[bonvo]

Well worked out for 5th at 60 @ 3,300 and 5th at 50 @ 3,000 and both work out to 5.12 and 5.26 ratio

 

The 4th gear @ 45MPH and 3,300 works out to 5.23

 

I used the 0.813 over drive 5th because it agrees closely with the 4th gear 1 to 1 tranny ratio.

 

I would have to say that 3,300 RPMs in fourth at 45 MPH is wrong. Speedo measurement is off or RPMs are off.

 

I would expect about 2,500 RPMs at that speed using a 3.9 rear axle. What do you think??

 

yeah 2500 @45 in 4th sounds about right