Checking RPMs

[vette]

While I was restoring the body on my BJ7 I had the rear gears changed to 3:54:1. Now that I'm driving the car alittle I am a bit disappointed in the crusing rpms that I am experiencing. I believe I was hoping for less RPMs at different crusing speeds but it has been so long since I drove the car that I don't remember what it was before the gear change. Can anyone concur with the engine speeds I am experiencing. I have to say these RPMs may not be exact because I didn't go out on the SuperSlab with the intent of recording this. I just remembered to look at it while running here and there.
- 50 mph, W/O Overdrive = 2700 rpms
- 50 mph, w/ Overdrive = 2200 rpms
- 60 mph, W/ Overdrive = 2700 rpms
- 67 mph ( i think, my speedo is fluxtuating alittle maybe as high as 70mph ) =w /overdrive,3000rpm
Thanks, Dave.

 

[EV2239]

With a 3.9 to 1 axle and overdrive, you should be doing 3000rpm at 72 mph or 1000 revs for 24mph. So using a 3.5 to one axle and overdrive you'll be roughly 10% higher geared, but your speedo doesn't know that. In fact your speedo is slightly confused without the change of axle, so may benefit from an overhaul and recal.

Back in the day we didn't mind if our speedos waved between 70 and 120 because we could assume the higher figure and boast in pubs.

 

[vette]

Thanks EV, Aside from the fact that the needle bounces alittle how do you get the speedo to be accurate with the new gearing? Dave.

 

[EV2239]

Dave

Best to speak to the people repairing and have them tell you what information they need. Some even sell a gearbox that goes in the cable. The difference is about 10%.

 

[57_BN4]

Dave- you need to figure out the TPM of the new setup. This is how many turns per mile the speedo drive cable does. Inside the speedo it requires 25 cable turns for each worm pinion gear turn which moves the ratchet one click. The odometer is calibrated by changing the ratchet wheel to one with less or more teeth. For a BN4 the standard calibration is 25 worm pinion teeth *53 ratchet teeth =1325 cable turns for one rotation of the ratchet wheel = one mile.

The TPM of your existing setup is on the face of the speedo, probably 900. Lots of methods to determine the new number via an online search. Then send the speedo to a repair shop along with the desired TPM and they'll be able to change the ratchet wheels for the closest matching ones to your new figure and if necessary replace the hair spring on the speedo.

I have changed from a 4.11 to a 3.54 diff and have worked out the new TPM as just over 1100 but it is affected by tire diameter and possibly the speedo drive ratio inside the OD housing so that might not be the same as you will get.

If you don't care so much about the odometer accuracy then you can do a slightly dirty trick which is to just move the needle to the correct place at your most common driving speed and accept that it'll be a little off at other speeds. Borrow a smartphone or satnav to find the real speed first.

Replacing the speedo cable inner will fix a wavering needle.

Andy.

 

[vette]

Thanks for the help EV and Andy. I also did go back thru previous post on this subject but guess i wanted to hear it from the current corporate wisdom in case anything had changed in the way to address this. I thank you both very much for the help and update. One point of note is that I did take my speedo cable apart and cleaned it and lubed it with white lithium grease I would be suprised if it is hanging up. But now with what I am reading I will go back and recheck everything. When I put the trans tunnel back on I forgot to hook up my speedo cable to the angled adapter before I bolted the tunnel down. As I am getting used to (not really) trying to reach something on this car after you forgot to do it at the proper time, trying to get the speedo cable connected to the angled connector was an absolute bear. And then I realized that the angled connector was loose enough on the trans that it easily swivelled up and down. Do you think that is loose enough to be causing the slipping. And then too I am now concerned about the speedo itself, I never did anything to it. Dave.

 

[57_BN4]

Hi Dave,

I recently discovered that the wavering needle is almost entirely due to the drive cable being not as rigid as it once was. Seems like the twisted wires wear or unwind a little so that you can grab the cable in two places and twist it a fair bit. A new cable is almost completely rigid end-to-end whereas my old one I could get nearly two turns on one end while holding the other.

The speedo needle wavers about because the odometer is quite stiff to turn and as it starts to advance, the springy cable winds up (needle drops) then after it has incremented the cable unwinds with the reduced load (needle pops up again) then it settles for a second or two before doing it again. Andy.

 

[glemon]

RE: resistance from odometer and trip odometer numbers causing speedometer bounce, I took the cover off and applied (very sparingly) some drops of this lube designed small model railroad engines (I figured the plastic compatible aspect would make it less likely to make the white on the numbers run or damage anything) and the bounce in my TR Speedo, although it did not go away completely, was substantially reduced. Link: https://www.amazon.com/dp/B001W8XD4...e=asn&creative=395109&creativeASIN=B001W8XD44

 

[TFR1]

A speedo from a non overdrive car is calibrated for the 3.54 rear gear. Maybe you could find one for your car.

TFR1

 

[vette]

Thanks everyone. I'm still evaluating what to do next. Im driving the car quite a bit lately to sort out any issues before I paint it. I have two issues with the new gears. Firstly I am disappointed with the gear reduction to the point that I am questioning whether or not I really have the 3:54:1 gears installed. Neither i nor the mechanic I gave them to to install, looked at the new gears very closely. Or we don't remember what we looked at, It was about a year ago that the change was made and the car not being driven till recently. The reason I am questioning the gear ratio is that my speedometer seems to be very accurate. On the interstate measuring between the mile markers, with the speedo at 60mph, it takes exactly 60 seconds. I have checked this about 5 times at different mile markers in the last two weeks and it always takes exactly 60 seconds. So I am really questioning what i have here. If someone with the standard rear end gears with an Overdrive transmission and someone who has changed to the 3:54:1 gears with overdrive could tell me what rpms their car is running at, at 55, 60, 65, & 70 mph I would really appreciate it. Also tell me what size tire you are running. Thanks. The other issue I have is that these new gears are howling ALOT. Between 50 and 60 mph the gear noise is drowning out the exhaust note. When I lift my foot off the gas at that given speed the howling stops. That tells me that it is the ring and pinion. This obviously can't stay this way. So I may be pulling the gears back out anyway. Dave.

 

[57_BN4]

Hi Dave,

Here's something a bit random I was thinking of last night on the way home. I was wondering what the simplest way to check the tacho accuracy would be as I have for a while suspected mine is reading quite a bit high. For example, I can easily hit 6000rpm if I stand on the throttle and wait long enough (Longbridge engine).

Here's the trick. Take a video of your rev counter while driving at different speeds [edit]or parked in the garage. It doesn't need to be recorded on anything fancy, I used a Sony point-n-shoot still camera that has a basic video function and it worked very well.

Next step is to download the free Spectrum Lab audio analysis program which is very simple and easy to use.

Then play the video with Spectrum Lab open and it will 'listen' to the sound and pick out the predominant frequencies. The lowest frequency is the exhaust note and the engine speed is 20 * that (Hz * 60 for minutes, * 2 because four stroke / 6 cylinders = rpm) e.g. 138Hz * 20 = 2760 engine rpm.



Above is a screenshot of playing back the vid I made on the way back home. Engine rpm is 2560 so the tach reads 440rpm too high and it gets worse the higher the revs go.

In OD top with 3.54 diff and Michelin 180HR15 XAS tires (rolling diameter 25.83"), 2560rpm gives 71.5mph.

Andy.

 

[Bob_Spidell]

... Neither i nor the mechanic I gave them to to install, looked at the new gears very closely ... The other issue I have is that these new gears are howling ALOT. Between 50 and 60 mph the gear noise is drowning out the exhaust note. When I lift my foot off the gas at that given speed the howling stops. That tells me that it is the ring and pinion

Maybe you need another 'mechanic?'

Where did you get the gear set? I put the Lempert 3.54 set in my BJ8 and, as expected, they reduced RPMs about 11% across-the-board. You can cross-check using a calculator like this: https://www.angelfire.com/fl/procrastination/rear.html

Have you checked your tach against a known accurate one? My diff whines a little, but not under load (my dad and I installed them).

 

[vette]

I bought my gears from Dennis Welsh,Uk. The guy that put my gears in is retired from a NASCAR team and his position on the team was setting up rear-end gears. I'll continue to make all the checks. But it looks alot like we will be pulling the gears again. Thanks, Dave. By the way my tire size is 165/15. But it looks like I am only getting 60mph out of 2700 rpms in overdrive. I'd swear the car still has the 3:90:1 gears in it.

 

[bob hughes]

I am looking into a differential change to give the car more legs and when enquiring about changes to the speedo I was told that they are sent up to Head4Healeys and they organise for the change.

There is also a firm over here that repairs automotive instruments in Swansea, South Wales - Caerbont Automotive Instruments -

Technical - technical@caigauge.com - they must be able to change the speedo as they strip them down and repair them as well as selling spares. Don't know if this helps but these companies are on this side of the pond.



Bob

 

[vette]

Just an update on what has been going on about the rear gears in my car. This has been frustrating for me because this car has been a very difficult build for quite a while and the rear gears are the only
thing that I have farmed out because I never did rear gears before and I thought It would be little less work just to give it to someone who has done alot of it. Anyway I have been able to verify that the new gears are the 3:54:1 which I specified. But in doing the checks I also noticed that I have excessive end play in the pinion shaft. It appears to be well over .010 end play. I would say that is why I have excessive gear noise. Don't know why this has come out this way, but in talking with the guy that installed them he agrees that I have to bring it back to him. So this has put another delay in the progress of the restoration. I have to wait for when he can get it into his shop. So I'm not driving it to do any more shake down runs. I probably don't need any more miles on it now, but the late fall weather is still nice here and I find it hard not to take it out. I am guessing that I have put about 300 miles on it with this loose pinion, I certainly hope I haven't damage anything or started to put a pattern in the gears yet. Dave.