I cleaned off my harmonic balancer and attached two reflective aluminum strips so an optical tach would work. I discovered my tach was a lot farther off than I thought.

My idle is set at 950 RPM which sounds right but if I floor it in neutral it revs to 3950 RPM before the govenor limits it? I set the tach to that value.

Can this be right? I assumed I have a stock govenor spring but now I'm wondering.

I also got my pyro installed so there will be a separate thread about that.

Edwin

 

What did your tach say when this optical dude read 3950, which I think is really high. For stock, I'd believe 750 & 2700/2800.

DBF

 

When I got my tach nearly a year ago I assumed the engine would max out at 2700 RPM so I floored it and set the tach to that. Now it tops out at 3950 the same as the optical tach. So I'm wondering it the PO has a GSK installed and if so which one.

Edwin

 

4000 RPM? I don't even think that you can get that with a 3200 spring, can you?


I'd check the optical tachometer on a vehicle with a good tach; see if you are still accurate. If you are, then check your valves.

 

Edwin, my 91 defuels at about 2800 on the factory tach.... floored thats all she wrote. The idle sounds right, but I think 4000rpm would just about blow mine to pieces..
We have similar trucks....

heres about where mine runs( aproximatley) if this will help:
Idle:700-900rpm
20mph in third-----1500rpm
40mph in 4th-------2000rpm
60mph in 5th-------1800rpm
75 mph------------2250rpm
80( top end, all there is) 2700 rpm

 

In neutral and under load RPM is different. You can rev the engine alot higher in neutral than you can down the road. Is your high idle screw backed all the way out? That would explain the 3900rpm.

 

Edwin, BTW my factory tach shows "redline" as being 3600 RPM.....not sure if that realy means anything or not

 

You could always do the speedo to rpm check IF you believe the speedo is accurate.

 

I am asking myself, why did he put 2 reflector strips on the dampner. I have always put 1.
I have a 3200 spring and can hit 3500, acording a photo tach, some math and dynoes.
At idle, I try and set mine where it idles the smoothest. I dont care if its 600 to 900, I want it smooth when Im setting at a traffic light.

 

Edwin,

Between these two posts (pyro & tach) it sounds to me as though your truck was bombed hard before you got it, and you have been living on time with a lien against it . . .

If you set up the optical tach correctly, then it will give you the true rpms. I don't see how you could have set it up wrong. You aren't idling at 475 and maxxing at 2000 rpm (not if you can hit 100mph), so it would seem your numbers are correct.

Bill is right, too, there is a big difference between max rpm no load and under lood -- the springs have a lot of droop, especially as you try to run them outside the stock range.

Since the truck is still running well, you should just grin and enjoy it!

If I had to guess, I would think you have the 3200 rpm spring in there, but it could even be the stock spring tweaked.

 

Thanks everyone for the comments.

The optical tach is correct. The reason for 2 reflective strips on the HB is because the tach is an aircraft type and is selectable for 2, 3 and 4 bladed props. I tested the tach on a florescent light and it reads exactly 3600 rpm. (Do the math)

I'm not real crazy about the temps hitting 1500 degrees though.

I was hoping I needed to add the 3200 spring because it runs out of rpms to quick in second gear when I'm trying to get away from the light next to the ferds and chebys. I wonder if their dually's can hit 100 though or if they dare?

Edwin

 

How do you tweak a stock spring? Do you trim some off of it to make it shorter? That doesn't sound like a good idea.

Edwin

 

I think it can actually done quite safely. All your doing is shortening it so that it it pulls harder to stretch the same amount as before.

 

If you're running to 3900 RPM you might (just might) want to get some 60 lb. valve springs, if you don't have them already.

 

I may do that if/when I decide to get a Jake Brake but I was just revving that high because I have to lay on the ground underneath to check the speed and at WOT is the best place to get a reading then go inside and tweak the tach. I can't ask my brother to do either task and it's best to calibrate it at a higher value than a lower one.

It seems like there are to many variables to get a definative answer as to what spring I have so I may get the 3200 GSK and install it to be sure. I think 3950 is to high also.

Edwin