Chris Fowler

Is it head/cam?

If you change them out, how do you know what your new redline is?

(especially if you're like me and a previous owner changed the cam out and you have no idea what it is... :()

Trying to figure out what my shift point should be at the track. I shifted at redline my last run, but that was the redline on the tach, which is for a stock setup...

79MakoL82

There are a lot of things that go in to establishing a red line. It can be heads (rocker arm studs, etc.) or bottom end. The red line should establish at what point your weakest link is apt to fail. This does not mean you can not go over the red line, and it is not necessarily the best place to shift. As I was told, your shift point should be at the level when you end up in the fattest part of your Torque/HP curve in the next higher gear. So your HP/torque curve determines your optimum shift point more than you red line. You're probably shifting too late.

Chris Fowler

Well...the previous run I shifted at the "yellow line" marker on the tach.

I gained .6 seconds by shifting at the red line marker...

79MakoL82

You've had some mods done to your engine haven't you?. Again, shift points depend on your Torque/HP curve. For your particular set-up, your curves may be higher and further up the rpm scale than on a stock motor. Red lines are determined by quality of the components in your engine, and how much abuse they can stand up to. The use of hydraulic tappets or rollers will limit your engine to <6,500 rpm's due to valve float. Also, if you have press in studs, you don't want to push your rpm's too high. Your cast iron crank is probably OK. The point of the red line is to give you a graphical place where you generally do not want to venture on a regular basis, as you could have catastrophic engine failure. Have you dyno'd your car? That would be the best thing to use to figure out where your best shift points are. If you need to shift higher because your curves are higher, by all means do it, but if you still have the stock L48 heads (push in studs) and bottom end, you may not want to go above the red line too often.


[Modified by 79MakoL82, 2:13 PM 12/3/2001]

Chris Fowler

Yup...dyno'd it.

ZD75blue

The red line is how high the engine can rev without damage, or shortening the lifespan. Also how many rpm the accesories can rev without damage (a/c Compresser, alternator)

Thats the basics

ZD :cheers:

BeaterShark

I'm no expert in this, but I would guesstimate your shift point at/about 5300 RPM.

I'm really suprised by a .6 sec improvement. What is the "Yellow" and "Red" line RPM on your tach? Also, where your 60 ft times about the same between the two runs?

Lohkay

according to your dyno charts, I'd shift at 5,500 with my 5spd but I guess you have a 4spd so it'd be more like 6,000.

Ad for the redline question, Most important is the type of lifters you have. Hydraulics will normally won't let you go over 6,500. Above that (solid lifter ground), the valve lift will be the next culprit...the lower the faster you can go. Then the next problem would be the crank, then again the lower stroke will let you go faster.

79MakoL82

Doesn't this relate back to the type of studs, etc.? With solid lifters, screw in studs, and a stud girdle, you can run the rpm's up pretty high with a high lift cam. But with pressed in studs (L48), the lift/rpm's become a problem. I would agree that with your curve, you probably want to shift up at your red line (5,500?) or a little higher, but unless you've done mods to your heads and studs, I wouldn't push it much past that. I think those are probably your weakest link. The way to increase your redline is to improve the quality of your components i.e. studs, lifters. If you have L48 heads, you might want to go to another head with screw in studs.


[Modified by 79MakoL82, 3:11 PM 12/3/2001]

BSeery

How many RPM's does the engine drop when you shift? -- or --

If you shift at 5700 RPM, what RPM are you running at after the shift ?

Looking at that graph I am thinking shift light at 5500 RMP, shift at 5700 RPM and hopefully that will drop you down in to the 3000 - 3500 RPM area where your torque and HP are still climbing -- maybe to the 3500-4000 RPM area, but after that your torque isn't getting any better.

Chris Fowler

Yellow line on my tach is 4500 RPM
Red line on my tach is 5200 RPM

My 60' time was improved by just .1 seconds...so I got the other .5 the rest of the way down the track.

Chris Fowler

The RPM drop is one thing I've gotta learn. I didn't pay attention to that one. I was concentrating too much on going straight and just shifting. :D

I might be going back Wednesday. I'll try to pay attention to the RPM drop.

The main thing I'm concerned with is deciding to shift too high and damaging the engine. Other than that, I plan on trying several different shift points.

BeaterShark

I came up with my 5300 RPM based on the shift drop. I also based this on the Muncie ratios (2.52, 1.88, 1.47, 1). I figured that the shift drops will be:
1-2: 5300 -> 3950
2-3: 5300 -> 4150
3-4: 5300 -> 3605

I don't know what tranny you have, but I would think it is close to what I have.

Again, I'm no expert, but I look for the shift point that will make the HP about equal before and after the shift. I don't even look at the torque curve.

Gameshow Bob

80's have a Borg Warner T-10 :yesnod:

Ganey

I wrote up something on this for bence13_33 in one of his topics if anyone can find it.

Chris

The Experienced Engine Builder determines the Redline which means Never Exceed. The yellow means Caution. If your engine is stock meaning assembled by Chevy then Chevy determines the Redline. Changing something does not change the Redline. It's the entire engine that has to be considered.

If an Experienced Engine Builder rebuilt balanced & blueprinted then the redline could be raised.

:cool:

Chris Fowler

Now...what if the engine has been rebuilt, cam changed, heads changed and intake changed. I seriously doubt it's been balanced and blueprinted. (rebuilt about 10,000 miles ago...just before I got it).

Does this still mean it's "stock"?

Techno

Crank material. (corvettes use a non cracked crank. Other chevys used the cracked cranks off the assmbly line)
2 or 4 bolt mains
connecting rod size
cast or forged pistons
rocker arm studs
All for whether or not it breaks

Cam - rpm range
lifter type -hydr. or mech.
valve springs -light or heavy
breathing, both intake and exhaust
Whether or not it can reach a red line.

You determine the redline by either breaking it or how high the engine can go with the parts that allow a high RPM
There were high redline and low redline BB.

If the cam was the only thing changed this means the rest of the engine is still under the stock redline limit. You shouldn't be there! If it was changed for a higher HP higher breathing cam (less low end) then the cam has a higher rpm ability or range. The rest of your possibly old engine isn't.

Ganey

Basically stock. Sounded like just the cam was changed. Was just trying to be clear so defined for this topic. Rebuild as stock & changing the cam would not change the redline. If enough cam was installed it would be Capable of a higher rpm assuming no intake restriction, etc.

Typically what happens is that valve float would limit rpm, if enough cam was installed so that there was no valve float like solid lifter or anti-pump up lifters, then typically it spins a bearing, throws a rod & wastes the block, etc.

Techno

Another thing the redline, actually just below the yellow line is for, engine life. Faster you spin it the shorter the time between rebuilds.
This isn't a big thing for cars but is for my Outboard. 200 hrs at 7500, drops down to one run at 10,000rpm. I'm sure its something the same for car engines. redlines for these are mostly how long between rebuilds.
The other parts to consider are your drive line system. I think the guy who originaly bought my 'Vette dragged it. high rev BB 4:11 posi 4 speed and only power windows. My left rear spindle broke in a parking lot.