Just wondering, after changing my crappy valve springs, what's the next good reason not to rev my engine too high? I mean, I know I wont be making any power up that hight, but I hate that 4500 RPM yellow line.. It's a completely stock lg4 except for the basic stuff like a carb, comp 981 springs, 1.6 rockers, etc. I'm betting the next thing to change so i can get some more rpms is the peanut cam.. but if I did rev it up past 4500, would it do any harm? Just curious.

Also, what kind of cam would you guys reccomend for me? I'm going to be getting headers as well in a while, but I'm at a bit of a loss for which cam I should get. I want power from like idle-5500 rpm, as it's a street car. 416 heads.. uh, that's about it. So what do you think? What should I look for?

 

Red line of an engine is just before you get valve float. Shift point is at the peak torque curve. They're never going to be the same. If shift point is above the red line then you've got problems.

Don't go by the factory markings on the factory tach. They don't really mean much unless the engine is completely stock. Even then the factory tach isn't very accurate.

 

well hopefully I wont have any valve float until I hit higher RPM, but the bottom end is still stock. I dont intend to take it up to like 5500 all the time, but I want to be able to rev it a little higher, you know, for showing off in traffic and stuff. As for shift points, I'll just have to experiment when I get the car to the track. But that wont happen until I at least get the proper carb on there.. Cam and headers are next.

 

I've shifted my stock bottom end 350 in my 69 camaro at 6200 or so, on a near daily basis for 5 years now. The 327 before that, I shifted at around 7000, but it was a little shorter stroke. A stock bottom end 305 in good gondition should go 5500 anytime without problems, assuming your vavletrain is in good shape. Of course on a stock TPI, there's NO power up there, so its a waste of time and gas ($$$). Of course, on my old iroc, it would take a long, long time for the motor to get from 4500 to 5500 even in low gear, so I guess the time@rpm should be factored in somehow. Har

 

87, I know this is what everyone says, but I am still trying to figure out why you would shift at torque peak and not go past it.

When you go from one gear to the next, torque at rear wheels is reduced by the change in gear ratio, so shouldn't you stay in lower gear even though you are past the peak if you are still producing more torque than what you would be making if you shifted?

 

This is a basic math issue. You want the HP vs. time curve to have the maximum area under it. When you shift, you want the engine to be at the max torque in the next gear. That moves the car the greatest distance in the least amount of time.

As for the defination of "red line", many different conventions exist. There is the power red line, and the physical capabilities red line. Poor valve springs is one example of a physical capability red line (my stock LG4 springs floated at 5200 RPMs). The next typical physical red line is the rod bolts in a factory engine. Floating valves typically don't hurt anything in a factory engine, but weak rod bolts will make for a bad day every time.

 

So, valve float basically killed the engine's ability to turn any faster, without killing anything. Almost a form of rev-limiter. Not quite, but almost. Improve the springs, and you now have to worry about the next-weakest link in the engine. A good balancing job and a quality vibration damper would do wonders for high-rpm operations, yes?

 

Yeah, probably. That's why it's a real good idea to get an engine balanced. I wont worry about rod bolts or anything until I get my new engine, but that wont be for a while, so I just wanna have some fun with the LG4. That being said, nobody has answered the question about what cam would go well with my particular engine and needs...

 

In modified engines, valve float can put you on the trailer in a hurry. Even stock engines under extreme conditions.

Engine balancing and high end crank dampers provide very little or no benefit on a stock engine with stock parts. Balancing becomes necessary when using non-stock parts which are lighter or heavier than the stock stuff. A crank, for example, is balanced from the factory for a fairly specific weight of rod and piston. If changing to significantly lighter forged pistons it would be advisable to rebalance the assemblies. Same with connecting rods, as overall and end to end weights vary between rods of different manufacture. Tearing down your stock street engine just for a balance job would be a phenomenal waste of money. As far as just spinning an engine, stock valve springs might be the first limiting factor. After that, probably the connecting rods. Stock small block chevy rods are pretty weak.

Put it this way, by the time you get enough RPMS on a stock assembly that it might need to be balanced, it will have already scattered.

 

Comp 262xe / 256XE

I'm most likely planning a comp 268XE. I have a stick shift, no OD, and 3.23 gears, so my cruising RPM is considerably higher then yours. I'll be in my powerband most of the time... -J

 

Yeah, ripping it apart just for balancing seems like a waste. But, if it's already apart, why not?

 

Valve float isn't what most people think. Weak valve springs is the main cause of valve float. Spring pressure is weak when the valves are closed. On a typical street car you can push the valves open with a simple tool. When the valves are open, there's a lot more spring pressure.

Because of the reduced pressure when the valves are closed, that's where float happens. When the valve slams closed, the weak spring pressure will cause the valves to bounce on the seats when they should be closed. This bouncing reduces cylinder pressures that the piston is trying to build up. Low cylinder pressure and you get low performance. The chance of a valve hitting a piston during float is slim so you're not going to cause any damage doing that. The valves and seats get the most damage plus if the valve is bouncing off the seat when it's supposed to be closed, it could cause valvetrain problems.

As for shifting at peak torque, it was explained above. Going through the lower gears of the transmission is only for getting the vehicle moving. The faster you get it moving, the faster you go. If you have enough hp, you want to be in high gear as soon as possible because going through lower gears although it increases torque multiplication, it doesn't allow the full hp to do it's job. Shifting at the peak torque allows the engine to build power in the next gear. If you shift after peak torque, you're over the powerband and not accellerating any more or at a decreased rate.

I use a powerglide tranny. If I run it out to 7000 rpm before I shift to second, I'm about at half track. If I shift early like about 5000 rpm where my torque peaks, I get into high gear quicker and my runs are quicker. Theoretically all I have to do is watch my tach to find my shift point. My converter stalls at 5700 rpm. When I launch, the converter will flash to 5700 and stay there until the car speed is going fast enough for the rpms to start climbing. Once the rpms start to climb above the flash point, I should shift to second gear.

Watch a Super Gas or Super Comp car. They'll launch then a couple of seconds later, the engine is throttled back on the throttle stop. During this time, the car is shifted to high gear and when they come off the throttle stop, they do a high gear pull to the finish line. They could be in high gear a couple of hundred feet off the start line. For them it's a little different. If the don't do a shift at WOT, they're more predictable but it's that high gear pull to the finish that does all the work. Gearing is just to get you off the line. It's needed to get all the weight moving. If you had a 500 hp, 1000 pound car, you wouldn't need gears in a tranny. There would be enough power to get you moving from a dead stop in high gear. Since most cars are typically less than 300 hp and over 3000 pounds, they need all the help they can get and that's where gearing in the transmission comes in.

 

So is the shift point at peak torque, or peak HP?

Sorry guys, I am still trying to make things add up. I took enough physics to be dangerous, but probably not enough to know what I am talking about.

Force = mass x acceleration. At any given point we want to maximize acceleration. Since mass is always constant (unless driver jumps out), we want greatest instantaneous force at all times. Torque = force x radius, so force at rear wheels that pushes the car is [rear wheel tq]/ [wheel radius], right?

HP is directly proportional to tq and rpm. HP is a good measure for comparison because for a given car tq can be altered using gearing but HP always stays the same.

But isn't torque that force that keeps pushing? If that's the case lower gear would always be better because tq is multiplied higher. I would think shift point would be after the tq peak where tq at the wheels and that engine RPM starts to fall below what the tq would be at the wheels in next gear when engine RPM is brought back into the power band.