Jim Stewart

First off I just want to say I am no expert on engines, so please forgive me if I sound like I don't know what I am talking about.

Let me describe my setup before I get into my question. I have a midyear that has what I call a mystery engine in it. The guy I bought it from told me that it is a 69 350 L-46. He said the cam was replaced with a "duntov" cam, but that it is not a solid lifter cam but hydraulic. The car has an MSD 6AL with a MSD billet distributor and an Edelbrock performer manifold with a Q-jet. To the best of my knowledge everything else is stock. There are no numbers visible on the block under the head to identify this engine.

My question is how do you determine what the redline on an engine is, my tach is a 6500 rpm redline tach (I know this has nothing to do with the engines capabilities)? When I get it up to about 5500 rpm it is still pulling hard like it wants to rev further. I never go beyond that because I am afraid I will damage the engine. Is there anyway to figure out what the redline would be?

Thanks.

Nowhere Man

http://forums.corvetteforum.com/zerothread?id=760293

i asked trhat question in the general corvette topics under engine mods :chevy :thumbs:

Jim Stewart

Thanks a lot for that is helpful.

Doug70vert

Where the engine blows apart and where it makes the most power are two different things. I just talked to my engine builder yesterday about setting the rev limiter. He said that there are lots of factors, but usually if you set the limit at 600 RPM past peak HP you'll get max power usage and long engine life. Of course you need a dyno run to find max power point.

JBR

Take it as high as you can very slowly, I mean very slowly. When you start hearing bad things from the engine, write that rpm down and don't go there again. :lol: I'm just kidding, I have wondered the same thing but if you don't know the engine or mods. How would you figure it out? :cheers:

Eric R

1000 rpm over peak torque.

This allows the shift point to fall back into the peak torque area of the power band. Torque is the motivational force that will get you from point A to point B. Once the torque stops, there is little reason to stay in that gear even if peak HP is yet another 1500RPM away. When I was young, I would just lay it to the wood until it would stop pulling hard then figure it from there. Looking back, that was no way to treat a Chevy DZ302. After a while, you will get the feel of the car and the pitch of the motor as it winds up, soon you will find yourself not even looking at the tach as you just "know" when to shift.

GDaina

It could be the the 350 horse cam that was used inte 327 350 horse motors

aharte

No. Lower gears multiply the torque to the wheels (what matters). Shifting up at peak torque reduces the torque to the wheels. You would do better not to upshift for a while. The correct point is however not necesarily at peak power. It depends on your transmision and your engine's precise torque curve.

This has nothing to do with when the engine will blow apart though.

Eric R

Doesn't matter what gears you have since your peak torque and RPM at the crank will still remain the same. While you are correct that lower gears will add in at the end, you still need enough motor to support your gear selection or you will be out of your power band before the end of the 1/4 mile. I am going to stick to the 1000RPM over peak torque unless someone can teach me a new way with data to support it.

aharte

The argument goes like this:

Remember the torque to the wheels is all that matters, so that's what we're calculating. Assume that you have a full torque curve as a function of rpm: T(rpm). You are in a gear with ratio R. The torque to wheels is then proportional to R*T(rpm), where the rpm is a function of speed and R in the usual way.

Now you always have the option of upshifting. Say the next gear has ratio R'<R. The torque in the next higher gear is therefore R'*T(rpm*R'/R).

Think of this like a game. You want to maximize torque, and you have two inputs: the throttle and the shifter. You obviously floor the gas pedal, but now you have to choose the best gear to be in at any given speed. You just have to compare expressions like I've written to see which gives the most torque (with your engine's health in mind too).

It is obvious that we only have to consider two gears at any one time. The others obviously aren't the right choices. It is also obvious that you should stay in the lower gear at least to the torque peak. After this point, T(rpm) is strictly decreasing, and T(rpm*R'/R) is increasing at least for a while. At some point the two expressions above become equal. This is where you should shift. If you stay in the lower gear longer, then upshifting will instantaneously increase torque to the wheels. If you don't stay in low gear long enough, downshifting would instantaneously increase torque. To reiterate, find the rpm where R*T(rpm)=R'*T(rpm*R'/R). This is in general different for each shift. In certain cases of people with really poor gear ratios (eg 1-2 shift w/700R4), your engine's redline is reached before this condition is ever satisfied.

I would bet you good money that this works better than your method in cases where the two don't give the same results (almost always). You can go try it at the track, or if you're bored right now, boot up a computer game or drag simulator.

btw, rear gears never enter the equation. This is a driving optimization, not a way of finding the best parts combo for the 1/4 mile.


[Modified by aharte, 12:04 AM 3/17/2004]

comp

5625 rpm's :lolg:

Vetterodder

:iagree: This is why shifting past the HP peak (not the torque peak) results in quicker accelleration. The next higher gear will have less torque multipication and therefore less hp and accelleration at a given rpm.

Shifting at 1,000 rpm above peak torque might be a good recipe for quicker accelleration for a truck engine but not for most passenger car engines, especially one that makes peak power in the higher ranges. If an engine makes peak torque at 3,500 rpm and peak hp at 5,500, shifting at 4500 is probably giving up 15-20 percent of it's available power and accelleration. Ideally, the shift point will be a good balance between the point that the acelleration rate decreases (hp peak), and where the the rpm in the next gear will be high enough to make enough hp to continue accelleration at the optimum rate.

BacaBill

All this chatter about torque & rear wheels & gear selection,,, what does that all have to do with ENGINE RPM (as it relates to the question)? He wasn't asking how fast he would be going when it blows, or what gear he would be in, or what part of the power curve,, he wants to know "when is the engine going to blowup"? I've only "redlined" once in a '86 Mustang GT, I knew by the sound of it, when I looked at the tach, that's sure enough where it was, at the RED line. Easiest answer is,,, You have to try it to find it. Once you find it, avoid it in the future. :nono:


[Modified by BacaBill, 10:50 PM 3/16/2004]


[Modified by BacaBill, 10:51 PM 3/16/2004]

comp

BLOW IT and TOW IT :nono: :nono:

Eric R

OK OK OK... I can see everyone is going to have their own opinion on this matter. Horse power is only a calculation based on torque as follows: Horse Power = Torque x RPM / 5252. Torque is what gets you where you are going, not horse power. If you plot out your torque curve you will see that horse power and torque will always intersect 5252. This is the most reliable way to cross check a dyno that I am aware of. The only reason that horsepower continues to climb is the fact that torque is dropping and RPM is on the rise. This does not mean that your motor is pushing you harder once you have passed your peak torque, in reality you are only gaining enough RPM so that your shift point will place you back in a reasonable level in the torque band. Example: If my peak horse power was 423@6000 rpm and peak torque was 403@4600 rpm, I will shift at 6100rpm. With a 3.73:1 ratio and a TH350 tranny I will loose roughly 2000 rpm at the shift placing me back at 371 torque @ 4100 rpm. Anything more than this will place me too far into the torque curve and there would be no gain. Do the math, shifting 1000 rpm over peak HP will kill you on your torque. Say for fun, 7000 rpm - 2000 rpm for the shift point will place me 400 rpm PAST my peak torque, not good, not to mention that the extra 1000 rpm past 6000 rpm my torque drops off the graph like a rock.
I stand corrected, I am shifting past my peak HP by 100 not 1000 rpm but only 1500 rpm past my peak torque.


[Modified by Eric R, 11:26 PM 3/16/2004]

comp

:thumbs:

Ganey

The knowledgeable engine builder determines the redline. Get on it in first, shift when it stops pulling. Probably a good conservative redline for you is 6000 or a little less. Depends on how well built. Use the rpm it stops pulling.

If it blows that was redline.

carnut114

On my camaro 350 engine, 373 gears, tci turbo 350 trans when i had 9 to 1 pistons and a 454 lift cam it stopped pulling at 6,000 rpms. When i changed to 11 1/2 to 1 pistons and a 486 lift comp cam i took it up to 6,200 rpms and it kept pulling but i was afraid to push it anymore than that. I put a rev limiter pill in my msd 6al of 6,000 since i don't race it anymore.
Eric

Fevre

Here is a pretty good article that points out the different factors used to determine redline:

http://www.canadiandriver.com/articles/jk/021002.htm

My definition is the highest rpm the weakest link of you eng can operate at and produce max hp or allow proper shifting for drag racing. Seems I read that the car makers do not use absolute redline but use a rpm that the eng can operate at for long periods of time without failure, something like 12 or 24 hours, not sure if that is true but does make sense since that would build in a safety factor.

Doug70vert

We're getting off track here but it's all fun.
this website will help you to find those exact optimum shift points.
It uses the dyno TQ numbers and your tranny ratios to figure things out. Not related to redline, but a good way to know when to shift for max acceleration.
http://www.prestage.com/Tech+Info/Drag+Racing/1121.aspx