Well, for one, it doesn't make much sense to spin much past where your powerband ends. I look at a redline as an RPM which it is unsafe to spin past for structural integrity reasons. The weight, and balance of the internals (combined with lenght of stroke, etc.) help determine the safe RPM limit of an engine. A lightweight, fully balanced engine is going to spin higher than a heavier unbalanced. Likewise, an engine with a shorter stroke is going to be able to spin higher than one of similar size and weight, but a longer stroke. This is do to the rotational forces that put stress on the crank and rods (causing them to stretch with up and down movement).

 

Red Line is when it flies apart. You only get to hit it once!

Manufacturers give you max rpm for intakes and cams. There is also valve float, head runner size, valve size, and carb cfm rating. An edebrock perf will max at 5500 rpm. An xe268h cam will max out at 5800 rpm and the valves will float at 6300 rpm.

What you should concern yourself with is shift point. Watch your tach under WOT. The car will climb very quickly in rpm then the climb will slow right down. You want to shift just before it starts to slow down the climb. This will keep you accelerating at the quickest rate.

If you have a rev limiter. Set it about 500 rpm above the shift point. If your late on your shift you will keep accelerating but not as quickly. If you hit your rev limiter you will slow down considerably.

 

RPM is dictated by the valvetrain with a iron fist, but there are some other factors.

 

What part dictates your red line?.......the weakest one!

 

piston velocity is a key factor I think. spacially depending on the alloy/weight.

If you can get everything else to keep up(ignition/valvetrain/breath/comp limits) I think piston velocity is the one that shouldn't be exceeded.

 

a belief in a higher power

 

i thought the redline was determined on how well the rotating assembly is balanced.

 

piston speed does it in. it will find the weakest link.

 

Like everyone said........it's the part that breaks first.

Larry

 

Malibu73... Dont think your question has been answered just yet...?

Bracketeer has good input to this...
The cam may pull good to 5800 rpm but the intake is limited to be effective to 5500 rpm... The engine will or should pull good up to the 5800 rpm of the cam if the other componets can handle it...? It might tend to flatten out past the rpm rating of the intake due to runner volume and or port size of the heads... A stock built or mild build SBC should have no problem going to 5500 rpm but much above that is where good parts come into play (forged pistons, ARP bolts in the bottom end and so on) balancing will make it live longer for sure at higher rpm`s...

A red line so to speak is determined by the quality of the parts and the assembly of such... BUT...!!! this is not to for tell the future... Even in good high performace parts there can be flaws... Many a good build has new windows in it for what ever reason and some times can be hard to find...


Build it with a target rpm in mind then build it for 1K rpm more and it will survive most likely for a long time... :thumbup:


BTW just saw a cool article on TV about Formula 1 engines... Damn things turn 19K rpm...!!!! No wonder they sound like a pissed off Hornet hive...

 

If manufacturers always list RPM ranges for intakes and cams then why doesnt that always apply in a real world application? Example: My friends (very ugly) 1977 camaro with a very mild 350 ran its fastest time shifting at 6000 rpm but the cam and intake are both rated at a max rpm of 5500. The engine isnt anything too special. Comp 268, 8.5:1 compression, performer intake, 600 carb and SR Torquer heads best time was 14.70. So if the cam and intake were rated to 5500 rpm then why was it fastest at 6000 rpm?

Also, my cam is rated at 2000-6000 rpm but my transmission routinely upshifts at 6400-6500 and it pulls hard all the way to the shift. But it doesnt even start to pull at all until almost 3000 rpm. I think this is largely because the rpm rating on the intake is 3500-8000 rpm. But...

It would seem to me that the cam would be more of a factor in setting the rpm band of an engine more than anything (provided that the springs and rod bolts and other like parts can handle the rpm) because it is what actually lets air into and out of the cylinders.

Furthermore, when you compare similar grinds from different manufacturers the powerbands on the cams can vary greatly. For example...A Comp 280H has a powerband of 2000-6000 and a Crane 286 hydraulic has a powerband of 3200-6200. Both have a 230 duration @.050, the Comp has a more lift but the Crane has a little more advertised duration. The biggest difference is that the Comp has a 110 lobe separation and the Crane has 106 lobe separation. What is the biggest factor in determining cam operating ranges?

 

Blazin72...

Bracketeer has answered your first couple of questions, also you answered your own question with your intake and the aftermarket heads will allow an engine to work better at a higher rpm as well... (read above post first)


As for what determins a cams useful rpm range is figured between the duration and lobe sep. with a simple thought that the lower the lobe sep. then more duration is needed to keep the rpm range... Of course this is combined with the proper lift... Lobe profiles are really wild and different between types of cams as well... Roller cams are just crazy but really get the job done... :thumbup:

 

I think Blazin72 almost answered his own question. You have to consider trans and rearend gearing and the torque curve. Shifting at an rpm above the advertised range may drop the rpm into a better spot on the torque curve, creating better acceleration in the newly selected gear. If you have the actual torque curve of your engine (from a dyno session), your gear ratios, and a pretty good idea of the converter slippage at certain rpm's, you could map out a starting point of where to shift the car. But nothing will replace test and tunes where you record the weather conditions and vary the shift points up and down.

 

Red Line is when it flies apart. You only get to hit it once!

That is the best answer I have ever heard.Great.

 

engine

Shift points and red-line is 2 different things in my book....

Shift point is where you want to shift the engine..

red-line ( as other pointed out) is where things start to come un-hooked...

Most engines will run out of usable power way before the red-line...

The only true way to find the correct shift point is to race the car and test different points untill you find the correct one....

red-line,,,,, well you really don't want to test for that one!!!!!

There is a list a mile long that will change the shift points for each engine combo,,,, some of them are, cam, heads,carb, exhaust, rear gear, weight of car,tune-up,,, and on and on.....

Keith

 

The quality of your rods and pistons play a bog role of how many RPM's your engine can saftly take.

 

FWIW the original post said that it seems to run best at 5500 rpm. I have the performer Intake and a Voodoo Cam with the good old Camelback heads. This combo likes to shift at 5500. Any more and the power just falls off like a rock. It will spin to at least 6000 but why spin harder when you go faster with less spin. I have no idea what my redline is but the parts I have are rated for 6000 as a safe redline with no problem of coming apart. That does not mean it won't take a rew more rpm, but I don't need to find out. I plan on swapping out my intake eventually for one that breathes a little better in search of the ever elusive horsepower.

I think I would not spin it too much harder than where it stops making power but a little more isn't going to hurt (much).

 

You will feel it in the seat of your pants. I have a buddy that likes to rev his 350 4 bolt with a 327 crank waaaay past its power curve. makes me cringe! I know desktop dyno (or any other software) is only for estimating and study purposes, but it gives you a graphic idea of how some aspects of cam timing and compression affect your power curve. I think it was bracketeer said a 5800 redline on the 268. That's about bang on with mine. Both on the software and in my car. It makes me a little nervous to take it higher in a BBC, but when I go for too quick a shift from first it'll stumble unless I do hit 5800-6000. I can only estimate my shifts as the car is a standard and I have no rev limiter.. ahhhh!!! blown shift!! BANG.....silence :drunk:

 

You will feel it in the seat of your pants. I have a buddy that likes to rev his 350 4 bolt with a 327 crank waaaay past its power curve. makes me cringe!

Well a 327 is able to spin a bit higher RPM due to its shorter stroke. Remember, a 350 with a 327 crank is the same as a regular 327. The shared a bore size but differentiated with the stroke.

 

Thanks for answering that guys, the engine is not together yet so I cannot 'feel' the seat of the pants effect until it is together and has the 500 mile break in/oil change.

Thanks for all the information, I will keep it in mind when I'm driving the car.

 

The whole story is multi-layered. First thing's first, combo is everything, I'll use the first post as a reference:

I have a 355 with a cam that goes to 5800RPM and an Edelbrock Performer that goes to 5500RPM. Will that be the redline? 5500RPM because of the intake restriction?

First of all you have to have a balanced combo, which yours seems to be. Rule of thumb on a shift point is 300 RPM above the HP peak of the motor. The reason for that is you're spending the maximum amount of time in the peak numbers, and you're hitting the "nearly peak" numbers twice (on either side of the curve). Further more, when you shift you're sending yourself back into a little bit better area of the powerband than if you shifted right at the peak. So I assume you're shifting btw 5800 and 6000. Do a couple runs, see what works best. Your shift point should always be very close to what I described above. Keep in mind the first thing I talked about is "combo." if you have a motor that's effective in building power to 6500RPM, but your bottom is really only strong enough to be spun REGULARLY to about 55 or 6000, you've built the wrong motor and have the wrong combo. Start over.

Redline is where the stress of RPMs is becoming too great for your parts to handle, whether it be floating valves, stretching rods or disintegrating pistons. The generally held convention on stock V8 motors is 5500 for a max RPM, but usually they don't even make power to that in stock form (more like 4800).

K

 

If manufacturers always list RPM ranges for intakes and cams then why doesnt that always apply in a real world application? Example: My friends (very ugly) 1977 camaro with a very mild 350 ran its fastest time shifting at 6000 rpm but the cam and intake are both rated at a max rpm of 5500. The engine isnt anything too special. Comp 268, 8.5:1 compression, performer intake, 600 carb and SR Torquer heads best time was 14.70. So if the cam and intake were rated to 5500 rpm then why was it fastest at 6000 rpm?

Also, my cam is rated at 2000-6000 rpm but my transmission routinely upshifts at 6400-6500 and it pulls hard all the way to the shift. But it doesnt even start to pull at all until almost 3000 rpm. I think this is largely because the rpm rating on the intake is 3500-8000 rpm. But...

It would seem to me that the cam would be more of a factor in setting the rpm band of an engine more than anything (provided that the springs and rod bolts and other like parts can handle the rpm) because it is what actually lets air into and out of the cylinders.

Furthermore, when you compare similar grinds from different manufacturers the powerbands on the cams can vary greatly. For example...A Comp 280H has a powerband of 2000-6000 and a Crane 286 hydraulic has a powerband of 3200-6200. Both have a 230 duration @.050, the Comp has a more lift but the Crane has a little more advertised duration. The biggest difference is that the Comp has a 110 lobe separation and the Crane has 106 lobe separation. What is the biggest factor in determining cam operating ranges?

Besides what other people have said, I think a big key factor here is cam timing also. If your cam is retarded then your power band will be moved up, regardless of what your cam is spec'd. So for example, if your buddy's 350 had it's cam retarded by a certain number of degrees, that 5500 rating can jump up to around 6000rpms, but will also pull the beginning of the bandwidth up with it. Same thing goes for your cam. You can do the opposite too by advancing the cam timing and bringing the power band in quicker

 

What part dictates your red line?.......the weakest one!

AMEN to that
i was going to say your wallet

 

My engine has 2-bolt mains, so I don't plan on revving it much past 5500. Maybe up to 5800 at the max but it all depends on where my powerband will be.

We're also planning on degreeing our camshaft so it sits at 0 degrees.

The motor's going to have a quadrajet carb on it for torque faster in the lower RPM's and we also have a distributor recurve kit for our HEI dist.

 

My engine has 2-bolt mains, so I don't plan on revving it much past 5500. Maybe up to 5800 at the max but it all depends on where my powerband will be.

We're also planning on degreeing our camshaft so it sits at 0 degrees.

The motor's going to have a quadrajet carb on it for torque faster in the lower RPM's and we also have a distributor recurve kit for our HEI dist.

You might also call your cam manufacturer as many grind a 4 degree advance already built into it.

Just be careful with spining it that high. If your setup isn't making power at 5800RPM, there is no need to spin it that high. You'll only be slowing yourself down.