Thread: Determining Redline...

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Ok, let's say I want to build up an engine. I buy / refurbish all my parts, spend some time carefully putting it together, and am ready to run...

But we don't want to make a window in the side of the block during a moment of Hot Rodding Bliss, now do we?

So what are some ways to determine the redline of my setup? Obviously, an assembly is only as strong as it's weakest part. Some manufacturers will give a max RPM you can run that piece at, but what about the parts that don't necessarily come with that info? I imagine there's a general formula for cast and forged parts, so that would be helpful.

Any other tips, tricks, or advice anyone would like to offer up?

And Thanks in advance for helping make this a GREAT WEBSITE!!!

Either the lowest redline point on any which one of those parts you use, and or whenever you power drops off, if you use quality parts and tune your engine properly i doubt that you will have any kind of power near to the rpm that could cause any kind of serious damage.

I agree about power drop, its like backing up when u get out of the powerband which is why dynos are so popular. Also a little understanding of blueprinting will be very helpful, I recommend reading anything written by Smokey Yunick on engines.

yeah dead bug you will have trouble with this one... since no one goes there.. The power is dropping when you get up there. You're supposed to shift man, SHIFT!

Originally posted by kcress
yeah dead bug you will have trouble with this one... since no one goes there.. The power is dropping when you get up there. You're supposed to shift man, SHIFT!

Ok, ok -- I'll shift! (LOL) Thanks to all who took the time to respond. I should have realized that I'd hit the top end of my power band before I hit redline....

So now that you all got me thinkin' (UH - OH!!!!!), what is the definition of REDLINE? I assumed it was the point where your engine would come apart (at least the point where damage would occur). I also know that there would be a difference between just touching a high RPM and SUSTAINING a high RPM.

When I first posted the question, I think I knew what I was trying to ask, but I didn't do a very good job asking.

When buying parts, manufacturers typically give limit recommendations (but when they do, do they mean peak momentary RPM or do they mean sustained RPM?). Staying within their parameters keeps rods from throwing, valve springs from weakening, etc. I also know there are general rules of thumb on cast vs. forged parts. But what about the small stuff like rod bolts and the like? Shot peening rods with strengthen them, but to what degree over the unmodified part?

I guess the best question is, if I do a reasonable job with part selection, pick forged parts and high strength fasteners, use a good machine shop, and torque EVERYTHING, do I have to worry about making my new engine a coffee table or a boat anchor?

In the mean time, I'll take the advice to get more reading material on the subject. I do like posting questions here, though, because more times than not, you guys fill in the gaps that the books and articles leave.

Keep up the good fight......

When i first started on my journey there was a lot of talk about a 50 cent part on a race car that lost a hi paying race. When engines didnt cost as much they rarely got tore down for a checkup. No body ever had a spare engine either. So when that keeper comes off at the end of the racewith out u knowing it, u start the next race with a engine set up for failure. I heard that the 3 car was running 1 million dollar engines, aaand they still ttook them apart after each race. The hotter the engine the more u need to keep an eye on it. Regardless of how much money u threw at it. Go watch some racing and see how much hi dollar stuff the waste. Hope this helps, there is a lot I still dont understand.

Originally posted by Ives Bradley
Hope this helps, there is a lot I still dont understand.

Yep, every little bit helps. Obviously I'm in my learning curve, too. I probably won't be tearing down the engine on a regular basis, but I do have a spare. I'd just hate to spend all that time and money and then do something stupid while running it. At the same time, I don't want to baby it, either. Why spend all that time & money and not use it to (close to) it's full potential?

That, my friends, is the key......

In my '92 E150 with a 351, it would pull real hard up until 5k, (redline was 5200, motor would explode at 5250 so it was limited at 5200) then it dropped off fast. It was tuned to shift at 5k under WOT... It would move too.

deadbugdug: Thinking more about this... I would check your documentation; cams, FLYWHEEL, CLUTCH, etc.,etc then figure your own rational redline guestimation.

Then armed with this info either take your engine to a dyno shop or take your vehicle to a dyno shop and have them run the curve on your setup. It is very straight forward and relatively quick. Then you will see the actual point that your engine peaks and starts to drop off. You can see if this is pushing your "redline" limit or not. And you will know when you are just thrashing your engine. This info will also tell you exactly what your gear ratio should be.


Seems to me you would have more interest in this info then usual bloke.

Works for me...

this is all good info they've given you so far... but one little bit people tend to forget to leave out.... try and get all your parts with the same matching rpm ranges.... it'll give you much better performance and consistancy.... if you dont match them up, your engine could end up just running like crap, even with high dollar parts

On that last post I mentioned FLYWHEEL, and CLUTCH because as a high school shop guy I had some experince with that..

I took a 283 and put powerpack heads on it. I added a radical cam, very radical! I even drilled and pinned the rocker studs. Then I dropped the whole thing into the only body I could lay hands on. BTW, I built it in my living room, what a great mom. Painted the engine metal flake fuischa. Then my buddy and I had to carry it out the front door and down three steps. Groan! Stewpid kids.

Well I stuck it into, (don't laugh), a 64 white Chevy Belair station wagon, with three-on-the-tree. Hey!, I couldn't afford a nice Hurst shifter!!

Well when you stomped on it, this thing really got with the program. It was my first experience with a major camshaft. It was truly amazing what happened when that cam came on... It just pinned you to the seat! It felt like two more engines suddenly came online! I didn't have a tack so I never knew when it actually came on.

Well one day my buddy with his stinkin FoundOnRoadDead Mustang (it was really nice actually) was ahead of me at the light, after school. We decided to have a quicky. We went across the main street thru the light, I took the right lane and we did a rolling start.

About two hundred feet down the road she came on the cam. Well, it was only a single block race so it was destined to be a short race. But this caused me to keep it in first just a little longer.

About three hundred feet down the road the clutch let go. The bell housing shattered. Chunks the size of playing card decks came thru the bell housing, thru the floor boards, thru the carpet, thru the glove box, thru the dash board, thru the windshield!!!
Other pieces tore seven chunks out of the asphalt, (they remained there for more than twenty years).

One of the playing card decks hit the road then flew 40 feet thru the air and hit a front porch ship-lap wall. It passed thu it like a hot knife thru butter came out the back ship-lap then went thru the front wall of the house stopping against the sheetrock on the inner wall.

The front was cut off the transmission. This caused it to drop on the ground..

Well luckily, no one was injured, very lucky!. A lot of friends appeared. After wiring up the tranny so we could at least push it, we pushed it 2 blocks to my driveway, . After inspection the body was deemed too screwed up to repair. Even the frame was damaged. So I pulled the engine. After a while, I put a stock came in it and put it into my Mom's 67 Impala, <sniff, sniff>

So... I beeeelieve in high quality flywheels, clutches, explosion shields, and REDLINES now. And I check those flywheel and clutch specs. I never use whatever is laying around. I make sure of clutch and flywheel pedigrees.

you got very lucky Kcress.... many people have lost their legs and even their lives to similar events.... most people dont know why race cars have that extra sheilding on the firewall/floor pans til its too late

Thanks to all for pretty good info.....many things I wouldn't have thought of!

Kcress, glad to hear you were ok!

When I get it built, it'll be off to the dyno.....

I like Kcress' story telling. It applies as much to automatic trans cars at high RPMs. A friend with a T350 in his Camaro has the scatter blanket on it, and it's been there for nearly 30 years (probably time to replace - age and improved technology). The C6 in the Shelby had one when it was being raced. I keep telling my son that, for the hot rod shoebox, we need to put the shielding in.

Question I'd like to hear addressed is what the practical limits are for factory balanced (ha) engines from the sixties and early seventies. My 428 was given the full balance treatment, but it always ran out about 6000 anyway. The SBC engines seemed to turn a bit higher without stress.

I noticed the increased smoothness of the engine (small, but there) after all the balance/blueprint work, and wondered where the top for a factory engine would have been, if no other limiting factors other than sudden catastrophic failure (grenading) were in place. And what a stock (non-balanced) SBC would handle if you went nuts on cam, heads, carb, etc.

Any thoughts on that? I can buy a cam for the 428 for up to 7000 RPM, and all the matching parts, but I don't trust the rotating and reciprocating assemblies to hold that.

Thanks!

dug, nobody is willing to step up and give you a definitive redline on a motor because nobody has access to a working crystal ball. It's all about the combination of parts you use. What's the weight of the crank, is it forged or cast, what's the stroke, what's the piston speed in feet per minute, what's the weight of the rods, forged or billet, what's the weight of the pistons. What everybody is saying about the cam is valid. Any cam you choose will have an operating range of about 3,500 rpm's, so if you choose a street grind, depending on your choice, you might be experiencing valve float at 5,000 r's, or 5,500 or 6,000 or whatever. Now, don't hold my feet to the fire on this, but I believe some of those NASCAR motors are buzzing up around 10,000 r's, so the cam they're using is probably making power from about 6,500 to 10,000.

I wouldn't have a problem turning a short stroke street motor to 6,500 or 7,000 r's if it had good rods and rod bolts in it and of course, a cam and springs to support those r's. I'd also prefer a 4-bolt main block to keep the crank planted. Now, since I've said that, I just know someone will come on here and say that they've turned their SBC to 8,500 or 9,000 and that's fine. The question is, how many times can they do it without running over the crank?

Hey Scooter, seriously, how tight have you wound a 283?