My 91 GTA has a redline of 5,000 RPM. That is for the stock motor. I have since rebuilt the motor with better components. Eagle cast crank, speedpro lightweight rods, speedpro hyperpuretic pistons. Granted, these are not great components but better than stock. Everything has been professionally balanced and blueprinted.

Since the components have been upgraded is my redline now higher? If so, what would a safe limit be? Is there a formula that can be used?

Thanks!
Josh

 

your redline depends more on reciprocating weight, pistons, rods, valves, and valve spring pressure and having proper clearances. but if the airflow capacity and cam profile are not designed for high rpm operation the additional rpm will do you no good. my $0.02

 

I have the Accel/Lingenfelter runners and manifold along with AFR heads. So, I am guessing I will be able to get a little more out of the engine than the stock TPI. It was more of a curiosity question than anything. I was breaking in the motor and did not want to redline it during the initial run, which made me wonder if my redline was different from stock.

 

Well it may be it may not be. If it was your bottom end that was what was limiting you then yea ide say it would increase it. However it may be the valve train and even if it did increase who knows to what? Basically ide keep it under 5.5k.

 

Under 5.5K? Maybe if he has stock valve springs, but if his valvetrain can handle it, 6.5K should be fine. My stock LG4 with 193K on it was never damaged by 6K, but it was somewhere around there or maybe just past that it would float the valves.

 

Correction not that you know of. See the thing about red lines that exceeding your red line wont instantly cause a problem. I mean really a rev limit is the RPM you cannot exceed without ill effects. I mean sure theres valve float that could drop a valve but it goes far beyond that. You can look at tons of videos and see where people put bricks on there gas pedal and wait till the motor blows and it takes upwards of 5 mins in some cases to really kill kill them. Why is this? Clearly the rev limit has been exceeded as it blew but why 5 mins? The thing is that motors dont wear at a linear rate. Meaning motors dont wear twices as fast at 2k as they do at 1k. Wear grows exponentially where there isnt w huge difference in wear over the normal rev range thats considered out of the norm. However once you exceed the rev limit you start to wear your motor out at an accelerated rate thats not considered normal AKA damage. If you exceed it by enough eventually you will break a rod or something and have some sort of instant catastrophic failure but those kind of RPMs arnt really attainable as other things like the valve train would fail first with valve float or heads not flowing enough killing the power so you physically cannot continue to rev up. So what im saying is taking your motor up to 6k for one second although may seem to take it like a champ may have also shaved 10k off its life. Try running it continuously at 6 k and things like a rod knock will develop much much sooner than if running at even 5k. I mean even things like balance GM knows how accurately the bottom end must be balanced to be reliable up to 5.5k and they build in a tolerance to guarantee the balance of the bottom end will be good for atleast 5.5k but pushing it beyond that who knows what will happen the forces may be so out of balance you start to have mettle on mettle contact in the bottom end. Its hard to know without really getting into the science and physics of it to back with testing as to know what the true rev limit of things are but GM set the rev limits at 5.5k for a reason and used weak springs to intentionally induce valve float as rev limiter of sorts. Pushing it beyon that can cause ill side effect that wont be known till later.

 

But we aren't talking about stock parts...
All the components he chose seem far better than stock, and for any motor under 6.5K or maybe 400 HP (it's hard to tell exactly) then even a stock crank and rods would be fine.
I'm hoping my stock crank, stock rods, unbalanced, small journal 327 will be good for at least 6.7-6.8K RPM, and will make at least 450 HP. But, I unfortunately won't get to even start it until spring.

 

Well the short stroke of the 327 helps a lot. Personally i dont know if ide trust it up that far but again i cant prove anything without testing or some actual numbers so your guess is just a valid as mine. However even in his case even with better parts who knows what its limit really is? No doubt the bottom end is better than stock but like the bottom end was balance but to what tolerance? The pistons are light weight how light? Theres many factors that go into it but unless it can be proven that his max rev is i cant suggest going over 5.5k. Maybe the machien shop that has direct knowledge of his build could recommend a max on the bottom end but that dosnt cover the oiling system or anything like that. I mean even the mechanical fans have rev limits and who knows what they are lol. Its just unless it can be proven otherwise i couldn't recommend exceeding the factory rev limit although again without testing who really know 100%? The only way you really know is when youve exceeded it but thats a hard way to learn.

 

He said he has aftermarket hypereutectic pistons, which will usually weigh less than half of what a stock cast piston does. With new bearings, even the stock oil pump should be able to make enough PSI at 6-6.5K RPM, but who rebuilds a motor and keeps the old oil pump?
He shouldn't have a mechanical fan, unless he traded his stock electric fans for it, so that shouldn't be an issue.

 

Are we taking redline to mean max rpm thats safe for the motor to spin? I always thought it ment max rpm that the motor is good to in terms of power production, and would be set by the rev limiter.

If a motor peaks hp at 5500 rpm, then shifting in the 5800-6200 rpm range would work well, so redline would be about 6200-6400 rpm. Redline would be about 100-200 rpm higher than max shift point as thats where the rev limiter would be.

My 383 peaked at 6300 but held to 6500+ rpm, possibly dropping off at 6600-6700, so i'd shift in the 6600-6800 rpm range. I set limiter to 7 grand. That was my redline

Going any higher than that will actually slow you down. I take redline to be dependant on cam/heads/cubes/intake etc which determines where power band will be. Components such as crank/rods/pistons may be good to 8K rpm, but doesnt help if the motor is done at 6500

Just my thoughts, maybe my definition is not right.

 

Thanks guys, I really appreciate your thoughts. I do not plan to run this motor super hard after all of the money and time that are in it. I was just curious as to what the limits of the motor are.

The oil pump is new and is standard volume/pressure.

I am running the stock electric fans.

 

Yeah, that's what I was talking about, but, I totally agree that there is no reason to ever do more RPM then a couple hundred past peak power.

 

Well the mechanical fan was really just an example as to how far rev limits go. And even with a new pump who really knows what its max potential is? Beyond all that though it really boils down to this im trying to say is he asked for a rev limit and I nor anyone one else can really say with certainty what that is. Its a real hard thing to prove with hard facts what the limit is. You can recommend up to 6-6.5k like i said i cant pull together any evidence that its not capeable of thoes RPMs just like i have no solid reason to believe that its safely capeable of that so i cannot recommend beyond stock. In the end hes free to do as he pleases and your free to have your oppinions on the matter and say that my estimates are conservative but again unless I have hard evidence to support that his motor can withstand thoes kinds of RPMs I cannot and will not recommend it as that would be pretty irresponsible of me because if i was wrong its his problem if the motor blows up not mine. Its always better to air on the safe side.

 

Ok. I feel rpms will still be somewhat controlled by the valve train, once they start to float I would consider that unsafe for the motor and consider that "redline". Weak springs floating valves at 5500 could happen, even tho the bottom end is good to 6500.

Bottom ends can spin up ok, even cast type stuff. Clearances are good, oil pressure is good, and no detonation they should handle the rpm load. Power load is abit different tho. May handle 6500 at 400hp but not 6500 at 750hp.

For the above motor, if all balanced and assembled correctly with good clearances, I dont see why that setup cant spin up to 6500 rpm. Wouldnt do it unless you had the heads/cam/etc to do so.

 

I am guessing my peak power is around 5,500 based upon the cam. So, not much of a reason to go beyond factory anyhow.

 

5500 peak you can go to 6000. Redline in my eyes would be 6200. No reason to stress it beyond that

I used to take my stock L98 to 6g when i had the HSR intake. It had 5200 rpm peak hp and fell off quickly after that but I still used to wind out at 6 just to hear the motor roar. At the track it ran best with a 5500 rpm shift

 

I think with AFR heads his valvetrain should be good for higher RPM, and his bottom end seems tough enough to, and if you think peak power is around 5.5K, I see no reason that you would ever have to worry about over-revving your engine.