Hot Rod Forum banner
1 - 20 of 26 Posts

·
Registered
Joined
·
328 Posts
Discussion Starter · #1 ·
When you buy a crate engine or car, GM or whomever might recommend max rpm to be say 5000rpm or so. Fine, i'm not arguing that.

When you start swapping components or building your own engine, how do you determine what the maximum/safe rpm is for the engine?
 

·
Race it, Don't rice it!
Joined
·
8,396 Posts
Experience and you're trust in the parts. Generally, no need to turn faster than it'll make power.
Boot77 pretty much has it down. There not a relaible mathematical calculation or anything.
 

·
True Hotrodder
Joined
·
1,954 Posts
Well, when a rod is sticking out of the pan, that's usually a good indicator that you found the upper limit!

There are some calculators online that can give you a number, although I think they are a little conservative. Rod ratio can play a big part and of course when the engine simply runs out of air that can be another indicator. Take a mid-60's small block chevy and normally people would say 6500 was the upper limit but a bunch of us used to turn those puppies to 7300-7500 pass after pass and that was with stock rods and bolts.
 

·
Registered
Joined
·
4,448 Posts
I have seen some bone stock small block chevy builds from the 70's with nothing more then old factory rods with factory bolts and were resized and cast cranks that were grounded under size and just stock cast hyper pistons and they were ran upwards to 6500 rpm on a regular basis with hydraulic flat tappet cams back in the 90's in dirt track racing with just junk yard car bodies while they were still available in my area. I don't know if that is what is called circle track racing or not but it was a dirt road circle track and just stock cars with nothing fancy and were beaters but it was fun watching when it was something that could be done with the common folks who used to like doing that.

I don't know if it still exists anywhere else in the U.S. but here were I am in Ohio I have not seen anything like that for over twenty plus years. The motors were nothing fancy and a lot of them was the same for Ford with a lot of 302 and 351 builds and the old Mopar 360 small block. They seemed to hold up pretty well. I can't say for how long but my Dad used to race them all the time and I never saw a failure on one ran for a whole season.
 

·
Registered
Joined
·
170 Posts
Shifted my 355 stock crank & rods with cheap Northern rebuild kit flat tops at 6000 mostly but 6500 many times when racing because I thought it had ARP rod bolts(young and didn't check the shops work). Cam went but otherwise I still got it and I'd run it with new bearings, a quick ball hone and some diff heads, fresh cam of course LOL
 

·
Registered
Joined
·
328 Posts
Discussion Starter · #7 ·
So.....it's basically use your best judgement? And, it's more a function of valvetrain than bottom end rotating assembly (crank,rods,pistons)?

Although, i'm generally more of a fan of longer(long as possible) stroke, which also tends to limit rpm?
 

·
Registered
Joined
·
510 Posts
from an engineering point, piston speed can be a factor. The manufacturer can tell you their limits. With rocker shafts and new spring technology 9k is not crazy. Nascar engines run 9+ for 500 miles. Formula 1 engines have run to 20k. Motorcycle engines have run up close to 20k. Thats probably close to the limit as there isnt enough time to fill the cylinders with air and fuel.
 

·
Registered
Joined
·
4,448 Posts
Good assembly and proper machining of the parts and good judgement with experienced eyes and hands can go a long way on a basic build. I have had several budget builds done back in the day and my Dad would always take my old 86s10 I used to own out and just romp the heck out of the thing and about threw me through the window into the bed lol as it would just take off so quick at just hitting it to 6500 rpm with no issue with tires a blazing all over the place.

It was just a bone stock bottom end with flat tops and a set of world product sportsman 2's with an old comp cam magnum 292h flat tappet cam and it held together the whole time I had that engine and it still exists today but not in a vehicle but still held together for many years after I sold it.

I also once had my one and only stroker build and was a 377 cubic inch aka 383 but it had a standard bore as the block was new and the rotating assembly was a used 400 small block crankshaft that was grounded under and the rods were the poorly made 5.565 factory rods that looked anorexic to a factory 5.7 rod in my opinion or it was just because it was that much shorter and I had TRW forged pistons on that rotating assembly and I had it up to 6000 rpm plenty of times with an old Crane flat tappet cam that was pretty wicked at 246/[email protected] with 500/500 lift and a 106 lsa and it had a ton of torque.

That engine held up pretty well but I can't remember what happened to it as I have changed things one to many times in my life.
 

·
Race it, Don't rice it!
Joined
·
8,396 Posts
Valve train:
Weight.....Lifters, push rods, rockers, keepers and retainers all weigh something.
Pistons:
Weight.....Pistons, rings, pins, bearings, all weigh something too. Then you have to find those parts that will stay together while changing directions every 2 to 4 inches. The momentum is what pulls the pins out of the pistons, pulls the caps off the rods, breaks the crank and breaks the main caps.
You can add weight to add strenght but the added weight reduces the maximum speed so what your paying for is someone who can figure out how to make things lightweight AND strong. It's not an easy feat and all the [email protected] costs money and time.
There isn't a straight answer to give you.

My 2.1cc single cylinder 2 stroke RC car engine turns 30,000 rpm and makes 1.1hp
It'll move a 6lb car up to about 45mph.

Generally speaking, Shorter strokes and smaller bore means more RPM capability.
 

·
Registered
Joined
·
4,406 Posts
When you buy a crate engine or car, GM or whomever might recommend max rpm to be say 5000rpm or so. Fine, i'm not arguing that.

When you start swapping components or building your own engine, how do you determine what the maximum/safe rpm is for the engine?
Two things, where the motor stops making power. Once the HP curve starts dropping, then there is no reason to continue trying to push it harder. When a crate motor ad lists that max rpm is 5000 rpms, then I'd safely assume that the cam and heads are all done by that point - quite typical for a 250ish HP crate motor.

Second, the limit of the parts. As mentioned, rods, rod bolts, valve train stability, heads, intake, exhaust, etc. I'm not familiar with any SBC that won't withstand 6500rpms occasionally and 6000rpms regularly.


Are you planning a build or already have a motor put together?
 

·
Hates: Liver. Loves: Diesel
Joined
·
6,111 Posts
When an engine says 5000 rpm redline, it's not necessarily because things will explode at 5500. It means that the cam, valvesprings, compression, and airflow are matched to make power in a certain RPM range, and revving it past 5000 is pointless and makes the car slower. No reason to risk the extra wear/fatigue on parts for less power.

Most stock SBCs will take 5500-6000 without falling apart. I had a Detroit 60-series that "redlined" at 2200, but it would be mechanically fine at twice that RPM without damaging anything.

where you get into trouble is if you choose components that move the powerband up to something above what is safe for the moving parts. If you put a cam and heads that would work well shifting at 6500 with cast pistons and rods, then you get things exploding
 

·
Registered
Joined
·
170 Posts
Lots of reasons why they may suggest 5k rpm Generally that low, it's not the bottom end for a SBC(other engines it may be)

I like to call the SBC a leaky dam VS the stock LS that is well built, plug all the holes and the SBC will perform(course the LS has a few holes as do most stock engines).

TPI intake is a good example, people have constantly said it's done by 5k or w/e. It may be peaked but it's not done if you plug the common SBC leaks. Then the power doesn't drop, it flat-lines and why would you want to shift at 5k n have less average power? I hate to bring it up because I know people will debate and of course the TPI intake is the limit and others will out perform it. But just because there is a limiting factor doesn't mean you can't shift higher IF the engine will hold, just look at Stock Eliminator.

edit: Corrected for the wrong type of racing.
 

·
Registered
Joined
·
2,013 Posts
Good assembly and proper machining of the parts and good judgement with experienced eyes and hands can go a long way on a basic build. I have had several budget builds done back in the day and my Dad would always take my old 86s10 I used to own out and just romp the heck out of the thing and about threw me through the window into the bed lol as it would just take off so quick at just hitting it to 6500 rpm with no issue with tires a blazing all over the place.

It was just a bone stock bottom end with flat tops and a set of world product sportsman 2's with an old comp cam magnum 292h flat tappet cam and it held together the whole time I had that engine and it still exists today but not in a vehicle but still held together for many years after I sold it.

I also once had my one and only stroker build and was a 377 cubic inch aka 383 but it had a standard bore as the block was new and the rotating assembly was a used 400 small block crankshaft that was grounded under and the rods were the poorly made 5.565 factory rods that looked anorexic to a factory 5.7 rod in my opinion or it was just because it was that much shorter and I had TRW forged pistons on that rotating assembly and I had it up to 6000 rpm plenty of times with an old Crane flat tappet cam that was pretty wicked at 246/[email protected] with 500/500 lift and a 106 lsa and it had a ton of torque.

That engine held up pretty well but I can't remember what happened to it as I have changed things one to many times in my life.
A 377 is a 400 cid ( 4.125 bore) with a 350 cid ( 3.48 stroke , while a 383 is a 350( .030 over) 4.030 bore block with a 400 cid stroke (3.75) . different block , bore ,& crank .
 

·
Registered
Joined
·
510 Posts
Boo77? what are you saying? Sounds like non sense (re tpi flat lining). If you rev the tpi to 7k while it remains at 220 hp your torque will fall off the graph so, YOU ARE LOSING POWER! Torque and HP are just numbers we use for discussion.

Super stock engines use tunnel ram intakes allowing (or forcing) more air into the engine. Lots of science in building a SS engine.
 

·
More for Less Racer
Joined
·
20,556 Posts
A 377 is a 400 cid ( 4.125 bore) with a 350 cid ( 3.48 stroke , while a 383 is a 350( .030 over) 4.030 bore block with a 400 cid stroke (3.75) . different block , bore ,& crank .
Actually, the 377 is 4.155" bore, .030" overbore of the 4.125 stock 400 block with a 350 crank.
At standard 400 bore of 4.125, it is a 372".

Most machinists call the standard bore 4.00" 350 block and 3.75" stroker crank package a 378,.... to keep it from being confused with the big bore/shorter stroke 377.
 

·
Registered
Joined
·
170 Posts
Boo77? what are you saying? Sounds like non sense (re tpi flat lining). If you rev the tpi to 7k while it remains at 220 hp your torque will fall off the graph so, YOU ARE LOSING POWER! Torque and HP are just numbers we use for discussion.

Super stock engines use tunnel ram intakes allowing (or forcing) more air into the engine. Lots of science in building a SS engine.
I never said 7k, the stock bottom end wouldn't hold. Stand by what I said if supporting parts are good power may stop climbing.

Meant Stock Eliminator racing, I'm not talking tunnel rams.
 
1 - 20 of 26 Posts
Top