Basically, what the title says. Obviously the 350 will need to spin faster, but I want to know what is the RPM beyond which the engine will not make any more power, for both a 350 & a 383.

Curious in terms of how strong the rotating assemblies will need to be.

 

I would say somewhere between 6700-6900 for peak hp using a solid with >250 at 050. For a 383, figure somewhere about 5 less. About 6400-6600.

Both will live at that rpm.. just go with light parts.

http://airflowresearch.com/articles/article32/A-P1.htm

 

If the 350 runs out of airflow at 6700-6900 (by accepting jcb999's number) then the 383 will do likewise from 6122-6305 rpm. They consume air at the same rate at those rpms because 350/383 = 0.914, so the 383 will always match the airflow of the 350 at around 9% less rpm than the 350.

If you want to know at what rpm either will cease gaining airflow, given the use of AFR heads, then you need to answer what cam will be used. IOW you can't really answer the question fully unless you know the flow of the heads plus the specs on the cam.

If you spin either motor at those rpms, you will need to be very careful with the rest of the engine components (i.e. mostly valve train because there is so much involved there, plus rotating mass of the pistons/rods vs the strength of the rods and rod bolts). IOW a lot can go expensively wrong if you don't know what you are doing.

Since you are asking questions about how strong these components need to be..... then you either need to spend a lot of money having someone with experience build the engine for you, or you need to do a ton of reading and be prepared to learn A LOT if you are really serious about this. Otherwise DON'T shoot for extreme rpms. You'll kill the engine and your wallet in the process. HTH.

 

Thanks for the input guys, pretty sure I'll just go 350 & PM rods with good bolts, just debating on whether to go with a cast steel or forged crank. I personally don't plan on going above 6000 RPM, I'd prefer not to have to go over 5500RPM and will probably cam accordingly, so I'm sure I'll be safe with the cast steel.

 

if you are looking for thosr rpm's(5500 or so) why not stroke it and make more power in the range you're working in?jmo.
Eric B

 

The motor will stop making power when the cam tells it to. If you're wondering when the A.F.R.'s will run out of ability to increase power, that depends solely on the airflow maximum, which should be around 0.600 inches of lift. At that point, the heads ability to flow air will begin to depriciate. A solid roller cam is usually the weapon of choice on max effort drag cars. For the street, a hydraulic roller cam is the way to go. It'll never compete with a solid roller due to the oiling problems that lead to valve float around 6000-6500 rpm's, but it's just what you're looking for in your intended rpm range. Sleeper makes a good point concerning the stroker set up, since it will increase your low-end torque output. Low-end torque is important on a street car, so keep that in mind.

 

There are other things to consider. One is the flow of intake and the other is how good the exhaust system is. The heads can flow great numbers but if you can't feed them or exhaust them then the maximum can't be reached.

 

1989GTA makes another good point that in my rush earlier, I forgot to point out. Maximum airflow is a combined package of parts that will help each part in the system breathe to its full potential. If that has been achieved, then the cam is the final decider on whether or not your combo reaches maximum airflow or not.

 

There are some laws of physics that come into play a little before these other "variables" guys.

Its called minimum cross-sectional area at the intake port.
Your average AFR 190-195, has about 2.1 square inches of intake port volume at the pushrod restriction. When the air-flow at that point nears supersonic, the HP will peak (regardless of intake manifold/exhaust or cam).

At that point, you might as well shift gears bc its not going any faster (it might rev higher, but its not going faster).

I dont have the math infront of me, but take my word for it, greater minds than mine have verfied this and for AFR190s, the hp limit is normally <7000 rpm on a 350.

 

Everyone has their own input. The most important thing is to help without letting the thread get overcomplicated to the point where it becomes useless. Bottom line is, the 190-195 heads reach their maximum airflow potential at 0.600 inches of lift. The intake manifold must have excellent airflow characteristics to maximize the heads ability. The F.I. or carb must also be able to feed the engine all that it requires. Beyond that, the cam will be the deciding factor on how the engine performs. If it does not lift the valves up to the 0.600 cfm limit the A.F.R.'s have, you will not extract every last cfm of airflow potential, therefore you will not unlock every last hp the heads can produce. Duration, lobe seperation angle and the intake centerline will all play a part in influencing the redline of the motor.

 

I agree completely, but it happens at more like half supersonic speed, near 550 ft/sec (1100 ft/sec is the speed of sound at sea level). Airflow will drop off (in any head, with any porting) when the Mach Index nears (or exceeds) 0.5

Here's one calculator for it, among others:

http://www.wallaceracing.com/machcalc.php

The original work that discovered the effect is in C.F. Taylor's book. maxracesoftware cites Taylor when he replied in this thread with a good summary:

http://speedtalk.com/forum/viewtopic...abe0f48927c4c1


You can also do a Google search using the keywords

"Mach index" engine

and you'll find others. HTH.

 

The source I was using I managed to find over the weekend. He does quote the same source.

But seems to use the rational that the AVERAGE airflow in feet second should be used to determine the RPM that the peak torque will be generated. It starts with a CID per cyinder number and reasons that the anything more than 240fps results in pumping losses that will prevent the torque being obtained at a higher number. It then adds approximately 2000 rpm for the Peak HP number and depending on transratios, figures what the shift points should be.

Dart and AFR 195 cc heads come in around 2.1square inches.
The AFR210s and Track1s are closer to 2.25.

Airflow wise, a 383 is about 8 percent more than a 355. However, with the additional friction from the longer stroke, peak hp will probably be a little lower than just the 8 percent would imlpy.
There will probably also be a slight narrowing of the range between peak hp and peak torque.

 

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The flow in a channel isn't parabolic (velocity plotted against location in the channel) but it's probably a good approximation here. So the peak flow at the channel center (symmetric channel flow assumed btw) will be some number higher than the avg velocity in the channel (peak at the center, zero at the walls, and sort of smoothly varying elsewhere). So using the avg flow speed * 2 (roughly) is an approximation for the peak flow speed, and thereby fits within the framework of the Mach Index of Taylor.

With 240 fps as the avg flow speed, the peak flow would be around 480 ft/sec, and the sound speed is around 1100 ft/sec. So 480/1100 is 0.44, and that's close enough to 0.5.... which is about where flows will stall. A well designed runner might go up to 0.5 or 0.6, and a poorly designed one at a Mach Index of less than 0.4. I think the calculator is a conservative one, because it tends twds assuming a poor runner. I'd have to get the references of Taylor's orig papers out of his book to see the details in the spread. His book shows only the summary.