A question for mechanical engineers (engine RPM related)

I would think that’s highly specific to components used. Piston weight, materials, rod composition, bearing type, yada, yada.

My guess is manufacturers run to destruction…

(rpm destruction)X0.8= redline…

 

Maximum engine RPM is to consider many factors. A basic rule of thumb is to calculate piston speed. For that, multiply the stroke by PI, and then divide by 12 to get your piston feet per revolution. Then take that figure and multiply that by maximum engine speed to get the maximum feet per minute . Typically, that will be some where around 4,700 FPM (feet per minute) piston speed. So, in reality, the shorter the stroke, the higher the engine RPM's. As an example, try turning a big block Chevy at 8,000 rpm's with out catastrophic results in it's stock form. But trying to do that with a Ferrari or similar motor with a short stroke, it will not be a challenge. Yes, other factors such as are the rods strong enough, such as Carillo's, and heavier springs in the valve train, and short skirt light weight pistons all are part of your maximum RPM factoring.
Gary Bobileff

 

No they dont. Redline is almost never due to structural limitations except in racing engines. It is far more commonly just a little above maximum power.
My supercharged Ford F 150 Lightning is around 5400. The engine mechanically is capable of far higher speeds but any faster yields no more power. I had a modified Datsun 510 many years ago. It would easily go over 10,000 but anymore than 8700 or so was a waste of time At higher speeds pistons and valve spring life was very short. Ferrari 355 was designed to be a 10,000 rpm engine but durability testing showed timing belt life was unacceptable so it was lowered to 8500. A lot of factors are considered but typically not material maximum limitations.

 

Isn’t there an rpm at which pistons hit valves… at least on interference motors?

Diesel truck motors have a redline under power, and a redline under engine braking… the latter from what I’m told prevents piston departure.

 

A. Yes there is but is is physics the limits it. Diesel is very slow burning so they simply make no power at a speed gas engines can make good power. Also due to the stresses involved they have very heavy reciprocating parts. So unlike gas engines they do have structural limitations and if your Cat diesel doesnt last 400,000 miles you'll be pissed so the speed limits are to assure that can happen.

Typically except on high rpm race motors there is no way a valve will hit a piston simply from floating. It needs to have a failure that causes a cam timing issue to allow it.

 

If you knew the details of the engine it would be easy to ballpark. Generally you would build something with goals in mind, such as how much power and where you want to make it, choose components accordingly.

 

This will answer all your quextions. Math is easy and the information is straightforward......https://www.amazon.com/Design-Tuning-Competition-Engines/dp/0837601401

 

Increasing rpm limits can yield more power but only if the design is considered 'as a whole', including flow through head and valves, cam profile as well as stroke length, safe max piston speed, etc. Ofcourse increasing rev limit can also place more stress on engine components such as bearings and hence when a high rpm design is being considered so is high up the list the minimizing weight of reciprocating masses, and it goes without saying that such engines should be extremely well balanced. Hence why some engines have hollow valves and titanium con-rods, lighter pistons, etc. How far you can or should go depends on so many design factors that its quite difficult to generalize.

The video in the link explains a lot of the questions being asked in an easy to digest fashion. Many argue you can make more power and torque say with an increased stroke length but ultimately that could increase piston speeds too much for your engine design so may require you to reduce maximum rpm, even though you'll be making more power. So max rpm isn't always the limiting factor to making more power, there are many ways to make power than just via revs. If you cannot flow enough air through the heads and intake valves or get the gasses out quick enough there isn't much point revving higher as an example and as others have mentioned there are other important considerations such what revving so high does to impact on say other engine system components such as cam belt tensioners, belts, etc.

 

On an old car you do also have to factor for ignition. Point ignition has some serious limitations at high rpms: the fixed dwell means the higher the rpm, the less the charging time for the coil (i.e not enough energy to produce the required amount of sparks). Driving a 50's V8 at high rpm, would certainly require multiple coils.

 

What are we considering "high rpm" for a 50's V8? As most would run out of breath long before a single point ignition couldn't keep up.

Most made peak HP well below 5k.

I did some head work and cam reground on my 59 Cadillac and still barely turn it over 5k.

 

I ran standard point ignition on a Datsun that went to 10,000 and had no point float. Ferrari ran points on 8000 rpm engines.

 

Instead of relative to max HP I would probably consider relative to max torque? I would assume that once max torque is reached we start to play with the physical limits of the engine (even if the efficiency loss is partially compensated by the higher rpm).

Assuming the torque curve is a perfect sinusoïd, at 2x you would have 0 torque left. So if your engine max torque is at 3000rpm, no way it can reach 6000rpm.

On my cars max rpm is rougly 1.5x max torque rpm, this seems consistent with other vintage v12. Maybe that one can play with a higher ratio but in any case it will be closer to 1.5x than 2x

 

Answer : No... (your engine is obviously not a common one)

Engine modelling is not only about mathematical formulas, it's also about simulations and iterative scenarii. You can get away with the later when you do use a "common" design. Not in this case.

What's your objective?
1) If you do want to add a rev-limiter in order to save a valuable vintage engine I would just suggest to follow the original manufacturer recommendation.
2) If you want to gain a bit of power-band, I would think about-it twice, as this engine is probably not as strong as what it was when it left the factory 50 years ago. Also based on your description, there is not much left to be reclaimed.
3) If you engine was recently upgraded, the mechanic who did the job should give you some direction.

Assuming you want to experiment: at first sight I would say that your engine heads are not overly optimized, so unlike a revy Ferrari that will happily drive itself to self-destruction, your engine should show the limits of its valve-train before you get a chance to destroy the block (i.e less costly in case of failure). So unless you want to invest in proper engine modelling you can empirically test a bit more rpm until you feel uncomfortable with the noise of the valve-train. Did this on my Renault 5 turbo cup, Not for the faint of heart (and unlike your car my "cleon" block was quite easy to source in case of failure).

 

I have a 1981 Ferrari 308 GTBi with only 13,600+ original verified miles. It has a blown engine from a mechanic not locking down the cams while doing a belt change. I have an estimate from Carobu Engineering, New Mexico for $25,000 to $35,000 to repair. Question will a 355 motor fit into a 308 engine bay. I know that the 308 is transverse and the 355 in longitude. But could it be done without messing with the frame? The 355 has 18000 miles and his car is as pristine as mine. Everything comes with his motor, engine, transaxle, radiators, air boxes, linkage, intake plenum, exhaust system wiring harness, everything attached to the engine comes with it for $18,500. What do you think?

 

I think you should start a new thread for this as it has nothing to do with this thread .

I think I recall seeing a 308 with a 355 powerplant here, but it's got to be a huge amount of $/work (in addition to the $18,500). I'd also guess that you'd lose the rear boot area with the gearbox behind the engine.

Edit: a couple of prior threads here:

https://www.ferrarichat.com/forum/threads/308-based-288-gto-with-355-engine.38256

but looks like you need to do the longer wheelbase change ala 288GTO too.