'fast-revving' short stroke is BS

[johnc]

no one in their correct mind thinks a 19,000rpm 3 liter formula engine will push a car nearly as fast as an intercooled twin turbocharged 555 cid displacement aluminum engine, running 80% NlTROMETHANE

20% toluene (those frequently make over 3000 hp btw)

 

Spoken like a true engine builder... You're forgeting about weight, packaging issues, chassis, etc. that have an ever larger affect on how fast you can actually go and whether or not you can brake for the turn up ahead and get the car around it.

 

You can't discount "rules" from these discussions. Rules are everywhere and are enforced by government (EPA, DOT) and sanctioning bodies (SCTA, SCCA, FIA). Engines and vehicles are not built in a vacume and as someone who's built a few I'm sure you've been "constrained" from building you're ideal 555cid engine many, many times.

 

And if you put a turbo on that 3 liter and ran it on something other then gasoline you'll probably get pretty close to the power of your 555. Remember, Renault was making 1,200 hp out of 1.5L Formula 1 turbo engines in the early 1980s - on gasoline.

[SpeedRacer]

As usual Grummyvette - you are right on!

 

And yes, I should not have said "everybody wants to rev their engines higher" I should have said HAS TO to rev their engines higher because of the restrictions in the rules.

 

BTW, I copied some interesting info from the Williams F1 site on their BMW engine:

 

Output of the BMW P83 is over 900 bhp.

 

Maximum engine speed is 19,200 rpm.

 

In a race, engine speed is limited to 19,000 rpm.

 

Idle speed is 4,000 rpm.

 

The engine weighs less than 90 kilograms.

 

It completes a distance of 500 kilometres before undergoing revision.

 

It comprises around 5,000 individual components, 1,000 of them different.

 

The air intake volume is 1,995 cubic metres per hour.

 

Maximum piston acceleration is 10,000g.

 

Piston speed peaks at 40 metres per second and averages 25 metres per second.

 

Exhaust temperatures of up to 950 degrees are reached.

 

Maximum air temperature in the pneumatic system is 250 degrees.

 

At the Monaco Grand Prix, the WilliamsF1 transmission and BMW engine have to withstand an average 3,100 gear changes.

 

Sorry, I know this it's Z related, but the technology in a modern F1 engine just amazes me.

[Guest Will]

A lot of knowledgible people have spoken on this, but I'm surprised no one has made much of the rotating mass of the stroker. A 383 has 17% more displacement than a 327, so its indicated output should be 17% higher. Since all the displacement difference comes from stroke, the math isn't too bad.

 

Math lesson: linear acceleration: F=ma (linear acceleration is proportional to the ratio of force to mass)

Angular acceleration: T=Iw (angular acceleration w is poportional to the ratio of torque T to moment of inertia I.

 

In the following discussion, I'm referring to an engine on a dyno, since it sounded to me like the original post was about revving engines by themselves.

The 383 has a longer stroke, which means more rotating mass than the 327. The moment of inertia of an object is proportional to the square of the radius of gyration of that object. The question is "how does the radius of gyration of a crank shaft vary with stroke?". If the radius of gyration is proportional to the square root of the stroke--which is entirely possible--then neither engine will rev faster, because the increase in torque is met by an equal increase in moment of inertia. If the radius of gyration increases faster than the square root of stroke, then the 327 will rev faster than the 383.

 

How fast an engien revs in the car goes back to linear acceleration: the ratio of contact patch force to vehicle weight, which is obviously dependent on gearing and tire diameter.

 

Now, here's a bit more complicated stuff. At a given RPM, the 383 will have 17% more piston speed than the 327, and thus higher friction. I don't know the power of velocity to which this friction is proportional, but if that power is unity, then the 383 will have 15% more bore friction than the 327.

Now, a lot of 383's use 6.0" conrods to improve rod/stroke ratio. If I use 383 pistons in my 327, I can fit a 6.250" conrod. The 383 has a rod ratio of 1.60, while the 6.250 rod 327 has a rod ratio of 1.92. This means that the combustion pressure of the 327 is more efficiently converted into torque and the ratio of IMEP to frictional loss is higher, favoring the 327's revving ability.

 

LOTS of other factors before the engine is even bolted into the car... but I felt like I couldn't leave the moment of inertia of the crank untouched.

[Guest Will]

I'd REALLY like to take the Dart high deck block (9.325" deck height), bore it to 4.125, install a 3.0" stroke crank, pistons for a 428 with 6.0" rods, and put 6.800" conrods into it. Nice cylinder heads, appropriate intake, and 2.7182818 assloads of boost inside said intake...

 

The rod ratio of that engine would be 2.27... extremely efficient, extremely tolerant of boost, and just begging for a combination of high boost and high RPM. It's not very widely known, but short blocks with high rod ratios make good use of combustion pressure later in the stroke than short rod short blocks do. This translates well to high boost (slower rise of combustion pressure and more pressure later in the stroke) and high RPM (slower rise of combustion pressure relative to how much time the stroke takes, and more pressure later in the stroke)...

[Heavy Z]

In the following discussion, I'm referring to an engine on a dyno, since it sounded to me like the original post was about revving engines by themselves.

 

 

My original post:

 

 

In two cars going down the road, at exactly the same rpm(lets say 2500), if both drivers step on it...

 

Good points you've made in great detail, definitely in an engine stand rev-off you've got me convinced the 327 may be very potent. On the road however I'll still put my money on a 383 to out-run(and thus out-rev)a same-geared 327. I'm not so hot on the 383 idea anymore because my 327 already gives me enough traction problems/fun to keep me busy, then again maybe next summer I'll change motors and see what happens.

[Heavy Z]

I like your thinking in your second message Will, that reminds me of a post a few months back about the benefit of long rods as something often overlooked.