Displacement VS RPM - What's Better?

[SquidBonez]

This could be an interesting thread...

I know the old saying is "no replacement for displacement", but what is true karts and minibikes? Is it better to have a built Predator 212 turning 7000-8000 RPM but with a short gear ratio, or a lightly modified Predator 420 turning 5000 RPM with a long gear ratio? Obviously the 212 has the RPM advantage and the weight advantage, but the 420 is putting out more power and torque, just at a lower RPM.

What do you think?

 

[Kartorbust]

I've seen a couple videos and one mini bike race in person. One bike had an actual GX390 and the other, a Predator 420, similar mods on both and the GX390 beat the 420. Theres a lot more going on than just displacement and speed. It's nice to have the displacement when getting out of the hole or out of a turn. It just really depends.

 

[anickode]

All depends what you want.

Realistically, if you want to get more than 35-40mph out of a go kart, you're going to need a built engine, not a bigger one.

If you want ground pounding torque, you're not going to get that from 200ccs no matter how much money you throw at it.

 

[JTSpeedDemon]

Another thought: bigger CC engines take up a lot more space, necessitating a bigger frame, which adds EVEN MORE weight, not to mention the increased weight of the bigger engine.

Also, if the counterbalance shaft on a big block has been removed, they'll vibrate a ton, so that also adds an inconvenience factor.

Also, a big torquey engine with a tall gear ratio will still be hard on the clutch.


Given all of the above, I'll take a built small block any day. Plus the feeling of an engine revving up high behind you is just SO COOL.

And with a low gear ratio, it'll still be quick out of the hole.

EDIT - However, if you want wheelies, get a big block.

 

[Kartorbust]

All depends what you want.

Realistically, if you want to get more than 35-40mph out of a go kart, you're going to need a built engine, not a bigger one.

If you want ground pounding torque, you're not going to get that from 200ccs no matter how much money you throw at it.

Generally speaking yes. However, you can get those speeds with an unmodified engine, but one will need a more expensive CVT. But those speeds are obtainable even with a 10:1 or 12:1 ratio and 20+ inch wheels. But it's about $600+ for the CVT to do it.

 

[SquidBonez]

Another thought: bigger CC engines take up a lot more space, necessitating a bigger frame, which adds EVEN MORE weight, not to mention the increased weight of the bigger engine.

Also, if the counterbalance shaft on a big block has been removed, they'll vibrate a ton, so that also adds an inconvenience factor.

Also, a big torquey engine with a tall gear ratio will still be hard on the clutch.


Given all of the above, I'll take a built small block any day. Plus the feeling of an engine revving up high behind you is just SO COOL.

And with a low gear ratio, it'll still be quick out of the hole.

EDIT - However, if you want wheelies, get a big block.

You could always run a CVT to allow for a taller gearing ratio without burning clutches. But then again, you can do the same to a small block. And now that they have that Juggernaut driver for the small blocks (supposedly rated for 8500 RPM), you can get best of both worlds; low end grunt with lots and lots of RPM. I'm not aware of any torque converters designed for the 1" shaft of a big block that are built for high RPM like 3/4" shaft Juggernaut/junior dragster drivers. And like you mentioned, there's something to be said about hearing clone engines wind up past 7000 RPM! They don't sound like lawnmowers any more!

 

[Kartorbust]

Depending on application, the 94c Duster was good for about 6500rpm roughly might be more or less, can't find the exact details at the moment. These were common on snowmobiles and were used on the FL250 (30mm tapered shaft on the Odyssey and sleds). The Comet 700 (770, 780, 790) were good for 16hp 4 stroke, 30hp 2 stroke. 4 stroke RPM rating of 5500, 10,000 RPM 2 stroke. The other more available option that I eluded to before was from CVTech, however as I've stated, minimum of $600 to get into one of those. But apparently they are very tunable, more so then Comets possibly. Then you also have the Junior dragster CVT clutches that can be had (summit racing sells them).

 

[Brianator]

As far as small engines and the toys they're attached to.... rev 'er out is my vote!

 

[Smerft85]

Why cant I vote both?

 

[Brianator]

Why cant I vote both?

Why can't I have my cake and eat it too!? Because its NOT FAIR! lol

 

[anickode]

Why can't I have my cake and eat it too!? Because its NOT FAIR! lol

Oh it's perfectly fair. Both have their place.

Better yet, big displacement that can rev like crazy. Like a top fuel dragster. 500ci big block spinning 9000+ rpm.
4.5 seconds light to line. 15 gallons of fuel spent. Less than 1000 revolutions on the engine, and time for a full rebuild.

 

[ol'joe]

Anything I build is for chugging through the desert, and with a manual clutch, so I need a gear ratio that will let me move at walking speed. Since I do not feel motivated to add different gears like a MC transmission, if I ever hope to get fast speeds on the rare occasions I get on the road or a nice flat terrain, I guess the only way I will get that done is with mods to the engine, allowing HIGH RPMs.


Mods not only increase top RPM, but expand the RPM range, which has value with a manual clutch.....Joe

 

[Brianator]

Oh it's perfectly fair. Both have their place.

My comment was directed at the question of why he can't vote for both, that's not how a vote works...

Rod ratio is what dictates a engine's potential rpm capability, that's why a 327 Chevy (or 350 for that matter) will outrev a 383 Chevy stroker by a long shot but that stroker is making gobs more torque AND they're built off the same platform! If you could build and engine with a 2:1 rod ratio and everything else was up to the task you could have a 10,000rpm motor on your hands! The shorter stroke (with higher ratio) generates slower piston speeds which allows the cylinder to fill properly at high rpms, the stroker has high piston speeds (with a lower ratio) and cant fill properly much past 5500rpm. That's my words of wisdom for the day!

*edit* in case you're wondering for a future build... you calculate rod ratio by dividing rod length by stroke. Piston height must be calculated into the equation in relation to the deck height and quench volume for piston to valve clearance. Okay I'm done now!

 

[jmaack]

idk i have seen plenty of strokers hitting high rpm. if you throw enough monies at it it will live and make serious pah. I personally know of an 8k rpm bored and stroked 410. originally a 360 magnum.

I know not a small engine. I'd rather have a slower turning engine for longevity just have to find your sweet spot that your wallet can handle.

 

[Smerft85]

My comment was directed at the question of why he cant vote for both, that's not how a vote works...

Since automotive came up, it's similar to asking if I'd prefer a 69 GTO with a 455 super duty vs a honda civic that gets better gas mileage but is built to smoke a GTO, in that case, I'd go with the big block and be sitting in style instead of following the heard.

Kart engines, well I don't really have preference. Have yet to find a running kart that didn't bring out my inner giggling child, I like the snappy fast of the small blocks, but I like the rattling my brainstem loose of the big block, riding harleys works up your immunity to it. I still cant vote with a definite opinion.

 

[Brianator]

idk i have seen plenty of strokers hitting high rpm. if you throw enough monies at it it will live and make serious pah. I personally know of an 8k rpm bored and stroked 410. originally a 360 magnum.

I know not a small engine. I'd rather have a slower turning engine for longevity just have to find your sweet spot that your wallet can handle.

"Speed costs money, how fast do you want to go?" (name that movie).

I didn't say all strokers can't rev I used the 350/383 as an example because the 383 has a poor rod ratio and can't rev as high as even the 350. I'd suspect that 410 has a decent rod ratio to hit 8k (maybe around 1.7:1 minimum) and speaking of Honda Civics.... certain ones can rev to 8k (stock) and just happen to have an excellent rod ratio!

Let us not forget the poll is for small engines, I only used automotive as real world example/comparison.

 

[Kartorbust]

Too bad this is only about piston engines. Rotary engines can rev much above 10,000 rpm.

 

[Brianator]

Too bad this is only about piston engines. Rotary engines can rev much above 10,000 rpm.

I love rotary engines, they can essentially rev until they fly apart!

I'm seeing more people talking than there are votes here.... c'mon guys pull up your socks! Lol

 

[anickode]

Since automotive came up, it's similar to asking if I'd prefer a 69 GTO with a 455 super duty vs a honda civic that gets better gas mileage but is built to smoke a GTO, in that case, I'd go with the big block and be sitting in style instead of following the heard.

Ahh the old boost vs brawn argument... I'd say there's a pretty wide gap to fill between the GTO (67 was a way better year btw ). A modern civic (or any car for that matter) has 60 years of additional engineering behind it. Engines are far more advanced now, with 2hp per cubic inch being quite attainable, even from a naturally aspirated engine, so it doesn't take a whole lot to make a small light car with a small engine "smoke" an old stock behemoth.

Likewise, going the other direction, if you were to build a big block Chevy to modern standards, with higher compression, stronger parts, tighter tolerances, bigger and more valves, variable timing, direct injection, 7000+ rpm redline, and a computer to control it all, stand back and look out. You ain't catching that with a civic.

But that's kind of like comparing a 6 horse hit or miss engine to a 6hp GX200

The comparison of large vs small industrial engines is more straightforward. It's equal technology across the board, and all of them are engineered for longevity over performance.

In my mind, there is no cookie cutter answer. What's best depends ENTIRELY on the application. I would put a built out 20hp 200cc on a race kart frame over a 670 twin. I would put a 670 twin on a 2 seat mini buggy over the angry mosquito any day. And then maybe build it too.

 

[Brianator]

In my mind, there is no cookie cutter answer. What's best depends ENTIRELY on the application. I would put a built out 20hp 200cc on a race kart frame over a 670 twin. I would put a 670 twin on a 2 seat mini buggy over the angry mosquito any day. And then maybe build it too.

Well said!