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Old 02-19-2020, 08:50 PM
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Default Predator 212cc Hemi, Max RPM?

Just wondering what the max rpm my engine can handle is...

What I really mean is, how high can it rev to? I'm not asking when it'll blow up, since I'm building it. I'm just asking what the point is when the engine just won't rev anymore. Could I go to 8k once it's built?
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Old 02-19-2020, 08:53 PM
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It's possible to break into the realm of 9-10K RPM.......
If your pockets are deep enough and skills good enough.
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Old 02-19-2020, 09:08 PM
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I ask because I'm trying to break the 70mph mark on a 60 tooth sprocket with a 12 tooth centrifugal clutch... And at 52 mph it sounds like I'm already turning 6500 rpms. I guess I need to make my pockets a little deeper but my skills are fine
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Old 02-19-2020, 10:20 PM
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70 eh...
What's the budget and vehicle...


Oh...
That's 70KPH downhill with a tailwind... right...
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Old 02-19-2020, 10:49 PM
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Now that would be moving and take some $$$$
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Old 02-20-2020, 04:32 AM
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70 mph.. aha
that's 31.3m/s
let's assume flat level road and a light kart

150 kg * 9.81 m/s≤ * 0.02 * 31.3 m/s + 0.5 * 1.2 kg/m≥ * 0.7 * 0.9 m≤ * (31.3 m/s)≥
means: 12512.263266 Watts

12.5 kW are about 17 hp

you want a bit extra to compensate for all less than ideal circumstances but yeah.
I've seen topped out gx200 in the 20 horse realms, so that'd be doable at least I guess.

Now you want to do that on a very specific chain ratio.. for some likely to be BS reason.

So let's cheat and use the wheeldiameter as our ratio compensation

with 16" wheels that'd require 7400 rpm
with 13" wheels that'd require 9100 rpm
with 11" wheels that'd require 10800 rpm

11k rpm from a stinking predator?
maybe from a tillotson block.. but never a predator..
9k+ rpm? no.. I'd still say no to that cheap chinese clone block I'm afraid

7500 rpm? well finally.. that might work
now the question is how to not only get to 7500 rpms (which is the easy part)
but also to wedge in 34Nm (25ftlbs or whatever) of torque into the engine
that has maybe 10Nm of torque when it left china (8 ftlbs ?)

trippling the torque will be a very challenging part I assume.

but hey, at least you're about to spend almost as much as Mommoth put in his gx390
(money AND effort...)
for maybe half the power
And I'd still vote for a tillotson block if you want that torque not cracking the engine case open on day one.

Maybe some nice cup of overthinking this first?

'sid
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Old 02-20-2020, 07:01 AM
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I turn my built predators 9500 every lap on race nights.
10k plus isn’t uncommon in open dirt racing anymore.
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Old 02-20-2020, 07:59 AM
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It's been done before. With some extreme modifying, I don't see why I couldn't do it also.

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Old 02-20-2020, 08:00 AM
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Why is 70 MPH always the magic number??? I just gotta ask.

Why do you want to go 70 MPH??????

It's not safe, unless it's a race kart on the Bonneville Salt Flats.

Also Carsandcameras is NOT a great example safety wise. Fun to watch, though!
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Old 02-20-2020, 08:52 AM
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Quote:
Originally Posted by JTSpeedDemon View Post
Why is 70 MPH always the magic number??? I just gotta ask.

Why do you want to go 70 MPH??????

It's not safe, unless it's a race kart on the Bonneville Salt Flats.

Also Carsandcameras is NOT a great example safety wise. Fun to watch, though!
Because 50 just doesn't feel fast enough

And because 80 just sounds too high


As soon as I get this kart built and done then I'll be buying another one. I want the current one to have a crazy top speed and the next one to have crazy acceleration.
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Old 02-20-2020, 09:12 AM
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Right now I believe the biggest limiting factors are the crank, and the head. A majority of people run stock cranks to 9k, but I wouldn't. ARC Racing does sell billet crank stroker kits, but they're $600. Other than that I don't know of any other performance cranks.
https://www.arcracing.com/6572-kit-stroker-kit/
I'd be worried about engine balancing and airflow. I'd say you'd want the rotating assembly to be professionally balanced at those speeds. And you'd need much more airflow than the factory head can provide. Also' you'll want to ditch the hemi head, the valves hit the piston at a lower lift than the parallel valvetrain of the non-hemis. Also the hemis have trouble with spring bind. And you'll want a billet sidecover to limit block flex. Should rev plenty, though you might burn up your clutch. 60t sprocket means at minimum a 12", probably a 13" tire, and with those tires, 5:1 is pushing it, considering that you'll need a higher stall clutch.
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Old 02-20-2020, 09:15 AM
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Quote:
Originally Posted by Flyinhillbilly View Post
I turn my built predators 9500 every lap on race nights.
10k plus isn’t uncommon in open dirt racing anymore.
Amazing indeed... well I'm amazed at least..
I'd guessed at that rpms it's life expectancy would be about 3 to maybe 5 hours of runtime before it bites..
I thought that was one of the main reasons you got yourself some of the nice tillotsons *shrugs*

Anyways
Could you estimate a hp and torque of such 9.5k predator..
and if the wife isn't watching, maybe add how much solo would have to put in parts to replicate?

just to get a good ballpark.
rpms are way easier to find in an engine like that than torque..
so more rpms means less needed peak torque (smaller wheels for better ratios)

'sid
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Old 02-20-2020, 09:32 AM
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Quote:
Originally Posted by J.S.@SMS View Post
Right now I believe the biggest limiting factors are the crank, and the head. A majority of people run stock cranks to 9k, but I wouldn't. ARC Racing does sell billet crank stroker kits, but they're $600. Other than that I don't know of any other performance cranks.
https://www.arcracing.com/6572-kit-stroker-kit/
I'd be worried about engine balancing and airflow. I'd say you'd want the rotating assembly to be professionally balanced at those speeds. And you'd need much more airflow than the factory head can provide. Also' you'll want to ditch the hemi head, the valves hit the piston at a lower lift than the parallel valvetrain of the non-hemis. Also the hemis have trouble with spring bind. And you'll want a billet sidecover to limit block flex. Should rev plenty, though you might burn up your clutch. 60t sprocket means at minimum a 12", probably a 13" tire, and with those tires, 5:1 is pushing it, considering that you'll need a higher stall clutch.
Funny, I can't find any cranks on ARC that say they're compatible with a hemi. Maybe some of the clone ones are. *shrug*

At any rate, a stroker would actually be counterproductive, as it would dramatically shift the power down. Plus it would result in faster ring and bore wear, since a stroker makes the piston move faster.


Honestly the easiest solution is just to get a bike engine!
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Old 02-20-2020, 10:22 AM
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Quote:
Originally Posted by JTSpeedDemon View Post
Funny, I can't find any cranks on ARC that say they're compatible with a hemi. Maybe some of the clone ones are. *shrug*
I put a BSP hardened heat treated crank in my hemi block, the rod and flywheel needs changed to regular honda/clone style, but no worries, if your replacing those anyway.
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Old 02-20-2020, 05:35 PM
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Quote:
Originally Posted by itsid View Post
Amazing indeed... well I'm amazed at least..
I'd guessed at that rpms it's life expectancy would be about 3 to maybe 5 hours of runtime before it bites..
I thought that was one of the main reasons you got yourself some of the nice tillotsons *shrugs*

Anyways
Could you estimate a hp and torque of such 9.5k predator..
and if the wife isn't watching, maybe add how much solo would have to put in parts to replicate?

just to get a good ballpark.
rpms are way easier to find in an engine like that than torque..
so more rpms means less needed peak torque (smaller wheels for better ratios)

'sid
Iíve got around $2K in the one I ran last year, Iím building a new one with the tillotson 72mm block that should be capable of touching 11k itís costing me over $3k to put together.
I wouldnít have any idea of the to and hp, but Iím certain that Iím pushing the limit of the predator block, in fact the block is the only predator piece left in that whole engine. Itís got a stroker crank, ARC rod, wiseco piston, custom billet tool steel cam, billet sidecover, lightweight lifters, chromemoly pushrods, custom built big valve head (32/28) with 55lb dual springs, stainless valves, split keepers, burris 1.3 rockers, raceng S1 flywheel, Honda coil, 28mm flat slide with an NR billet intake, and this whole mess runs on 25% nitromethane. I believe that itís one of the nastiest honest to goodness 212cc engines out there. We have a lot if guys at my local tracks that run engines from big name builders and my little predator will eat them up. I canít wait to see what my new 14.5 cubic inch motor will do. The parts list on that one is crazy. The head alone set me back $700
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Old 02-20-2020, 08:55 PM
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Quote:
Originally Posted by Flyinhillbilly View Post
Iíve got around $2K in the one I ran last year, Iím building a new one with the tillotson 72mm block that should be capable of touching 11k itís costing me over $3k to put together.
I wouldnít have any idea of the to and hp, but Iím certain that Iím pushing the limit of the predator block, in fact the block is the only predator piece left in that whole engine. Itís got a stroker crank, ARC rod, wiseco piston, custom billet tool steel cam, billet sidecover, lightweight lifters, chromemoly pushrods, custom built big valve head (32/28) with 55lb dual springs, stainless valves, split keepers, burris 1.3 rockers, raceng S1 flywheel, Honda coil, 28mm flat slide with an NR billet intake, and this whole mess runs on 25% nitromethane. I believe that itís one of the nastiest honest to goodness 212cc engines out there. We have a lot if guys at my local tracks that run engines from big name builders and my little predator will eat them up. I canít wait to see what my new 14.5 cubic inch motor will do. The parts list on that one is crazy. The head alone set me back $700
Holy moly. 2k? Nah I'm good. I could have a 10 second car for that much.

Looks like I'll stick to whatever I can buy from GPS and see where that gets me. I will update as I go.
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Old 02-21-2020, 08:55 AM
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Quote:
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Holy moly. 2k? Nah I'm good. I could have a 10 second car for that much.

Looks like I'll stick to whatever I can buy from GPS and see where that gets me. I will update as I go.
I get that. I never dreamed that Iíd sink as much money into these engines as I do. I race in unlimited modified, so I have to swing for the fence to even stay competitive. I like building hot rod motors, and these are waaaaaay cheaper than my drag car was. I had over $3k in the cylinder heads alone on that one. I was a sad guy when it fireballed and totally yard saled the engine. I couldíve bought a nice truck for what I had in that engine.
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Old 02-21-2020, 09:11 AM
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2k is too much... OK by how much then... My first comment tried to assess that???
Then maybe a 10sec car is your thing then...

Why are people surprised about the cost of these performance engines.. Ya sure it maybe partly Chinese.. the word performance say "money" right there....

If fast "cheaply", or cheaper than that, is definitively your goal.... then get a motorcycle engine...
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Old 02-21-2020, 10:01 AM
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Camshaft for the most part determines your peak RPM. You can’t turn a certain RPM unless you have a camshaft that has a peak rpm at the range you want.

Unfortunately the highest RPM cams are the ones that require the most supporting modifications to achieve your target max RPM.

For these small blocks, a stock cam will go 6000 +rpm, a solid torquey go kart or mini bike cam revs to 7500 rpm. A midrange cam revs 8500 rpm, and the high revving cams may get 9000+ RPM.

I’ve been 65 mph on a mini bike and that felt like way too much.
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Old 02-21-2020, 12:01 PM
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I had McGee cams custom grind one to match my engine combo. That’s the only way to go in my opinion.
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