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Old 05-02-2018, 07:31 AM
pearl111 pearl111 is offline
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Default Looking for more RPMs

I have a couple of hemi pred. 212s with the stage 1 upgrades (header, air intake with filter. gas jets) and governor removed. This gives me a little more RPMs (51000) I'm guessing. So for more RPMs If I install billet flywheel billet rod and stiffer springs will this improve my RPMs? I would like to get close to the 7000 range. Or is this an unreal goal? Thanks for any help.
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Old 05-02-2018, 08:09 AM
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Well, you'll need the flywheel and rod to run those kinds of rpms. But, you'll never get there without a cam.
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Old 05-02-2018, 08:17 AM
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Gotta run a hot cam and a big carb if you want to make any power at those RPMs.

Your stock carb, regardless of jetting, simply won't flow enough air.
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Old 05-02-2018, 09:22 AM
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Quote:
Originally Posted by pearl111 View Post
I have a couple of hemi pred. 212s with the stage 1 upgrades (header, air intake with filter. gas jets) and governor removed. This gives me a little more RPMs (51000) I'm guessing.....
I'm quite sure you're guessing WRONG
(well I'm guessing you have a typo there *giggle*)

Anywhoo.. 7k is in fact achievable and doesn't exactly require a cam at all (it'll help alot though)
But it indeed requires some durability upgrades (conrod, flywheel are HIGHLY recommended here! everything else is considered stupid tbh)

The revlimit you are facing now is mostly due to valve float.. stiffer springs can overcome that (18 or 21lbs I'd say should do)
that might even already get you up there.. but you indeed need to make sure the engines not choking up there..
gx390 carbs help.. but chances are you can get very close with the stock carb already
but it's indeed much easier to just upgrade the carb to a larger size and not with flow optimisation and jetting

Now, in order to maintain some power up there you want to advance your ignition timing
(generally that's done with the billet flywheel itself.. modern ARCs are either adjustable or have an inbuild 8 timing advance)

And on some engines that's already all there is to it.. (no cam!)
some are a bit harder to push over the cliff,
and you indeed need to stiffen the pushrods in order to not constantly bend them,
and some indeed want a cam (duration should suffice, timing might move the powerband to your desired rpms but if you're after mostly low end power and just the occasional speed run.. there's no real need to move the powerband IMHO)
And depending on build quality of the engine
some even need their heads to be cleaned and worst shaved...

I'd say with advanced timing , conrod, flywheel and stiffer valve springs alone you'll at least reach 6.2rpms 6.5 is likely... and with a little luck maybe even more ...

Cam.. yeah makes sense to actually get some performance out of those upper rpms..
picking one is a science in and off itself.

'sid
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Old 05-02-2018, 10:22 AM
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Thanks everyone for the help. I saw that Go Power Sports has a stage 3 kit (minus pushrods) that would get me to the RPMs I'm looking for. However they state the some times the cam may need to be grinded a little. When do you find this out? After the motor is completely built? Or can you discover that this needs be done during the building process? And what determines this? I know a lot of Questions.

I would like to learn to build my own and not have to pay for a prebuilt motor. Any and all advice is welcome.
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Old 05-02-2018, 11:59 AM
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And also need to make sure gearing and tire size allow you to use those RPMs.
If 7000 RPM is 60 MPH, how much time will you spend at 60MPH?

If the engine is making peak power at 5300 RPM and you spend most of your time between 30mph and 40 mph, then I would gear so 5300 RPM is 35mph, which would give you 46 mph at 7000 RPM.

Gearing so 5300 RPM is 40 mph would give you ~53mph at 7k RPM.
Gearing any faster than that and I doubt you will reach 7k RPM very often.
But that depends on where you ride I suppose.

---------- Post added at 12:59 PM ---------- Previous post was at 12:56 PM ----------

Quote:
Originally Posted by pearl111 View Post
Thanks everyone for the help. I saw that Go Power Sports has a stage 3 kit (minus pushrods) that would get me to the RPMs I'm looking for. However they state the some times the cam may need to be grinded a little. When do you find this out? After the motor is completely built? Or can you discover that this needs be done during the building process? And what determines this? I know a lot of Questions.

I would like to learn to build my own and not have to pay for a prebuilt motor. Any and all advice is welcome.
On some hemis I've had to grind the block (bottom of cylinder) and the cranksahft journal "lips" to make sure the camshaft lobes cleared. Other people grind the actual cam lobe, which seems like a more delicate process - so I don't do that.
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Old 05-03-2018, 03:27 AM
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Thanks for the help. Now I think I will attempt to build my own motor. I believe for me the best parts kit would be the DJ-1024 Builders Prepared Parts Kit from ARC RACING. I will also ask them for a set of chormoly push rods. For a beginner like myself I think this will work. I know a lot more can be done but I am new at this and I don't want to get in over my head.
Any thoughts or advice is welcome.
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Old 05-05-2018, 07:26 PM
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Quote:
Originally Posted by pearl111 View Post
So for more RPMs If I install billet flywheel billet rod and stiffer springs will this improve my RPMs?
Yes it will, but a stock cam and stock head is still going to make peak power at about 5500rpm.

Focus on a power goal rather than an arbitrary RPM number. The power you shoot for will determine how high you have to spin the engine.
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Old 05-06-2018, 05:19 AM
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Quote:
Originally Posted by 65ShelbyClone View Post
Yes it will, but a stock cam and stock head is still going to make peak power at about 5500rpm.

Focus on a power goal rather than an arbitrary RPM number. The power you shoot for will determine how high you have to spin the engine.
not really.. I think peak power will still happen earlier than that
(even with advanced timing.. 4.8k maybe 5 is a more reasonable guess!)
since these engines are meant to peak @ 3400 rpms roughly and by advancing the timing you can shift that peak only so much.

BUT and that's the point.. that doesn't mean that you don't have
any power at higher rpms, it just fades off.
if too quickly or not is unfortunately not something you can say withou testing.
given the engine is allowed to breath freely, chances are it's just the sweet spot
for most power at cruising and just enough for the occasional floor kicking..
So if you listen to Bob above it might be alright.

Personally I wouldn't install any timed cam at all, a duration or lift cam.. sure.. timing NOPE!
here's why.. on recreational karts/minibikes (i.e. no competetive racing)
it's much nicer if you at least have the option to go slower than 30mph
And with certain timing cams you MUST up your clutch engagement to 3500rpms (or worse)
in order to get any noteworthy power from the engine...
And that gets old quickly on your driveway or your neighborhood streets ...

So choose wisely.. stock duration/lifting cams are mostly cheaper too so yeah win/win IMHO

'sid
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