Do I Really Need an ARC Rod?

[BAMBY]

Ok, let me just get this out of the way 1st.
I know its bad to rev an engine without a load.
but i did it anyway.

Briggs 5hp
Gov removed
Valves lapped
Ported in/ex
Shaved eyebrows
All else is stock

ok i have a tachometer and this past weekend, i took my chain off and tuned the carb (pulsajet) to see how high i could get it to rev. during normal minibike riding, the highest rpm i have gotten is 5380. this past weekend, i revved to 6544 no load.

Is my engine invincible or what? Why hasnt the rod blown? oscaryu1 says they should blow around 5k rpm

my friend thinks that since it was built in 73' that they proabably used a better aluminum alloy for the connecting rod, that is stronger and is harder to break.

Im not trying to blow the rod, i just wanted to see how high i could rev.

so, I'm thinking i want more power outa this thing, why not put a cam in it? well, a cam would also make me rev higher... which means i should prolly get a ARC Rod... Right?

well here's my reasoning. ive heard people say its easier to blow a rod with no load. so if thats the case, i should be safe under 6500rpm WITH a load right?

PLEASE dont tell me i shouldnt have revved it like that... i know it was bad.

Just give me your opinion on whether i should get a arc rod if i get a cam.

Thanks, BAMBY

 

[theo]

I might not blow the first time you rev it up to 6500rpms but it might go the 3rd or 4th. Or you might have damaged the rod.

theo

 

[oscaryu1]

6500RPM? Really? I'm suprised the dipper hasn't broken. One of the guys here's 5HP's dipper broke off after a high RPM run...

I'd check it if I were you. An ARC rod is safest, if not, at least a Raptor 3 rod.

I'd also be careful about the flywheel now.

 

[kart_racer20]

if your going to turn the motot 6500 rps you'll need to get the arc rod or it will shatter and send it through the block and depending on what cam you get you might have to get the lifter carrage welded so the cam doesn't bust tthe lifter carrage.

 

[BAMBY]

theo, good point.

oscaryu1, nope, my dipper is fine, its one of the sheetmetal ones that is triangular shaped and connects to both rod bolts.

kart_racer20, the cam i would get really isnt that aggressive, but where would these lifter welds go anyway? Ive heard about welding the lifter bore, but i really dont know where exactly that means. inside, outside,...???

thanks guys, BAMBY

 

[kart_racer20]

they go on the inside of the block where the valves move up and down but if the cam isn't that agressive then i wouldn't worrie about it that much

 

[BradenM]

The question is how much you value your safety. You may have a phenomanal engine that has the best alloys; maybe the stock parts are good enough.

But maybe the next run you do your rod will let go sending hot oil and shrapnel on your back. Or your flywheel might let go and paralize/maim you/others.

I'm not sure what the statistics/evidence are on B/S motors, but If you were putting cams, e.t.c. in it I would first get a Billet flywheel, THEN a rod.
If you get the rod and cam, your engine will be able to spin even faster-upgrade the flywheel and rod at the same time, and you know your safe. Upgrade the rod and cam, neglecting the flywheel, and nobody knows for sure what will happen.

 

[mattgr]

horizontal shift!, that's fast 6544 rpm
hax?

 

[kart_racer20]

the stock flywheel will be fine i ran the b/s stock flywheel on my flathead motor last year on my racing kart and i was pushing 6500 rpms but anything more than that i would have put a billet flywheel on it

 

[BradenM]

I wouldn't go as far to say that "it will" be fine, more that "statistically, it should be fine". There are no guarantee's with these things.

 

[BAMBY]

yeah i hear what all you guys are saying about mine/others safety. i feel like what yall are saying is that its all or nothin.
money is the problem here: billet flywheel- $$$$$$$$, Rod-~65, cam-60-80.

but to be safe means to get a billyflywheel and a rod. that costs wayyyy too much.
i guess i should just stay a.s.a.p. (as stock as possible) for now in the interest of my saftey and others.
thanks guys,
BAMBY

 

[Jerryburger]

Hi Bamby- hope you don't mind 2 cents from this corner....

A couple of things come to mind with your question:

1.) If you're hitting 5400 rpm under load with a relatively stock engine, you need a bit taller gear. B/S flattys in stock form might rev that high, but don't make their power up there. The meat is in the 3600- 5000 range.

2.) You're on a budget (aren't we all) and I'm guessing by "more power" you mean higher top speed? If so, keeping your motor in it's natural powerband by higher gear ratio will net higher top speed.

3.) You CAN get a B/S to make power in the higher rpms, but you're talking $$$. Lots of it- and why spend money to make a small motor more powerful, when for the same $$$ you may be able to shoehorn in a larger (or higher stock hp) motor into the same place?
A stock HF Clone "Blue Terror" is a NICE upgrade from a B/S 5hp Flatty, and should fit in the same space. Parts are starting to become available for it, and I'll predict that the GX200 clone is the modern-day "5hp B/S Flatty" of the karting world.
I think (but has it ever been made official?) that a head-to-head build up at various levels of $$ input between the 5hp flatty vs. GX200 Clone would show the clone whomping the flatty just by superior flow design alone.
I'm sure I'll get jumped by the "Flatheads-Forever Camp" somewhere for saying that, but even Harley Davidson, who's meat and potatoes is "Retro Engineering" gave up on flattys because... they needed MORE POWER.

More than 2 cents worth. Sorry.

 

[BAMBY]

jerryassburger! your 2cents are greatly appreciated

I mostly want more power so i can accelerate faster, but a higher topspeed would be nice too. i already have a 60tooth in teh rear, do you think i need a 54 or someting? i think thats what you mean by a "taller" gear.

maybe your right about the bluclone... it would be a nice change of pace, and my flatty was built in 73 according to the shroud, so it burns oil and is getting old in general. ez to fix i know, but i really dont like torquing the rod bolts back on.

B/S flattys in stock form might rev that high, but don't make their power up there.

that was a really good point. totally validates your different gear ratio theory.

hmm... a sprocket is cheaper, and maybe, i will give my 60 tooth to my friend, MATTGR, who needs a 60tooth for his martyrcicle.

thanks jerryassburger and all other replyers
BAMBY

 

[Rotore]

doesnt the arc rod have the dipper integrated into it?

 

[jr dragster T]

Yes, The ARC rod has the dipper integrated into the rod end cap. This is a before and after picture of a blockzilla that snapped the crank. Pretty self-explanatory.

 

[freakboy]

wow that sucks. And is that your nitro truck? i got a traxxas tmax and stampeed, hpi rs4,hpi rs4 3 evo, and hpi rush.

 

[oscaryu1]

Me likey!

 

[Rotore]

Yes, The ARC rod has the dipper integrated into the rod end cap. This is a before and after picture of a blockzilla that snapped the crank. Pretty self-explanatory.

yea becuz i was wondering if i could use one in a vertical engine..

 

[jr dragster T]

In a vertical shaft engine the rod isnt even sitting in oil. If it was it would smash into the oil very violently and cause fatigue. It uses a gear on the cam or a oil pump to spray oil around onto these parts. So basically the dipper aint gona dip much. And Yes that is my nitro truck, Team Associated MGT 4.60 .28 big block SE.

 

[freakboy]

Nice and the question is does it run....... none of mine run cause i havent goten time to work on them.