Anyone "HOP" up a crf450?

[KB]

I'm talking cam's, higher compression piston, run race gas??


Chime in if you have and your results. I need some more umph to get my fat ass up over the 3 into 4 hill.


I have already gutt'd the ehxaust, running a 172 main, 48 pilot, airbox completly opened up.

KB

[JeffL]

Dont forget to replace your connecting rod.

[Awesome]

The more pounds you lose the faster you bike'll go.

Juss sayin'......

[R7]

Leave it stock and you'll be fine. Build it up and it will blow up

[Paul_E_D]

You should have said, "hop it up and it will blow up sooner"

[TheIglu]

Here is a question me and KB were duelin about last night:

Is it true that a longer duration cam (intake) will increase the compression of the bike?

I say yes. Longer duration = more air = more air in the same space = higher compression.

[Paul_E_D]

No it will LOWER dynamic CR. More time for charge to escape out either side before being squashed. I think there will be a moment in the rev band where compression may get higher, but it will be fleeting at best. I think that's why a lumpy cam almost alweays accompanies a higher compression piston (or a head shave)

[Honclfibr]

Originally posted by TheIglu
Here is a question me and KB were duelin about last night:

Is it true that a longer duration cam (intake) will increase the compression of the bike?

I say yes. Longer duration = more air = more air in the same space = higher compression.

Well, PV=nRT. So if the valve is closed before the piston completes it's downward stroke, the volume of the system after that point would increase with no way for additional air molecules to enter. Since the equation needs to be balanced and V increased while n decreases, either the pressure needs to decrease (below standard atmospheric) or the temperature needs to decrease.

Once the piston began it's compression stroke and got to the cylinder volume at which the valves had closed, the pressure in the cylinder would be back to atmospheric and the compression cycle would start from there. So you could roughly estimate the effect on compression by taking the volume when the valve closes during the intake cycle as the uncompressed gas volume, and go from there.

I suppose there is an effect at the top of the cycle too depending on when the valve opens, since the vacuum inside the cylinder is resolved in finite time, but assuming that the lift is reasonably long I can't imagine that the difference between cylinder pressure and atmospheric pressure at the point when the valve closes should be that significant. But I could be wrong about that, valve timing and lift is a pretty small number.

[Karaya One]

My dirt shops spend more time fixing CRF's than anything else. The parts are always on b/o with Honda for a reason.

And when you start going crazy with it, forget it. Your just waiting for that pinned to be pulled...

They all usually replace the rod with anykind of work since it seems to be the weak spot with a racer on this bike.

[Evil_Weasel]

what good is a hopped up CRF450 if it leaves it's guts in the T-1 braking zone?

i'm not a motard guy so i won't pretend to know much about them. i will say i've seen afew not stock CRFs blow up this year. keeping that in mind i'd say the best way to make a CRF450 faster and still reliable would be to trade it for a KTM 560.

[TheIglu]

My assumption was that the modified cam actually allowed more airflow into the cylinder than the stock one during the intake stroke of the piston. Why would they make a cam that allows reverse flow into the intake manifold instead of closing in time to prevent any loss of fuel charge? I always thought that a modified cam would be changed to open the valve eariler to attain max airflow. I've never really gotten into the theory of it so I'm curious to hear.

[Paul_E_D]

I *think* honclfibr just agreed with me, but I'm not a math guy... I think what he said translates as: The dynamic compression ratio doesn't begin until the valve is closed, so the later you close it the lower the number. It's my understanding that the reason to do it is to benefit from tuned scavenging from the exhaust side, which has an effect through to the intake side. All parts must be matched well to benefit from this. Also, as I said before, static CR must be increased to surmount the losses in dynamic CR. Advertised static CRs in the high 12:1 range (and higher) are really not achieved while running the motor.

[Rick Lee]

KB im going with the info i've seen at the track and on the boards don't build it up. The things you have done are good and race fuel does give you 2 or 3 more HP. I tried both one weekend (thanks heath) and have felt the difference. Do what im trying to do and lose some weight. Besides chicks look at a fat racer and say gross...do it for the pussy!!!

[SilverDragon]

Agreed. The reason for a longer duration is scavanging which helps the motor breathe at higher rpm making more power up high at a loss of low end. Another way to make up for it? Boost.

[SVRACER01]

KB.. i wouldnt go crazy with it. a cam would be good but once you start putting the HC piston there your reliability goes down. use some small tricks like getting rid of gaskets and hondabonding the cylider on. or leave it stock and change your gearing around so that your power is in a better spot up the hill

[R7]

Originally posted by Paul_E_D
You should have said, "hop it up and it will blow up sooner"

Being a honda, that is what I should have said

Though if he wanted to run a season or 2 without blowing the motor, the stock Yamaha is the only option

[a13x]

Food for thought:

Jeff and Eric Wood's 2003 CRF450 - Pipe, filter, jets, port and polish, cut head (lil more compression). Run for 3 seasons without a rebuild. Felt a bit 'weak' (according to Jeff) during the race against Doug Henry in October, doing 15's. Being rebuilt currently. Mind you those 3 seasons included LRRS + two years of AMA Supermoto.

Jeff's RMZ - same build as their CRF (mild). Run's 16's. 4 speed gearbox. No motor issues.

Most other motards at Loudon that are running 'really fast' (sub 20) - Stroker kits, High compression work, cams, etc etc. LOTS OF BLOW UPS.





Personally I wouldn't build my motard up past what Jeff recommends. We had a long talk about this last Thursday since motards blowing up bothers and confuses me and I noticed the CRF450 had the motor out and on the stand.

[Paul_E_D]

Hmmm... Maybe he likes to exaggerate? I remember him blowing up the DRZ right out of the box? Well, I don't know what the problem was, but I remember him parking it.

[a13x]

Originally posted by Paul_E_D
Hmmm... Maybe he likes to exaggerate? I remember him blowing up the DRZ right out of the box? Well, I don't know what the problem was, but I remember him parking it.

Mean the RMZ (see avatar)? Dunno, know it was rebuilt by REP going into 06, I'll be sure to ask why.

[WebCrush]

I have a big bore, Stage 2 hot cam, STM slipper in my 04 CRF. The main is up to a 192, goes pretty well, no issues. I'd like to get the head done and maybe up the comp some.

[Awesome]

first mod, slipper clutch.

[a13x]

Slipper clutch affect the ability to back a motard in? Not so much for Loudon but actual SuMo tracks..

[WebCrush]

Banging three downshifts into turn 3 is a lot smoother . . .

[a13x]

Originally posted by WebCrush
Banging three downshifts into turn 3 is a lot smoother . . .

Yea I rode Jeff's RMZ450 (avatar) with no slipper, then rode Mike Green's KTM with a slipper, and ya, banging downshifts was pretty awesome with the slipper!

Turn 3 wasn't even the most impressive (even thou it was very), I got a kick out of the parts you'd be banked over banging a downshift. smoooth brother smooooth.

[Paul_E_D]

Mmm... That's one of the secret pleasures of a GP bike. clutchless downshifts at 45 degrees of lean!!!