Cams vs Big Bore

[M5-]

Since my 2018 YZ developed its tranny problems I have been riding my "B" ride the 2011 YZ450 with the GYTR CNC head and cams. I swear this bike pulls longer and harder than my stock 2018. Back when I was riding orange bikes I had the 520 big bore Thumper racing fiasco so I permanently swore off big bores and after market engine parts and went for the cheaper to maintain blue bike. I'm going to tear down my 2018 from top to bottom and rebuild it, everything new, I can't sell it wounded to anyone in good faith so I have to do it anyways. If you watch the Airforce Velocity Intakes guys video for the YZ he states that the YZ450 made the same power as the YZ500 when they dynoed them both with his stack. I'd do the YZ500 but I'm a little gun shy about the big bore. I'm thinking that what I'm feeling with the 2011 is the benefit of the race cams, probably 75% cams 25% porting. It makes power right where you want it on the rev limiter. I'm leaning towards stock 450 bottom end, GYTR race cams for the 18, any real world experience out there or thoughts?

M5

 

[wwillf01]

On ktms we have been always happier with standard bore and max out everything else through cams, porting, stacks, dyno and good fuel. They seem to be super reliable!


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[eric82930]

Skip the big bore and do the cams. it's all about getting air flow at 9-10k rpm. Big bores usually boost 5-6k range
Big bores can work too with the cams but reliability is hit our miss... Wasn't the gytr head from the early yz what they used to model the 14 Stock head? then in 16 they changed cams and started to neuter it for more linear power. I haven't paid attention to the blue bikes much since then but that seemed to be the trend that worked better on dirt....

 

[jim]

Depending on elevation, I'd focus more on optimizing your compression with higher compression piston and cam timing to match.

 

[byeatts]

The High rev cam on the ktm 450 is magic, huge RPM gains

 

[eric82930]

Even at low elevation you may need more compression with a hotter cam because the longer duration/overlap causes the intake valves to close later which bleeds off a lot of compression. My rfs450 is running a 13.5:1 piston with a thinner BG and tightest squish possible and still likes low octane because of the cam is so radical. The consensus of bike tuners on the forums all think my cam is too big for a 450 on the the dirt because it blows of too much low end compression but it is what a snow bike needs. If ktm ever builds a 450sbs (snow bike special) it will have a big cam that I guarantee won't work on a motocross track.

 

[M5-]

I'd have to look as I don't remember off the top of my head but I think the 2018 YZ is 13:1 stock. I know my 2011 with the GYTR head is 12.5:1 stock which is how I left it. One thing I don't want is to be married to race gas or high octane fuel. For the 2011 Yamaha recommends race gas if you run their head and cams with the GYTR high comp piston which is 14:1. I'm leaning towards head work, cams, stock bore, RP pipe, Airforce stack, tune, done.

M5

 

[NEWDUCK]

Even at low elevation you may need more compression with a hotter cam because the longer duration/overlap causes the intake valves to close later which bleeds off a lot of compression. My rfs450 is running a 13.5:1 piston with a thinner BG and tightest squish possible and still likes low octane because of the cam is so radical. The consensus of bike tuners on the forums all think my cam is too big for a 450 on the the dirt because it blows of too much low end compression but it is what a snow bike needs. If ktm ever builds a 450sbs (snow bike special) it will have a big cam that I guarantee won't work on a motocross track.

There is an online calculator for figuring out compression with the exact cam you're running.

Reminds me of the hotcams back in the day. I had an 03 crf 450. Hot cams stage 2 was all the rage. So of course I had one. I started talking to custom cam builders and they showed my compared dyno sheets. The stage 2 actually made less hp up top than a stock cam. I got told they did that because very very few people actually rode the bike to the full potential wide open. I turned that bike into a 490. Was a great mid range torque track bike but everywhere else stock was better.

 

[eric82930]

Yes you need to ignore the stage name of the cams and focus on the specs if you can find them. Mine is a stage 4! impressively big number! Also on some bikes you can dial in the degrees of advance to get the intake closed as soon as possible depending on how deep the valve pockets are in the piston but you need to pay attention or things might collide inside your engine. Also the duration specs vary between brands some measure it at 1mm lift and others at random lifts from barely off the seat.

 

[jim]

Agree on being married to race gas...it sucks. But bumping up compression with head and cams does not necessarily put you there. I run a PG turbo...can detonation pump gas. I add a bottle of octane booster, royal purple, every 10 gallons...works great. Also has good stabilizers and corrosion inhibitors. And good quality octane booster helps stop detonation very well. Good luck!

 

[tchriste]

so who does the best head work, cams. piston combo? From my research there doesn't seem to be a lot of options.

 

[M5-]

I've had good luck with my GYTR CNC head and race cam setup. It has just under 170 hours on it right now and has been solid. There are some great deals on the GYTR set up for the 14-17 YZ out there right now if you happen to own one of those bikes. GYTR blows out the older parts usually at 40-50% off every couple of years. So far the 18-19 head is still full price.

M5

 

[M5-]

Anybody have any experience with the RP straight exit Snowbike pipe for the YZ? Specifically I'd like to know if its obnoxiously loud.

M5

 

[AllHatNoCattle]

A buddy has the RP on his Husky. So loud.

A couple times I have been standing at the back of the bike giving him a push after getting him unstuck and he revved it full out. With my helmet on I still think it damaged my hearing LOL

When sitting on his bike it is a bit louder, but not too much more. Maybe 15-20% louder than an FMF.

 

[byeatts]

Anybody have any experience with the RP straight exit Snowbike pipe for the YZ? Specifically I'd like to know if its obnoxiously loud.

M5

they are loud if ewer behind juan. No doubt many wont like it however performance is top dog

 

[cbc76am]

I have 7 yz450f and fx on the mtn in my crew. We have all going to Athena 102mm big bore with hotcams stage 2 IN and EX. Plus 3 fuel trim and 2 degrees on ing adv across the board. the 18 motor rips with this combo. azquadparts sells a CP piston kit with the cylinder that is amazing. I'm doing a pair of 21s now for this season - no cam options from hotcams yet... gytr don't really add much as they are meant to smooth the power out on the 13+ motor.

These bikes absolutely rip - no reliability issues. be sure to install the gytr dual nozzle oil sprayer.

 

[cbc76am]

Anybody have any experience with the RP straight exit Snowbike pipe for the YZ? Specifically I'd like to know if its obnoxiously loud.

M5

it's throaty and deep, i don't feel like it's louder at RPM than a full FMF system. I do feel like it looses some bottom end power with the RP straight personally

 

[M5-]

Your post is actually pretty good timing. I'm just in the middle of ordering parts for my 2018 YZ for a top to bottom rebuild. At the end of last season my 2nd gear went south and I had to ride my backup bike for the remainder of the season, so it's time for a complete rebuild. I have all the new updated trans parts coming, a new crank, every new bearing in the motor, timing chain, water pump, the works, the motor has just under 170 hrs. I'm just in the process of seeing what parts I need for an FX 5th gear with the YZ 1-4. I've been down the big bore road before back when I rode orange and it didn't go well. My number one concern is durability. How many hours have you gotten on the Athena big bore? Any of them blow up yet or take out any con rods?

Thx M5

 

[cbc76am]

Other than a guy that ran without coolant for a day and warped a cylinder I've seen no failures. My previous personal 2015 YZ (now my dentists ride) has 75 on the top end. My buddy Ryan's now spare bike has 120+ hours on the Athena/CP top end - I've warned him it's about time on a single ring piston to swap it out - I doubt he will as it's the backup bike.

The rest are in the 30 - 50 hour range and running strong. The ring gap is cut to max range on all of them ( .004 per inch - so .016 on these 102mm) We're also running the 12.8:1 gasket set for pump gas on them azquadparts sells - I'm going to 13.4:1 on the 21 bikes - will likely need some blending of race gas.

I've only got one KTM left in the group, he's got the Thumper Racing 490 kit, which is now out of business and Built Racing took a year to get him a piston and rings, haven't seen issues with that 16.5 450 motor bored out either though.

 

[byeatts]

BB adds hp but do not add RPM,so works great with a CVT clutch to harness the extra ponies, high rev cams let the motor scream and i tap rev limiter in 2nd and occasionally 3rd while climbing. I also have the airforce intake and RP exhaust , You need to free up air flow to hit this high of RPM. RPM is what spins the track with a gearbox. HP does not add significant RPM, The stock bore with these mods is an total screamer. it outclimbs the 490, keep in mind the larger the bore has less capable RPM.