I use .003 for both intake and exhaust. You made a good decision upgrading the rod and flywheel for your Predator 420 but higher RPM alone will not increase horsepower with everything else being stock. The camshafts on the predators are made to provide low and mid-range power up to around 4,200 RPM. After that, the HP curve rapidly flatlines as you run out of lift, duration, and flow. To really get more power:
Required:
Larger carburetor (Mikuni, Keihin, bored, Etc.) - allows for more airflow. Stock carbs on Predators are very restrictive and will NOT support high RPM power.
Free flowing exhaust - either straight-pipe it or get a better flowing muffler. Stock exhausts are meant to be used on generators and lawn equipment, compromising power for noise reduction. They will bog your engine down, especially at higher RPM.
Higher duration and lift camshaft - as I said, the stock camshaft will run out at around 4,200. You need more duration and lift. I recommend the CM/CS cams as they provide respectable top end and low end.
Timing advance - Likely built in to your flywheel already but 6 degrees of advance will improve top end while not killing low end power.
Gearing - Your gearing needs to take into account that your engine will be revving at 7,000+ instead of 3,600 (governed). If you're hitting 50 MPH at 3,600 RPM than you'll need a larger rear sprocket or your engine won't ever be producing its peak power and all your upgrades will be useless. If your desired top speed is 65 MPH then gear your kart/minibike so that your engine will reach 7,000 RPM (or whatever your rev limit is) at 65 MPH. If you mess up your gearing than your engine may only be theoretically achieving 5,000 RPM or so at 65 MPH and you will not be taking advantage of the extra power you spent so much time and effort to get. Even worse, you may end up REDUCING your usable power since these sort of mods make high-end power while reducing low-end. It is always a compromise between the two. If you stay below 4,500 RPM all the time then you would have been better off with the stock setup since its made to make max power under 4,000 RPM. I use this gearing calculator and a small tachometer to achieve the optimal gearing ratio.
www.bmikarts.com
Optional:
Porting
Bigger valves/head work
Increased compression
Good luck and happy modding!
A post I think is a good read for everyone modifying these small engines.
I 100% agree with what you said about blueprinting. That's why I came here to ask questions and plan ahead before I started slapping on performance parts and messing with the engine. There are several websites and YouTubers who will sell/promote "Stage 4" kits or whatever "Stage" BS and claim you can make 20+ horsepower with their stuff just by bolting everything on, no planning/measuring/testing/tuning required. Two examples are Red Beard's Garage and CarsandCameras. I honestly despise both of them for lying so much about HP numbers and trying to scam people with their "Stage" kits. Both are heavily sponsored by GoPowerSports (not even a dedicated engine performance website, they sell parts for cheap Chinese dirt bikes and ATVs!) and make wild claims of 22+ or 15+ HP by buying their Stage 2, 3, or 4 kits. Never ONCE have they tested their engines on a dyno, I wonder why. It's an amazing marketing gimmick. Their videos are actually very useful if you're a newbie (like I was) and want to learn about how to pull apart and modify an engine. They are relatively short and make the process seem incredibly easy when, in reality, it's much more difficult and complex. Of course they always begin the videos by saying "I got this Stage 3 kit from GoPowerSports to make 15 HP". So, someone who has no clue how an engine even works goes and spends $600 or more on one of their kits after watching the video to get "15 HP" then ends up with the engine probably making less power than stock since nothing is set up or tuned properly.
Here is a perfect example that proves they are BS.
This is a chart of their drag race and lap times with a Predator 212 at different "stages". Notice the
stage number on the far left column and the
CLAIMED power on the 3rd column from the left. I believe that the only "stage" which actually increased HP was Stage 2. The predator will respond well to removing the governor, re-jetting the carburetor, using a higher flow air filter, and opening up the exhaust. The power claims for the rest of the stages (3-5) are complete and utter BS.
How are you only shaving off
0.19s on your lap times when you increased
HP by 6 (Stage 4 vs Stage 2)
, while you reduced your lap time by
6.47s with a 2.5 HP increase (Stage 1 vs Stock)???
The drag race times provide even stronger evidence. Somehow,
increasing HP by 2.5 in produces a
1.17 decrease in drag time (Stage 1 vs Stock) but raising
HP by 11 only results in a
0.52 decrease in drag time (Stage 1 vs Stage 4)??? This is some serious BS here. Even if the gearing wasn't correct, it wouldn't explain anything considering that the Stage 1 engine cannot rev beyond 5,200 RPM while the Stage 2+ engines should be capable of at least 7,000 RPM. More RPM = More Wheel Speed regardless of what size sprocket was used.
The people who get these sponsorships from GoPowerSports (who also put their name on everything imaginable) are some of the dumbest people I've ever seen. If you're going to lie about HP numbers than at LEAST make your lap times/drag times reasonable. The two nitwits in the video couldn't figure this out
.
