whats the highest compression for pump gas?

I ran 13:1 in my TRX450R for two seasons on 93 octane and never had issues, but I did dial back the timing a couple degrees and kept the jetting rich. Every engine is different though. Make sure your cooling system is spotless.

12.5:1 is a safe bet for pump gas if you have a good tune and don't ride in extreme heat. I've seen engines melt pistons at 13:1 with stock ECUs. Honestly, the gains from 12.5 to 13 aren't worth the risk on a trail quad.

I disagree with the previous comment. I've run 13:1 on a modest cam for years without issues, but I use a programmable CDI and run ethanol-free pump gas. The key is proper squish clearance and quench. Don't just slap a high-comp piston in without measuring everything.

From building dozens of YFZ's, I'd say 12.5:1 is the max for pump gas without detonation, especially if you're not racing or running a standalone tuner. Even then, be prepared for harder starting and more heat. Go 12:1 if you want reliability.

What elevation are you at? At sea level 13:1 is sketchy on pump gas. At 4000+ feet you can often run higher comp because the air is thinner. My buddy runs 13.5:1 in Colorado on 91 octane with no ping.

I tried 13:1 on my LTZ400 with 93 octane and got pinging under load after 15 minutes of hard riding. Switched to 12:1 and it's been perfect for three years. The tiny power difference wasn't worth the worry. Listen to the engine.

Have you considered a stroker instead of just high compression? A longer stroke with moderate comp (11.5:1) can give you more low-end torque and still run pump gas easily. Just a thought before you commit to a piston.

I'm a mechanic and I advise 12.5:1 max for pump gas unless you're willing to data log and tune constantly. Over that and you're playing with fire. Also, check your cylinder head for warpage—high compression puts stress on the head gasket seal.

FWIW, I run 12.8:1 in my KFX450 on Shell 93 and it's fine. But I have a high-flow head and aftermarket fuel controller. If you're stock ECU and stock cam, stay under 12.5:1. No two builds are the same.

Detonation isn't the only worry—high compression also means harder on the starter and battery. My 450 with 13:1 would barely crank hot. I went back to 12:1 and starter lived longer. Something to think about if you ride alone.

I've been running 13:1 on a YFZ450 with a stage 2 cam for two years pump gas only. No issues but I always run 93 from a station I trust and I let the quad warm up fully before any hard pulls. Chances are you'll be fine if you're meticulous.

Don't forget about fuel quality variance. Not all 93 octane is equal. I had detonation on one brand and switched to another and it was fine. Also, ethanol additives lower effective octane. Try to find ethanol-free if you go high compression.

The real question is what gas do you have locally? In some areas 91 is the best you get. I'd never go above 12:1 on 91. Check your local stations before ordering a piston. Don't assume all pump gas is created equal.

I'd argue 13:1 is fine for pump gas on a properly built engine with quench pads. But most people mess up the squish clearance. If you don't know how to measure and set quench to 0.035-0.045 inch, stay at 12:1. It's about the whole combo.

I went 12.5:1 on my Raptor 700 and it transformed the power without any ping. I used a JE piston and ran 93 octane. Starts fine hot or cold. I think 13:1 is overkill for trail riding. Focus on cam timing instead.

Have you thought about a thicker head gasket to lower compression slightly? Some guys run 13:1 and then use a 0.040” gasket to drop it to 12.6 or so. That buys you safety margin but still a bump over stock. Just an alternative idea.

If you're on a budget, stick with 12:1. Higher compression means you'll need a remapped ECU or fuel tuner, which adds cost. For the price of a piston plus tuner you could get a cam that gives similar gains with no gas worries.

I know a guy who ran 13.5:1 on pump gas by adding water/methanol injection. That's an option if you're set on high compression but not willing to buy race gas. But that's a whole other system to install and maintain. Probably overkill for most riders.

I ran a 13:1 CP piston on my Predator 500 for a season. It started to detonate after 20 hours—small hole in piston. I went back to stock compression and never looked back. The power difference was not huge. Learn from my mistake: 12.5:1 is the ceiling.

One thing no one mentioned: if your engine has carbon buildup on the piston or head, that effectively raises compression. So even if you build to 12.5:1, after a season of carbon it acts like 13:1. Keep your top end clean if you run high compression.