can you run pump gas on an 11:1 high compression piston

[Mattman88]

hey guys, i found a 400ex im interested in buying and it has an 11:1 high compression pistion. im just wondering if you have to run race gas in it or can you can straight pump gas in it

u can run 91 octane in it and be ok...premium unleaded

[chad502ex]

Originally posted by Mattman88
hey guys, i found a 400ex im interested in buying and it has an 11:1 high compression pistion. im just wondering if you have to run race gas in it or can you can straight pump gas in it

Matt, actually 10.8 is the absolute highest compression you can run on pump 93 without bumping up octane requirements.

chad502ex.com

[quadryda]

u use 93 seems to ork fine i have tha same comp to

[stonerider250x]

darrell has a 11:1 in his 330, and uses 93 and it runs good. But he said that he is gonna start running 104 octane

Yes the motor will run.


Of all the owners of 11:1 pistons on the site that have ever answered this question, 90% said there quad predetonated and ran like crap on 93 octane gas. I know my 10.8:1 runs a heck of a lot better on higher octane compared to 93 but i don't have any predetonation on mine running lower Octane.

If your gonna run pump gas mainly get a 10.8:1. it's not like there's a 15 HP difference between the 2.

[ranger400ex1994]

i am building a 416 that has a 11:1 compressoin and i was told that i could run 92 no problem now is that wrong and should i be able to.

Originally posted by ranger400ex1994
i am building a 416 that has a 11:1 compressoin and i was told that i could run 92 no problem now is that wrong and should i be able to.

Yes you can run 92, but your newly built motor will not run at peak performance like you'll be expecting it too..

After hearing the info from the horses mouth (lots of 400ex owners with 11.1 comp. pistons), all pump gas works but not near as good as a higher octane.

I can even remember when one user stated he took his newly built 426 to the dunes and thought he had something jacked up in the motor because it would barely pull itself up a dune. He then put a mix of 110 and 93 in it and said the mofo was pullin wheelies up it. :eek2:


I can't say from experience that 93 and a 11.1 piston won't work well, but being on this page for almost 3 years and seeing this exact same question asked many many times. The answer has always been 11.1 and pump gas don't work well together, whether that person is riding at glamis or Michigan. take it for what it's worth.:cool:

[Mattman88]

thanks for the advice guys

[muff]

if you're looking to save some $ just run a 50/50 mix instead of running straight race fuel..you may not be having the top peak performance but you'll be saving like $4+ a gallon

[carlj75]

When I built my 426 with 11.1 comp, I ran a 50/50 mix, but I found that it runs a lot stronger on straight race gas.

[Aceman]

I have a 330 kit 11:1 and it's a pump gas kit. But since the highest octane is 92 around here I mix one gallon of 110 with four gallons of 92 just to be safe. A few extra dollars spent on fuel is worth knowing I won't have predetonation.

[ranger400ex1994]

so would i get more power from a 11:1 compression or from 10:8 compression on pumped gas? but i might just have to go get some race gas to make the power any way.

the 11:1

[airheadedduner]

Originally posted by ranger400ex1994
so would i get more power from a 11:1 compression or from 10:8 compression on pumped gas? but i might just have to go get some race gas to make the power any way.
The difference would not be noticeable.

I have a 440 that is 11:1 and it runs great on pump gas. I know I would probly benifit from some race gas. It is hard to get a good answer on the question cause of the area of people you are dealing with. Pump gas is not very consistant, it also veries through the year. Like now around here they oxygenate it some to keep polution down, not good for quads. Quality seems okay over though.

[wilkin250r]

Obviously there are more factors involved other than just compression and octane.

For example, Wiseco compression numbers are specified with the use of an XR400 head gasket, which is a little thinner than a stock 400EX gasket. So, it could be possible that you don't have a true 11:1 compression.

Generally, liquid cooled engines can handle higher compression with pump gas, because they run a little cooler. Less heat means you are less likely to experience detonation.

With a properly set up squish band, it helps cool your fuel/air mixture as in compresses. Less heat means less likely to detonate. However, with the squish band, we are now in the realm of very detailed and expensive engine modifications, which doesn't really apply to the recreational duner.

So, it's possible to find a combination with a 11:1 that runs perfectly fine on 87 octane, but not likely. I only bring that up because we are talking possibilities, not realities. In reality, to answer your question, if set up properly and you have a true 11:1 compression, you're much better off running a 50/50 with race gas and pump gas than just pump gas alone.

[airheadedduner]

Originally posted by wilkin250r


With a properly set up squish band, it helps cool your fuel/air mixture as in compresses. Less heat means less likely to detonate. However, with the squish band, we are now in the realm of very detailed and expensive engine modifications, which doesn't really apply to the recreational duner.


How I understand the squish band is when the combustion chamber is filled with fuel/air it is not equally distributed, spots accross the chamber could be lean and detonate.. So when the piston comes up with a properly set squish band it forces most of the mixture into a more concentrated area(usually the center) of the piston. With the fuel charge in the center when it fires the outer area is not getting direct heat, acting like a heat sink it pulls heat away to thoose areas reducing chances of detonation. It is damn complicated from what I have read. But it also does not apply to 4 strokes as far as I understand. No room in the head to properly do it with all the valves. If you tried you would just be wasting the fuel charge. With the squish set wrong some of the fuel would just linger on the outskirts of the piston and not be burned till after TDC. All that does is produce excess heat and with the lingering fuel burning after TDC, it don't produce power. With the squish set wrong you can loose around 5-8% across the RPM range, depending on the the size of band you set.