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Non restrictive by law? The spark arrestor in both my YXZ and Raptor are highly restrictive. That is why companies make ones that remove that restriction.
 

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'89:

Been too long since I did all that research, but yes. As I recall it was the US Forrest Service, and I think it was actually 1PSI for 4 stroke. 2 stroke, sometimes benefiting from back pressure, were allowed a higher value. As I recall.. Digging deeper in the memory, the rated flow, in CFM for a SA was to not exceed 1 PSI.

Now I don't know if a US FS approved SA regulation is a "law" once the USFS says something, so I've perhaps over-stated thing sawyng law or legal. I know I can get a ticket without one, and that every state I know bases their laws on USFS regulations, but in truth I don't know the mechanics of how USFS regulations are or become law when written or are merely guidelines that effectively become law when states adopt them.

1) I suppose it's possible that said back-pressure / CRM rating guideline can be violated and a mfg get away with it. You seem to think this is so (I'm assuming, please forgive me if I'm wrong). I don't know why any MFG would so something that stupid, but I've long since given up trying to understand marketing compromises mfg make.

2) I also don't know the process that happens in guidelines becoming laws, be it federal or state. Might in practice it be legal to accept a screen based spark arrestor that doesn't meet the 1 PSI guideline?

I see no other wiggle room for a muffler to be stamped USFS approved. Mind you I'm thinking "Boeing" right now too, so there's that. There is also something called "Confirmation Bias" to consider as regards marketing of "HP improvements" in general.

Let me offer this thought: IF a MFG were to get an exhaust stamped USFS approved that didn't meet USFS pressure guideline, that seems more an indictment of said manufacturer, not of spark arrestors, the USFS, or SA performance. There are plenty of SA's out there the do meet the 1 PSI guidelines and thus have no tangible impact on our performance. You've seen the insignificant dyno results on this very thread showing just that.

In any event, this is all memory. I spent a lot of time in this like a year ago, and it's just too much reading and digging to redo over something as irresponsible as not running an SA. I hope I've clarified a bit more, but otherwise running sans SA isn't a topic worthy of more discussion on my part.

Best of luck to you though!

-d
 
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More food for thought directly from the Forestry Service:

Do Turbochargers qualify as effective Spark Arresters?

Turbochargers
qualify as effective spark arresters when 100% of the exhaust gasses pass through the turbine wheel. The turbine wheel must be turning at all times and there must be no exhaust bypass to the atmosphere. Depending on design, small enclosed system bypasses that reroute some exhaust back through the engine may be allowed. The illustration of the exhaust-driven turbocharger shows the air intake and the exhaust path through the turbine wheel. The action of the rotating turbine wheel causes carbon particles to remain within the confines of the turbo-drive section until they are reduced to a harmless size by attrition.
 
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