Temperature output of carb heat muff

[wjohnson]

Hi,
I have a question about what the max temperature output of a carb heat muff may be. I have the standard VariEze muff for a Herb Sanders exhaust system with extra coils.
Looking at max temps for red (Scat or Sceet)  Aeroduct are 450-500 deg F. I have thought of using max temperature indicator tape that I have seen on the internet. The temperature range of the tape I have seen is limited enough that I don't know which temperature range to start with.
My interest is to improve the heat muff (and possibly the ducting and air box) with Fiberfrax insulation.
Next question: Am I going to get to close to the max temp (1120 deg F) of aluminum on the heat muff also? Not likely?
I flew for years with the standard Rutan induction system with the round air cleaner on the firewall and had no carb icing problems (Cont engine). The ram air box has changed all that! Since I didn't have a carb temp gauge at that time,  I can't compare carb temps.
My carb temp gauge is not calibrated, but at 40 deg F  OAT at altitude the carb temp is about 12 deg C with full carb heat...two degrees above the caution range.
Any experiences or thoughts? Thanks!

Wayne Johnson VariEze N725EZ
Lake Stevens, WA

[Radioflyer]

[/quote]I flew for years with the standard Rutan induction system with the round air cleaner on the firewall and had no carb icing problems (Cont engine). The ram air box has changed all that! Since I didn't have a carb temp gauge at that time, I can't compare carb temps.
My carb temp gauge is not calibrated, but at 40 deg F OAT at altitude the carb temp is about 12 deg C with full carb heat...two degrees above the caution range

Interesting comment. I did not realize that a ram airbox would exacerbate carb icing. I too would  like to hear commentary on this since it is a mod I will be considering.

[maprather]

CP32 recommended a minimum of 90F rise in temp from the carb heat system.  In other words, if the OAT is 120F, the air temp delivered by the carb heat system should be 210F, which is well below the 400F limit that the various hoses can handle..   It seems that your system isn't getting the required 90F rise - if it were, the carb temp would be warmer.  

I think I'd check to make sure that there isn't a cold air path (leak) that reducing the effectiveness of the heat system.  Is the carb heat valve fully closing-off the cold air path?  Is there a leak in the carb heat muff that is allowing cold air to bypass the heat transfer area?  Is some part of the induction system deforming under engine vacuum while the engine is running?

What are the OAT's you are operating in?


Regards,

Matt-

[wjohnson]

Matt,
Good point about closing off the intake air with the carb heat on. I wil see if I can improve that part of it. I know that there is some clearance to the flapper in the ram air box.
That last test flight was 40 deg F at 4000 ft and don't want to have to fly with full carb heat to get above the caution range!
Wayne

[wjohnson]

I am still interested if anyone has insulated (partially or completely) the carb heat muff or any of the rest of the hot air induction system. Whether I find too much cold air getting in when the carb heat is on, I'm still interested in any thoughts or ideas.
Thanks,
Wayne