Gentlemen,
Having attended both rallies as well as the Indianapolis GT gathering, I've had a chance to examine several hundred GT's. I noticed both top and side vents were warped on the engine covers of a lot of GT's. Yet my stock yellow GT, with 3000 miles on it that has been regularly driven in our 120° heat here in Arizona was fine. My friend Larry (AZGT) had 4000 miles on his stock Arizona driven GT and his vents were fine as well. The weekend before the California Rally we had the GT guys come out and set up shop in Larry's garage. Larry and I both had the Accufab exhaust installed as well as a 91 octane Heffner tune and pulley. Within 500 miles both of our cars had warped both the top and side clamshell vents. The dynamics of this cause and effect came into clear focus when I was flying my Cirrus SR-22G2 home from Corona California yesterday. The Cirrus has a very sophisticated engine monitoring system that displays, among other things, both the cylinder head temperature and exhaust gas temperature of each individual cylinder. At altitude, with the push of a button, the system will help you find the optimum fuel air mixture for either best power or best economy. Takeoffs are generally made with the mixture control at full rich. Even at sea level this setting is much too rich to produce maximum power, but this setting keeps exhaust gas temperatures almost 300° cooler than they would be with an optimum fuel air mixture. Lower EGT's keep cylinder head and oil temperatures correspondingly lower as well. As the aircraft climbs and the atmosphere thins the mixture gets progressively richer and must be manually leaned. During this process fuel flow is cut with airflow remaining constant until exhaust gas temperatures peak and start to decline again. As I pulled the mixture back gradually and watched my exhaust gas temperatures climb nearly 300° during the process with a corresponding increase in engine power, those warped clamshell vents directly on top of my Accufab exhaust popped into my head. Unless one is flying above 15,000 feet or thereabouts a piston powered aircraft engine is never operated at peak EGT because this would put cylinder head temperatures above safe operating levels and would shorten engine life. So the best power setting for Continental Aircraft engines is 65° rich of peak EGT.
As soon as I got home from this flight I went straight to my garage and sure enough, the section of my vents that were warped most severely were directly above the top curve of my new Accufab exhaust. That portion of the vents closest to the rear windshield directly above the engine but away from the top curve of my exhaust was fine. Three components are coming into play here. 1. The smaller pulley forces a denser fuel air mixture into the cylinders producing more power and more heat. 2. The Heffner tune moves the fuel air mixture away from the very rich (low exhaust gas temperature) stock settings to a more optimum (leaner) mixture that produces more horsepower. This leaner mixture, just as it does in an aircraft engine, will raise exhaust gas temperatures by 200+ degrees. 3. Last, we have the Accufab exhaust itself. The stock muffler, at least with the stock tune, defuses the relatively low exhaust gas temperatures feeding into it over a large space with a lot of surface area. Evidently, the peak temperature radiating up from the stock muffler when a GT is either stopped while idling or shortly after it has been shut off, is not high enough to warp the vents. At least not in my case nor in Larry's case. The Accufab exhaust, when it is being fed by a pulley and tune equipped GT, has much higher exhaust gas temperatures feeding into it and a very small area that all that heat is radiating from. At speed there is no question that the Accufab exhaust will allow far more airflow past it and out the rear vents than the stock muffler. It is at idle and at rest after shutdown and there is no airflow through the engine compartment that this comes into play. Now this much hotter exhaust system radiates its heat from a much smaller concentrated area straight up into those vents. And the plastic doesn't like it.
I am not an automotive expert but I have worked as a test pilot evaluating many different systems on aircraft. And although I'm not in possession of the equipment I would need to prove what I've just laid out here, I'd bet the farm on it. I have noted in a few other posts forum members who mentioned that they had replaced their stock exhaust with either the Ida or Accufab units to reduce under clamshell temperatures "and their vents still warped". I believe their vents warped BECAUSE they changed exhaust systems.
Please note that I enjoy my Accufab exhaust, the additional 100 hp provided by my Heffner pulley and tune, and I have no intention of removing them. I am making an assumption that the experts at Heffner, when they developed this tune, did measure exhaust gas temperatures and that they are in a safe range for long engine life. I do wonder what the effect of this high peak temperature will be over the long term on the composite clamshell itself. :ack At the minimum, I must have both the top and side vents made up of a different material as replacing these with the stock plastic units is whistling in the wind. Cost be damned, I would like machined aluminum or stainless steel powder coated black. Cheers. :cheers
Chip
Having attended both rallies as well as the Indianapolis GT gathering, I've had a chance to examine several hundred GT's. I noticed both top and side vents were warped on the engine covers of a lot of GT's. Yet my stock yellow GT, with 3000 miles on it that has been regularly driven in our 120° heat here in Arizona was fine. My friend Larry (AZGT) had 4000 miles on his stock Arizona driven GT and his vents were fine as well. The weekend before the California Rally we had the GT guys come out and set up shop in Larry's garage. Larry and I both had the Accufab exhaust installed as well as a 91 octane Heffner tune and pulley. Within 500 miles both of our cars had warped both the top and side clamshell vents. The dynamics of this cause and effect came into clear focus when I was flying my Cirrus SR-22G2 home from Corona California yesterday. The Cirrus has a very sophisticated engine monitoring system that displays, among other things, both the cylinder head temperature and exhaust gas temperature of each individual cylinder. At altitude, with the push of a button, the system will help you find the optimum fuel air mixture for either best power or best economy. Takeoffs are generally made with the mixture control at full rich. Even at sea level this setting is much too rich to produce maximum power, but this setting keeps exhaust gas temperatures almost 300° cooler than they would be with an optimum fuel air mixture. Lower EGT's keep cylinder head and oil temperatures correspondingly lower as well. As the aircraft climbs and the atmosphere thins the mixture gets progressively richer and must be manually leaned. During this process fuel flow is cut with airflow remaining constant until exhaust gas temperatures peak and start to decline again. As I pulled the mixture back gradually and watched my exhaust gas temperatures climb nearly 300° during the process with a corresponding increase in engine power, those warped clamshell vents directly on top of my Accufab exhaust popped into my head. Unless one is flying above 15,000 feet or thereabouts a piston powered aircraft engine is never operated at peak EGT because this would put cylinder head temperatures above safe operating levels and would shorten engine life. So the best power setting for Continental Aircraft engines is 65° rich of peak EGT.
As soon as I got home from this flight I went straight to my garage and sure enough, the section of my vents that were warped most severely were directly above the top curve of my new Accufab exhaust. That portion of the vents closest to the rear windshield directly above the engine but away from the top curve of my exhaust was fine. Three components are coming into play here. 1. The smaller pulley forces a denser fuel air mixture into the cylinders producing more power and more heat. 2. The Heffner tune moves the fuel air mixture away from the very rich (low exhaust gas temperature) stock settings to a more optimum (leaner) mixture that produces more horsepower. This leaner mixture, just as it does in an aircraft engine, will raise exhaust gas temperatures by 200+ degrees. 3. Last, we have the Accufab exhaust itself. The stock muffler, at least with the stock tune, defuses the relatively low exhaust gas temperatures feeding into it over a large space with a lot of surface area. Evidently, the peak temperature radiating up from the stock muffler when a GT is either stopped while idling or shortly after it has been shut off, is not high enough to warp the vents. At least not in my case nor in Larry's case. The Accufab exhaust, when it is being fed by a pulley and tune equipped GT, has much higher exhaust gas temperatures feeding into it and a very small area that all that heat is radiating from. At speed there is no question that the Accufab exhaust will allow far more airflow past it and out the rear vents than the stock muffler. It is at idle and at rest after shutdown and there is no airflow through the engine compartment that this comes into play. Now this much hotter exhaust system radiates its heat from a much smaller concentrated area straight up into those vents. And the plastic doesn't like it.
I am not an automotive expert but I have worked as a test pilot evaluating many different systems on aircraft. And although I'm not in possession of the equipment I would need to prove what I've just laid out here, I'd bet the farm on it. I have noted in a few other posts forum members who mentioned that they had replaced their stock exhaust with either the Ida or Accufab units to reduce under clamshell temperatures "and their vents still warped". I believe their vents warped BECAUSE they changed exhaust systems.
Please note that I enjoy my Accufab exhaust, the additional 100 hp provided by my Heffner pulley and tune, and I have no intention of removing them. I am making an assumption that the experts at Heffner, when they developed this tune, did measure exhaust gas temperatures and that they are in a safe range for long engine life. I do wonder what the effect of this high peak temperature will be over the long term on the composite clamshell itself. :ack At the minimum, I must have both the top and side vents made up of a different material as replacing these with the stock plastic units is whistling in the wind. Cost be damned, I would like machined aluminum or stainless steel powder coated black. Cheers. :cheers
Chip