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Among all fuel system cleaners I'm aware of, Redline SI-1 contains the highest quantity per dollar (based on manufacturer MSDS) of the critical fuel-system-cleaning compounds known as polyether amines (PEA). Primarily for this reason I believe SI-1 to be the best available and most cost-effective fuel system cleaner product at retail pricing. At the current price per bottle when bought by the case right here on Amazon ($5.30) it is a bargain (12/15/09 edit: the price here has almost doubled since I wrote this and it is now, unfortunately, far from a bargain).
In my case it has not been necessary to use Redline's recommended quantity of approximately 3 oz per fill-up. The effects of 1 oz per fill-up are indistinguishable in terms of tested results (see my basis for this statement below). This results in an approximate cost of $0.35/tank or less than a tenth of a penny per mile. A full case at this usage rate is enough to treat 180 full tanks of fuel or to last approximately 80,000 miles. (Figures based 15gal/tank, 25-35mpg).Long version:
As far as I know, every effective fuel system cleaner on the market uses a class of compounds known as polyether amines (PEA), in varying proportions, to effectively clean deposits from fuel system components, and particularly from fuel injectors, which can quickly impact engine efficiency and performance when not operating correctly. My understanding is that these compounds were first developed by Chevron and sold under the Techron name, and have since been made available to other blenders of fuel treatment products. Until recently BG 44K, Chevron Techron Concentrate, Gumout Regane Fuel System Cleaner, Amsoil P.I. Performance Improver Concentrate, and Redline SI-1 (among others) listed polyether amines on their Material Safety Data Sheets (MSDS) in proportions varying from approximately 25% to 50%. Most of these products no longer list PEA on their MSDS, having changed their terminology to disguise the precise nature of their products' constituent components. Whether PEA is still contained in those products is a matter of uncertainty. As of this writing Redline's SI-1 MSDS still specifies that the product contains 30-50% PEA. Consequently it is one of the few remaining fuel system cleaner products that undoubtedly does contain PEA in significant quantity.
I have been able to test the efficacy of fuel system cleaners in one of my own vehicles using a method I stumbled on after having its injectors professionally cleaned while out of the vehicle. The car in question uses a Bosch engine management system that reports fuel injector data to an on-board computer, which then uses that data to calculate and report instantaneous and average mpg to the driver. I noted after the professional cleaning that the accuracy of the reported mpg, when used to measure average mpg over each full tank of fuel, improved suddenly from a prior error of around -3% - -5% (that is, the reported mpg was ~3-5% below the actual mpg, or typically just over 1mpg low) to an error as close to zero as I could measure (that is, usually between -1% and 1%, fluctuating above and below a perfect "match"). This can be explained on the basis that the engine management system will compensate for a dirty injector by holding the injectors open slightly longer during each combustion cycle, in order to admit the proper amount of fuel. The on-board computer interprets that as a slightly higher rate of fuel consumption, reporting a lower-than-actual mpg figure.
After several thousand miles - not long - the perfect mpg accuracy I'd noticed began to deteriorate, likely indicating that the cleaned injectors were beginning to suffer from some sort of renewed impedance to fuel flow. Out of a desire to retain near-perfect injector performance, and also out of curiosity, I started experimenting with various commercially-available fuel system cleaners and keeping records of the results at each fill-up.
Leaving out the long details, I'll simply say here that the results were surprisingly clear and unambiguous. Each time I went several tanks without using a fuel system cleaner (usually as a result of simply forgetting to use it or not taking the trouble), the on-board computer (OBC) accuracy would deteriorate. This would fluctuate from tank to tank, of course, as a result of inevitable measurement errors, but the trend was very clear even over a small number of fill-ups. Returning to the use of a fuel system cleaner (Chevron Techron Concentrate, Gumout Regane or SI-1, all of which at the time did contain PEA), the accuracy would improve again very quickly - within 2-3 fill-ups. I began using the SI-1 exclusively seven months ago based on its apparent cost-effectiveness and since then I have reduced the quantity I use in each tank to the current 1oz per ~15gal fill-up. The results remain unambiguous. If I use this small amount of SI-1 consistently, the accuracy of the on-board computer is excellent, with an average error of below 1%, or a fraction the error rate seen when not using such a product.
Based on the above I feel I can confidently conclude that SI-1 works very well, even at reduced treatment rates, at keeping fuel injectors clean. Fuel system types and injectors will vary, and other parts of the fuel system - for example intake valves and combustion chambers - might benefit from higher treatment rates (or, conceivably, might not benefit at all). Actual engine efficiency will not vary nearly as much as injector cleanliness, since the engine feedback system normally corrects for imperfect injector flow rates. However, as the flow is more greatly impeded, or impeded differentially among the individual injectors, mpg will be affected to some degree. I feel it is well worth the tiny cost to consistently use a small quantity of SI-1. Other benefits, such as to combustion efficiency as a result of combustion chamber cleanliness, to volumetric efficiency as a result of intake valve cleanliness, and to fuel system lubrication, probably exist as well although I can not evaluate them and have not attempted to do so. Other fuel system cleaners may work as effectively, or nearly as effectively, but I do not believe them to be as good in terms of value per dollar spent.
In my experimentation I did try some less expensive fuel system cleaners, those not containing PEA. They appeared to have no effect. I can not categorically state that only PEA-containing fuel system cleaners work, of course, but my observations did match the conventional wisdom on this point. I also experimented with using top-tier fuels only, without any additional fuel system cleaners. The results were the same as when using non-detergent (Costco and others) fuels. I don't doubt that top tier fuels contain small amounts of cleaners and will keep a fuel system functionally and acceptably clean, but the quantities involved are reputed to be tiny and my observations seem to indicate that even a small amount of additional additive is far more effective.
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