Gibson Muffler Questions?

[rlitz]

Originally Posted by RockyMtnRay

And even better, the muffler had the unexpected effect of lowering the peak torque RPM by about 100~200 RPM: my engine is now making peak torque at 3200 RPM and the point at which the torque increase from the headers starts is closer to 2500 RPM than the 2700 it was before the muffler. And, of course, the lower the RPM that peak torque is obtained, the better for towing.

Ray - Did you dyno the before & after of the muffler install? Can you share those charts with us, or if you've already posted them, can you point me to them?

-Ron

[RockyMtnRay]

Originally Posted by rlitz

Ray - Did you dyno the before & after of the muffler install? Can you share those charts with us, or if you've already posted them, can you point me to them?

-Ron

Nope, did not dyno, partly because the closest one is in Denver (about 90 miles each way) but more importantly because it's nearly impossible to get reliable dyno tests at RPMs below about 3200 RPM because of torque converter effects. If you look at nearly all published dyno tests, they typically don't start until at least 3000 RPM, more often around 3500 RPM. For towing, IMO all the important torque issues happen at or below 3000 RPM.

So instead I did a series of real world towing tests over Colorado's high passes...keeping track of ambient temperature on each climb; trying very hard to insure that the weight of truck and trailer were the same (or that I knew the exact difference) and making careful note of exactly how far (to the tenth of a mile) into each climb I could get before the transmission had to downshift to 2nd gear, and finally, what my speed was at several different points during the climb (as the altitude increases, the speed gradually bleeds down). The main test climb was the western approach to the Eisenhower tunnels on I70...an 8 mile long grade of nearly perfectly steady 7% with an altitude change of 2400 feet (start elevation is 8700 feet, tunnel entrance is 11,110 feet above sea level)...with a start speed always of 65 mph in 3rd gear (OD off). Because of the exceedingly long length of this grade (it takes about 10 minutes to climb given the inevitable speed bleedoff), and that's got 3 lanes so I've never had to let off the throttle due to traffic, it's actually quite good for detecting even minor towing performance differences. So good that Subaru uses it for real world testing of their supercharged sedans.

I did 6 measured towing tests. My first two tests were with the stock truck, the next two tests were with just the headers, and the final two were with headers and muffler. The performance gain from the headers (measured in both how far I could get before the tranny downshifted and my speed (at the various points) was quite consistent with the torque gains reported elsewhere by those who actually did dyno tests with JBA headers...namely about a 7% gain (~22 ft-lbs if measured at sea level). The performance gain from adding the Gibson to the headers...again measured by distance to shift down point as well as road speed at various points in the clmb...was roughly half that of adding the headers or roughly 3 to 4% (around 10 ft-lbs if measured at sea level). Several members have reported that a good performance muffler (e.g. SpinTech) produced roughly a 10 ft-lb gain when used with headers so I feel my results are reasonable. The difference is that a Gibson is rather quiet at full throttle as compared to the noise of most performance mufflers.

As for the difference in torque onset point and peak torque RPM, those were observed in both non towing and towing circumstances. A goodly number of people who have installed JBA headers have noted there's a very distinct noticeable push or rush that begins around 2800 RPM...and, as a person who's trained to closely observe events (I have a MS in Chemistry), I too definitely could sense a definite increase in the rate of acceleration right around 2800 RPM after I installed the JBAs. And then after I added the Gibson to the JBAs, the RPM where that noticeable increase in acceleration would occur actually did lower a bit to around 2600 RPM. This actually surprised me a bit because I was predicting it rise to around 3000.

And finally, to determine the effect on peak torque RPM, I observed the tach very closely when towing on a couple of long 4% grades out on the high plains (elevation around 5000 feet). What I noted after several runs on this road (I25)...after the muffler was installed...was that the truck would steadily accelerate to about 3200 RPM (OD locked off, 3rd gear) and then wouldn't accelerate any more. If I managed to get the RPM higher (say the stock peak torque of 3400), it would slowly bleed down to 3200 RPM. Not 100% scientific (since higher RPM also incurred a slight increase in aerodynamic drag) but an indicator that peak torque was mostly likely closer to 3200 RPM than 3400 RPM.

The bottom line is that these are mostly not "butt-dyno" results but rather fairly closely measured real world numbers taken during very challenging towing situations. I chose my mods very carefully and am quite satisified with their effect on towing performance. If I hadn't been satisfied, I could certainly have easily afforded to try other approaches. And there's now a substantial amount of anecdotal evidence that my mods have also worked out very well for several other TS members who wanted better towing performance without much increase in noise.

If you want traditional dyno charts, you're probably pretty much SOL...especially if you want numbers that apply to towing.

[rlitz]

Originally Posted by RockyMtnRay

Nope, did not dyno, partly because the closest one is in Denver (about 90 miles each way) but more importantly because it's nearly impossible to get reliable dyno tests at RPMs below about 3200 RPM because of torque converter effects. If you look at nearly all published dyno tests, they typically don't start until at least 3000 RPM, more often around 3500 RPM. For towing, IMO all the important torque issues happen at or below 3000 RPM.

So instead I did a series of real world towing tests over Colorado's high passes...keeping track of ambient temperature on each climb; trying very hard to insure that the weight of truck and trailer were the same (or that I knew the exact difference) and making careful note of exactly how far (to the tenth of a mile) into each climb I could get before the transmission had to downshift to 2nd gear, and finally, what my speed was at several different points during the climb (as the altitude increases, the speed gradually bleeds down).

Ray - it sounds like you did a great job of comparing each of your mods, and you can definitely conclude that your mods gave you more torque where you need it. I guess we'll never really know how some of these different mod packages compare to each other. Maybe the difference between your system and the other permutations are only a few ft-lbs. With only a few choices in each category (header, muffler, pipe diameter, air filter) there are so many permutations!

-Ron

[cupidstoy]

i have to chime in and confirm the results of the gibson/jba combo. i was running a straight pipe with no muffler from the resonator back. that was pretty good and i enjoyed playing around with it.

couple of weeks ago, had the muffler man install the gibson muffler in place of the stock muffler and reuse the 2 3/8" stock piping. there is definitely more towing torque with the gibson. i also had him remove the resonator. i'm not sure this has much of an effect after looking inside the resonator. i do have a mild drone, more of a whoosh or reedy sound actually, at 60 mph, that was not present with the resonator and straight pipe.

but, something semi-magical happens with the gibson. there is a real nice push from the low end when accelerating either towing or not towing. it's not a lot, but about like ray states, it adds another 5% or so to what the jba cat-4wards will do for you.

i really notice it most in around town driveability. also, i seem to be getting about 1 mpg better when towing. that is something i never thought i would see.