I would like to here from some of you about ETs with open exhaust vs full exhaust. Comparison with ETs not your seat of the pants feel please. I have a well set up camaro, that runs high tens low elevens and was wondering what you may have picked up.

Billy

Here's a guy that did some testing:

http://www.pontiacstreetperformance.com/psp/exhaust.html

The difference between open exhaust and closed exhaust can be quite substantial depending on the physics and tuning of each setup.

Mine picks up .25 with the exhaust off. (11.15 with exhaust, 10.90s no exhaust)

My Camaro is 0.15 secs slower with the exhaust disconnected at the collector. 11.50 with full exhaust verses 11.65 without.
MPH showed almost no change.

I put it down to the X-pipe helping scavenging or improving flow in some way or another..
There is the possibility that A/F ratio was effected with the exhaust removal, however, I would not expect a slowing with the system removed as in almost all cases I know of, and when the A/F ratio was as close to correct as it was in the first place..
More testing will be done when I get the chance..

Thanks Guys,
Just thinking about uncapping mine once to see what it would do, I guess the biggest thing is how well your exhaust already may be. I think that I have to big an exhaust system already but may try to uncap it just to see what it does.
We have a pinks all out coming to Houston and was thinking about what I could do to pick up 3 tenths.

Bill

Keep in mind you'll need to re-evaluate the carb jetting with any exhaust changes to truly be able to tell if the exhaust vs open headers made a measurable difference. The best way to tell is to use some sprt of wide band O2 logging to make sure the A/F ratio is the same between both open and capped exhaust.

If you have a well thought out exhaust system, you won't gain anything and will probably lose ET. If the exhaust is just thrown together without wave lengths calculated and/or back pressure is high, it may show a gain. When I set my exhaust up, I tuned the collector length with PipeMax and then tested at the track starting at 20" and cutting half-inch off until best time was run. PipeMax was pretty much dead on for 17.9" collector. That's where I mounted my chambered muffler to "act as atmoshere". With straight headers, my collector is 10" long and I lose .2 ET. Low/Mid range is dead. I have an O2 sensor and the jetting didn't require much change. I'm running 72F/84R

Uncapped for Pinks all out in Houston and the et changed from 10.95 Friday night to 10.72 @ 124.8 Saturday during the day with no changes but uncapped using 18 inch extensions. I guess I need to re-think my exhaust system due to what I gained in the open vs close. My set up is 2 inch hedders into 3 primary with 3 inch exhaust with cross over into 2 chamber flows with turn downs. The exhaust system is not mandrel bent but is what I thought was a pretty free flowing system.
I may have been able to tune more out of it but didn't have time with only the two time trials for the show. The car to me didn't sound like or fell like it was running on all cylinders but the et was improved. It felt like I had more vibration like the engine was not running on all cylinders. Anyone have any ideas or experance with this?

Billy T

Merely uncapping your exhaust (as in using dumps) may not help ETs much if at all (uncapping my car was worth a couple hundredths). However, if you remove your exhaust system you are shedding weight -- and some exhaust systems can weigh a lot.

My set up is 2 inch hedders into 3 primary with 3 inch exhaust with cross over into 2 chamber flows with turn downs.

2-chamber Flowmasters just sound fast, they don't flow all that well. It'd be interesting to see what you run with a set of Dynomax Ultraflows.

On the chassis dyno my friend's 383 blower motor made 450'ish to the rear wheels through 2.5" Flowmaster pipes and 2.5" Dynomax Ultraflows. Uncapping the exhaust only picked it up 5 HP. I have exact numbers somewhere . . . but I think you guys get the point -- the Ultraflows are a very efficient muffler.