[DeTomaso] Free Flow Exhaust Runs Hotter?

GW gow2 at rc-tech.net
Mon Jun 6 18:52:44 EDT 2011


Some time ago I was wrote a computer program to read values from various 
circuits from EGT, O2, timing etc. I had built my headers which were 
quite a bit larger then stock. I made a mistake though in that they were 
paired 2 x 2 and in a pairing that did not work. If 2 cyl were paired, 
one would fire, an unrelated cyl would fire, the second cyl would fire 
then 6 more un related cyl would fire and then it would start all over.

In a static no load run at about 2500rpm, the first cyl had no back 
pressure. The second had back pressure. The first cyl ran cool, the 
second cyl with no back presure ran almost 2x as hot. Each of the cyl 
were paired and had the same effect. The cyl WITH back pressure ran 
completely clean and cool as the charge burned completely in the 
chamber. The cyl with no back pressure expelled burning gas into the 
exhaust as it was not done burning and the exhaust on these chambers 
were all sooted up. This also caused some backfiring in the exhaust. 
Here is a screen shot of just the EGT:

http://www.rc-tech.net/cars2/ts/eh/12082008.jpg
http://www.rc-tech.net/cars2/ts/d2.jpg

The egt is hotter because the fuel is burning outside the chamber near 
the egt but it is also making the header and associated are hotter and 
is not aided by the same cooling the heads are. I would like to 
experiment with cam timing (among other things) to compensate when 
opening exhaust.

Gary



Lee, Patrick Q wrote:
> After replacing the restrictive 1 3/4 in pipe Borla's on my 393 with the
> dual 2 1/2 inch Magnaflows,
> the engine seems to run hotter. I though it was only my crappy
> temperature gauge but my IR gun confirms
> that the engine is a good 10 degrees hotter with no changes to the car
> other than the new exhaust.
> Also some popping and backfiring on off throttle declaration. Could this
> be a lean condition caused by the new exhaust?
> Solutions? I just spent last summer getting the car cool enough!
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