Back Pressure Discussion

[DanStokes]

I have the results of a test and now I'm trying to understand what I learned - or didn't learn.

I ran Mom's Celebrity ('87, 2.5l 4 cyl, auto w/o lockup, 4 door sedan) at Maxton in September with a best run of 92+. I brought it back in October with the following changes:
1) Full tune-up with plugs, filters, and plug wires
2) Oil & filter change. I wouldn't expect that to have an impact on performance but the stuff in there was old
3) Cut the exhaust right ahead of the cat and supported the pipe so it could exhale freely. The cat seems to be an old GM pelletized unit and my nose says it still works
4) The aero of the car was unchanged although the tires were pumped a little harder

The difference in performance - 95.3838. However, the air was considerably cooler and I'd expect that to have some impact on HP.

So the discussion point is - Did cutting the exhaust system out of the loop make any difference or is the slightly faster speed just run-to-run variability? Three mph isn't a lot but is a significant uptick in the LSR world.

Please discuss
Dan

[Beagle]

3% says to me you made some more hp, which I don't think surprises me with an open exhaust v. 80's tech cat and muffler that could stifle a frog fart.

Cut the cat out of the loop with a test pipe ? That'd say if it's the cat v. the muffler? Guess it's too late to test that at Maxton (tear.Boo.)

Cooler air = could be most of it? My junk LOVES colder air.

btw, 95 out of that car is smoking! I'm actually impressed by that.

[Barry Donovan]

a cat won't do anyhting.
and 1987?
change that crazy cam.

I learned to actually like the cat local, ends up a heat pump, and the comb keeps stuff from climbing back in as a scavenge (carb engine).. the header stays sterile, builds a bridge between tough alloy and t304...

if a cam and air/fuel is big enough.. a cat is not part of the race engine anyway.

[DanStokes]

I was AMAZED at 95! Even my first run in September was 90+ and I was expecting maybe 82 or so. Peppier than I would have thought. My initial thought is that most (maybe all) of the improvement is the cooler air.

Boxer - no honeycomb. This seems to be a GM pelletized as it's flat and has a drain plug. We just ran the car for fun because it was Mom's. It'll go up for sale next week, so no cam change or such. It'll help finance the new race engine for the Buzz Bomb.

Dan

[SpiderGearsMan]

bead converters were low flow

[TC]

Though colder air may make more HP, it is also more dense and I believe that can hamper aerodynamics at speed...........Meaning you have a thicker soup to cut through.......

And are you still running the stock air box??

[DanStokes]

No air box. It's TBI and has what looks pretty much like a regular old air cleaner but with a cold air intake. I put in a brand new AC element! Also painted the can. I would have done the "flip the lid" thing but the holes are asymetrical and it won't flip w/o modification. So I pissed away 20 HP right there. Remember this car will get sold ASAP now that the grudge match is over.

Dan

[Beagle]

What would whizz me off is ... how close did it get to the Camaro? I'm honestly still kinda shocked it went 95 with an Iron Duke. That is freaking awesome. Imagine if you had put the Type R or Spec V stickers on it? Holy crap man, it would warp time itself!!

[BigBlockMopar]

I would think it's a combination of things.
First the cooler air and reduced restriction in the exhaust helping the motor make power, and 2nd the loss of weight by removal of the rest of the exhaust. The aired-up tires should've been good for some less rollresistance aswell I would think.

[CaminoKid]

Dan,you know that you can not judge what made the difference without making one change and running two times per change.Or so I have been told by the twins.

[DanStokes]

True enough, Woody. Normally that's what I would have done but this was our last hurrah so it was "do it and make the runs".

BBM - Weight loss is not significant in LSR. Less weight only helps in accelerating the mass not keeping it in motion. The fast guys actually have to ADD weight to keep 'em on the track (I've never been fast enough that I had to worry about that). Besides, I didn't remove the old system, just pushed it to the side.

The Camaro has run 107 in basically stock configuration but it's a generation older than the Celeb. Still, the Celebrity's performance was, to say the least, impressive. A little tweaking and it would be possible to hit 100, I think.

Dan

[Stich496]

if you advanced the timing 4* and closed the spug gap to 25 it hit 100
pontiac 6000 is the same car and I got 100 out of it..

[DanStokes]

Stitch - I'd have turned the distributor up a couple of degrees but there AIN'T no distributor! This is the first year for coil packs. Changing the plug wires was tough enough with the coil packs wedged between the back of the motor and the firewall and under the intake/throttle body. I tried turning the coil packs but it didn't seem to help. If I'd had more time I could have taken it to my friend Dave's shop and maybe he could have tweaked the timing curve in the 'puter but I ran out of time and it was what it was.

Still, I got the free dinner so all is well.

Woody - as far as running a change twice. Here's how we did it in the data business (what I did for a living in the Gov). "Once is an occurrence (or anomaly), twice is a coincidence, and three times and you can BEGIN to say something about the data." That's why we would sometimes run as many as 50 repetitions of a test point - our data had to hold up in court and the statistical analysis had to be absolutely bulletproof. But with two data points there is at least SOME reassurance that the event (in this case, a change) may repeatedly have this impact on the test subject.

Dan

[Stich496]

haha you think changine parts of the hei was easy back there..

[Caveman Tony]

Wonder how it woulda run with some synthetic 0-W-30 oil in it and a couple of rolls of duct tape on it.