Cross over and down pipe upgrade question

[coxfmly]

Hello,
I want to upgrade my factory crossover and down pipe for better flow. here are the 2 choices and a question.. Sub already has a "cat back" exhaust upgrade....

#1, a kit from Flowmaster system #17220 with a 2.25" crossover and a 3" down pipe. This kit is running around $130 with free shipping...

#2, a cross over pipe from IMCO #339786 with 2.5" dia. tube. its about $35 with shipping.

Q: should i go with the kit which has both or just the crossover with a larger diameter and keep the factory down pipe? Not sure if its better to go with the larger crossover or is the kit overall better??

thanks....

[DmaxMaverick]

In the case of the cross over, bigger is not necessarily better. Larger diameter means slower fluid velocity, and greater heat rejection opportunity, neither being beneficial. The OEM pipe is more restrictive, due to poor bends and crimps, but not nearly as bad as believed by many. Of course, it can be improved, but not necessarily by just making it larger. One bank only has to allow 1/2 the collective flow of both banks, with similar heat rejection, for ideal efficiency. Efficiency is power. Power is economy. All else being equal.

[DmaxMaverick]

With the downpipe, it's the opposite. Quick cooling and reduced fluid velocity is desired, best accomplished with a larger cross-sectional conduit. The gas has finished its work, and needs to exit with the least resistance possible. Reducing the temperature (greater gas fluid density) and increasing the conduit volume expedites this. Bigger is better, in this case.

[coxfmly]

Hi Dmax,
Would the 3" flowmaster down pipe be better than the factory unit? Not sure what the factory pipe diameter is? I have found a few larger down pipes (4"), but they don't have the factory 4 bolt pattern on the down side. It looks like the flowmaster would be my only choice with this connection?

Thanks again, always appreciate your words of wisdom.....

[DmaxMaverick]

It comes down to cost vs. return. What's your intention, or goal? If you're planning on staying around stock power, then there's no gain. Conditions can be better, but not enough to show a gain, even over the long term. If you're planning on increased power, then it may be beneficial, depending on the end goal. Some upgrades do improve economy, but the expense involved often precludes any return. Unless you're planning a program (chip) and other upgrades, it's usually fruitless, economically. The ROI just never shows up.

That said, larger is better, and not, depending on the part of the system. Efficient flow isn't necessarily "bigger", but may be only more efficient (less restrictive). There'll be almost NO gains, at or around stock power levels. A better (not bigger) crossover, and a larger downpipe can increase economy and performance some, but you must weigh the expense vs. the gains. You may not gain any economy, but may get a bit more power available. Or, the opposite may be true. It all depends on your intent, and your driving habits.

[coxfmly]

At this point I have installed a turbo master, relocated PMD, OPS saver set up, new heads and a performance chip from Heath. Amazed at the power and drivability over my 84. .....I'm sure the previous owner installed an upgraded cat back system as well. Its my daily driver with 230K so I'm trying to keep it around for a bit longer. I tow every now and again so I want to do all i can to keep thinks cool and under control. Looking for the best set up to help things along. With my set up, do you think the 3" down pipe would help ?

Thanks..

[DennisG01]

Just to add another thought regarding the crossover... regardless of the size, it still necks back down to the size of the manifold/donut gasket. The factory crossover is a double-wall construction and sometimes the inner wall gets over-crinkled. If you can pass a golf ball through it, it's likely good enough. I still have my original crossover - but when I do change it out, I'll keep things simple and get it from Kennedy or Heath... they've used and abused the stuff they sell and know that it works.

The downpipe, if stock, is usually an odd looking thing. Below is a picture of mine (and also the piping where it passes the AC lines). I think the stock DP is about 2-3/4". I think my new downpipe (mandrel bent) is 3" and I have NO problems with EGT's and I've done lot's of stuff to my engine, as well. When I tow, it's about 10,000lbs.

http://www.thedieselpageforums.com/photopost/data/519/medium/downpipe.JPG

[coxfmly]

Update...
I went with the kit from Flowmaster with a 2.25" crossover and a 3" down pipe.
Here are a few things I have noticed since the install...
Cons:
It feels like i have lost some bottom end torque...
I'm sure my mileage has dropped a bit, will have specific numbers this week.
Going easy on the throttle to see if its my right foot or the new exhaust.

Pros:
with the factory set, the Sub seemed to like 68 mph when cruising down the road...it felt like it was being pushed a bit past that.

with the new set up, the Sub would push easily to 70 and likes to cruise at 72 mph.
doesn't it felt like its being pushed at all.

Thinking of going back to the factory cross over to get the bottom end back and stay with the larger down pipe.

Any thoughts......Thanks

[racer55]

Not really worth the effort IMHO,you likely have heavy right footitis since the install and improved mid range likely translates to a perceived loss in bottom end that is really not there.