OverdoneRX7

I was told that I can NOT put just any high-flow cat in the car because I need to maintain the proper back pressure so I won't overboost. Anyway I don't have the stock one anymore, I have a 3" downpipe and a Greddy Power Extreme cat-back. So my question is what is the correct cat for this car???? Thanks, Jim

dgeesaman

iTrader: (7)

To keep your boost in check, you want to avoid a midpipe.

High flow cats are usually similar in their backpressure characteristics, the difference is whether they can withstand the heat of the exhaust (FDs run really hot)

If you run some searches you'll see what others are using - Bonez is one brand that works well.

Dave

7racer

It seems like you are asking the right questions. However, the person giving you answers is leading you astray. Dgeesaman is right.

You CAN run a more free flowing exhaust but you do need a boost controller to maintain boost.

I would spend ALOT of time reading the stickies above and then searching to clarify things that you don't understand.

If you don't already, make sure your car has a boost guage.

axr6

Who ever told you that was RIGHT. My boost was perfect before with DP, Stock Cat and CB.... then... I just replaced my stock cat with a Metallic Cat that I build myself. My boost is pretty much out of control right now, just like it would be with a straight midpipe. My PFC can not effectively controll it at this point. It seems that these Metallic Cats just don't provide much backpressure.

I cut up an E-Bay midpipe and welded in a standard 300 cell/sq inch Met. Cat. with air pipe fitting. Now I got to figure out how to get my boost back under control. Could just return the stock cat but, giving up all that power gain is not easy...

dgeesaman

iTrader: (7)

1) You started with a stock cat. He's starting without one.

2) The question was a comparison of high flow cats, not high flow vs. a stock cat.

3) Yes, boost control is always a question when adding flow to a system. Every car is different.

7racer

axr6, overdone.....

sorry it was late at night and I don't think my post was clear

here are some threads about controlling boost with a more free flowing exhaust. (basically boost control and wastegate porting)

https://www.rx7club.com/forum/showth...+boost+control

https://www.rx7club.com/forum/showth...+boost+control

Sgtblue

iTrader: (16)

^Second on the Bonez.
https://www.rx7club.com/showthread.p...&highlight=cat

axr6

Your point #3 is where the answer is. Everyting that I'd read on this forum stated that people who used high flow cats, ceramic or metallic, seemed to avoid the dreaded boost problems associated with straight MPs. Well, my car have proven an exception. In that context, I think it was the proper reply to the original question weather ANY high flow cat may be used without upsetting the boost pattern.

Now, my car seems to be a unique case and I need to find out why. For a given set of mods and PFC boost duty cycle and fuel inj. map settings, I seem to run much higher boost and require much more fuel than others. That is probably why my car reacted so radically to the met cat. I have not tried any other high flow cats so, don't know if it would overboost with ceramic types, or not. From my research on the subject I find that metallics produce considerably less pressure differentials (back pressure).

Thus, I think, we can let this case stand as an other example for how each car may react differently to a particular set of modifications.

dgeesaman

iTrader: (7)

That's an interesting piece of data.

Dave

Sgtblue

iTrader: (16)

I really haven't checked into them much, but I thought that the main advantage of a metallic substrate cat was that it didn't need the extra O2 (as in no airpump needed) like the ceramic ones. ???

axr6

That may be true for longevity but, if you need to pass CA emission tests you probably should have the airpump. I have read one post on this forum where one FD passed with the airpump operating and did not pass when it was disconnected. That was with a metallic substrate cat.

I saw the main advantage of the metallic ones that they create less back pressure, therefore, more performance. But, as it turned out today after removing the pill from my wastegate line, it caused boostcreep in my car. So, not enough backpressure, it seems, for boost control.

adam c

iTrader: (2)

I see lots of answers, but no questions. The first question should be: What do you want from your car? If he wants to keep it pretty stock, then he should get a stock cat. This would resolve any major overboosting problems. If this is to be an all out racing machine, with no regard for smog issues, then a different course of action should be taken.

So, what sort of car do you want to have?

purerx7

iTrader: (61)

Well if your have erratic boost patterns with the midpipe, you should port your wastegate. By porting your wastegate that should help your boost problems and it will stabelize after being ported, all assuming you have the stock twins.

OverdoneRX7

Thank you guys very much for all of your info. As for the question from Adam, The motor is not stock at all, it is custom build by Pettit Racing, but the twins are new/stock turbos. Using an Apexi computer and either the Apexi boost controller or Greddy Profec B, not sure yet. So the goal is around 350hp, thats plenty for me. It used to have a much different set up with a T-78 single. That was just too too much for the streets in NY. Also, there is no more air pump and running the simplified vacuum routing sytem. Jim