Thread: Headers without Hi Flow cats?

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  1. #1
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    Default Headers without Hi Flow cats?

    I have a 2.5" Corsa cat back on a otherwise stock 1996 GTS and am thinking of adding headers to the cylinder head upgrade. Been convinced that JM Stage III heads with stock manifolds isn't really an optimal strategy. I'm sure the same could be said for not adding hi flow cats, 3" exhaust, a better than 708 cam, and so on.

    My issue isn't necessarily money but rather decibels. I like it right where I'm at. No drone and conversation-albe. That said, if I'm going to make the header investment but the cats just choke them out, I could save a few bucks and pass. Live with what whatever horsepower I end up with.

    So the question: Does anyone have dyno numbers that document the added HP of headers with stock cats or a pre-high flow vs. post hi flow dyno numbers?

    Thanks

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Headers won't add much power (well you might have larger gains due to the heads) but keeping the factory cats with your heads and headers seems like a huge step backwards. HoFlow cats will be slightly louder, make way more power, and is probably your best route.

    0.02

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    Default Re: Headers without Hi Flow cats?

    Thanks Stuntman -

    Logically I concur but would love some numbers to back up our assumptions. Has anyone flowed a factory cat? No doubt Hi Flows are way less restrictive versus stock, but is it the next bottleneck after the heads and headers are replaced? Changing only Hi Flows in an otherwise stock car seem to be more about cooler sills and louder exhaust rather than power increases, meaning (no surprise) that in a stock system their flow is matched pretty well to the rest of the exhaust system.

    On the intake side, how much does all this cylinder head porting matter if the weakest link on that side, flow-wise, is the intake manifold?

    If only I had a flow bench!

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Items like the intake manifold and cats are restrictions but are not true "bottlenecks" by definition. Despite the intake manifold being a restriction, gen 2s can see 15-22hp gains from smooth tubes and K&N panel filters. Same for the exhaust where hiflow cats or removing the factory cats all together will see 2-digit gains as well. Someone is back to back testing headers alone (w.o cats both before & after) to see the difference which might not be much over 10whp.

    Your heads should flow much better than stock and should make more power (than stock) with a stock intake, exhaust manifolds, and cats. However you may see even greater gains from the above modifications because your heads flow better. I dont see any reason not to go to at least Hi flow cats since they tend to free up more power than headers alone. Sorry I dont have any useful flow #s for you.

    EDIT: (Another good reason to get rid of the cats):

    http://forums.viperclub.org/threads/...My-ACR-today-(
    Last edited by Stuntman; 03-02-2015 at 03:15 PM.

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    Default Re: Headers without Hi Flow cats?

    Headers without hi flo cats are sort of choked with good HP gains. Going with hi flo will allow the full potential of the headers plus a better sound and cooler temps around the sills.

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    Default Re: Headers without Hi Flow cats?

    Decided to go headerless at first, dyno then decide if the added investment is worth it. Not logical for maximum power and more work but also more fun for me as I enjoy finding the true value of each mod.

    Thanks for the comments.

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Ron, I've noticed that a new generation of forum members has brought on plenty of misinformation. The factory intake and throttle bodies are not a restriction. The factory cats may kill a couple of horsepower, but it's negligible. The real issue is in in the design of the stock cylinder head. At damn near 600 rwhp, the difference between stock cats and hi flows was 5 hp and the same between hi flows and straight pipes.
    The nice thing about a properly designed header is that you can bolster the power under the torque curve. Everyone always talks about the big number rather than the whole picture. Smooth tubes are a cheap and easy addition to a non ACR Viper, but they won't yield you more than a few hp"s.

    With respect to the guy whose sills melted. That's a clear case of a modified car that"s been tuned completely wrong. It's got major issues and changing an O2 sensor isn't going to fix it. Stock Paxton units meet CARB emissions standards, that doesn't happen without extensive testing to ensure emission equipment durability. God knows what kind of engine damage is occurring due to improper calibration. The thread is creating a false impression that the CAT is the problem.
    Last edited by Camfab; 03-08-2015 at 03:44 AM.

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    Default Re: Headers without Hi Flow cats?

    Quote Originally Posted by Camfab View Post
    . The factory cats may kill a couple of horsepower, but it's negligible.
    I was surprised how clogged my factory cats were at 27,000 miles and how hot they got. Went with M&M headers and then hi-flow cats to supplement the 3"inch Borla exhaust that came on the car. Big difference you can feel at the pedal on my 96 GTS . Factory cats are junk imo...el

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    Default Re: Headers without Hi Flow cats?

    Sybil, I can't comment on your particular scenario because I really don't know the particulars of your car. Let's just say that stuffing those CAT's in the confines of the side sills is a bad design solution. Moving past that, the CAT's get super hot because they are actually doing their job. The more unburned fuel ends up in the exhaust, the hotter they are going to get. Any CAT that's actually effective at doing its job will meltdown when subject to massive amounts of fuel in the exhaust. I tested my car, both built engine V's. Stock with OEM CAT's V's. Hi quality metal matrix CAT's.

    http://forums.viperclub.org/threads/...ight=emissions

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    Default Re: Headers without Hi Flow cats?

    In the end and as cash flow allows, I'll might end up with all of the above anyway. My spare ECU re-programming will be a compromise between with and without headers so post heads & rockers I'll be able to just change the headers and compare, then the high flow cats and compare.

    Where I'm not going is a better than the stock 708 cam, forced induction, or any mechanical intake / throttle body / fuel system changes. Got way to many expensive hobbies to get into this one that deep and I prefer as stock looking and driving car as possible. Just want a more modern power / weight ratio without the reliability / noise / drivability issues mods can sometimes cause.

    I've noticed that a new generation of forum members has brought on plenty of misinformation
    Camfab - I've observed the same, but unfortunately that's been an issue from the beginning of the internet... Something as simple as what is the chamber volume of the GENII head can't reliably be answered with the search function of this board or Google. I found numbers ranging from 73cc up to 76cc and every number in between. How can the chambers be that different if the compression ratio of all year GENII's is 9.6? Not possible. So I cc'd a late model (2000) stock (AB) casting (76cc's) and when I take my '96 GTS heads off, I'll cc them (an AA casting). If both are 76cc's, issue resolved. It's the same in chasing down most any Dodge undocumented spec. If you want fact not opinion, you pay to get the fact.

    Part of the fun I suppose but time and dollar consuming.

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Camfab - I just checked out your link. You did Stryker heads, cam, headers and a tune with stock cats and produced 585 RWHP?

    Since you had hi flow cats 2,000 miles prior, why did you revert to stock cats?

    Fascinating...

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Quote Originally Posted by Camfab View Post
    Ron, I've noticed that a new generation of forum members has brought on plenty of misinformation. The factory intake and throttle bodies are not a restriction. The factory cats may kill a couple of horsepower, but it's negligible. The real issue is in in the design of the stock cylinder head. At damn near 600 rwhp, the difference between stock cats and hi flows was 5 hp and the same between hi flows and straight pipes.
    The nice thing about a properly designed header is that you can bolster the power under the torque curve. Everyone always talks about the big number rather than the whole picture. Smooth tubes are a cheap and easy addition to a non ACR Viper, but they won't yield you more than a few hp"s.

    With respect to the guy whose sills melted. That's a clear case of a modified car that"s been tuned completely wrong. It's got major issues and changing an O2 sensor isn't going to fix it. Stock Paxton units meet CARB emissions standards, that doesn't happen without extensive testing to ensure emission equipment durability. God knows what kind of engine damage is occurring due to improper calibration. The thread is creating a false impression that the CAT is the problem.
    The Gen 2 intake manifold's long, small-diameter runners and under-sized plenums do restrict the motor at higher RPMs which is why the power and tq tank after 5,500rpm compared to the Gen 3 (more so for a 4/5) intake manifold, which breathes better at higher RPM due to larger diameter, shorter runners and a slightly larger (but still undersized plenum).

    Admittedly, an intake manifold will not yield anywhere near the gains of a good set of ported (or newer gen) heads, but regardless, the Gen 2 manifold is restrictive at higher rpm.

    I agree there is a ton to be gained with head work, which is why I said the intake manifold was a 'restriction' (because like the restrictive air filters or intake tubes, improving them will yield gains) and not a 'bottleneck' (which by definition, is a choke point where any other changes to improve airflow would not make a difference until the bottleneck is addressed).


    Last edited by Stuntman; 03-08-2015 at 10:35 PM.

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    Default Re: Headers without Hi Flow cats?

    Quote Originally Posted by Ron View Post
    Since you had hi flow cats 2,000 miles prior, why did you revert to stock cats?

    Fascinating...
    I'll take "Because he lives in California" for $300 Alex

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    Default Re: Headers without Hi Flow cats?

    I'll take "Because he lives in California" for $300 Alex
    I'll be betting with you not against you but regardless an impressive number with stock cats

    Weather's not as nice but much more car friendly government in Indiana

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Headers are the best mod you can make to a Viper. Cats totally unnecessary from a performance or sound standpoint on a Gen II. Just put in a piece of pipe and save some $$$. Just be sure to heat wrap everything in the engine bay in the vicinity of the headers, use good heat shields and spark plug wires, and then ENJOY!

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    Default Re: Headers without Hi Flow cats?

    Stuntman, I like your charts. I wish I could overlay my dyno sheet over them. The only issue is that my sheet is RWHP, and those are crank numbers. My numbers exceed both torque as well as HP numbers on those graphs. Translated to crankshaft numbers my engine is producing roughly 667 HP and 668 LB/FT of torque. You are absolutely correct in your assumption that the Gen II manifold runner design does not lend itself to high RPM usage. That being said, and remembering that the Gen I and II engine displacement is less than the Gen III or IV your not comparing apples to apples. The Gen II should not be spun hard anyway. It's all in at 5500 and spinning it beyond that is setting yourself up for a spun main bearing. So I say again, the engine is producing over 150 HP over stock and bigger numbers from idle to 5500. NO changes to the stock throttle body or intake manifold. Jeff Mory's told me specifically that changing to the Gen III intake manifold would show up 80 LB/FT of torque loss through the mid range with a very small HP gain on top.

    Ron, you are both correct I changed back to the OE CATs due to emissions testing. I switch between the OE and high flows because of the heat issues. My car has a seriously wicked exhaust note with those heads and cam, but sometimes I really prefer the OE CATs as they make the car somewhat civil.

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    Default Re: Headers without Hi Flow cats?

    Cam, its not really "my" graph since another posted it originally and I'm aware its crank HP. What are your mods and can you post your dynosheet?

    Why can't the Gen 2 bottom end and main bearings handle more rpm (like a 4/5) if you improve the top end to handle it? I think there are pretty big gains from the 4/5 manifold over the 3.

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    Default Re: Headers without Hi Flow cats?

    The Gen I and II have large OD main journals which are great from a strength stand point but bad from a lubrication stand point at high speed. The GTSR race motors of the day had reduced journal sizes. I'll pull up my graphs and post them, they are somewhere on the site from back about seven years ago.

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    Default Re: Headers without Hi Flow cats?

    Stuntman, I have Striker Heads and the matching cam that was developed for those heads. Jeff Morys worked or works at Chrysler in the head shop. He developed those heads and they flat out work. He sold the company, from what I heard Chrysler felt there was a conflict of interest. Anyway when speaking with Lee at Arrow Racing, he noted that the Gen IV heads were very similar to the Strikers (um, I wonder why��).

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    Default Re: Headers without Hi Flow cats?

    Ron,

    If you want a little more seat of the pants feel without increasing how load the car is, why not go a different route? Consider doing a high axle rear end ratio, a set of pulleys and an aluminum flywheel.

    The only negative about the aluminum flywheel is that the car may be a little jerky and low speeds due to 708 cam, but my understanding is that the 708 cam cars can be a little jerky anyway with stock flywheel.

    As far as the stock cat versus high flow cats, there are definitely differences in them. To stock cat has a very good flow rate, I considered keeping them when I modded my car, but I found a set of barely used Random Techs for less than what it would have cost me to modify the inlet/outlet openings of the stock cats. If you went with aftermarket headers, you would want to modify them to match the header outlet piping. I remember a while back there were some who swapped out the manifold and went with the newer header design that the 2000-2002 cars came with. They event gained some power. If you are going to do all that work though, you might as well go aftermarket.

    one final comment, as far as drone in the cabin is concerned, Corsa is the best at eliminating drone in the GTS. So I would change anything there. As I have an RT/10, I went with B&B as cabin drone isn't really an issue.

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    Default Re: Headers without Hi Flow cats?

    Thanks John -

    I went the aluminum flywheel route years ago and felt no SOP difference except that it annoyingly chattered a bit. When I did my rear main seal I reverted to stock and sold it. Had to change the pinion bearing in the rear and decided to keep the 3.08's as I like the loafing 6th gear for long highway cruising I tend to do. I've never driven a non-stock geared Viper so maybe if I had I would have done otherwise. As far as pulley's never gave that any thought.

    The header / hi-flow issue is only a consideration because of the ported heads. Seems logical to optimize the entire path within my constraints (primarily sound) but by doing it incrementally, I get the benefit of documenting the value of each.

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    A good 80rwhp can be gained on the 1996-1999 models with simple bolt ons. The camshaft on these year models responds well to minor mods.

    Back in the day, my 1996 GTS went from 400rwhp to 480rwhp with K&N replacement air filters, smooth throttle body inlet tubes, 1.7 ratio rocker arms, stiffer pushrods, headers, high flow cats and a Borla cat back exhaust.

    We added a set of ported heads and raised the compression to 10.2 to 1 (milled heads .020 and used a .030 head gasket) and gained another 40rwhp (520 rwhp). Later we changed to a 228/232 cam and gained another 40rwhp (560 rwhp).

    Over the years we have found some better cam profiles and headwork, now seeing 600rwhp on the same combination of parts.

    Understand that many of these mods may not pass an emission test in some states.

    Hope this helps.

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    Default Re: Headers without Hi Flow cats?

    Quote Originally Posted by Dan Cragin View Post
    Back in the day, my 1996 GTS went from 400rwhp to 480rwhp with K&N replacement air filters, smooth throttle body inlet tubes, 1.7 ratio rocker arms, stiffer pushrods, headers, high flow cats and a Borla cat back exhaust.
    Hmmm, this is what I have except for the 1.7 rr's. Well I have them..... still in the box so hopefully I will get these numbers eventually lol

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    Default Re: Headers without Hi Flow cats?

    Thanks Dan, that's impressive. No emissions issues here in Indy thankfully. Ultimately I'll probably do everything but the cam, but still excited to see what the heads, rockers and tune do for starters. Comp ratio is still a bit of a mystery and will remain so until I take off my stock heads and cc them. I suspect the early GENII casting were different from the later ones but time will tell. There is so much bad data out there, including the 708 cam specs, so everything has to be checked. I'm pretty fortunate as my 1996 GTS baseline is 435RWHP with smooth tubes and K&N's.

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    ^Can you post your dyno?

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    Default Re: Headers without Hi Flow cats?

    ^Can you post your dyno?
    Can do.....
    Attached Images

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Awesome thanks. Are you on the stock 17" wheel?

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    Default Re: Headers without Hi Flow cats?

    Yup - currently all stock but for:

    Smooth Tubes
    OEM 2000+ oil pan
    98+ OEM shorty exhaust manifolds
    2.5" Corsa cat back modified with OEM exhaust tips
    Tom's 40mm rear brake calipers
    Snake Oyl cruise control
    Entire exhaust system from manifolds to rear muffs including cats are plasma coated with some type of zirconium ceramic mix - An experiment in heat control.

    Ron - '96 GTS

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    Default Re: Headers without Hi Flow cats?

    Quote Originally Posted by Ron View Post
    Yup - currently all stock but for:

    Smooth Tubes
    OEM 2000+ oil pan
    98+ OEM shorty exhaust manifolds
    2.5" Corsa cat back modified with OEM exhaust tips
    Tom's 40mm rear brake calipers
    Snake Oyl cruise control
    Entire exhaust system from manifolds to rear muffs including cats are plasma coated with some type of zirconium ceramic mix - An experiment in heat control.
    How do you like the Snake oyl cruise control? Easy to install?

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    Default Re: Headers without Hi Flow cats?

    How do you like the Snake oyl cruise control? Easy to install?
    It works very well and I've had it since 2002 or so. Installation instructions were OK but some of the wire colors they said to splice into were wrong. I hope by now there are long since corrected but it's worth double checking against the service manual. Just don't test the clutch disconnect for 50 miles or so as it doesn't function until it learns something. Hand on key when you do....

    Don't think there's another option out there so if you do a decent amount of highway driving, it worth it. A real knee saver.

    Ron - '96 GTS

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