4" too big?

[Guest 280ZForce]

Then you've read wrong. Sorry no way around it. 2.5" is good enough for the hottest n/a l28. Good luck

it's been dyno proven that 3" on a n/a l28 gives better power than 2.5", even all stock.

[johnc]

Then you've read wrong. Sorry no way around it. 2.5" is good enough for the hottest n/a l28

 

Every single top level SCCA ITS racing NA 2.4L 240Z over at least the last 20 years ran a 3" exhaust. 2.5" might be "good enough" for some folks, but not for guys that are winning races.

[Thumper]

it's been dyno proven that 3" on a n/a l28 gives better power than 2.5", even all stock.

 

Sorry I didn't make my self clear. Yes you can run a 3" exhaust and yes it will make more power if you have you want it at 6000+rpms. But if you want to drag race it, autox, or even drive it on the street then a 2.5" true mandrel bent exhaust will flow plenty and make you overall faster.

[Thumper]

Every single top level SCCA ITS racing NA 2.4L 240Z over at least the last 20 years ran a 3" exhaust. 2.5" might be "good enough" for some folks, but not for guys that are winning races.

 

I really don't see a reason why. Unless they were running lots of rpms 7500+ they weren't pushing enough air at low rpms and were hurting their exit speeds. I've seen 300whrp v8's upgrading from a 2.5" single to a 3" single and only gaining marginal power up top but lowering their torque peak and moving it higher in the rpms.

 

But I guess its two schools of thought I say its too big stick with a 2.5", the experienced engineer/racer johnc says 3" I would go with johnc if I weren't so damn stubborn.

[johnc]

I really don't see a reason why. Unless they were running lots of rpms 7500+ they weren't pushing enough air at low rpms and were hurting their exit speeds. I've seen 300whrp v8's upgrading from a 2.5" single to a 3" single and only gaining marginal power up top but lowering their torque peak and moving it higher in the rpms.

 

There this misconception on the Internet that having good low and mid range torque will make your car faster around a road race track. It does not work out that way very often in the real world. Corner exit speed has everything to do with handling and how soon you can get on the throttle and very little to do with horsepower or torque. 140hp Spec Miatas and 110hp Spec Racer Fords have amazingly fast corner exit speeds, they just don't have the horsepower to build on those exit speeds down the next straight.

 

On most road race tracks you are using about a 2,000 rpm range in each gear from 3rd through 5th, except for maybe one corner where you might need a 2,500 rpm range. If you gear the car correctly it really doesn't matter where that 2,000 rpm range is in the engine's rpm curve. That's why Honda S2Ks are so fast around a race track with only 164 ft. lb. of torque at 6,000+ rpms.

[Tony D]

On most road race tracks you are using about a 2,000 rpm range in each gear from 3rd through 5th, except for maybe one corner where you might need a 2,500 rpm range. If you gear the car correctly it really doesn't matter where that 2,000 rpm range is in the engine's rpm curve.

 

1) The 2000 rpms range is very true! Even in land speed racing, you are only below 'peak power range' when you push off from the line. Otherwise, you are shifting to have your engine at/near peak torque...on built N/A's that's usually 4000-4500...pulling to horsepower peak somewhere between 6-7000. On our L28 LSR L28 we rarely let the rpms drop below 5000 between shifts, and on the L20A closer to 6500.

 

2) Our dyno testing showed some very interesting results regarding 'Internet Flow Potentials' that would seem to support John C's experience as well. When dynoing our L28, we found that we required two different ECU calibrations for when we were dumping straight off the header to atmosphere, and when running a full DUAL 3" exhaust to the back of the car... When we run the belly pan, we have to run the full exhaust, and when we do, the car is down on HP around 8-12HP depending on where you are in the usable RPM range. We of course remapped the ECU to minimize these losses, and to optimze each situation. Thosed lossed are the 'best case' scenario.

When we went to the L20A (1998CC's) similarly we expected to have to recalibrate the engine when running header-dump versus full exhaust. In this case, with theengine making it's power peak closer to 8800 rpms (as opposed to the L28 at 8300) we noticed no difference in HP levels with or without the exhaust in place. So somewhere around less than 1400CC's at 14:1 CR and 8000rpms is where you will start to see a decrease in performance unless you are using a 3" exhaust!

 

The L28 runs 14:1 CR, power peak at 8250. 320 HP

The L20A runs 12:1 CR, power peak at 8775. 205 HP

 

I could say that DUAL 3" exhausts on an L28 will show benefits... so a single would be 'shy'!

 

Curiously, the Z432 came from the factory with TWIN 60mm pipes on it's exhaust system, and the original L24 Sport Exhaust was Twin 50's...when you do the cross-sectional comparison the equivalent is a minimum of 2.78" diameter for the L-Engine, and 3.34" for the S20 DOHC setup. Something to keep in mind for what Nissan Engineers thought was an 'optimized' setup for a STOCK engine!

 

N/A versus Turbo are two different thoughts according to most, but for a Turbo 'Bigger is Better' is the simplest way to put it, the less backpressure you have the quicker you spool. And that is the name of the game. There, now I at least touched on the original topic!

[HowlerMonkey]

Dan bisone put down 240hp (crank) in his ITS car in 1987 with a 2.5 inch and he passed tech. with it which was rare for him.

[johnc]

Bullshit! Or that was to loosest tech I've ever heard of.

[(goldfish)]

From my understanding, once you are past the header length, then bigger is better and you can never have to little backpressure for any situation. Of course there is a point of minimal gains from going larger. Where that point is depends on the CFM the engine exhausts.

 

I think most agree that on street driven Z's 2.5" on an N/A is good enough.

[boardkid280z]

The Japanese were routinely running 125mm exhaust systems on Twin-Turbo High-Horsepower (444Kw to the rear wheels) S30's in the 80's.

 

Sure, 75mm Single or Twin 50mm was what all the N/A guys were running, but the Big Turbo Guys were fitting those sewer pipes under there...

 

The only real tight spot is near the differential, really. Everyplace else you can fit just about anything else you want.

 

Just to put Tony's comment in perspective, 125mm is 5" exhaust (not sure whether the TT setups are sequential or what). And 444kW is about 600hp.

I work at Lindsey Racing and on the 4 cylinder 944 Turbos we build (2.5L-3.0L), we recommend 4" over 3" on anything over 300hp because you will notice a difference. Around 300-400hp you're not going to have as much of an advantage in peak hp and torque so much as how soon it comes on. Ref: http://www.lindseyracing.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=951EXHAUST4INCH

We noticed an increase of about 10ft/lbs of torque at 390 peak and the horsepower didn't change much (it's actually lower with the 4", but it was 7 degrees hotter that day). BUT - "by 4000 RPM, the engine was making 75 more Ft. Lbs of torque at the wheels. It makes more torque all the way up the graph. Also note that the engine reached it's peak torque 350RPM sooner on the graph."

So, with a larger exhaust - on a turbo, you'll have more power lower in the rpms and you'll increase your peak power output by a marginal to significant amount.

[Tony D]

Thanks for the translation Greg! I forget I am 'metric disabled' and many people wouldn't realize how big the JDM was actually running. Yeah, the RS Okinawa 77 S30 was cranking 444Kw on the same dyno my L20A put 97PS (I chose "PS" for my sticker, instead of the lower Kw Number! LOL). The Twins were not sequential, but parallell, eachbeing fed by 3 cylinders.

 

JeffP's testing with the 3" downpipe and exhaust showed no loss of down low power or torque, and in fact showed a 20 HP boost on the top end. Overall the whole dyno curve was better than stock with only the exhaust being changed!

 

N/A and Turbo are two different animals, but John C brought up a very valid point of operational rpm range. For a track car for sure 'bigger is better' but on a turbo car, no matter what, "Bigger is not big enough!" should be the rule!

 

But again, someone stating that 2.5" 'is good enough' really has to look at packaging! The twin 50's will flow like a 2 3/4" pipe, and FIT like a STOCK system (actually it IS a stock system, hanging from stock hangers replicating the 60mm Z432 Setup!)

 

I know some people have had issues with the 2.5" near the diffy, unless you use a later crossmember which makes packaging easier. But twin 50's fit like the stock setup on the early 240/260 diffy crossmember. Sure twicepipes cost more, are more complex, but they do also flow better. And if you package 60mm tubing, you flow BETTER than a single 3" pipe!

 

It's just another option. I recently sent someone the parts page from the Fairlady 432 Parts Booklet so he could compare the stock single tube system to how Nissan hung/packaged the dual system in the same chassis. The only real difference is hangers and how they routed it. Twin 60's / 50's are no more susceptible to bottoming on lowered cars than properly-hung stock systems if you follow Nissan's designs and engineering!

[Careless]

i'd like to see a car that has an underbelly pan thats just a huge separating wall from the turbo to the undercarriage, and it just spans out at maximum angle, bolts to the downpipe flange, and separates exhaust fumes straight from the turbo, with no exhaust at all. That way any gases don't come out of the hood.

 

If bigger is not big enough, the none should be the biggest!

 

following competition rules, however, would not be an option in the case of my "super non-pipe" design. LOL

[X64v]

i'd like to see...

 

http://forums.hybridz.org/showthread.php?t=126299

 

Not quite what you had in mind, but one could easily run a 4" or 5" pipe, pretty much making it just as good.

[Careless]

http://forums.hybridz.org/showthread.php?t=126299

 

Not quite what you had in mind, but one could easily run a 4" or 5" pipe, pretty much making it just as good.

 

sounds nice, but i meant something more like just a bunch of sheets of metal that serve to separate the gases from the rest of the car. almost like those rear under-spoiler venturi tunnels, but in a different fashion.

 

just a big aluminum welded half-box that serves to span out the entire width of the car and separate turbo poo from the good stuff.

[RIP260Z]

Tony,

I agree with your thoughts on the twin pipe system. I was lucky enough to be given a Japanese aftermarket Z432 sports system to copy (which I had done in stainless steel). All I had to do was put a " small downward kink" into one of the pipes so that it would not bang on the diff/ rear suspension. The reason was since the system was designed for a slightly smaller diff, it could catch onto the larger R200 diff, especially if running lowered suspension. Oh, and pipe size is 60mm, will be running on a Rebelllo stroker.

Cheers

Ian

[Thumper]

So with all this talk of bigger is better it looks like the 3" exhaust for my turbo setup I was planning would be better if it was 3.5"/4" The elbow off of my turbo is 3.5" and I know I can build a dp and piping all the way back to diff. The problem is going through the diff area.

 

Has anybody run 3.5" or 4" back through the diff area on a street car?

[HowlerMonkey]

Sorry John C. I just checked with my Improved touring buds and Bisone's 240 put down 220 and not 240 to the crank around the time it lapped the field at the blue grey classic at summit point in the early 90s.

[Thumper]

So with all this talk of bigger is better it looks like the 3" exhaust for my turbo setup I was planning would be better if it was 3.5"/4" The elbow off of my turbo is 3.5" and I know I can build a dp and piping all the way back to diff. The problem is going through the diff area.

 

Has anybody run 3.5" or 4" back through the diff area on a street car?

 

Looks like I should have read better. Post #6 in this thread shows 280zforce with a full 4".

 

 

280zforce, Have you had the car on the ground yet? How does ground clearance look? Thanks

[Tony D]

I was lucky enough to be given a Japanese aftermarket Z432 sports system to copy (which I had done in stainless steel). All I had to do was put a " small downward kink" into one of the pipes so that it would not bang on the diff/ rear suspension. The reason was since the system was designed for a slightly smaller diff, it could catch onto the larger R200 diff, especially if running lowered suspension. Oh, and pipe size is 60mm, will be running on a Rebelllo stroker.

Cheers

Ian

 

At last, someone else who can speak on the 'twice pipes' workability.

 

In the JDM, as many cars already had the 'header exhausts', HKS made a Downpipe for the stock turbo that hooked to the 50mm HKS Sports system for the S30, I ran that 2.5" downpipe into twin 50mm pipes for years on that system making 350hp. 2.78" flow equivalence.

 

Now Ian has inspired me to make a twin 2.5" setup (making the closest approximation to 60mm tubes this side of the pond).

 

Why fight trying to fit a single 3.5" pipe back there, when twins package so much more efficiently.

 

As I recall, there was an S130 Exhaust that was popular that ran 90mm pipe in the center section of the tunnel, then branched out to twin 60's on each side of the car...easily passed above the rear subframe just like the stock muffler tubing! I believe the front section was set up at owner's discretion to either accept dual turbo downpipes or a 90mm turbo downpipe.

 

They had such cool stuff in the JDM, and they weren't trying to fit it around a steering shaft!

[Guest 280ZForce]

Looks like I should have read better. Post #6 in this thread shows 280zforce with a full 4".

 

 

280zforce, Have you had the car on the ground yet? How does ground clearance look? Thanks

on my old susp and wheels (tokico hp shocks & springs and 16" wheels w 55 series tires i had about 3.5-3.75" ground clearance at lowest point right behind the diffy. not sure how it'll be with the new susp and wheels yet.