hashman626

I was wondering if there is any necessary tuning with the intake section of the car after a free-flow exhaust system is installed and almost all back pressure eliminated? I have a '91 coupe and the exhaust system I'm thinking about buying is the corksport header back, this replaces everything, the stock reactive exhaust manifold, number 1 and 2 preconverters, main converter and the rest of the piping. Would a haltech engine management be the cure, when set up correctly, if any tuning at all should be done?

littlemimus

is this a turbo or non turbo car? on a na car you generally need back pressure otherwise you will lose hp. it creates kind of a suction to get all the exhaust gasses out.

MadScience_7

Wrong. Back-pressure is always bad for power. On S4 (86-88) N/A cars, the aux ports are actuated using exhaust back-pressure, so freeing up the exhaust can cause the ports to not open. On S5 cars though, the ports are actuated by the air pump, so you won't have that problem.

The last bit of suction you mentioned is more related to tuning the headers. Properly designed headers will direct positive and (more importantly) negative pressure pulses to the exhaust ports. If a negative pulse reaches the port just before it closes, it will pull the last bit of exhaust out, which I believe is referred to as scavenging.

To take full advantage of a high flow exhaust, the intake will need work too. However, slapping it on without intake work won't hurt anything, and will very likely give you some more power. The exhaust is one of the the things that can make a decent difference on it's own.

Maybe I'm just splitting hairs here, but the whole car is an inter-related system, and changing 1 thing will change the behavior of other things.

hashman626

It is a 1991 coupe NA, I have apexi power intake, I was speaking about the intake system/manifold in general. The headers I will be getting are corksport power series headers, they are listed to have added 14 horsepower on an rx7 which was completely stock (except the headers) and had over 100k on the motor. http://www.corksport.com/corksport-p...r-for-rx7.html
I have also considered racingbeat headers

MadScience_7

I wouldn't put too much stock in the numbers, but it will work fine on the car. I have those on my S4 and I like them a lot. You might have to extend the wire for the O2 sensor though.

littlemimus

k. so upon further research for a na car backpressure is bad. and this is because the exhaust gasses lose velocity it travels through the exhaustwhen u "upgrade" to a 3in or bigger than stock piping?

hashman626

I don't exactly understand this statement from mad science 7, "I wouldn't put too much stock in the numbers"
sorry, have no idea what you meant, if you could tell me i would appreciate it

hashman626

i think the 3 inch tip and larger piping from a performance exhaust will eliminate backpressure and allow the exhaust to flow freely at faster rate

littlemimus

the thing is how big piping? from what i just read if its correctly sized it is made to increase velocity of the gasses moving out and decrease backpressure.

hashman626

most exhaust systems have a tip of three inches or not too much bigger

MadScience_7

I meant the 14hp number. All of these cars are a little different by now, even if they haven't been modified, they're all 20+ years old. You may put it on and get 4hp or 20hp. I'm not saying that it's bad, just that performance stats are not perfect.

also:
exhaust design

hashman626

gotcha, i have a rebuilt motor from the previous owner (im the 3rd) it has about 30k, forgot the exact number but yea, hopefully i have the 160hp which my car was advertised at before any mods

rx7racerca

iTrader: (1)

Too large an exhaust is what hurts torque, especially at the low end - this is due to the scavenging effect already mentioned - which basically consists of the momentum of the airflow already in the exhaust helping to pull a little more exhaust out of the engine behind that mass of moving air. This is lost when exhaust diameter is too large on a NA engine, because the air essentially can "stall" as it expands into the exhaust's large volume, and scavenging is lost.

IIRC, 2.5" is usually the largest diameter that makes sense on a NA, unless it has some pretty serious porting.

oakback

You're over-thinking it. Just install the exhaust, you'll have more power than the stock system.

hashman626

well the 3 inch tip of the exhaust I want and potentially a "large steet port" by pineapple racing in portland, OR pushing about 40 hp which is advertised by a place in CA, tells me the possibilities of what a street port can do (25% actually but of 160hp of a s5 na) and maybe that would work well with the 3 inch exhaust

rx7racerca

iTrader: (1)

At 8000+ rpm, yes. Below 6-7K, likely noticeably less. Which is fine if the car is destined for primarily track use. But killing all the torque on a relatively torque-less engine doesn't read as an improvement to me if the car's going to be streeted. But dropping it to third to maintain speed on a moderate grade at 50mph, and gunning it to keep up with Corollas at lights gets old fast.

FWIW, I've the 2.5" RB downpipe and cat replacement midpipe/resonator - great power adder, people who should know better have mistaken the pull for a TII (maybe S4, at any rate) - but I definitely noticed some power loss below 4000-4500. The pull from 6000-8700 is great, and pretty worth it, but I track the car a lot. Even so, around town, it means holding gears longer than I'd like in moderate driving, and stirring the gearbox more to have some power in the ebb and flow of traffic.

TheAbsence

iTrader: (19)

He meant stock as in the stock market.


Bigger exhaust piping = slower exhaust velocity


Go with a 2.5 inch unless you're going peripheral port or bridge at the least.

hashman626

so 2.5'' piping after the race pipe or high flow cat is ideal for overall performance on an s5 NA with the prospects of a street port?

TheAbsence

iTrader: (19)

2.5 inch piping is ideal everywhere after the collector.

walken

iTrader: (6)

spend the money on a haltech and gut your cats. have the same power with better gas mileage after a good tune.

RockLobster

iTrader: (7)

+1

The first mod on an N/A car should be to get some way to pull a ton of fuel out of the upper load cells from 5500 RPM up. No drop in low end torque just 10-15 top end HP after a good tune.

I prefer Rtek to accomplish this but, to each his own.

RockLobster

iTrader: (7)

To expand on this idea....the reason you want to avoid an exhaust velocity that is too slow is because the exhaust gasses cool and become more dense, when they do they actually cause more back pressure.

hashman626

Can Haltech accomplish the tunning required for this or is it something a shop should do, or a combination of both?
I've never heard of this, if there is a thread on this site that you know of explaining this in fine detail please link me, very intriguing 10hp with no consequence sounds like, why not?

TheAbsence

iTrader: (19)

A Haltech is a fully programmable computer so yes, you will need to have it tuned specifically for your application.