decat....higher mpg? surely that can't be right!!!!

Hi

I've got an N/A and have been driving with the 2nd decat and resonator on the HKS HiPower backbox removed for months which was fine and sounded nice and throaty. The 1st cat starting rattling so was definately on the way out. I drive about 80 miles each day to work and back and £70 was lasting me 4 days with mainly motorway driving.

Anyway the rattling got worse so i finally had the 1st cat took out on Saturday. Results were excellent, the throttle response is as different as night and day kicking in almost straight away all and going all the way through the rev range rather than 4000rpm like before. There also seems to be a hefty chunk of extra power. I was expecting a bit more but christ...im wondering exactly how restrictive the 1st cats are. I was however expecting/dreading a drop in mpg also.

The weird thing is, after a bit of playtime at the weekend listening to the amazing noise i now had coming from behind (loud but not horrible and raspy...sounds like a lambo V10 in 2nd gear holding at 2500rpm) i put £70 in as usual and topped up for the next 4 days. Well here we are at Friday night and i have a quarter of a tank left and wont need to put any in until Sunday night so a whole week of fuel rather than 4 days.

Is this possible to happen? have i witnessed some kind of miracle or was the cat that knackered that it was affected MPG without me even realising?

Answers on a postcard..lol

400 to a full tank is usually about right for an NA on motorway. Thats what I used to get in mine.

So you seem to be getting much more than that. Seems like a happy miracle

I must have been getting around 320 to a full tank before. I mean i dont drive like a saint but i dont thrash it around everywhere either. I've actually done more 120mph (i mean errrr 70mph) blasts this week than normal as i've been obesessed with the exhaust noise and still managed over 400 to the tank. Something weird going on there i feel.

Can a failing/blocked cat cause the mpg to drop? The car seemed pretty quick before and doesn't seem noticeably different from when i bought it 5 years ago but with all cats out it just seems to have really sprung into life and feels so much better to drive even normally. I get more excitement now than when i first got it..lol

Of course removing back pressure in an exhaust will increase MPG. The engine is basically an air pump, and it will pump more freely if the intake and exit pipes flow as freely as possible. There's one thing for sure, car makers do not fit catalytic converters through choice. They reduce engine performance and decrease MPG. There is however a big difference between back pressure, intake de-restriction and affecting resonance at a particular load and RPM.

My na will do 370ish to a full tank, that's with no cats, race manifold a middle box and a back box

I've been considering taking out the second cat on my na, as I already have a first decat... But mine is already a little raspy, I'm worried taking the second cat out as well it'll be even more raspy!

Mine is on stock manifolds, first decat, second cat, and stock separated mid section (with no boxes) and a blitz nur spec s back box.

Dunno If it'd be worth fitting a boxed mid section and second decat?

To OP, are you still running stock manifold?

Fill your tank, set the trip, do your weeks driving, re fill the tank then you can work

out what its doing accurately

400 miles using every drop in your 70 litre tank is 26 mpg

400 miles with 1/4 tank left (17.5L) is nearly 35 mpg

Fill your tank, set the trip, do your weeks driving, re fill the tank then you can work

out what its doing accurately

 

400 miles using every drop in your 70 litre tank is 26 mpg

 

400 miles with 1/4 tank left (17.5L) is nearly 35 mpg

I thought J-specs only have 60L tanks and UKs had 70L. Or have I got that wrong?

I thought J-specs only have 60L tanks and UKs had 70L. Or have I got that wrong?

I've had over 70L in my UK before. Most ever was 71.54L when I was on the light.

I'm running a stock UK spec with a HKS superdragger, just had both O2 sensors changed so I'm keen to see how many miles I can do now. Never got more than 282 miles before the light came on from a full tank before.

Encouraging to know that a de-cat may increase MPG somewhat.

See I had 61.45L from the light being on a while. I think i'm right, im sure I read it on here somewhere

As far as im aware the UK spec has an 80 litre tank and j specs have 70 litres

As far as im aware the UK spec has an 80 litre tank and j specs have 70 litres

Sounds right on the money tbh.

As far as im aware the UK spec has an 80 litre tank and j specs have 70 litres

I'll happily go along with that then as I have no confidence in my own knowledge of these things

any videos of the noise it makes?

Recently had to change may cat back exhaust due to rust issues (the aftermarket back box fell off) and i am told it will be quieter.

I currently have a 2nd decat and was thinking of removing the 1st but worried about how loud it will be

(BTW its an NA also)

Of course removing back pressure in an exhaust will increase MPG. The engine is basically an air pump, and it will pump more freely if the intake and exit pipes flow as freely as possible. There's one thing for sure, car makers do not fit catalytic converters through choice. They reduce engine performance and decrease MPG. There is however a big difference between back pressure, intake de-restriction and affecting resonance at a particular load and RPM.

Removing the some of the backpressure from the exhaust gives you the scope to have a more powerful, thirsty engine, but if you don't drive it to exploit that extra power, then you will reap efficiency benefits from the lower pumping losses.

In for the video of the sound as Kniff_666 said! I also have an N/A with second decat...it already sounds kind of raspy, not sure what would happen to the sound with the 1st cat replaced with a decat too...unless i go for a manifold which replaces the first cat...but would have to hear before i commit to buy, hate that honda civic rasp!

I have some good vids but my reg will defo need deleting

In for the video of the sound as Kniff_666 said! I also have an N/A with second decat...it already sounds kind of raspy, not sure what would happen to the sound with the 1st cat replaced with a decat too...unless i go for a manifold which replaces the first cat...but would have to hear before i commit to buy, hate that honda civic rasp!

Paul whiffin's twin box 2.5 got rid of my rasp on a fully decatted Na.

Here's a vid of mine.

Mines na,

Stock manifolds, first decat, second cat still in place, stock twin midsection without boxes and a blitz nur spec s backbox

Go to about 1minute in. And at the end.