4-2-1 manifold mod - what is the benefit then?

[matt uk]

On my ever-growing wish list of mods for the MX5, I've come across this one a couple of times.

Doesn't cheap to buy and I can't see it being cheap to install.

So what is the benefit of this mod? Or is it just a replacement when the original reaches the end of it's useful life?

Cheers,
Matt

[MX-5 Lazza]

A good manifold will allow better airflow and should give improved mid & high range power. However, you won't get much benefit from it unless you have replaced the rest of the exhaust system (always modify exhausts from the back forwards). To get the best from it you would need to improve the airflow through the head too. As you have a Mk1, fitting a Mk2 head is a good upgrade.

[matt uk]

Lazza, you really do seem to know your way around these cars mate

Getting the car to 'breath' properly is what I'm trying to achieve.

I'm going for a S/S exhaust from the cat back and am now thinking about the manifold as well.

So, two things I'm now thinking about;

Firstly, the cat in the middle of it all. I've not decided what to do on this front. Having had a look at it when up on the ramps it doesn't look in the best condition and will no doubt end it's useful life sooner rather than later. Should I de-cat and keep it in the garage for MOT? (not sure if this would make the car a bit loud? – also I don’t want to move to far away from that ‘British sports car’ noise and stray into chav-lad territory) Or would you recommend getting some sort of upgraded cat vs the norm?

Secondly, you talk of a putting a Mk2 head on. I know nothing of this and would be farming the work out. Would you put on a new head? Or hunt down a re-con. Secondly, what sorts of costs are involved here?

Looking at your car (I was there at the Brands day on 5th Dec) you've done some pretty neat stuff to yours - not sure if you do it yourself or pay the labour for someone else?

I'm at the stage of weighing all these things up and costings will, like most things in life, be part of the final decision as to how far I go with it all. I'll give Mark at Freelance a call next week to try and get some figures on all this stuff.

Your advice is very much appreciated.

Cheers,
Matt

[maz8062]

The best way to get advice is to give all the information upfront. |I've no idea, for example, what model 5 you have, age, your goals etc.

[matt uk]

maz8062 said:

The best way to get advice is to give all the information upfront. |I've no idea, for example, what model 5 you have, age, your goals etc.

OK, good call - here goes..

MX5 1.8i NA
1996
77k ish
No PAS, or air-con, or EW - just a basic model

I've had the car a couple of months. It's a weekender, not a daily driver. A hobby car to be used for sunny day drives to the pub and a treat for the kids to go for a spin – but mainly for me to blast the cobwebs away and to get on a track half a dozen times a year.

It’s in good nick mechanically and hasn't used a drop of oil during the 800 very hard miles I driven (150 of those have been on two track days)

What I am trying to acheive? Good questions that and one I probably can't give a very good answer to I'm afraid. I've not had sports cars before and hence the whole modding scene is new to me. Hence the questions.

But at this stage I want it to:
breath nice and freely
find and release or replace any hidden BHP that may have wandered off over the years
sound nice and vocal and urgent without being overly loud of chavvy
handle well on road and track whilst retaining the run-factor of the std set-up

[Howard-]

Those are pretty much my goals, too.

Fortunately I have no issues replacing the entire exhaust system including de-catting cos it's a 1990 model

[Fruitcake]

The 4-2-1 exhaust manifold will allow you to achieve the goal of getting the car the breathe more easily.

It'll allow gas more freely into the manifold and more freely out. It'll also enhance the vacuum effect.

As long as it's a good one, though.

[kevham]

Matt,
Best thing to do is drive the car for a year and then decide where you think it needs tweaking. You can spend an awful lot of money and make things worse if you're not careful. Lazza spent years driving his Mk1 before deciding to tweak it - ie replace it with a supercharged Mk2.5 monster ;-) I've had three MX-5s and only started modding when I decided to start competing in sprints and hillclimbs. The standard car is GREAT FUN as it is.

Having said that, the exhaust is a good place to start on a Mk1. You won't see any significant power gains but you can make it sound and feel faster with a sporty back box. You'll probably need to retain a cat for the MoT emissions and if you go for a 4-2-1 header, you'll risk making it very loud without the cat.

As for costs, expect to spend a couple of hundred on a back box, about £2-300 on a header and if you're not fitting yourself, allow another £100 to get them fitted (back box is easy though).

As for Mk2 head - this will set you back a fair bit more. Allow £3-500 for a refreshed head (you'd be talking £££££ for a new one!) and then 5-7 hours labour to fit, I guess. You'd do the timing belt and water pump at the same time so you'd need to factor this in. Probably allow £800 - £1000 all in? Bear in mind that the changes Mazda made between Mk1 1.8 and Mk2 1.8 achieved an increase from 130bhp to 140bhp and you'll have to decide whether that sort of expenditure is worth the effort.

You see - all that cash could pay for a hell of a lot of petrol - and fun!

[Phil @ P5]

matt uk said:

maz8062 said:

The best way to get advice is to give all the information upfront. |I've no idea, for example, what model 5 you have, age, your goals etc.

OK, good call - here goes..

MX5 1.8i NA
1996
77k ish
No PAS, or air-con, or EW - just a basic model

OK, that means a 'normal' manifold/header isn't going to work. You have a 'long CAT' on this model, which means that each side of the CAT the system is different from eariler models. If you want the freedom to choose any system, you need to look at replacing the entire system.

[maz8062]

Does that mean he can buy a 1.8 JDM 4-2-1 header and fit a catback system with a 370mm de-cat i.e the complete exhaust system

[Phil @ P5]

Yes. Or a US 1994-97 header with 440mm 1800 CAT and CAT-back. Or swap the lower part of his existing downpipe with either a JDM 1993-97 or UK 1994-95.5 unit with relevant CAT and CAT-Back.

[PushedDover]

• *BUMP***

So the search of PH can occasionally work.

OP, are you still an MX5'er ? with that NA '96 model ?


I am at a similar crossroads with my long termer. It is no longer a DD, but is a keeper for life. A 1.8i non PAS etc of 1996 vintage.
What road did you take ?