4-2-1 manifold - any gains to be had?

Hi all

Would welcome thoughts and experiences of stainless steel 4-2-1 manifold, for road and track. (1.8 mk1 133bhp model)  Much of a power hike? Better flow?

I’ve got a standard exhaust system now, but with a Cobalt 3 inch (ish) from after the cat, backwards. Standard air box and filter. Runs and pulls well, but looking to eak a bit more!








No power hike. It might shift the torque curve around, thats about it. If fitting to a Japanese import, you either have to source a Japanese made manifold, or swap the catalytic converter to a 440mm EU/US spec cat. MX5parts used to sell a german made adapter piece to enable their manifold to hook up to a Japanese cat (I had the prototype), but it was a terrible fit, and gouged my PPF.

If you have the UK late model, with a 470mm cat, well, to fit the Cobalt correctly, you must have already changed out the cat and downpipe for the earlier model.

Is the Cobalt 3", I thought that was just the tip. Rest is more or less standard Mazda.


Hi mate

thanks for the comments, useful. So you think a bit of extra torque in the mid range? Is it worth doing?

It’s  a Eunos, 97P plate, 1.8

i’ve bought the car with the exhaust fitted so I don’t exactly know what cat or manifold I have! It’s not a stainless steel i know that.

the cat back is stainless, the Cobalt. It measures 4inch outside diameter, but there is an inner pipe that measures 3 inch!



If a a late Eunos, it will have a 370mm cat and standard length cat back. If you fit a header from any US/UK/Europe based supplier, you will need a new cat to fit. All of these manifolds will come with a port for EGR; hopefully they should have a bung, as the Roadster won’t have EGR. Don’t get a Mk2 header; these won’t fit.

The “rule of thumb”; 4-2-1 will shift the torque down the range. 4-1 will move it up; hence 4-1 is more suited for the track, where you spend more time banging off the rev limiter. Thats presupposing the manifold has actually been designed by anyone. Racing beat have spent some time with their manifolds, getting the flows good, and that reflects in their price. Who knows with the Japanese headers; you kind of think they should , given the extortionate pricing. Chinese headers; which is pretty much everywhere else. Well, they are vaguely copies of Racing Beat headers. But the bends are quite right, the welding is scruffy. No one has optimised these. They might make car go better, they probably won’t. If you are unlucky, it will make the car drive worse.

Having said that, I have Chinese headers on my cars. One is from MX5parts, been on c. 14 years. It buzzed a lot compared to stuck, but is ok now. It went on at the same time as other parts, so can’t tell if it made a difference. After a couple of years, it cracked around one of the headers, and was welded for beer money

The other car has a no-brand header from a Sheffield supplier, but likely Chinese made. It went on to replace basically an identical header that had cracked through on the downpipe. It was cheaper to buy new than to get the old one welded up. I treated it as bling, making sure the header was alchol washed before it was fired up the first time (during the first time it warms, the stainless steel will passivate, or blue. If you have left greasy fingermarks on it, the grease will make into the steel, and it will look terrible. Clean, it will go a nice golden colour, and purply around the welds, and then stay like that.

When fitting a header, you will need to extend the wiring for the O2 sensor. Racing Beat sell a plug n play extension.

So, if you are buying one to make more power, buy from a supplier who can show you dyno charts, otherwise you are buying because you just fancy a stainless steel header.

Racing Beat has a nice article and charts:

One side effect is that a 4-2-1 header will quieten down an exhaust.

Busted v new (1.6)

New MX5parts 1.8 v OE

New 1.6, unheated


MX5parts adapter, required to mate manifold to Jap cat. Now ditched.

MX5parts manifold fitted, unheated. But I didn’t clean this one. After a year or so, not really golden.

Warning about used Jap manifolds; to reduce the buzziness of 1-piece headers, the Japanese often fit a piece of flexible tubing (Goodrich tubing). This stuff wears out and blows. Some sellers will ask a lot of money for a busted Japanese header because it is Japanese. But you really don’t know if in fact its just junk.

The MX5 already has a 4-1 manifold, this is one reason why the accepted viewpoint is that a manifold upgrade yields marginal gains. I have followed all the well designed headers for Miatas for more than 20 years; none of them yields a decent power increase thats outside of the error range of a dyno. And thats with a well designed system, such as Racing Beats. But most manifolds are not well designed; they’re Chinese and copies of American designs, but without the benefit of the American knowhow; I’ve met the designer of RB manifolds. Clever bloke, with an incredible Bonneville RX7.

Although a Mk2 manifold can be made to fit, its a pain. The downpipe won’t mate up with the Mk1 cat, so that has to be chopped, welded and whatnote. The EGR fitting fouls on the Mk1 dipstick tube. I paid £150 for the Mk1 header from MX5parts.

Without going for a boosted option, the well trodden route for a power increase that can be felt is the Jackson Racing cold air intake, with are fiarly unobtainable these days.

Years ago, Grassroots Motorsport published a series of articles with sequential upgrades of a 1.6 Miata, using mostly Jackson Racing parts. And also; Mazda produced the 1.6 M2-1001, which had raised compression, 4-2-1 manifold, hot cams, and a HKS catback. It made 130hp. So about £1500 worth of upgrades yielded 15hp.