joefish1298

Firstly, is there any point to a 70mm exhaust (or for that matter anything bigger than stock) if you don't change the header? Isn't the gas only going to flow as fast as the smallest restriction will let it, ie bottle neck at the collector on the stock header?

Secondly, heaps of headers in the official header thread are listed as having a 50mm-60mm collector? Is it 50 or 60 or anything in between? How does that even work, isn't the end of the header a fixed diameter?

Thirdly, there's plenty of 70mm exhausts, why are there no 70mm headers? This kinda ties back in to the first question.

MAD828

Re headers, no point running large diameter headers if the exhaust ports on the engine block are less than 70mm. That could be one reason.

jbird

From my understanding a header is designed to scavenge the exhaust gases coming out of the engine. Sizing is really important here to promote optimum scavenging, so most likely a 70mm header would be too large to be optimal.

It's like trying to blow darts with a 10cm PVC pipe versus a 3cm sized pipe. The expelling of your breath would be faster/higher velocity out of the smaller diametered pipe.

IMO After the header collector, size of the exhaust is less important which is debateable. Some people say 60mm is optimal, 70mm is best, 80mm for max power etc. I will leave it at that because that's a whole new topic.

joefish1298

I guess what I'm asking in one question, is if you just change the exhaust, how have you actually made it any less restrictive, because you still have a bottleneck at the header collector.

vyets

Bottleneck is more like the other way around. IE Header but stock exhaust. kinda like the inside of the bottle its big but has to squeeze out a tiny hole.
If you just upgrade the exhaust then the header flow will be the same, but everything else is abit more freeflowing. It's better to get a exhaust and run stock header, then the other way around. Either way you will still get gains with a 70mm exhaust

joefish1298

But a system can only flow as well as it's most restrictive link right? Header and no exhaust vs exhaust but no header seem the same to me. There's a still a restriction in the chain which brings the whole chain down.

Imagine a single lane road can carry x amount of cars. Then at a point in that road you turn it into a 3 lane freeway. The freeway can carry 3x amount of cars, but the single lane road can still only feed it x amount of cars so nothing has been gained. This is how I see a free flowing exhaust added on to a restrictive header.

Maybe that example doesn't apply for gas flow, I don't know, that's what I'm trying to understand.

jbird

The stock header is not that restrictive and upgrading the exhaust from the header back e.g test pipe/hfc and catback will free up some power and make some more noise.

So in short you haven't removed all bottlenecks but you'd rather a bottleneck at the start of the system than during the system.

Going with your analogy, if all cars on that single lane travelled at 200+km/hr entering that 3 lane in an orderly single file that is not considered so much a bottle neck now is it?

vyets

Well, in that freeway case then, as soon as the road opens to 3 lanes, everyone merges and flies at 100km/h making their way to wherever.

Think about a 3line freeway, then they block off 2 lines to make roadworks, everyone has to squeeze in and merge to one lane. When that happens traffick backs up all the wayy down.

D&G

It choose to stay with a 60mm when I ordered my j's racing one. Felt that the 70mm would apply more for track use where you can make the most of the gains, but in more streetable conditions the 60 would maintain more of the low-mid range torque.