I want to upgrade to a extractor on my a15 i want to know what sizes the pipers are to be,It will be for a streeter i'm using bigger cabby 32/36.Does the collector size has to be presice or important? Changing to 1 3/4 after the extractor to the back of the car I dont want to get it made like #OOPS# an i could have got it done better.What are your suggestions and tips

It does not have to be precise to run well. Header are typical added for looks and sound.

To obtain maximum HP, you need to know:
* which RPM range the engine will be revved to, and size the collector length appropriately. Whether it is equal-length or not is unimportant for a streeter, but only the collector length is significant.
* the HP output of the engine,and size the diameter of the runners appropriately.

If it is a stock A15, is it an 80HP A15 or a 67HP A15? Generally the ones after 1982 are low power and do not make as much HP when revved high.

The rpm Range would be around 3,000 to about 4,500.Yes it is hi hp a15 80hp!
Does it need to be equal lenght for street?And what size does the colloctor pipe has to be? What size does the runners usually be.

3-4500l range seems. Little low in the top end, I would have thought at least 6500rpms.

Here in Aus I run a set d Stewart Wilkins extractors, sound great good figment, on a mild a12 and 1 3/4 exhaust, the go well.
Check out JTS for extractors too.

The 80HP A15 makes peak power at 6000 RPM. So save yourself some money and instead of buying extractors, just start revving the engine higher. Instant HP gain.

Because im looking at low rmp power i dont have access to cams an double valve springs etc how else could i get it to rev higher?

Also on a low budget system/engine.

You don't need any of that stuff. It will still rev to 6000 RPM.

I did a post on this a few years ago - I dunno if it is still easy to find.

In very basic terms, THE single most important dimension of extractors is primary pipe diameter.

Since the dawn of working on engines for more power, a bunch of guys smarter than me discovered that the headers/extractors work most optimally when the gases inside them are flowing at an average speed of X (I forget what it actually is, and the actual figure isn't as important as knowing that such a phenomenon exists). So then they worked out that if each cylinder was a certain capacity, then at a certain rpm, it'd take a specific pipe diameter to attain that average speed.

In practice, yes every engine is different, rod length, port efficiency, etc etc etc, so you'd actually find, if you could test it, that a particular 0.25mm difference in pipe diameter would be a little better for some engines, and worse for others.

THe thing is, short of a million dollar budget, you really only get pipes made (that you can then make extractors from) in 1/16th of an inch increments or 1/8th or whatever. As an example, most street hemi 6 cylinder engines could actually work pretty well with 1 11/16th primaries. But the tooling to bend pipes in this diameter doesn't exist (or isn't common) so the two main sizes are 1 5/8" or 1 3/4". Since the hemis have a (relatively) short rod for their stroke, the 1 3/4" is the go for anything going above about 280-290bhp (actual hp, not claimed).

But anyway back to the a series. Oh, and a big point to make here - just because the math pinpoints "X" rpm, well that's the theoretical 'sweet spot' where the pipes will give their biggest boost to torque, but the pipes will tend to work VERY well as much as 2000rpm either side of that. Certainly they'll work over 3000rpm (1500rpm either side of said sweet spot).

Note that this isn't where the motor would make peak torque, it's just where the extractors are giving the biggest boost to torque. IF you have a cam/port combo that makes peak torque a lot lower or higher, it will still tend to be the main factor as to the torque curve (unless you run an exhaust so restrictive that it is choked to death by mid range rpms or something) the extractors won't magically shift this powerband a few thousand rpms either direction, they'll only be working at _their_ best at those particular rpms.

You have to allow for the pipe thickness, as it is the inner diameter that counts for the math, but the pipes are measured by outer diameter. I generally round it off to 1/8" less than the outer diameter which isn't perfect but it'll do.

With an a-15, std bore, the main primary pipe sizes going around tend to be 38mm and 35mm which are basically 1 1/2" and 1 3/8" . the smaller of the two has its sweet spot (for a std bore a15) around 4700rpm (and looking at this I might have mistakenly plugged in the numbers for 1 3/8" pipes in a post I made about 88mm stroked a15 buildup stuff, hte perils of posting without ever getting much sleep!) - which means it'd work from about 2700rpm to 6700rpm - not a bad street/mild/stock range. For 1 1/2" primaries you would be looking at 5770rpm or from 3700-7700rpm (or maybe 4200-7200rpm) that'd be very decent for a mild-more serious streeter.

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Primary length does have SOME effect but it's far lesser than that of diameter. Basically longer will make them work a little better at lower rpm and shorter will be better at higher rpm. There's (again) somewhat well established and tested lengths to coincide with the sort of rpm range you are chasing. FOr a streeter possibly as long as 34-38 inches would be good, but don't lose sleep if it's shorter than that, the effect is icing ont he cake, not 'end of the world' stuff. Equal length is 'nice' but the thing is, they might all be 'equally wrong'! So again, a few inches difference from one primary pipe length to another - just don't lose too much sleep over it.

FOr an all out racer, sure, but for anything else where you don't spent a few grand just chasing 1-2% power increases, just don't worry too much.

Collector size. There is certainly such a thing as too big. BUt there's also 'adequate' and 'on the restrictive side' . Generally you'll find most come with 2inch main collectors. Not a bad place to look for an a15, esp a street one. You could start to make a case for 2 1/4" at really high power levels but again, unlikely on any streeter. A different engine, I'll grant you, but 2 inch collectors work on race mini engines up to around 140-145bhp, so it's a good enough size for that.

Where it can all fall down is the muffler itself. THe problem is the right sized pipes and collector will not work well at all with a restrictive muffler. you have to make a call on just how quiet you need it because generally good flow and low noise aren't easy to combine.

There is a way you can 'trick' the exhaust gases into thinking they are exiting from the collector into open air (which when done right - i.e. the collector into actual open air = the best exhaust scavenging effect at the most power). and that is a pressure wave termination box. All it is is a big pipe/section that the collector feeds into, where the cross section area increases dramatically. It needs decent cross section area and volume (about double the total engine capacity at a minimum afaik and there's almost no such thing as too big) and then it'll work like an open exhaust. At the other end of this big 'bucket' or cannister, you simply funnel it back down to go into the rest of the regularly sized exhaust pipe and on to the rear muffler. Done right this lets you get the exhaust scavenging effects and still run a muffler that quietens it down enough to keep you safe from unwanted attention (be it the epa, the boys in blue, whatever) and make it liveable for daily driving.

As far as mufflers go, there's massive difference in flow and noise elimination from brand to brand, and I honestly don't have ANY decent advice on what brand is currently a good option (I'd also add that you can find similarly named brands that are possibly low buck copies from who knows where being sold as the real thing, so it is a battle to get the best outcome)

Another option you can look at is brazing a copper pipe into the main exhaust just in front of the muffler, and routing that (with enough loops so that the temperature doesn't potentially reach the cockpit/guage/whatever ) to a pressure guage. You are going to have to accept some backpressure (though for optimal power zero is the go) to get the noise down. If it is around 2-3psi, that's a very good place to aim. If it is 5psi or so, it's certainly still a reasonable enough compromise. Above 5psi, esp over 10psi, and it's costing you power serious power.

I'll do a diagram of a pressure wave terminator box if you wish, it can be easily enough made with a welder, a hammer or two and some time. In theory some 'resonators' do the same job, but most of them don't do it anywhere near as well if for no other reason than they just don't have the size/volume to achieve it. Depending on the precise shape you aim for (it doesn't have to be perfectly round, you could oval it for better floorpan/ground clearance) it may or may not require a bit of floorpan massaging to clear it without it being too low to go over speed bumps and the like.

Cant even get a lenght on the collector anyone

Photos of extractors are a start on collector length. First choose whether you're going Tri-Y or 4 into 1. Tri-y collects 1 and 4 together and 2 and 3 together before merging the two secondary pipes.

My extractors (on a A15) have primary pipes of about 400mm and the secondaries would be around the 1 meter mark. You'd probably be happy with a length around 1.5m for 4 into 1 extractors (based on observation this is about how long they are). YOu'll notice the GX exhaust also has about 1.5m before the 4 cylinder pipes merge.