gullys01

can anyone tell me what size should the ends be on a manifold as i am getting an up rated manifold.

harvey if your there you said that you had some dimentions or something when we met at well lane.

or any one out there who has some advise.

if all goes well and i get info i might be able to get manifolds made for around £470

thats a big saving off the big companies which are around £900

thanks guys up front

every one contribute please

gully,.,,,,,,

harvey

iTrader: (48)

I assume you are referring to the tubular headers which from memory on my car are from 43mm dia stainless. The problem is getting the tight radius 180 degree bend on the driverside bank without it rippling or colapsing.
You need a set of headers to start with to make a jig with the exact positions of the four exhaust ports and exit to up-pipe so even for someone working with pipework it is not an easy task.

As an aside I am presently modding a cast set of headers and O/E up-pipe and hope to show useful and measurable gains so that info will be available in the next couple of weeks.

gullys01

chears harvey

i look forward to those results.
if the pipe size is bigger does that make it better as more gas can flow through to the turbo. or does that make the gases too loose (expand) and slow the pressure ?

the bend shouldnt be a problem as if it is possible this man can do it.

gully

nom

Pipe size is fairly critical as you want to keep the gases moving as quickly as possible (small diameter) but not be restrictive, which they would be if too small . Depends on what sort of power you're looking for/turbo to be used with/where you want the powerband.

gullys01

im hoping for 380bhp with about 350 Ib torque

what would you recomend

tanx gully

Katana

The trick is to get the material right. The guy I bought my car from did experimented with this and it kept cracking after a few miles.

pat

Header design will be very much dependant on your intended output characteristics, ie ultimate power, torque spread, intended power band, etc. In all cases the aim should be to maintain high exhaust gas velocity without sacrificing flow. I'de suggest primaries of about 1.375 inch diameter for lower power outputs, up to about 450 BHP. For power outputs up to 650 BHP you'll be fine at 1.5" but beyond that you'de ideally need 1.626 inch diameter. I'de recommend that a 4-2-1 be constructed, with the final 2-1 happening at the turbo entry, this minimises cross-port pressurisation and maximises the pulse energy extracted by the turbine wheel... it's no coincidence that the STi VIII manifold is contructed like this

It's a lot of work to make a full manifold, and changing the up pipe is a pain, but it's worth it, IMVHO. People have reported issues with spool up on such headers in the past, but this is probably down to the relatively large tube diameter used. For example, a standard exhaust manifold has a 48mm ID up pipe, or 1.88". This is only 58% larger than a 1.5" tube, yet it is being asked to carry the gas from 4 cylinders!

A 1.5" 4-2-1 would have a single 1.5" pipe for every 2 cylinders in the twin up pipes, so they'll be 27% larger than the stock headers, no wonder they don't spool quite as well, especially since there's much more pipework volume to pressurise. Now compare this to a 1.375" header... that's about 7% bigger than standard, so it will spool similarly, however each part of the header will have much better flow characteristics than the stock ones... using a set of headers of similar design resulted in maxing out 550cc injectors with increased fuel pressure at only 1.5 bar; that engine never ran on the dyno with this setup but it would have been around the 400 BHP mark.

Now consider a 2.5 litre engine which will naturally flow 25% more gas anyway... suddenly the 1.5" headers seem much more appropriate. Interestingly, dyno testing revealed a flow restriction in the twin up pipes, and these were changed to a pair of larger ones with an impressive increase in torque, without slugging the turbo; in both cases, before and after, the boost threshold was around 3600 RPM (not bad for a turbo that dwarfs Andy's, Bob's, Harvey's etc). This is probably because only a relatively short section was enlarged, not the whole headers... it's always a good idea to increase the pipe diameter slightly when two pipes merge into one.

A good example of the general design is the Maxim and Hayward & Scott equal length headers... have a look at http://www.subarureview.com/showreview.php?id=716 for a piccie of the Maxim ones, can't find a piccy of the H&S ones at the moment...

Hope this helps,

Pat.