2" primaries too big?

[Aus450zx]

I have been searching on the net and it seems that 2" primaries are too big for an N/A as the powerband will be shifted too high. but i'm just wondering if this holds true for the VH45 considering how big the exhaust ports are, would the benifit of keeping the exhuast gas flow even from the head into the primaries outweigh the negative effect the large primaries have on scavenging?

The reason I ask is because I can get stainless bends in 2" REALLY cheap, but thats the only size available.

How would 2" go at making turbo manifolds on the VH? again the consensus on the net seems to be keep turbo manifold primaries small, but could they allow better breathing on the VH45 because of it's massive exhaust ports?

[Wes M]

I've been looking to make my headers from 1.5 or 1 5/8 primaries. From memory John used 1 5/8 on his 5l and I think that was working quite well for him. The increase to 2" would give a very substancial increase in cross sectional area (squared relationship) and a drastic drop in exhaust velocity and scavanging benefits. Not to mention if you have limited space to work with it could be a physical fitting challenge.

[Aus450zx]

cheers for that Wes, I'd already told the guy I couldn't use them because they were too big but thought I'd double check on here first. In a turbo manifold you would want good velocity as well so no good on that front either. From memory the factory manifold is 1.5" and john used 1.7/8" primaries.

[Chrispy300]

I've got 1 5/8" and that is really tight. I just can't see you being able to make the drivers side fit with 2". The collectors will be pretty big too.

[gs14racer]

i used 1 3/4 on my first one and it worked well, my fathers big block chevy with blower has 2 1/4 primarys.

My vote if you can fit 2s in there id do it.

[Aus450zx]

in a N/A setup or for turbo headers? or both?

[SuperHatch]

i used 1 3/4 on my first one and it worked well, my fathers big block chevy with blower has 2 1/4 primarys.

My vote if you can fit 2s in there id do it.

I used 1-3/4 on my first set as well. I don't know if I'd go as big as 2", but Jerry and I can agree to disagree

[Orphan]

Unless you have some crazy big block I don't think anything much more than 40mm ID is of any benefit, for reference 1 3/4" is 44.5mm. 2" is definately excessive - even more so on a 4.5L N/A engine. Its only 562.5cc's a cylinder so a larger than 40-45mm ID primary is really only going to start to have negative impacts performance wise. 2" or 50mm might not be THAT bad but as gs14racer said his father has 2.25" on a supercharged big block so if that size will support that capacity + supercharger the much smaller 4.5L doesn't need anything near that size. Headers for N/A applications should be designed based on the target for the engine and peak torque as exhaust pulses and velocity have a big impact in N/A applications compared to in turbo applications though it is still important.

[gs14racer]

I used 1-3/4 on my first set as well. I don't know if I'd go as big as 2", but Jerry and I can agree to disagree

i agree... wait or disag.... im confused.

2 inch is probably to big, unless you had 12.5 to 1, then for sure...

[Aus450zx]

and if it was supercharged running ~10psi?

[Orphan]

With only 10psi I still think 2" would be a bit big but you can generally go a larger exhaust piping with a supercharger as the headers don't contribute as much as they do in N/A, your real aim is to get the exhaust gas out of the cylinders as fast as possible to force as much air into them as you can unlike in N/A where you have to considering many aspects that affect cylinder filling and the torque band a lot more than in forced induction situations. I can't see 2" piping hurting performance much if at all with a decent supercharger set up.

For reference a friend of mine who builds custom steam pipe turbo manifolds uses mainly 40mm ID steam pipe for the primary runners and always works out the total area and has a secondary or collector to match the flow of the individual runners and has always gotten very good results but then again in turbo applications the velocity of the exhaust gas is important in spooling the turbo and doesn't really play much into the supercharger application.

You could always consider sending an e-mail to a drag team using a supercharged v8 and see if they will give you any suggestions or thoughts regarding the best way to size exhaust runners in supercharged applications. I actually find out a lot of information talking to race engineers or people who work with specific automotive applications, as long as your not taking up heaps of their time and research before hand to have the best grasp on the topic I have found most to be very happy to help but you have to remember that many of these people have builds that are competing in drags or racing and won't tell you detailed info about their vehicle in competition as many teams are very secretive about their development.

[hannibal]

I think Dixon's were 41mm=1.61".

Ive looked at the 4.6L Mustang motor. Similar displacement and but lower redline. Anyway, most of those headers are 1 5/8 or 1 3/4.