Headers for Blown 3.9

[cammmy]

Hi Guys

Can anyone give me advice on header dimensions for a blown 3.9 with mildly worked heads and running up to 400hp?

I've been looking at Bell's forced induction book, Hammill's Rover V8 book and Vizards How to Build Horsepower book.

Bell says I should be looking at 1.75" primaries and 2.0" secondary's for blown 500cc cylinders, Hammill says 1-5/8" Primaries into 1-3/4" secondary's for worked V8 and Vizard is saying 1.5 - 1.625" primaries based on a guestimate of flow with 2.5 - 2.73" secondary's (Which seems massive)

Vizard says V8's are primary length insensitive but secondary and collector sensitive. He says primary lengths between 28 - 40" will net all show very similar output but that I should look to have about 13" collector. He makes no mention of how to work out secondary lengths (despite saying they are very important) and that up to 60" secondary's are good for power around 3000rpm; so I can only guess at 40" for my build.

I have to say I am EXTREMELY disappointed with Vizard's book. I know it's a generic book but he glosses over almost everything making excuses about lack of space and at other times gives no information and says get on the dyno and work it out for yourself.

Anyhow. Can anyone help me decide on what headers might be suitable? I'm thinking 1-5/8" primaries between 28" - 40", 1-3/4 secondary's at about 40" and a 13" collector (not sure what diameter).

Any advice appreciated

[kiwicar]

Hi
With a blower I would go for 1 5/8 or 1 3/4 primary don't think it will matter that much, you could even step them half way down so you can nod knowingly in the pub! length about 36" +- 6 inch or so (so basically what you said) and 2.25" or 2.5" secondarys. As for length if they are stainless then make them well over length polish the pipe and mist washing up liquid and water over them with a plant spray bottle let dry. warm the engine up on manifolds only. Stick on the first silencer box you intend to use on the end of the secondary pipe and bolt on the secondary onto the manifold fire up the engine and accelerate under full throttle in 3rd gear from about 2500 revs, do it 2 or three times and driving it the minimum in between. Now take the secondary off (wearing gloves) and you should see stripes on the secondary of light and dark where it has got hot (and not) align the point where the silencer goes to maximum diamiter just after the joiner bit to one of the dark stripes and cut the secondary so when it is all joined up the length to this step in diamiter will be maintained.
Best regards
Mike

[cammmy]

Awesome Mike, thanks. I'll talk to you a bit more about this soon

[stevieturbo]

I assume by blown in this instance you mean supercharged ?

1.75" would be more than you'd ever need at 400hp. But at the same time, with a blower, I really doubt there would be any negatives
Of course, even the smaller sizes will still yield good results.

IMO it really isnt as important or will make much difference as a n/a setup.

[mgbv8]

I'll have to measure my primaries I reckon. I'm not sure what they are?

I bought a set of MG RV8 stainless manifolds and chopped the collectors out to 3" and fitted short 3" pipes to exit just behind the front wheels.

6 psi of boost from the 4/71 made around 150 extra horses for me on the 4.6 and the modified 3.9 which I now have fitted. Both engines run carb and harcourt single plane inlet manifiold.

[bigaldart]

With a blower you don't need gas speed in the exhaust to empty the cylinder, a wacking great pump on the other side does that for you. Bigger is better, I would have no hesitation running 2" primaries, 1 3/4" at a minimum. The blower takes power to drive so gas flow at a net 400 bhp will be more like a 500 bhp N/A motor. For a turbo it's very different as you want to maintain velocity and heat.I think ours are 1 7/8" ID zoomies and you really feel the blast from 6 feet away.
Alan

[cammmy]

Thanks for the replies guys.

The motor will be supercharged but the chargers have electric clutches and a bypass so that they can be turned off.

I'll look a 1 3/4" primaries and 2.5" secondary's then?

Also, 4-2-1 vs 4-1. Thoughts? This motor will be running N/A part of the time and unlikely to see the high side of 6k rpm. Mike says 4-1 is easier to get right and package. What kind of difference will it make on something like this? Is it worth trying to get a 4-2-1 system in?

[topcatcustom]

I believe I did 1.5/8 primaries, 2" secondaries and 2.1/2" main pipes, with a proper X over underneath the gearbox. Worked well, around 370bhp and that was 3.9/4.0.

[cammmy]

Cool. Was that blown or N/A?

For reference, i'm using the Real Steel Blower cam.

@.050" in 112 deg, .465" lift, ex 218 deg, .480" lift, LSA 114 (13 degrees overlap)

With so little overlap and only running up to 6k; i'll be wanting to ensure good scavenging before the inlet opens. Would I be safer sticking to 1 5/8" over 1 3/4"?

As it's blown as opposed to turbo'd, I'm essentially designing the exhaust as if it's a larger N/A motor?

[topcatcustom]

Blown!

I used the same cam. A few people told me just make them big and don't worry about it as the blower just forces the spent charge out- which is true- but still wasteful and doesn't mean you shouldn't think about design just because the extra power from the charger will disguise any non-efficient parts of the system. If that makes sense.

I looked at it like a NA system- if you can get exhaust scavenging just like a nasp motor- it will make it more efficient. Anyway that doesn't really answer your question, just that 1.5/8 worked well for me. If I wasn't running a full system I'd probably use larger headers. I'd say not much point going bigger on a 4.0 at less than 6k- but I'm no expert.

edit- just found old pic of the front of my system- it's not ideal as it was so ridiculously tight I couldn't get equal length headers- I think the X worked well though, the rolling road plot should be somewhere else in my thread- the mx5 rv8 thing (or something like that) in the garage area of this forum.

[cammmy]

Ah, I've read that thread. Very impressive build!

Yeah, that's how I see too.

Apparently equal length headers don't make much of a difference on a V8, it's the secondary's and collector that are important.

I'll stick on the smaller side I think. Seems that going slightly too small won't make a massive difference but going slightly too large will. Will merge them both into a tuned length collector.

[bigaldart]

Before you do the rest of the system the easy way to tune the collector length is with a crayon along the length, run the motor under load on just collectors, see where the crayon burns off to, then chop them right there and build the rest of the system. It will be longer than you expect sometimes over 24 inches so space becomes an issue. Chinagraph pencil works well.

Alan

[mgbv8]

Before you do the rest of the system the easy way to tune the collector length is with a crayon along the length, run the motor under load on just collectors, see where the crayon burns off to, then chop them right there and build the rest of the system. It will be longer than you expect sometimes over 24 inches so space becomes an issue. Chinagraph pencil works well.

Alan

Thats what I was told about fitting Zoomies to my engine Alan. The guy said make them quite long, at least 2 feet. They will look silly at first, but paint them with some hammerite and then race the car. Where the paint stops burning off is where you take the saw to them

[topcatcustom]

This is the secondary length I assume? I take it it is best to try and work out ideal primary lengths- then into secondaries and cut them where the heat stops then into full system, assuming this is 4-2-1 system!

[cammmy]

Sounds like what Mike was explaining.

That technique is for the collector? Anyone now how to work out how long I should make the secondary's if I go 4-2-1?

Do I need to pay special attention to where the primary's meet the secondary's or is it just a case of blending them in?