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Ok.....
So I have a stroker in a front engined trike.
There is nowhere to put a traditional 4 into one merged header.
I have been running straights like motorcycle drag pipes for many years. But now I am building my motor again and want to tune it better. It is going 2387, fuel injection and edis ( an old megasquirt).
Seeing as there is no place to put a 4 into 1, I was thinking of a 4 into 2.
Pair up 1 and 3. I my be able to sneak one pipe across the flywheel end. Then out the side.
Then pair up 2 and 4 around the pulley end and out the other side.
Wish it could be a tri y. But it would be super long by then.
I have not found much on a 2 into 1 header tuning. I know it is done on a 2 cyl motorcycle when performance is important.
But info is thin to find. What do yall know or think of the idea?

Hi mate, you are correct in that 421 systems are not available, mostly because they got a bad rep from supposed expert commercial mfrs pairing them wrongly, as you correctly state the pairing is 1/3 & 2/4, leaving them just paired thus would be an improvement over your straight stacks, but the final hurdle, as you have found, is the hard part, in joining the 'two' into one, but the gains are well worth the effort.
My length calculations won't help your install, just physically getting it in there will work well for you, too long being better than too short, have a look under a vanagon to see how VW do it, not Aircooled, but Wasser vanagone have 421 primary pipes, they did it for a reason, to promote the low and mid range performance the heavy bus needs, your flyweight big motor trike will pull a house down at 4000 rpm with a 421 system, you will have to break rules to do it, don't worry too much about ideal equal length primary's, you can use half the ideal length on one side of each pair without losing anything to worry about, just go for the easiest packaging to get it all in there, I did one with a cross pipe under the bell housing, I made a flattened oval shape chamber under the b/ housing to gain ground clearance with good pipe volume. Good luck on it, post up your progress please. Regards BBB.

Thanks for the reply and vote of confidence.

I can not go under. My drain plug (no add on sump) is only 3.5 inch off the ground. The transaxle is upside down formula v style. But i have a drive extension made of an empty case to move the engine forward. I think I may be able to pass one pipe through the drive extension.

I am thinking of 1.75 pipe for my 2387 and fk10.
Or do you think stay with 1.625?

Yours sounds an interesting device with Big cam big motor and the inverted box, why? Ring gear flip/swing axle or IRS?
Exhaust dia' ? Well You will be fine with the small pipe, it must be a wheelie beast just now anyway,,,, it may be over camed a wee bit, you have a seven grand motor there, that thing will take some hanging onto in top gear lol, don't say it's got a 5 speed also.......
Wait until this 421 kicks in, I keep to 1"5/8 right up to 2.5 ltr motors for torque on my circuit cars were you want gas speed for throttle response, I could run a two inch system and pick up 4/5 hp on the hp high, but the thing would be useless in a dog fight, most Aircooled VW speed equipment available has only drag racing as its development platform, other disciplines have to DIY everything, but that's the appeal of it all for me.
Yeah, try through the extender unit, You will find the bell housing area pretty tough with plenty of wall thickness, try some bore holes in the required area and you will see it's very strong, my trans has windows cut in the bell to cool the clutches, it's still strong, but Remember its magnesium lol...

It is a swing. Several reasons.
I wanted it small. Hard to get irs this narrow.
It is 48 inch wide to the outside of the tires.
I did not want squat (camber).
And I wanted it low like a chopper.
Inverting the trans put the engine much lower.
It has the normal 1st and 2nd
1.26 third
.82 fourth
3.44 ring and pinion.
Welded syncros
4 spider diff
Billet side covers.
Short axle minus 6 inches a side.
Disc brakes.
Then the oiling mods to run upside down.

So nope not a 5 speed.
It blows through the gears so fast I do wish I could have an even bigger 5th gear. It blew through the gears so fast it was not unusual to hit 8500 before you could let go of the handle bar to shift it again.
That is with the 2110!

All apart right now.
Going 2387 fuel injected.
the fk10 is not much bigger than the previous cam. Just a little more cam for the little more displacement.

I am thinking right through the area where the diff would be.
Just under the starter.
The coil goes away anyhow.
The oil filter is on the other side.
Will hafta find a place for it to move to.

43mm Mikuni throttle bodies.
42mm intake valves m
Porting not huge.

I am thinking you may be right about the 1.625 exhaust.

You will enjoy the ride not having gas fumes in your face percolating from the old carbs. EFI is much cleaner. Just parking in a closed garage after a run EFI makes a big difference in evaporative emissions. This is the coolest trike I've seen.

Steve Arndt wrote: This is the coolest trike I've seen.

Isn't it just, COOL, looks like a shop bought one too I get some in to fiddle with, but,,,,,,,.. Well I could be rude...
My favourite project line at the moment is side drought intakes, I see you are heading that way, some Folk have beaten me to it in F Vee, they win races already, Tillotsen carbs are used, Fi not allowed, for me with a saloon it's difficult with clearance issues, it needs really wide track width.
Yes, keep the 42 intakes and no drag race ports, just 3 cut or radius the seats, I can get 175 hp on carbs from Only 40x35.5 valves with killer throttle response easy, as Steve says, your planned injection will keep the traffic manners and emissions nice and kind. Do keep an interesting build up on the forum please, best regards, BBB.

Ok then.
A pic or two.

Size of the frame.
Cut up a gold wing

Drive extension made from empty case.

New front end I am building for it.

I have clearance the case more to fit the 86mm crank.
Same Chinese forged unit every sells.
H beam rods with chevy journals.
Mahle piston and cyl.

Still have to find the time to do the hoover oil passage mod.

This time around I am going to use cerakote.
They have the conductive coating that is supposed to increase thermal transfer some 15 to 30%.
Then for the inside they have the thermal barrier that is supposed to block heat from entering. Coat the piston tops, chambers and exhaust ports.

Anyone try this stuff? Sounds to good to be true but I ordered it anyway.

Cracking build, Original ideas, nothing wrong with swing axle either, lighter unsprung than IRS, possibly weak at the spade ends were as IRS handles power better, but your light weight so will be fine, I can't comment on the coating but hear good things, somebody will chime in I'm sure, The Hoover mods are easy apart from some cases break out under the #2 cylinder, get the TIG out time if that happens, I think my bit is 6mm x 11"1/2 long, take the centre shoulder out from the lifter groove also, Something tells me your running hot on the oil? So this will help that, next, you need a cunning foot change shifter, how about a "stirup" arrangement for your foot to enable it to shift across the gate, this new beast of a motor will need all your steering input powers, at least when the front is on the ground...