Cummins 4BT & Diesel Conversions Forums banner

1 - 16 of 16 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter #1
Hey all! I've been kinda lurking for a month or 2 now, getting ideas for 4bt conversion into a K5 Blazer that I hope to do someday (far into the future). Anyway, I noticed something about the 4bt manifold that I figured I'd bring to everyone’s attention.

Everybody knows that most turbos have a divided inlet to the exhaust housing. I can't remember the exact reason turbos are made this way, I think it has something to do with pulse tuning or something like that. Anyway, I do know for the divided exhaust housing to work the way it should, the pulses from the exhaust valves opening on different cylinders have to alternate between the two sides of the exhaust housing. So for example, on a 6bt, with firing order 1-5-3-6-2-4, the exhaust pulse from the number 1 cylinder goes to the front half of the turbo exhaust housing, the pulse from the number 5 goes to the back half, the number 3 pulse to the front half, alternating back and forth. This is why the 6bt exhaust manifold is designed so that the front 3 cylinders feed one half of the exhaust housing and the back 3 cylinders feed the other half.

However, with a 4bt things are a little different. The firing order for the 4bt is 1-3-4-2. Notice that instead of alternating between the front cylinders and the back cylinders like the 6bt, the 4bt alternates between the cylinders on the ends of the block and the ones in the middle. What this means is that the exhaust manifold must be designed differently to get the same pulse effect, and it is. If you look closely at the different factory 4bt exhaust manifolds, you'll notice that they're designed so that cylinders 1 and 4 feed one side of the turbo exhaust housing, while 2 and 3 feed the other, getting the desired pulse balancing into the turbo.

I bring this up because I notice many people are designing their own exhaust manifolds to fit the 4bt (often modifying a 6bt manifold) and coming up with manifolds that divide between the front and back cylinders. The effect of this is to send 2 pulses to one half of the turbo, then 2 to the other, instead of alternating pulses 1 for 1 between the 2 halves of the turbo. I don't know how much of a detrimental effect this has on turbo performance, but its something I thought I'd point out to everyone.
 

·
Registered
Joined
·
126 Posts
Wow 4BTblazer,

This is a very good point. The pulse-tuning of exhaust systems has become quite an art. A common tuning method we all see is done with "H" and "X" pipes on many gas V8's (albiet further "downstream"). Much of it has to do with keeping the flow in the forward portion of the exhaust system laminar- thus producing optimized flow. Another reason for tuning is acoustic, non-turbulent flow under the floorboards is much quiter than turbulent. Cancellation of harmonics at certain RPM's may also be another benefit and this can occur anywhere from the turbo housing all the way back to the tailpipe. There is also the thermodynamic aspect of the turbine itself but I do not know too much about the specifics on pulses and pulling heat out of the blades. I do know it is possible to melt the blades on an over-taxed turbo...

The funny thing is the 6BT manifold will work, just as an untuned header will work, so outward signs would be, "Well, it runs, it must be okay!" The chance is it could run better with the correct manifold splits. I'm hoping someone who has done a 6BT modification can comment in order to confirm/dismiss.

I've considered getting a spacer/shim made to go between the head and manifold to re-aim the turbo outlet, but pulse tuning was one of the issues I did not have enough specific diesel knowledge about.

Thanks for sharing that insight,
George
 

·
Super Moderator
Joined
·
8,847 Posts
A divided exhaust manifold's aim in life is to keep the cylinder pulses seperate and feed them right into the turbine. The pulses spin the turbine more efficiently than say a log style manifold which makes each pulse dissipate it's energy before getting to the turbine.

It's not like a non-turbo petrol engine where the low pressure zone behind the pulses are used to suck exhaust out the cylinder, increasing volumetric efficiency.

The "Diesel Engine Reference Book" has a whole chapter on turbo exhaust manifolds, covering the length ratios and cylinder combinations for different engine configurations and firing orders.
Maybe your local library has a copy, if not a university library will.

The isuzu 4BD1T (japanese 4BT copy) exhaust manifold I have was split pulse 4-2-1 design. Cylinders 2 and 3 had runners approx half the length of cylinders 1&4. Cylinder 2 runner joins cylinder 1 runner halfway along, then feed into the neck with the turbo flange.

Unfortunately this manifold was modifed by cutting the neck off, rotating it 180deg and welding it back on. So it no longer works as a true split pulse.

But even so it combines the pulses from each cylinder, resulting in two strong pulses instead of four weaker ones. At idle out the end of the exhaust these pulses can be felt and heard.
 

·
Registered
Joined
·
1,638 Posts
Gonna bring this one back up for further discussion.
When I chopped my 6bt manifold down, I cut 1 ex port off each end, leaving 2 ex ports feeding each side of the scroll. I am considering trying to build a twin set up but to fit it I would need to chop the 5th & 6th ex ports off and then flip it over to use as a top mount for the smaller turbo. This would mean I have 3 ports feeding one side of the scroll and 1 port feeding the other.
Now, the smaller turbo would be an HX30 which is not a twin scroll turbo so all exhaust coming out the manifold would dump into the center of the ex housing. Does this sound like it might be an issue to anyone?

Dave
 

·
Registered
Joined
·
2,103 Posts
Gonna bring this one back up for further discussion.
When I chopped my 6bt manifold down, I cut 1 ex port off each end, leaving 2 ex ports feeding each side of the scroll. I am considering trying to build a twin set up but to fit it I would need to chop the 5th & 6th ex ports off and then flip it over to use as a top mount for the smaller turbo. This would mean I have 3 ports feeding one side of the scroll and 1 port feeding the other.
Now, the smaller turbo would be an HX30 which is not a twin scroll turbo so all exhaust coming out the manifold would dump into the center of the ex housing. Does this sound like it might be an issue to anyone?

Dave
I did same with one feeding one side 3 others feeding other. Has a definite tone b4 pump was cranked up.

Nothing now but the 3 are feeding small end of compressor and one into large end. Spools quick like now.

Been on my mind to do twins and may go back to the twin scroll stocker as I do believe it is better design.

I could have flipped stocker and spaced it and still may do so with twins but have a few 341's laying around and

just may do a single for now. Hate to redo all lines and stuff and not do twins while at it though. Another thought

would be to gut inside of manifold with mill so divider is gone??
 

·
Super Moderator
Joined
·
8,847 Posts
Another thought would be to gut inside of manifold with mill so divider is gone??
Definitely. You don't want 3 cylinders fighting for half the turbo and one having the other all to itself. Simplification I know, but taking out the divider would be a minimum IMO.
 

·
Registered
Joined
·
1,638 Posts
Remember guys, the hx30 is not a twin scroll turbo. All exhaust only goes through one port.
Dave
 

·
Registered
Joined
·
2,103 Posts
Remember guys, the hx30 is not a twin scroll turbo. All exhaust only goes through one port.
Dave
Yes but with 2 lopped off one end the scroll is lost and one pulse is feeding one side and three pulses are feeding

the other side. Even with the hx and hy undivided I do believe there is a difference as I could hear it in exhaust

as a blurp in note as it changed from when stock manifold was changed out for cut 6bt. Not as noticeable now with

fuel cranked to runaway but still can hear it. 4bex 004.jpg

Probably cant see it too well in pic but hi mount works good tons of room for twins..
 

·
Registered
Joined
·
1,638 Posts
Thanks for the pic crow, that looks sweet. I think this should give me plenty of room. BTW, is that a flipped 4bt manifold or a modified 6bt with the tilt?

I think I will take your advice and mill out the divider between the scrolls. I understand what you're saying about 3 pulses coming down one hallway and just one on the other, might make for some sort of disturbance.

This all depends on whether or not I make the deals on the parts that I am working on. I'm usually really good at scoring parts cheap. It's all in where you look.:eek::grinpimp:

Dave
 

·
Registered
Joined
·
2,103 Posts
opped 6bt the stocker hits the valve cover but have spacer to move it up to clear. I tried up down center back

but front was the best for room. Also the alternator bolt would not clear the drain tube so had to just use dowel

with hair pin. Right now ac is the priority and am going to copy thinders bracket. I dont care what I have to move

or fab tired of no ac and if twins are in the way then ac wins..
 

·
Premium Member
Joined
·
389 Posts

·
Registered
Joined
·
1,638 Posts
I've seen those Wolf and they are very impressive but I'm only doing this if I can do it on the cheap. I really love my hx30 but there is so much debate about what a smaller set of twins can do on our motors, that I figure I'll give it a try if I can get everything on the down low.

So far my tally is this:

96 6bt ex manifold = $53 shipped

94 Wh1c (basically an Hx35) with comp elbow $120 shipped

I have an hy35 for mock up in the garage. (exact same length as my hx30 which will be the smaller turbo)

Made my own hotpipe $0

Made my own dual Y drain so both turbos drain back into the block. I didn't want to tap into the oil pan like most people do. $0

will make my own downpipe out of current pipe $0



Still need,

Second oil feed line $30ish

Need to fab a brace to help support larger turbo. $0

The setup,

Smaller turbo - Hx30 = 40mm comp- 6cm ex house w/wastegate

Larger turbo - Wh1c = 56mm comp - 12cm ex house W/wastegate

I've read on here somewhere that a member did an hx30 over an Hx40 and had a hard time getting the larger turbo to light.

I figure this should just make a fun powerful combo. I'm not looking for 40/50psi. I just want to see instant boost like I have and see what more air up top will feel like.

Plus, I've run out of things to tinker with and I just can't sit still.

Dave
 

·
Registered
Joined
·
784 Posts
I have been thinking of the same set up for the same reasons, I'll keep watching for your's.
 
1 - 16 of 16 Posts
Top