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74K views 402 replies 52 participants last post by  mukle 
#1 ·
Do we really need them?
This is the question that I am researching right now.

Turbo or Supercharged piston engines typically use many cylinders arranged in-line and one or two superchargers. Superchargers deliver air at a relatively constant rate, while cylinders demand it in a varying manner, as the valves open and as piston speed varies through the stroke. Simple direct ducting would give problems where the nearest cylinders received more airflow. The pulsating demand from the cylinders would also show problems of either pressure waves in the duct, or a shortage of inlet air towards the end of the inlet phase.

The solution is to provide a large-volume plenum chamber between the inlet and the cylinders. This has two benefits: it evens out the difference in path restriction between cylinders (distribution across space), secondly it provides a large-volume buffer against pressure changes (distribution over time
 
#2 ·
Finally you ***** guys have started to think about the plenum capacity issue.
Have you ever logged Your boost separately from front and rear cylinders?
 
#3 ·
Finally you ***** guys have started to think about the plenum capacity issue.
Have you ever logged Your boost separately in front and rear cylinders?
We have been discussing for quite sometime but cant seem to find the exact answer were looking for. Opinions are all over the map.
Have not logged cylinders seperately, it would be interesting

Sent from my SPH-L710
 
#5 ·
It would seem that the larger the plenum, the longer the boost lag. But the larger the plenum, the more stored boost available as RPMs increase. I remember having this discussion when a friend was adding a lot of large aluminum pipe to mount a custom intercooler to his Buick Regal GS Turbo. Naturally, he had a capacious engine bay with which to work compared to us.

I can't even imagine how you can expand the plenum to a meaningful increase without expanding the air box. But I've always thought that we could benefit by moving the throttle valves out of the intake runner air stream. Maybe you could do both?

The balance is prolly exacerbated by the relatively slow pulsation of having only two cylinders. The pressure fluctuations in that plenum must be wild!

Good luck with the experiment.
 
#6 ·
Correct about the bigger plenum will increase the lag.

I feel that Trask went with the smaller turbo so as not to have a lag.

Yes it is there but not as much as the bigger turbo.

There are other turbos out there with true ceramic bearings in a bigger turbo and will spool up trwice as fast as the 25.

Well the second option would be a Y set up. I was thinking about this before I saw Karzza's intake.
I think we could do this pretty easy. I would like to put a spike in the center of the inner Y to devert the flow of air.

If we was to go with they Y set up then there would be no need to change the air box cover.

I am hoping that Karzza can post up some pics and give some more insight to this.

The timing of the valves openiong and closing are just creating all kinds of problems.
How many time have we heard of the front cylinder going out. This whole time thinking the boost was being pushed in the front cylinder when in reality it was not.

This is just my opion. When looking at the flow of the air it takes the least amount of resistance. The rear cylinder was taking it all in and leaving the front to be lean.

This is going to lead to some people viewing this.

I do not by any means permit the use of my ideals and thoughts.
This is to protect my future projects.
 
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#7 ·
Hmmmmm. Interesting. Why could'nt the stock airbox be used? Might leak, but, so what? My SC just uses rubber elbows. No seal around the bolt heads. Hmmmmmmmm. Joe
 
#10 ·
#8 ·
I'll be the first to admit I don't know crap about boosting Twins. However I am well versed in boost. I would be curious about boost pressure being different between the cylinders. Flow is another matter entirely. If you are measuring boost in plenum, that pressure is going to be equal as it is volumetric. But flow can change and not be affected by pressure.take for example Turbo size / efficiency. At the same pressures they flow differently.

All this being said, I would monitor EGT's more then boost pressure going to each cylinder unless you have a way of measuring flow to each one. EGTs would give you a closer idea of what the cylinder was doing. While it's not as noticeable in a "normal " engine it will be more noticeable in a Twin and that is the power difference between the two cylinders. The opposing force you would want to be much closer in the twin.

I have read a crap ton of your guys posts and seen the engines explode here. I will give a +1 to the front cylinder being lean and or the rear. Cylinder pushing the front to help it pop.

Like I said I'm dumb when it comes to the twins. This is all food for thought.....
 
#9 ·
I installed egt sensors for the 1350 and the rear cyl temp was always cooler up to over 100 deg difference but not always consistent. Not sure if longer exaust to turbo vs shorter front had anything to do with it or not?
I did have some extremely high egt temps especially during cruise (16 to 1800 deg.) which is now believed to be caused by to much back pressure from a turbo that is supposed to be good for 200hp. Temps would drop down to neighborhood 1450 while boosting.
At the time i made some calls to figure out what was going on but noone seem to know what exactly was going on.
When the turbo went and some input from forum members we are getting a clearer picture to what's happening.
Like Rod said, this has been a learning experience and I totally agree for sure.
Sent from my SPH-L710
 
#12 ·
Good and equal WOT air flow in plenum is important to get maximum power.

There exists another point in a asymmetrinc V2 (one crank pin).
Stock camming and stock timing

Front cyl in TDC.............0 deg
Front intake opens........342 deg
Front intake closes........590 deg

Time for fill plenum.........52 deg

Rear intake opens.........642 deg
Rear intake closes........890 deg

Time for fill plenum........172 deg

Front intake opens.......1062 deg

What I have been trying to say, in case of asymmetric V2 engine a turbo has much more time to generate boost for a front cylinder than for a rear cylinder. That's why a relative high plenium volume is needed, to reduce difference of conditions for cylinders. All volume between compressor wheel and butterfly is countable.
This is raising up while more boost, rpms or cubics.

There is a little bit same situation on exhaust side.


While you have VRFI or T-max, autotune is taking care of different need of fuel to maintain a target AFR individually (trim) for both of cylinders.

I do not have fact, how much more power a front cylinder is producing in your case, 10hp I guess.
 
#13 ·
I think Nick did a great job for a hobby kit to keep prices in line for his R&D & manufacturing costs. At this point I'd like to go in with you guys & design something that optimizes the setup. If we can agree on design parameters & have someone build 5-10 at once we could make it more affordable to each of us & have an end product better suited to us than any individual effort. Agreed ?
Maybe we could start at the TB & work backwards ? (Boots can be addressed as we progress ) Can we design around 58mm units ? Our ID going up to the plenum would be 58mm and flared at the top. What length do we need ( TB flange bottom to top of velocity stack ) ? Looking at the Trask set up ( most common, but very similar in most ) in appears that a center inlet, between the cylinders coming from the right side of the bike would be most efficient. It would entail relocating the coolant hoses but that seems a minor consideration. A Y shape might enhance air balance & physical positioning of necessary periferals. Are we heading the right direction ? Who's in ? THX
 

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#14 ·
If someone has the time do a rough measurement of both headers from head to turbo flange. I'm curious. Karzza brought it up when he mentioned exhaust, and me forgetting how much a Twin relies on exhaust performance for overall running.

Those EGTs are curious Lane. The 100 degrees between front and back I'm thinking could be caused by a runner length difference. Or the flow problem showing up. I think if a perfect condition were to exist you'd shoot for within a 20-25 degree difference. 100 sounds a bit steep considering were only working with two cylinders here.

The 1450s are about perfect. Its funny they only existed under pressure. Inherently boost creates higher temps especially when coupled with such a high compression ratio. I would assume your a/f was correct or at least enough so for you not to have mentioned them. I could see this happening if you were running about 13.5 to 14 during off boost cruise. Or maybe even a little higher. Then as boost came in and you added fuel to obtain an 11 or 11.5 target could very well be part of what dropped the temps so drastically.

Back pressure being too great would also increase temps... and you would maintain elevated temps during boost. This could be tested by trying a larger a/r on the exhaust side. But keep in mind you may sacrifice a shade of spool time.

There are so many variables to look at... even intake flow, even exhaust flow, a/r sizing... you can go bald! Lol
 
#17 ·
If you had one single TB you would not need a Y!

So now the Y would be below the the TB's. Correct.
You would still have the same timing of opening and closing of the valves.
 
#16 ·
Karzza,

The Y set-up is a way of providing a chamber for each cylinder.
Thus allowing the turbo to fill the chamber above the TB's.

I understand what you are saying.

The more volume over the rear is better. Just trying to minimize the difference.

I suppose if you where to tune the runners at different length you could greatly minize the difference. But this is not good. Long runners do nothing but hurt the intake of boost.

So to find the sweet spot that takes care of both the front and rear would be seperate chambers period.

Since we only have one turbo we have what we have.

Do you have any pics of your plenum?
Did you reduce the front inner diameter of the chamber over the TB?
Is your Y set-up offset any at all?
Is the inner diameter tapered to a smaller diameter?
I can see that the diameter is quit larger then the Tb openings.

My head hurts!
 
#21 ·
air flowing at the speed of sound...:D

wish the rod went that fast.
 
#22 ·
A new plenum design!!!!

I hope :deal:
 
#20 ·
Just talking out loud here...

have enough volume not to hurt the engine as it daws it in.
to have a limited amout above this so there is always enough there
if to big it will hurt on the engine response (slow)
if to small it will increase velocity of the incoming air.
high velocity will lower the boost...Not good.(faster response)

as the engine is boosted it rotates the crank faster allowing the engine to pump more air and faster.

this is where you need to have a plenum to store more air then what the engine cd is.
but how much more...
more then the amount of air passing through the engine at WOT!
so at this speed of transfer the runners are not going to big deal.

but to have a turbo that can fill the plenum as fast as it is removed.

air has to flow smooth.
so any air moving across a wall will have restriction
restriction slows the air down.
no restriction means smoother flow.
Y set-up straight into TB's

air turbulence...
No turbulence means less atomization of the fuel and mixing the air
More turbulence means more atomization and mixing of fuel
once the valves open the force of air is moving into cylinder bouncing off the piston, mixing up in the chamber. very nice...

Y set-up vs trasks

better overall flow of air.
less dead space inside.
 
#23 ·
You guys are thinking to much....

You want EQUAL then make things EQUAL.

To do this you simply do twin turbos. You can make the front and rear exhausts the same diameter and length. You can make the intakes each the same volume.

There would be no interference from one cylinder to the other in regards to airflow and pumping action timing. As long as the cams were timing identically the cylinders should perform equally with the only real variable being coolant temps on each half of the engine.

DONE...that was easy!
 
#24 ·
You had to make it simple like That didn't you
OOOOooooBoy no sleep tonight ;)

Sent from my SPH-L710
 
#32 ·
You guys are killing me!!!!

I should have known all the BB guys would be all over this.lmao

At least I know someone is undserstanding what I am saying.

So now you all put your heads together and solve the issue with one turbo hahaha.

Jan yes I do agree with you on this.

I was thinking about putting a expansion chamber before the split in the Y set-up.
split the expansion chamber in half and drill holes in the part that splits the chamber.
 
#35 ·
Yeah I know...

:stooge:
 
#37 ·
This sounds like a job for DMX Motorsports. . .
 
#42 ·
PM sent Greg
 
#43 ·
Ok I've been running a trask turbo for a year now and been reading what's here so I put a boost/vac gauge on. Found trask gauge in plenum reads 8lbs and boost/vac under throttle body reads 5lbs, was abit surprised. Thought was getting a turbo kit that put 8lbs into engine.
 
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