1835cc plenum advice needed -> Grelland's 1835 MS in Ghia

grelland · Post by **grelland** » Wed Nov 08, 2006 11:27 am

Dear all.

A few months back I posted a question on TB size, and I got some great advice.

In the meantime, my engine plans have changed slightly, and I need advice as to whether I am heading in the right direction...

My current engine is an 1835cc with 69mm counterwighted crank, a w110 cam, heads with 37x42 valves, and generally overhauled and blueprinted.

It currently has a pair of Dellorto 40mm on it, but looking at the state of the car it will go into, it will probably not be runnig this side of Easter (07)...

:

So, since I am assembling a MS II V3.0 kit during the long, cold nights up here in Norway, I may convert this 1835 engine to FI before I put it into the car in time for summer (My original plan was to use the engine as is, and build a new 2174 engine where the FI would go).

For the HW for the FI system, I plan as follows:
- MS II v3.0, also controlling ignition
- single TB
- CB or similar end castings
- plenum located under/behind original/modified generator stand
- VW wbx distributor incl. hall sensor
- various TPS and temp sensors
- narrow band O2 sensor

I am planning a plenum design along the line of this:

(Forgive me for borrowing your drawing, Mabbo)

My targets for the project are:
- To understand and get to know the MS system
- To learn how to tune the MS properly before letting myslef loose on a REALLY expensive engine
- Equal or better performance than the dell's
- Better driveability than the dell's

Now to my questions:
1
What guidelines exist for designing plenum and runner lengths

2
What TB size would suit (I have a 45mm from a wbx lying around)

3
Will I achieve my targets?

Any feedback is highly appreciated on this matter

KDF Lad · Post by **KDF Lad** » Wed Nov 08, 2006 1:38 pm

I've picked this up from elsewhere on this site:

Plenum = 2.25 - 2.5 times a runner volume

Runner volume = cylinder volume

Runner dia = inlet valve openning dia less dia of valve stem (more important to get good flow from runner to head)

TB dia = 1.25 - 1.5 runner dia.

Ray Greenwood is very knowlegable and will probably add, clarify or correct what I have listed.

grelland · Post by **grelland** » Wed Nov 08, 2006 2:27 pm

Thanks a lot; just what I was looking for.

Please feel free to add or comment further

raygreenwood · Post by **raygreenwood** » Wed Nov 08, 2006 3:14 pm

Just be sure not to make dead end areas where you transistion from the larger to smaller diameter. Also...since you have an entrence from TB making a 90* turn and trying to split in two directions...there may be a fair amount of turbulence. I would suggest a small splitter pyramid under the TB entrance. Ray

grelland · Post by **grelland** » Fri Nov 10, 2006 2:18 am

Thanks for your advice.

In my search for inlet manifolds (as an alternative to the CB ones), would these work:

Will standard injectors fit?

They are said to come off a 75-79 T1 car.

Roy

grelland · Post by **grelland** » Fri Nov 10, 2006 2:24 am

or these:

Which ones shoud I use?

raygreenwood · Post by **raygreenwood** » Fri Nov 10, 2006 4:01 am

Actually they both worked well. The first picture being type 1 and teh second type 3.
There is the great possibility that both of those sets will have runners a tad small in volume for the 1835. That can be made up for (though not completely) by making a larger plenum and then being very careful in selecting the TB size.
In this case, if the runner diameters are just about equal...use the siamesed type 1 runners in the first picture. Also do not get the valves too large.
The good part about having runners just slightly too small is that they will have excellent velocity. A cruderule of thumb for valve sizing would be to have intake valves that are just at or slightly smaller (in open area ) than the area of the runner cross section....minus the diameter of the stem.
So.....if you have a 30mm runner....and a 36mm valve...minus a 7mm stem...you are close to a good starting place. Not scientific.....but similar to the correllations in a lot of successful factory engines. Ray

mschilling · Post by **mschilling** » Fri Nov 10, 2006 8:17 am

It's not easy to find injectors to fit those manifolds.

Steve Arndt · Post by **Steve Arndt** » Fri Nov 10, 2006 9:07 am

They are very small inside diameter without a lot of room for port matching. They are very high quality though.

Steve Arndt · Post by **Steve Arndt** » Fri Nov 10, 2006 9:08 am

mschilling wrote:It's not easy to find injectors to fit those manifolds.

I think Ray has posted about using a CIS o-ring kit that makes normal injectors fit into the old small o-ring german manifolds.
Ray?

s

raygreenwood · Post by **raygreenwood** » Fri Nov 10, 2006 9:34 am

The CIS seals fit better for the stock injector. Nice and tight. The stock D-jet seals were junk. They would also fit tight on smaller injectors...so they may indeed help.
On the other hand...it should not be too much of a problem finding a flanged teflon bushing at someplace like mcmaster carr supply...to drop right into teh D-jet hole to fit a smaller injector. Ray

grelland · Post by **grelland** » Fri Nov 10, 2006 11:32 am

Thanks again for all the good advice.

So to sum up:
- I should go for the T1 variety
- Make sure there are no dead end areas
- Verify the diameter of the runners vs my valves
- Maybe some fabrication is needed for fitting the injectors

As soon as I get some measurements on the runners, I will keep you posted.

Roy

grelland · Post by **grelland** » Sat Nov 11, 2006 12:22 pm

OK, so my friend here measured a set of the T3 runners, and they have an ID of 30 mm and length of 400 mm.

My intake valve size is 37mm, and stem dia is 7 mm, so the runner diameter seems spot on.

But working out the runner volume, yields some 283cc per runner, while my cylinder volume is 1835/4 = 459cc.

If I were to have the same volume as my cylinders, the 30mm runners need to be some 650 cm long (more than 2 feet!).

Given that I use these runners, should I aim for a plenum 2.25-2.5 times bigger than my actual runners, or should I use the desired runner volume as starting point?

Also do I include the flkange where I will mount the TB in the plenum volume (all the way to the butterfly?

Or will I be tatally off track anyways, since the runner volumes are this much off?

Please guide me ....

Roy

KDF Lad · Post by **KDF Lad** » Sat Nov 11, 2006 2:54 pm

From what I can work out, if the desired runner volume can't be achieved then the plenum volume can be increased to compensate. How much? I doubt there would be a difinitive answer, but closer to 3-times the runner volume. Remember that the plenum is where each runner gets its supply from. So if a cylinder is supplied by 1 runner and the plenum (rather than just the runner) then the plenum size would need to be able to take up the slack.

In the case of the plenum design in the 1st post it appears that the plenum size could already be over-size for an 1800, and due to the orientation of the plenum and runners (in-line), a reasonable compensation has been achieved.

As I understand it, the plenum volume is calculated from the TB.

raygreenwood · Post by **raygreenwood** » Sun Nov 12, 2006 11:55 am

There really are not any rules at this point. There is math...that points out perfect runner lengths and diameters for expected velocity at a given rpm band....but thats not the whole story. Too many people who do nothing but race...write a lot of the books.

But a good factory correlation is about 2.5-3 runner volumes per plenum. Bear in mind thet in a lot of successful factory plenum vehicles...the runners are right at one cylinder volume each. As long as the runners are not so small in diameter that the y are a restriction (yours are good for a start).....then all the smaller diameter will do is increase velocity and vacuum signature. But...from that point....if a normal guestimate for plenum is 2.5 times runner volume, which would also be 2.5 times cylinder volume..... Yours would probably be fine to be 2.5 times cylinder volume. If you get too large in the plenum, the velocity in the plenum drops too far and you get a throttle that is peaky. Ray