TheSamba.com :: View topic - D-Jet PhD Sticky. Newbies Need Not Enter. Advanced FI Tuning

Sat Jun 04, 2011 11:10 am

Here it goes…

I wanted a smooth running, torquier than stock, easy to build, possible to tune, engine for my daily driven ‘68 square with factory d-jet. I drive 64 mostly freeway miles round trip 5 days a week. I also wanted to test the tunability of the factory d-jet, and perhaps find work arounds to its limitations.

1776(?), built on a dual relief case, oil filter take off from the side, DPR C/W 69mm crank, Mahle 90.5’s, stock 034 heads (step machined off, rebuilt locally, fluffed and buffed by me following Wilson book but maintaining the round port, single springs, stock rockers), 7.8 c/r, custom grind cam, reground stock lifters (the red ones).

Cam was built locally by DR Camshafts in Single Springs, CA. He listed 914 d-jet cams on his website, and was interested and engaging on the phone regarding my build. We discussed my expectations and my application. I took him a couple of cores, and he reground a cam similar to one of his 914 d-jet cams. 254’ advertised duration, .426” lift with 1.4 rockers (.334” with 1.1’s), 109’ lobe centers. By widening the lobe centers, it allowed more duration without changing the intake valve opening point at all. Wider lobe centers also allow a smoother idle (more vacuum), and lowers the peak torque RPM as compared to narrower lobe centers ( I read too much).

The biggest issue that I am having is the early (B-ECU) d-jet does not allow for adjustability of the full load enrichening (mentioned by Ray G. a while ago, but not taken to heart). I think that the full load enrichening is not activating, as when I accelerate at highway speeds, I get a ping. The switch does not engage in the driveway (I do understand that there is no load to speak of in this configuration).

I have run a 50 ohm resistor at temp sensor 1, and unplugged temp sensor 2, and jacked the fuel pressure up to 35. I have tried timing it at -5’, 0’, and +5’. It is rich at idle, there is much carbon in the exhaust, and nothing feels hot. I think that I have too much vacuum in the plenum, that is not allowing the switch to move.

I had the same ping under full load issue with the 1600 (fluffed and buffed heads), but only when really trying to kick its ass. I am in the middle of trying something really dumb to make this work.

Sat Jun 04, 2011 11:52 am

Measure your vacuum at idle. If the vacuum is too high...you actually run lean...not rich. What is your fuel pressure? Is it stable? this is really common when going up in size but changing little else. Measure these tow things and get back to us. Ray

Sat Jun 04, 2011 5:20 pm

I'm also going to be bumping up to a 1776. Right now I plan on using a stock cam, but possibly running some larger valves, maybe something that is 40 x 35.5mm. If the larger valves are going to be a huge issue, I'm not opposed to running stock heads. I was going to keep the runners stock, and maybe polished, but I plan on increasing the plenum size. I'm hoping to get some weekend access to my buddy's CNC machine so I can make a new back plate that will increase the plenum volume. There isn't much room back there, but is it possible to go too big? How many cc's would you think would be sufficient?

Sun Jun 05, 2011 9:12 am

voeltzwagen wrote: I'm also going to be bumping up to a 1776. Right now I plan on using a stock cam, but possibly running some larger valves, maybe something that is 40 x 35.5mm. If the larger valves are going to be a huge issue, I'm not opposed to running stock heads. I was going to keep the runners stock, and maybe polished, but I plan on increasing the plenum size. I'm hoping to get some weekend access to my buddy's CNC machine so I can make a new back plate that will increase the plenum volume. There isn't much room back there, but is it possible to go too big? How many cc's would you think would be sufficient?

Before you go too far.....measure what you have. The two greatest points of rectriction are going to be the runners and TB. You are effectivelt building a 1.8L. The 1.8L type 4 engine (which came with L jet)...I have run with D-jet as well. Its plenum was taken from the 1.7L (1679 cc). It is right at 1 liter. It ran well but was slightly restricted...partly due to the runners being slightly smaller than the 1.7L....with basically 42 x 36 valves.

The valve size you are planning is fine.....especially since you cannpt do anything about the skinny type 3 runners. To offset that with a larger plenum....a good starting point is roughly 2.5 cylinder volumes (444 x 3= 1.1L) You could do fine with a 1 liter plenum.
Actually...more precisely...in most systems the runner volume is typically about 1 cylinder and chamber volume....so really the plenum should be 2.5 runner volumes. I do not know what type 3 runner volume is.
If you can do that with the plenum (2.5 runner volumes)....and have it be slightly undersized with reference to cylinder volume....you will get high velocity ...maybe to high.....but you wil definately need a larger TB to control that.
The trick is going to be making a slight;y larger TB to replenish that larger plenum. It should all still run OK...but I;m not sure if it will run or tune better. Ray

Sun Jun 05, 2011 2:58 pm

A couple pages back, there was some pretty good discussion on plenum and runner volume. I think the runners clocked in at 250cc each, and the plenum was 550-580cc (someone had been drinking :wink: ).

Pending these numbers were accurate, it was determined that the 1776 is going to be the absolute max the stock intake could run.

I was thinking I could get away with a 15-20% increase in plenum size by building a new end cap that could accomplish this, but trying to get 1L out of a 550cc plenum may get a little fugly, and sourcing a 1.7 T4 plenum may be the way to go.

Sun Jun 05, 2011 3:14 pm

Nope! no one had been drinking! The original plenum is sized about right for the stock runners.....but the runners themselves were always a little skinny on the type 3. It kept the velocity up high. Bear in mind that the port areas in the heads count as well.

The problem with sourcing type 4 plenem is that it does not fit the runner spacing and diameter. I think your idea for expanding the stock plenum would really help. You couls...also get some more plenum...if you have room....by removing the throttle shaft and plate...install a tube of a few inches...and putting a new TB on the end of the tube.

I think it will work with a 1776.....but I'm betting you will need to retune the MPS. If you are doing all of this...do yourself a favor and build a new harness now so that any gremlins you end up chasing are from new changes...not old issues. Ray

Mon Jun 06, 2011 7:45 pm

Ok did some preliminary measuring and depending on the machining I should be able to fudge 125cc out of a new end cap on the plenum, which could expand the stock plenum to 2.5x runner volume(stock runners 250cc x 2.5=625cc) , or somewhere between 625cc and 655cc depending on whether the stock plenum is 550 or 580cc or somewhere in between.

Now for the throttle body. I couldn't quite tell for sure what size it is, 38 or 39mm (lost my calipers). There seems to be enough room in the neck of the plenum to bore out a couple millimeters and then have a custom butterfly built to attach to all of hardware (modified hardware) already there. The hardest part seems to be the idle adjustment port, which needs to be in front of the TB, there isn't much room. I suppose one could tap or press in a tube that could seal against the TB, but I'm not sure if that port diameter has significant effects on idle adjustment (it only needs air, right?).

Anywho, I'm learning a helluva lot and I apologize in advance if I ask a stupid question or miss something. I love this thread and I'm getting more and more addicted to learning how to take d-jet to the limits. This is way more fun than messing with my SC300 back in the day. After this...I guess its MS. :twisted:

Sun Jul 24, 2011 5:42 pm

rosevillain wrote: Here it goes…

I wanted a smooth running, torquier than stock, easy to build, possible to tune, engine for my daily driven ‘68 square with factory d-jet. I drive 64 mostly freeway miles round trip 5 days a week. I also wanted to test the tunability of the factory d-jet, and perhaps find work arounds to its limitations.

1776(?), built on a dual relief case, oil filter take off from the side, DPR C/W 69mm crank, Mahle 90.5’s, stock 034 heads (step machined off, rebuilt locally, fluffed and buffed by me following Wilson book but maintaining the round port, single springs, stock rockers), 7.8 c/r, custom grind cam, reground stock lifters (the red ones).

Cam was built locally by DR Camshafts in Single Springs, CA. He listed 914 d-jet cams on his website, and was interested and engaging on the phone regarding my build. We discussed my expectations and my application. I took him a couple of cores, and he reground a cam similar to one of his 914 d-jet cams. 254’ advertised duration, .426” lift with 1.4 rockers (.334” with 1.1’s), 109’ lobe centers. By widening the lobe centers, it allowed more duration without changing the intake valve opening point at all. Wider lobe centers also allow a smoother idle (more vacuum), and lowers the peak torque RPM as compared to narrower lobe centers ( I read too much).

The biggest issue that I am having is the early (B-ECU) d-jet does not allow for adjustability of the full load enrichening (mentioned by Ray G. a while ago, but not taken to heart). I think that the full load enrichening is not activating, as when I accelerate at highway speeds, I get a ping. The switch does not engage in the driveway (I do understand that there is no load to speak of in this configuration).

I have run a 50 ohm resistor at temp sensor 1, and unplugged temp sensor 2, and jacked the fuel pressure up to 35. I have tried timing it at -5’, 0’, and +5’. It is rich at idle, there is much carbon in the exhaust, and nothing feels hot. I think that I have too much vacuum in the plenum, that is not allowing the switch to move.

I had the same ping under full load issue with the 1600 (fluffed and buffed heads), but only when really trying to kick its ass. I am in the middle of trying something really dumb to make this work.

Just for the sake of continuity, the above mentioned engine now resides comfortably in my '70 westy. The limitation of the early full load system is my excuse for not running it in the squareback.