Dyno testing

[Phil West]

Any thoughts on how best to cure rich condition between 2,800 and 4,000 rpm at WOT, looks to be the likely cause of drop in torque (and power) as indicated.  1956cc street motor C35 cam and twin 40 DRLA.  Mainjet fairly good at top end.

Cheers
Phil

[spanners]

Don't forget the idle jets have a big say in progression fuelling, that and too much pump jet fuel may be worth looking at, around 50 for both should be near,  be sure the pump circuit check valves are in good order, they can act up if the fuel has had water contamination at some stage, then they can dribble fuel and its hard to see it happening with the squirt hole on the underside of the nozzle, I've also had DRLA's in with the idle set up on the throttle stops/ linkage so badly that the first progression hole was already un covered, this caused mayhem to the idle/progression/mid range fuelling. VW's seem to love .2 emulsions, If the carbs came used from an Alfa they will likely have .3s, so have a look there also.

[Bruce]

What are your main and air jets?
Drop the main by 5 and increase the air by 20.

[modnrod]

Do you have a large-collector-volume 4into1 with a "Fatboy" muffler?

[Phil West]

40 Dells fitted with CB horizontal discharge update kit which comes with the special 36mm chokes.  So the carb idle air bleed is blocked off and uses their tall idle holders.

60 idle jet (1 2/3 turns out on the mixture screws give me 14.0:1 on the LM1 while idling)
120 idle air (see below)
185 main jet
165 tall air jet
42 pump jet (see next post)
9164.2 emulsion
Pump check valves all nice and free, I rebuilt these carbs fully.
1 5/8" merged exhaust with CSP dual mufflers
1.25 rockers on the C35 gave me 504 thou lift on the 40/35 valves in 044s.

Most of my driving is on the idle circuit which was not running that well before dynoing.  Various stumbles through the bottom end and was lean on cruise at the top of the idle circuit.  The main issue turned out to be the idle air.  The CB kit makes idle air tuning tricky but the dyno guy came up with a good solution, using DCOE jets that happen to fit perfectly snugly into the top of the tall idle air supplied with the CB kit.  Which meant we could try any idle air we wanted.  With the 120s the bottom end on part throttle is now really excellent, all general cruising and driving is much improved.  However we didn't get time to spend on the torque at WOT.

185 main seems larger than what I have been used to and 165 airjet seems smaller than what I am used to.  Again the 165 tall air is what is supplied with the kit.  I'm thinking maybe try a 180 main and a 185 air as per your suggestion Bruce?

[Phil West]

Pump jets:

The Dyno guy said that the DRLA is based on the IDF.  The IDF is based on the DCOE.  On the DOCE the pump jet fires ***all the time*** on WOT.  I did not know this.  Therefore the IDF and also the DRLA pump jet also fires all the time at WOT.  I have no test data to back that up.

I tuned the pump jets on my LM1 while driving, over a period of many years - but only on gear change when I came on and off the throttle.  With any sizeable jet I could see the fuel dump when I got back on the throttle - went horrid rich in the 10s for a fair few seconds every time.  Eventually I got down to the DHLA size 30 pump jet (or was it 33).  Also backed out the pump jet squirt time to the minimum.  Perfectly smooth still on gear change yet the tiniest amount of richness for only a split second.  Basically doing their job really well.  However during dyno he said to go up to the 42 (possibly to help the transition/upper idle if I recall correctly).  Ultimately we spent pretty much the whole time sorting the bottom end driving.  That was about all I could afford on that session.

However - I did not consider the impact of pump jets at WOT.  I'm not sure how the carb is operating there - there's only a certain amount of fuel available within the pump jet well but maybe it can draw fuel through constantly at WOT like on the DCOE?

We didn't do enough testing to see if those larger pumps are contributing to the rich WOT condition from 2800 to 4000.  Could be the case.

[spanners]

Hmm, Phil, Some huge sizes jets your running there, somethings way off, my circuit motors run 180x200 on 40 chokes X 60 idles and they are on the fat side for valve cooling reasons, and I run a huge 60 pump jet, a circuit only thing, given throttle action may be frantic for a lap or two, then just flat WOT for Many laps, but given you can only go 36 chokes in 40 dells, it should gas the workshop out on your sizes,, now I no naff all about how the CB kit would affect sizing, but normal thinking suggests even 160 mains would be plenty 'safe', as to the pumps fuelling 100%, not so, tho I have had cup race motors with centre mount dells do it occasionally, symptoms were no pump jet response out of high speed turns etc, we modded the jets and went to 'end squirts', the under slung squirts 'could' pull a vacuum under the jet, empty the well and leave no reserve for when needed, but it would not overcome the check valve, out of 30 odd cup motors out there, just a few front runners had that problem, and those motors tended to use bigger chokes than others and had plenum intakes and solid mounted motors, maybe even vibration was a factor, maybe others had it but never picked up on it, but I've never seen it with dual carbs, your thoughts welcome.

[Phil West]

Ok thanks for that.  I would say I don't have that same issue with pump jets that you saw, so I don't think that would be the cause of the torque dip on the graph.  However I might try going down a bit on the pumps and see how idle circuit driving is.  Also a step down on the mains, they do seem big although I do tow with it and find the extra load requires a step up in mains but surely a 180 would be enough.  Still an odd shape to my curve, pretty sure I've seen similar shape before on other VW dyno printouts although not as pronounced.  Idle circuit jetting just fine but it's that low to middle mainjet circuit I need to work on.

[richie]

Phil

do you have stacks on those carbs? if so how close to the filter tops are they?


cheers Richie

[dannyboy]

Efi 

[Phil West]

Phil

do you have stacks on those carbs? if so how close to the filter tops are they?


cheers Richie

Yes I have fitted some full radius stacks, they are a tight fit front to back on the air filters (had to bulge the filters a bit there).  So they almost meet over the jet stacks.  Anyway there is a height clearance of 37mm to the filter tops

[Torben Alstrup]

Hello.
Your Dyno guy needs to read up on the varius carbs and how they work.
The Dells DO NOT squirt all the time at WOT from the acc pumps. Nor does the IDF´s. That is ONLY for accelleration in the lower to mid rpm area. .33 is normally right for 40 Dells.

Next. You are overlifting the valves. ,504" lift with the C35 is something like 1,35 ratio. That´s pushing it. It may be good for peak power, but it slows down the incoming mix in the lower midrange. It also makes the exhaust side too efficient, so you experience some overscavenging, which again results in some odd jetting to try to overcome the rich condition that is developed.
Also your intake length is not exactly perfect. A little longer would have been good. But you can get around that with jetting.

And last. you ar not the first, and you will most likely not be the last. BUT. The update kit does nothing good to the carbs and or mix if the engine doesnt need the extra air. If you were in the 140 -155ish hp area where the carbs are getting close to be maxed out THEN the kit will help with a little extra power.

My best advise to you is to dry your eyes and eat your cookie, and get out in the garage and build the carbs back to normal (You can leave the jet doctors in. They are allright) An engine like that should be in the 32 - maybe 34 mm venturi size range. With the 009 dizzy I´m tempted to stay on 32 mm. If you had an SVDA it would handle 34 mm no problem. - I doubt it will pull more than 130 hp anyway, so there is not really any requirement for larger than 32 mm. BUT, sometimes the power delivery gets more steady and even with a venturi that is on the large side. I´ve seen that a few times, especially with cams that are advanced a lot, like for instance Engle W cams
Did you tell us which muffler system you are using? That may be able to tell us more about what may be teasing you.

T

[Phil West]

Hi Torben,

Exhaust is a 1 5/8" merged header and the CSP stainless duals:

http://www.csp-shop.com/cgi-bin/cshop2/front/shop_main.cgi?func=det&wkid=10727189250&rub1=Exhaust%20%2F%20Heating&rub2=Exhaust%20System%2DModified%2CMufflers%2CStainless%20Steel&artnr=23345b&pn=0&sort=0&all=

Heads are 044s, have very nice port and chamber work, they are quality so flow nicely.  Ports are not big.  I ended up with 8.27:1 compression.  The Scat 1.25 rockers worked out at 1.34, a bit more than I had planned.  I'm doing a full rebuild this winter for various reasons so I will have the opportunity to make changes.  To be fair I am mainly concerned with low end power around the 3,000 region, on heavy loads.  I'm interested in any recommendations you might have for cam/rocker ratio.  009 has the Ignitor 3 package, and 29 degrees all in.

I fitted the update kit many years ago.  I will admit to being seduced by the Trends article although I have no test data before and after.  As you say, not the first or last person!

This was the article:
https://www.cbperformance.com/Articles.asp?ID=297

Here's a pic of their graph.  Again I don't have my own test data so it was a gamble.  That motor spec wasn't a million miles away from mine.  Anyway, as you say the jet doctor is good, plus I do like that I can tune the idle air now.  So it may be time to go back to trying vertical discharge and stock venturis.  I was out testing yesterday and it still does have a small bit of hesitation down low, I think from those bigger venturis gulping in air when I open the throttle.

Cheers
Phil

[Torben Alstrup]

Hello.
Yes I saw that too back then. I have never been even close to duplicate those numbers on mild engines. I think the figures have been, - shall we say commercially enchanced 
OK with the header. That explains a good deal. Your exhaust side is way too efficient, and the large collector makes it even harder to jet right in the lower to midrange rpm.
What you can do here and now to improve your lower to midrange torque and easyness of jetting is to replace the Scat rockers with stock rockers on the exhaust side. That will balance the I/E ratio to some extend and keep the engine from letting unburned fuel out of the exhaust during the intake stroke. To put things in perspective we can put it this way: You have an exhaust that is capable of 180-190 hp and you are imost likely in the 120 to 125 hp range. See.

By doing the split lift trick you may actually be able to make the carbs deliver a better fuel pattern as is and you will most likely also see an improvement  in fuel economy.

T

PS. Let us know what happens, right.

[Phil West]

Ok thanks for the tip, it's not something I've experimented with before.  Will put on a 1.1 on the exhausts.  Once I've done the rocker shaft end float and swapped the carbs back to stock chokes I will get out and test.  Will let you know.

Cheers
Phil

[Torben Alstrup]

Any progress ?

T

[Phil West]

Hi Torben,

Just had a race meet so have been on the racecar the whole time.  Had to tow with the bus as it was.  Now I can get in there with the first job and get the carbs off to swap the chokes.

Still on the lean side at low RPM (i.e. most of the time), those big chokes aren't helping.  Will get back to you

Cheers
Phil

[Phil West]

Hi Torben,

I've swapped the carbs back to good old fashioned vertical discharge, I've put in some 32 chokes although I do also have 33 and 34 to maybe try in future.

As for the rockers - not done yet - the 1.1s will give me 90 thou less lift than the 1.25s.  Ex lift will be 414 thou with inlet lift 504 thou (assuming the 1.1s do work out at 1.1 exactly).  This will affect my rocker geometry - ideally I'd like to get the motor on the stand, I'm not sure if I will need to cut some new pushrods for the exhausts to correct this difference.  In which case I may have to wait until winter when I was going to pull and rebuild the motor.

Or shall I just install them and use for this year?

Cheers
Phil

[Torben Alstrup]

I would see how it looks with the pushrods you have. You may be able to adjust your way out of it and still be in the OK zone.

T

[max Der Bahnstormerz]

I'd try a 55 or 58 idle. Remember when playing with these things only change one thing at a time! Otherwise you won't see the full effect of the change