Great info. ..any other specs on the venture sizes? 38 ,42...? Thanks

Hi,
Great build!
I'm curious why you stayed away from twin plugs and 40mm carbs. All that I've read seems to point to both being better for torque.
I do get the 46's if you're on a track most of the time ;)

I'm planning a similar build with 40mm PMO's, SSI's and twin plugs @ 10.5/1 CR
I'm wondering how different the numbers would be.

That little car must be a blast to drive...

...and what a little sleeper :)

oh yeah, Venturis... mine are 38mm I completely forgot to even look at which Etubes I have in there too. This is all kind of my point... there is a ton more to play with in there. Now I'm all excited to try more.

78scrsman- I didn't really shy away from twin plugs, just didn't feel like dealing with it at the time and didn't feel like spending the money. I have a VERY well built set of heads on my car. It has a lot to do with why it makes as much power as it does and why my 3.0l did when these heads were on there too. Truth is, I haven't seen any dyno numbers on a similar build to mine WITH twin plugs that made me want to get them. I stayed as is because of money at the time and would have been happy with whatever the motor wound up becoming. I had no idea it would become this.

Maybe when everyone on this thread with twin plug clones of my engine changes their jetting I'll think otherwise but for now, I just don't see the point. Making great power and didn't drop a ton of cash for the twin dizzy, machine work, more plugs, more wires, another coil, and another MSD.

Between that and the huge differences in what I saw with changing jetting I am beginning to realize the things we read here in PP world are not always the law and are usually just starting points hugely based on the individual motors. Actually... SOLELY based on individual motors.


40mm PMOs better for torque with little SSIs? Better than what exactly? Stock? sure-of course. Not discounting you, but just trying to think a little deeper into it. I've got around 260tq at the crank and the power comes on in the low 3s... Plus if I leave my phone on the dash and punch it, it can almost hit the back window. I like to think I have enough torque. :-)

Nice numbers for your 3.2SS for sure. Also goes to show what a difference fine tuning the jetting will indeed accomplish! That said, going from a 155 to a 170 main fuel jet is a BIG jump. Also something to keep in mind is a main jet step change is generally like making a 3 step air corrector change- in the opposite direction. So your main fuel jet change going from 155 to 170 is like changing your 185 air corrector to a 140! What that means is that for fine tuning, the air corrector has a smaller effect if you want to further fine-tune things.

Twin plugging is only a must-do when you venture beyond the 9.5:1 compression ratio so you can better control the ignition of the air fuel charge. Basically the higher compression can mess with the portion of the air-fuel charge opposite the plug, when the piston comes up and "blocks" that side of the combustion chamber from the single spark plug. If you can get away with the single ignition, there's nothing wrong with that. Just have to be sure you're getting good octane fuel so as to not have air-fuel ignition occur prematurely.

Good on you for sharing, as it helps a lot of people recognize where they can improve their setup. Richard is an incredible resource but there's only so much he can do over the phone to get you dialed in. Altitude, fuel quality, timing, proper carb operation and various other things make it nearly impossible to get you started off spot on with the jetting. You absolutely need to be willing to tune on the carbs to get your settings optimized.

I feel the key is having an air fuel ratio measure to get you within the ballpark before you go to the dyno. A wideband air fuel ratio gauge goes a long way toward providing you that info you need to refine the tune. Then when you get on the dyno you can be ahead of the game so to speak by having your jetting more closely dialed in when you're seeing what's going on via the dyno graphs. Pre-tuning will let you make an informed decision on what jets you or the shop need to have in hand to make your adjustments most effective.

Emulsion tube is the piece that brings the air bleed/correction & main fuel all together and sends it on to the secondary venturis to feed the carb throttles. So having the right one is indeed key. By the looks of your graph, you've got a good emulsion tube because your AFR is steady from the get-go when you give it full throttle. Sharing which one you have in there would indeed help others know what's a good choice for an engine with the type of tune/use you have.

One area I bet you could further improve your engine's performance down low is an adjustment to the amount of accel pump fuel delivery you're getting. See that big dip in the air-fuel graph at 3K rpm? That's when you mash the gas on the dyno and the accelerator pump squirts more fuel into the carb throats. I bet if you adjust the accel pump linkage to reduce your accel pump delivery volume, you can flatten out that curve and not go so rich when you push the loud pedal wide open.

I have a 3.0L twin plug i'm building that will be an interesting comparison when I get it on the dyno. I have to dyno it anyway to certify my race class designation. From the top down:

>PMO 46 w/40mm chokes and we'll see what jetting PMO starts me with
>46/40 "tall" manifolds
>SC 78-79 large port heads port matched to manifolds, EBS valve spring kit w/Ti retainers
>Electromotive XDi twin ignition
>WebCam 120/104 cams
>10.5:1 JE pistons in Mahle 95mm cyls
>Pauter rods
>SC crank cross drilled and lightened (slight knifing of various counterweights)
>MSDS 1-3/4" headers & Y-pipe merged single exit

It has to pull to 7500+ rpm so I don't run out of gears at long racetracks like Road America. I'm actually hoping for no more than 230hp at the wheels or else I have to throw weight on the car to stay in my class.

Kevin,
Your posts are always informative (and I'm pretty sure I have something from you on my car, lightened flywheel maybe).

Can you elaborate on this -
I bet if you adjust the accel pump linkage to reduce your accel pump delivery volume, you can flatten out that curve and not go so rich when you push the loud pedal wide open. What are your plans for linkage going from CIS to.... well, not CIS?

I haven't tackled this yet (haven't even looked at it) on my conversion.

Todd

Great post Kevin. I have never seen an twin plug explanation like that. Really helped me understand.

You are dead on with the delivery volume. That was something we were discussing after the fact as well.

I need to go out and check one of my venturis for the sake of the whole setup.

It's funny, I spent 8 years in Spec Miata. Testing and tuning and testing some more, all kinds of things on the dyno for that extra 2-3hp. Every successful idea opens up new possibilities. This is starting to feel the same but without all the secretive stuff. I can actually tell people what I discovered because I don't have to worry about them driving past me on the back straight. It's nice not to care about that stuff once in a while. Makes it fun. Like an arms race.

Hey Todd, are you going down that throttle linkage rabbit hole we were discussing offline?

He was referring to the accel PUMP linkage, not the accel PEDAL linkage. The amount of squirt. If not I totally missed the point.

The linkage is the next (and last) hurdle. Unfortunately I've been home a total of about 6 days since the new year :-(

So, I'm still drawing a blank on the accell PUMP linkage. Pump turns on, supplies a constant PSI, carbs swallow the gas. (it's been a long day ;-) ).

Todd

take off air filters. Look down barrels. push down throttle rod. see 3 squirters squirt in fuel. The doodad that makes that squirt is the accel pump. Square-ish boxes on the sides of the carbs by the cross linkage mounts. Has a diaphragm and spring in it. Threaded rod on there too.

Squirt make car go vroom-vroom. :D

throw a vial in and check the squirt volume, then adjust the rod for more efficient squirt. can't remember the volume range off the top of my head but going to add this to the list for my next dyno day.

I am determined to get another 10hp through serious tuning vs. spending. Could probably get it by going open exhaust but that is too easy. I'm convinced there is some kind of magic sweet-spot combo with acceptable trade-offs in there somewhere.

I have no actual reason to do all this other than to see if I can. Once it's all done I can proudly say... well, nothing. It might just prove I'm insane and I should keep it to myself.

Mazing3. Thanks for sharing your carb settings. Once things warm up in the frozen north I'll use them as a reference point while continuing my carburation tuning quest.

Robbie, I got it going, here is my thread, it's probably something you will want to do:

http://forums.pelicanparts.com/porsche-911-technical-forum/795056-msd-6al-2-digital-programable-install-distributor-lockout.html#post7891696

Looks great, congrats!

Glad to help out. These types of threads are really helpful for people trying to dial things in. Collectively the group throws their 2 cents of experiences in the discussion to create a pool of jetting info that others can use to theoretically get their setup dialed in closer right from the beginning.

Robbie is correct about the accel pump description. Here's a tip direct from PMO on that accel pump injection adjustment- see item #3 Bulletins

So what PMO is saying is that there's a discharge valve fitting that controls how much fuel is sucked in by the accel. pump assembly. You can change the accel pump suckage by selecting how much that valve dumps back into the float bowl. You can also adjust how much fuel the accel. pump squirts by adjusting the length of the tiny linkage rod under each trio of carbs.

Pegasus has a good listing of parts that fit the PMO (which uses the Weber 48 IDF jetting system & other various parts) and they have a very good description of what that valve does

https://www.pegasusautoracing.com/group.asp?GroupID=WEBERIDFPJ

The accelerator pump dischage valve (also called the intake and exhaust valve) bleeds off some of the accelerator pump output back into the float bowl. This controls how much fuel is actually delivered to the pump jet. Once you have selected your accelerator pump jet size, you can use this valve to fine-tune the output of the pump. If the accelerator pump jet is a little too large, one of these valves can get you to a setting "in between" jet numbers.

Discharge valve sizing, similar to Weber jet sizing, refers to the bleed hole size in .01mm increments. The 40 discharge valve bleeds fuel back into the float bowl as if it had a 0.40mm hole. The 100 size bleeds like a 1.00mm diameter hole, delivering the least fuel to the jet. (The 100 size makes a 50 size accelerator pump jet behave more like a 46 size.) The 000 size has no bleed hole, so it forces all of the accelerator pump volume to the accelerator pump jet. The closer your accelerator pump jet is to being too large, the larger the discharge valve required. The closer the jet is to correct, the smaller the valve required.

This valve is used in the DCOE and IDF series, as well as both the 48 IDA (Dual) and the 40/46 IDA (Triple) carburetors.

The discharge valve is a little fitting on the bottom of one of the two float bowls serving the carb trio. So in the case of our dual triple carb PMOs, there are two of these valves you would replace if you choose to adjust the accel pump output that way. The pump jet that Pegasus refers to is the little nozzle squirter that points into the carb throat. We typically don't change those, so consider that jet a fixed size. Just messing with the tiny linkage rod and/or discharge valve is how we typically tune the accel. pump delivery volume.

Regarding the throttle linkage, Steve Weiner has told us that the original CIS or Motronic bellcrank on the trans is OK to use for carbs. The key is we have to shorten the rod coming from the gas pedal to the trans bellcrank.

Also while you're messing with the throttle linkage, it's a good idea to look at the clear plastic bushings that carry that long throttle rod from the gas pedal to the bellcrank. They're really old by now and probably cracking apart (if gear oil gets in there, they're toast) which creates a lot of slop in the rod travel. Part number is 901.423.227.00 and you need three. Also take a look at the bushings in the bellcrank. Those are usuallly junk by now. Canton makes them in bronze & Pelican sells them. OEM part number is 914.423.211.00, quantity = two

Here's some good pictures of the carbs (Weber, not PMO but very similar ) to give you an idea of what the parts inside these crazy things actually do. I'm constantly amazed how the carb functions solely on air pulling fuel thru passages via the venturi effect.

Here's the accel pump assembly and note #36 is the discharge valve (called suction valve in picture) #25 is the pump jet and #33 is the infamous accel. pump linkage we can adjust to vary the pump output

http://forums.pelicanparts.com/uploa...1391533535.jpg

Here's the accel. pump assembly taken from the real deal Italian Weber Service Manual

http://forums.pelicanparts.com/uploa...1391533978.jpg

Here's how the carb works in general from a side view perspective. Cool stuff. Yeah, i'm a carb nerd.....

http://forums.pelicanparts.com/uploa...1391534176.jpg
http://forums.pelicanparts.com/uploa...1391534184.jpg

Something worth noting about the last two pictures. See how both the idle circuit (#19 thru 24) feeds off the #12 emulsion tube well (#12) as does the #10 main fuel jet? That demonstrates how the idle circuit pulls fuel from the same point as the main fuel jet but its just fuel. It's sort of "stealing" from the main jet circuit because the main jet doesn't come into play until the air velocity is high enough to activate/pull air & fuel mix thru the air corrector + emulsion tube combo (#3 plus #12) via the secondary venturi/pre-atomizer (#1 with #2 inside it).

Just goes to show how the "idle" tuning via the idle jet and the mix screw is critical in getting the engine to build rpms in order for the main circuit to come on-line and feed the high rpm demands. When the main circuit comes on fully, it basically shuts off the idle circuit because the supply of fuel from the emulsion tube well is supplying all the air-fuel mix to the main carb throat via the secondary venturi. The idle passage is much smaller and can no longer keep drawing fuel from the emulsion tube well so its basically "off." The main circuit coming on-line and the idle circuit turning off is what's typically referred to as "transition."

Kevin, that is just an amazing education. I've never really known the right questions to ask to get an explanation like you gave. For the 100th time in my 5 years on our forum... I want to say thank you.

Subscribed!

Kev, one more thing... "(which uses the Weber 48 IDF jetting system & other various parts)" ----So are you saying we can use parts for the 48IDF Webers, jets etc..., for our PMOs? Nothing against our usual source and process for getting these things but I don't always have the time an patience for that process. Being able to see the variety available in front of me is always helpful as I don't always even know what I am looking for and what the options really are.

Hope that made sense. Was trying to be a little intentionally vague because the usual source has been very, very, helpful but not always ideal in the way I like to buy things as I am still learning as I go.

Robbie-
Thanks for posting your story, sounds like a great build. I'm curious, did you ever find out exactly why you lost a rod bearing?

That became my single biggest question for months after that happened. The engine only had 1200 miles on it at the time and was otherwise running very strong.

This happened at the track and this was the first time id had this car out there. Saw temps getting weird on the 3rd session and 10 mins into the 4th I head the knocking and the power seemed off so I brought it in.
I mention all that because it led me to believe whatever happened, happened pretty quick and didn't just deteriorate over a long time.

I had mentioned that a supposed local expert had built that part for me back then but everything else he had touched along the way had been crap in one form or another. Just so we are clear, this WAS NOT Jack Lewis.

When I got the engine apart down the the crank all the rod bolts were still in place and tight and nothing looked bent so I started removing the rods and think I found the problem...

I can't remember tq. Specs for oem rod bolts off the top of my head but think it's 55ftlbs. Even if that is correct, they were a hell of a lot tighter than that when I tried to remove them. It took a breaker bar and some serious effort to loosen them up to remove. WAY more than it should have if they were torqued properly. After struggling to remove a few I tested one and it was torqued to 85lbs!! And that was one of the easy ones. I know there was a lot of heat on that crank during this meltdown and if just one or two were extreme I might have chalked it up to that. But all 12 were that bad or worse.

I never thought more about it after that but I'm of the opinion the dufus "expert" either didn't use a tq wrench or grossly over torqued them. My thinking was by doing this he pushed the intended friction load of the bearings way past normal to a point where they were eating material and eventually wore through.

The other thought was that he just did a crap job preparing everything prior to assembly and they might have gotten scratched or contaminated in some way leading to the failure.

When I rebuilt it all myself I spent days cleaning the case and treated it like a sterile environment every step of the way. 2400 miles on it so far and not even a whisper of a problem down there.

Granted, I replaced the crank, rods, bearings of course, and used ARP rod bolts this time. Even used a new case. But at this point I'm still pretty convinced everything was just detrimentally over torqued.

Could be something other than that but there wasn't anything else going on to indicate some other issue.