Dyno results, tuning help needed

[boogiedude]

Update!

I ended up ordering a new head gasket to replace the one that had fitment issues, and ended up receiving a "performance" gasket(the thicker, more heavily reinforced one) instead of the standard one i intended to order, but oh well. Got it back together and its come back to life! BUT thats where the relevence to my original post is going to end. Because....

I decided to switch my 38ADL for a single 45DCOE combined with a Lynx crossflow intake manifold that i had recently purchased, and dive into getting that setup working instead of fiddling with the 38.

And now we're back in with needing tuning help

The car runs with the DCOE, but not up to the level that I would expect from this large of a carb with such prestige.

The idle is slightly rough, and and will vary by up to 1500rpms sometimes(for example, I come to a stop and the idle will drop to a reasonable 900-1k and while remaining stationary it will climb up to ~2000 sometimes as high as 2500! the opposite will also happen sometimes, where i come to a stop and it will hang at 2000 and then eventually drop down), there also seems to be an issue with rpms not dropping very quickly between gears. With the 38, between shifts the rpms would drop down quickly enough so that when shifting up, and letting the clutch back out, the rpms would either be exactly where they needed to be to match road speed, or would be slightly below. With the 45, by the time i let the clutch out after shifting, the rpms will have only fallen a few hundred or so instead of the near 1000 that it would with the 38. And lastly, I seem to have lost power in this swap although that could be down to it needing some jetting alterations. It's most notable in the bottom end and when going up hills below 3.5k-4krpms

Here are the specs for the 45 currently:
32mm chokes
f16 e-tube
135 main
155 air
60f8 idle

Also regarding the cam timing that was brought into question earlier, It is set as close to spec as I can get by doing it my self. A dial indicator was used along with degree wheels to be as accurate as possible.

[131]

Why bother going to the extent of purchasing a manifold and 45 carb if you're only going to run 32mm chokes? They're the size used on a 1600 and they use 4 chokes!

[boogiedude]

That was the recommended choke size by pierce manifolds, I thought they were on the small size too but didnt 2nd guess the experts. What size should i be running?

[boogiedude]

That was the recommended choke size by pierce manifolds, I thought they were on the small size too but didnt 2nd guess the experts. What size should i be running?

[131]

There is a weber graph around that shows choke sizes for one choke per cylinder, taking into account cylinder size and rev range. I haven't seen a chart for 1 choke feeding 2 cylinders. What's the car going to be used for? 304 deg. cams are pretty wild for a daily driver given that the compression isn't that high?

[124ADDHE]

A single 45 with the autocross cam is a bad combination; you need a better exhaust and to keep that 38 OR get another 45 and manifold for the dual carbs, or just go the established route of the IDF's. I dont think that single 45 will have the flow that you want for those cams...and you definately need at least a 2" and up to a 2.25" exhaust, I think that is why your HP peaks @ 6000rpm.

[boogiedude]

There is a weber graph around that shows choke sizes for one choke per cylinder, taking into account cylinder size and rev range. I haven't seen a chart for 1 choke feeding 2 cylinders. What's the car going to be used for? 304 deg. cams are pretty wild for a daily driver given that the compression isn't that high?

I know i've seen the chart you've mentioned in a workshop manual before, just need to figure out which one it was in and go find it. But a quick search came up with an alleged rule of thumb regarding chokes which basically claimed that the choke and throat diameters should be no less than 7mm apart so that seems to suggest a 38mm choke at the largest for a 45, but no indication of how small it can be taken. Does that seem right?

The car is my daily driver, but will be running autocross once the engine is sorted, and rallycross and small track racing once the local track is finished with construstion.

Also, compression is somewhere in the very high 9's to low 10's, which is about as high as you can take it on pump gas to my understanding. Without adding xylene at every fill up to avoid pinging.

A single 45 with the autocross cam is a bad combination; you need a better exhaust and to keep that 38 OR get another 45 and manifold for the dual carbs, or just go the established route of the IDF's. I dont think that single 45 will have the flow that you want for those cams...and you definately need at least a 2" and up to a 2.25" exhaust, I think that is why your HP peaks @ 6000rpm.

The exhaust is coming along, money is tight when you're a 20y/o college student paying for everything yourself. I'm doing what I can for now in terms of tuning while I wait for the funds to accumulate for the header and exhaust system.

Could you explain the reason why the 45 wouldn't provide the flow i need, and why its a bad combo? I'm a novice when it comes to carburation theory but my naive logic says that a straight shot, direct fed carburator with larger throats would flow better than an upright more restricting carburator with a cumulative feed.

As for the HP numbers, while they are stil technically applicable since its the same engine, I feel that the peak and curve numbers are entirely different with this carb based on my "seat of the pants" feel.

[124ADDHE]

[fiatfactory]

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[fiatfactory]

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[boogiedude]

Lynx manifold isn't a straight shot, it's a crossover manifold. Cylinders 1 and 3 draw from 1 choke, while cylinders 2 and 4 draw from the other, to give a 180 degrees pulse, pause, pulse, pause for each choke...great in theory... BUT ...it ends up with a very convoluted flow path, where the lengths and restrictions are far from even across each port. Really it's for an improvement over stock but not for high revs, more for torque.

45 will have slow air speed, despite the small choke size, this added to the convoluted flow path, means you won't be getting a very strong signal at the primary diffuser at low revs...

Go back to the 38ADL, or go for a 40DCOE on the lynx....

Max and min choke sizes to suit a particular carb certainly isn't a one figure fits all (like 7mm) it's based on a %age of butterfly size... I'm not at home right now so I dont have the exact info handy, but I'll dig it out and post when I'm home in a few days time...but I can tell you that with 32 chokes in a 45 your running at about 70% ... from memory the ideal is around 80%, max is around 85ish% and min is around 75ish% ...basically if you need to fit chokes larger or smaller than these %ages to get the tuning right you need to rethink your carby sizing

"compression is somewhere in the very high 9's to low 10's"

Well the euro spec CC 1756 used 5.2mm domes pistons with quite a high compression height (can't recall off the top of my head right now) so if you have 4mm domes (and unknown CH) your going to be around mid 9's...post 1974 australian spec 1756 CC's used a 4mm dome and were just shy of 9.4:1


SteveC

At this point in time, I'm going to keep tuning with the 45. Why? Because I'm stubborn I do have a set of 40dcoes mounted on a manifold(sweptback) that I could take a single and swap onto the engine, but that would require a rebuild of the carb and jetting etc..

I had always heard that the 4mm domes gave ~9.8:1CR? And in the machining process, my block and head were both shaved and I think it was by .010 each. But don't quote me on that. So I had been assuming I was closer to 10:1.

From the Weber bible... the maximum recommended choke size should be between 82 and 85% of the carb butterfly size, any more than this and the power at lower engine revs will almost certainly drop. If the engine needs a larger choke size then it means a carb too small for the job is being used. The best atomisation and airflow combination occurs when the choke size is 80% of the butterfly size, and the best working range is from 72 to 80%

85% is a 34mm choke in a 40 carb, and 38mm for a 45
80% is a 32mm choke in a 40 carb, and 36mm for a 45
72% is a 28.8mm choke in a 40 carb, and 32.4 for a 45

Dave Vizard uses the exact same formula in his Pinto4, Leyland A series and SBC books ...there isn't much that he doesn't know and hasn't tested on a variety of engines.

SteveC

I'm planning to go back to the dyno in the next few weeks and doing a fair number of runs and trying a few different things.

45 with larger chokes, Experimenting with the cam timing, and depending on how those numbers look, I'll probably throw on the 40dcoe for a couple of pulls as well

[manoa matt]

Boogie,

Keeping that 45 and moving to bigger chokes is two steps in the wrong direction, stubborn..yes. Think about it this way: With the 38 you had good power and torque. When you switched to the 45 with 32mm chokes you experienced less power and torque in the lower RPM range. This is because as others have stated, the air speed through the carb is too slow, even with the 32mm chokes. In order to increase the air speed you would need to go even smaller on the choke tubes. Given that the throat diameter and choke diameter are no where near the optimal percentage it indicates that a 45 is too big for your engine and intake manifold.

A 40mm DCOE is only 2mm larger than the 38ADL in terms of throat diameter, so if the 38 worked, a 40 should too, and the 40 DCOE is infinitely more tune-able than a 38 ADL will ever be. Go with a 40mm DCOE with 32mm choke tubes and tune from there. If you decide keep that 45, buy bigger chokes, and make "a fair number of runs" you will just waste "a fair amount of money."

Keep the cam timing where its at, fooling with it will only make things worse. At this point in time your compression ratio is mearly a guess without actually knowing the very specifics of the piston dimensions, crushed head gasket thickness, and what was done to the head and block.

Dyno runs are for changing a jet or two, or incrementally adjusting distributor timing...in a systematic and planned method. Changing too many variables at once without knowing how one affects the other will have you chasing your tail and be a huge waste of time, money, and possibly lead to engine damage.

[baltobernie]

I've recently purchased and installed a wide-band O2 sensor to aid in my tuning efforts, and highly recommend this to engine builders using aftermarket or non-OE application Weber carburetors. The ability to visually see A/F changes as the carb goes in and out of transition from low to main circuit, and through various load conditions, is unavailable by any other means. Webers are so incredibly adjustable that there is virtually no chance that you can stumble upon the correct combination of jets, tubes, chokes and venturis for optimal street use. I should have spent dyno money on this gauge long ago; I think I would have been further along the road to somewhere if I had.

[WYSpider]

Balto, do you have a link to what you purchased?

[baltobernie]

It's an AEM "UEGO" unit - completely self-contained. They sell online for $200 new, $150 used. I plan on selling mine when (or is it "if"?) I get my 38DGS tuned. I attached a cigarette lighter plug to the +12 leads, taped the gauge and wires to the dash, exiting thru the vent window and under the hood. Bung in the collector of my stock 2/1 downtube. This gauge has an RS-232 output for data logging, but I use a pencil and notepad while driving

The "sniffer" that a chassis dyno uses is worthless below 2,000 RPM. A whole lot is happening at low RPM inside a Weber, and with big cams, there's not a lot of engine vacuum to help things along. There's no discernible difference in idle quality with 80/40 cams (let alone with wilder ones) between 12:1 and 14:1 mixture, but the "idle" circuit of a Weber affects everything.