Winter wonderings: DCOE40s on an AQ171 16v.

Ok, I'm going bonkers and I know if it wasn't winter here in Canadia, I could easily answer this for myself by finishing the swap and driving the car. Since I can't do that for another couple of months...I have a question for the ether: The stock AQ171 16v carb setup was dual single-barrel 44mm carbs with 32mm chokes. I have a set of DCOE40s, with a couple sizes of chokes available to me (34 and 36), and I'm really wondering if I could be happy without going to DCOE 45s.

I've read so much about 36mm chokes not being really great in a 40mm carb, because of the lack of airspeed/turbulence/mystic science of the choke being very close in diameter to the internal diameter of the carb, which will allegedly affect driveability. I am very much a 'mid-range' driver, and have no real interest in maximum power. I am going with the 2.5L for the midrange torque that larger displacement engines provide, and with a maximum HP figure coming in at 5700 rpm (PZ penta cams), I don't need to rev to the moon.

What I am trying to figure out is if the stock setup used two singles choked down from 44 to 32, could I get away with the DCOE40s with 34mm chokes, or is there anyone out there who's run 36s in theirs and haven't found any real issues with driveability?

I hate winter.

I'm far from an expert on DCOE's, but I've been running DCOE 40's on my 2.1L B20 in the PV for a long time now. And what people who I think would know say is... even I need more carb (it's making about 160-ish whp). And I'd possibly/probably be making some more with 45's.

I am running 36mm chokes in mine, and it works fine. But I will say that I tried them earlier on in my motor's slow 'evolution' when it still had a stock F head and it did NOT work well, it was a bit too much for the motor at that point, and it was boggy on throttle open, you could get it to run OK if you 'snuck up' on full throttle. So I swapped the 32's back in until I piled some more mods on it (R-Sport head, 'bigger' cam, etc) and tried the 36's then, and they worked great. So there's no real issue with 36's in 40's, as long as your motor needs that much air flow. Which a DOHC 2.5L will CERTAINLY provide.

I would really suspect you'd be leaving some HP on the table, though. Just not enough carb for that much motor.

TBH, the point of view of someone who is actually using 36mm chokes in a DCOE40 is more helpful than all the charts and gobbledegook on the net, so thank you. I think the listed power level (~165 ish) was done through the two little factory carbs with 32mm chokes, so as long as the dual 40s don't leave too much behind vs. that rating, I'll be happy. The boat cams should provide all the mid-range torque I'm after, hopefully. Maybe one day I'll grab some 45s, but the genuine 152Gs are not as cheap as they used to be, so it might be a while. I've tried the chinese 45s...and no amount of modification could make them work well (porous castings, leaks, etc) so I'm very averse to going with anything other than genuine.

Thanks again for the help .

If you already have the 40's, I'd go ahead and stick them on and enjoy the noises and the throttle response.

That's always been my stance on the PV, I stumbled across the set of 40's really cheaply on eBay. Something like $300 shipped with the Volvo manifold and linkages, just missing some jets which wasn't a problem because they were set up to run a stock-ish B18.

So it runs great, sounds great, is all sorts of fun, why spend a bundle on swapping to some bigger carbs? And go through the hassle ot rejetting/tuning them again? Nah....

Same thoughts. All the sidedrafts I've ever run have been jetted for fun midrange with a decent cruise. Usually I take the 'appropriate' size chokes and downsize one, and just burble my way on the road to happiness. I was all ready to use the 36s in my 40s, but then of course...winter brain kicked in. I can't do ANYTHING car related except sit and think, so overthinking is usually always sitting in the wings waiting. I'm still months away from being able to take the B21 out, and the parts are starting to pile up so my impatience is rising, lol.

Broke4speed said:

I can't do ANYTHING car related except sit and think, so overthinking is usually always sitting in the wings waiting. I'm still months away from being able to take the B21 out, and the parts are starting to pile up so my impatience is rising, lol.

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It's going around. Except in my case it's 3 projects plus 2 daily drivers that need some work. (and too much to do at school) I'm slowly shaking off the Paralysis By Analysis. Maybe next year I can work on some motorcycles.

I agree, slap those 40s on and have fun.

And if at some point you stumble across a nice pair of 45's, snag them. And if not, 40's are just peachy.

PS: I do keep thinking about sticking my 16V B230 in my PV, perhaps with a pair of DCOE's to keep the old school flavor going.

I am perfectly happy with the B21/D-cam I have currently...but the old-school DOHC look won me over immediately. Exposed cam gears, timing belt, yum .

I just wish the chinese DCOE clones weren't so bad, haha.

copied from this site:
http://www.s262612653.websitehome.co.uk/DVAndrews/webers.htm

personally i'd be wanting to have 45's at least, But with the stockish cams you could get away with 40's and 36mm venturi's.
Why not bite the bullet and get yourselve 45's or maybe even 50's so you'd be able to swap cams some day and make some proper power with that awesome cylinderhead. if that's not going to happen, 45's will be the best compromise.

Our shop runs a LOT of vintage cars on DCOEs, and I'd want a set of 50s on that, at least. On the 8-valve Datsun motors we run we put Mikuni PHH40s on the 1600s (150ish hp), and 44s on the 2 liter cars (around 215). On the Lotus 1600s, we use 2 45 DCOEs, and make about 190hp out of 1600cc. Honestly when you look at carbureted FIA 2 Liter cars (2000cc, 16-valve head), they usually have 55s, and even then they aren't as fast as the MFI cars, which are easily over 300 hp. These are obviously all high CR race motors, but they are also all a lot smaller than 2.3 liters...

Maybe get a set of 45s and machine them for the bigger chokes if you're on a budget? Or find a Corvette vintage racer and see if they have some 50 or 55 cores for cheap?

msweeney said:

Our shop runs a LOT of vintage cars on DCOEs, and I'd want a set of 50s on that, at least. On the 8-valve Datsun motors we run we put Mikuni PHH40s on the 1600s (150ish hp), and 44s on the 2 liter cars (around 215). On the Lotus 1600s, we use 2 45 DCOEs, and make about 190hp out of 1600cc. Honestly when you look at carbureted FIA 2 Liter cars (2000cc, 16-valve head), they usually have 55s, and even then they aren't as fast as the MFI cars, which are easily over 300 hp. These are obviously all high CR race motors, but they are also all a lot smaller than 2.3 liters...

Maybe get a set of 45s and machine them for the bigger chokes if you're on a budget? Or find a Corvette vintage racer and see if they have some 50 or 55 cores for cheap?

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Woah, is this thee actual Mike Sweeney?

My experience with DCOE's on an early B series engine is consistent with John. I currently run 34s in my 45 DCOEs on my mild big bore 2150CC engine and throttle response is crisp but ultimate RPM is noticeably limited. 36s provided more RPM and stronger mid and top but you needed to roll into it. Too much cam can really exacerbate intake vacuum pull as well which will especially effect low end response. Always consider the system as a whole and then consider how that system will effect each system of the DCOE's, lots of variables at play.

R32RennSport said:

Woah, is this thee actual Mike Sweeney?

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Haha, hey dude. I needed the board to remember what I had done to my car last time i worked on it, so here I am, lol. That Amazon has come a long way, dude!

Anyways, I'll say something on topic to keep from cluttering up OP's thread...

I should mention that all my data is going to be off of engines most here consider pretty radical, but I still think most of that is going to show up in jetting and chokes. That 16v needs to breath to be worth the work of having on there! On similar motors we usually see peak power somewhere north of 6500 rpm, running out to at least 8200.

Left internally stock, even a set of 40s would sound and look really great, but you'd be spending thousands for a few horsepower and several headaches. That may be totally worth it, I had 2x 40 DCOEs on my 1600cc 510 in college! It made like 10 more horsepower than stock, was a tiny bit hard to start when cold, ran like **** below 3k, and I loved every second of it.

SkeTchy-MechAniC said:

copied from this site:
http://www.s262612653.websitehome.co.uk/DVAndrews/webers.htm

personally i'd be wanting to have 45's at least, But with the stockish cams you could get away with 40's and 36mm venturi's.
Why not bite the bullet and get yourselve 45's or maybe even 50's so you'd be able to swap cams some day and make some proper power with that awesome cylinderhead. if that's not going to happen, 45's will be the best compromise.

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Funds, that's the limitation. I have a bunch of 40s, but have never had a need for 45s and can't justify spending the money currently. I have no need to extract maximum power out of this car, but I also don't want it to run like poop, which is why I asked if my 40/36 combo might work-ish. If it's truly horrible, then I suppose I will have no choice but I am going to try the smaller ones first and see.

msweeney said:

Haha, hey dude. I needed the board to remember what I had done to my car last time i worked on it, so here I am, lol. That Amazon has come a long way, dude!

Anyways, I'll say something on topic to keep from cluttering up OP's thread...

I should mention that all my data is going to be off of engines most here consider pretty radical, but I still think most of that is going to show up in jetting and chokes. That 16v needs to breath to be worth the work of having on there! On similar motors we usually see peak power somewhere north of 6500 rpm, running out to at least 8200.

Left internally stock, even a set of 40s would sound and look really great, but you'd be spending thousands for a few horsepower and several headaches. That may be totally worth it, I had 2x 40 DCOEs on my 1600cc 510 in college! It made like 10 more horsepower than stock, was a tiny bit hard to start when cold, ran like **** below 3k, and I loved every second of it.

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That's the beauty of it...I got two AQ171s for $500 CAD, and sold the spare head for $500. I'm revenue neutral, engine wise. The 40s currently live on my B21, and I've got a fairly complete jet kit already, and a bunch of chokes, so it's not really going to hurt me to try. It's definitely not for power, just a bit more oomph in the midrange. It'll probably hit WOT near redline once per year, lol.

I am not blind to the ridiculousness of 40s on a heavy-breathing 2.5 16v...but it's my toy car so I have to at least try .

Broke4speed said:

That's the beauty of it...I got two AQ171s for $500 CAD, and sold the spare head for $500. I'm revenue neutral, engine wise. The 40s currently live on my B21, and I've got a fairly complete jet kit already, and a bunch of chokes, so it's not really going to hurt me to try. It's definitely not for power, just a bit more oomph in the midrange. It'll probably hit WOT near redline once per year, lol.

I am not blind to the ridiculousness of 40s on a heavy-breathing 2.5 16v...but it's my toy car so I have to at least try .

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Then order a manifold and make the world a better sounding place already!

Per that chart, my 2.1L B20 should be about out of puff with the 36mm chokes at 6000 rpm, but it pulls nice and hard up to 7500 rpm, I do that pretty frequently, and the power isn't really dropping off much. Even if it's a little hard on the cast B21A pistons.

Although I guess that is really saying I'm losing some HP above roughly 6000 rpm? Even if it seem to have plenty, it could have more?

msweeney said:

Then order a manifold and make the world a better sounding place already!

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Way ahead of ya



Just a few more things to finish and then I can swap the DCOEs over from my B21...and when it's not -20*C out, of course .

JohnMc said:

Per that chart, my 2.1L B20 should be about out of puff with the 36mm chokes at 6000 rpm, but it pulls nice and hard up to 7500 rpm, I do that pretty frequently, and the power isn't really dropping off much. Even if it's a little hard on the cast B21A pistons.

Although I guess that is really saying I'm losing some HP above roughly 6000 rpm? Even if it seem to have plenty, it could have more?

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I've got soooooooooo many weber/dellorto tuning documents/books and I find the info is...good...but it is always decades old. That airflow chart that seems to be everywhere is definitely from a time that didn't include modern head design, wideband O2 sensors, etc. I find that a lot of the documented setting lists can always be improved. Many of the DCOEs of the early days before fuel injection had fewer progression holes than the modern versions, which meant the idle circuits were jetted crazy fat (and gas was CHEAP), etc.

I've always run a choke one size smaller than what conventional knowledge suggests, and have had a lot of fun. The top end may be limited...but the sound is definitely still as lovely .