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Throttle response - 1 vs 2 throttle bodies

Michael Moore

Tuned and Synch'ed
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I was looking at some of the posts on the newer (and new to me) EFI bikes and noticed that some are running two throttle bodies up near the head, while others are running a single throttle body (controlling air to both cylinders) located under the seat.

I'm curious if there's any significant difference in throttle response between the two methods. With injectors at the head/port there should be no concerns about fuel drop out with a long and curvaceous inlet runner between the head and single TB (which can be a problem in some automotive applications).

Does the single TB free up an appreciable amount of knee/shin space vs having dual TBs at the head? The smaller TBs look pretty compact, certainly much more so than most carbs.

I've got a new set of FCR41s for my 1000SP project. I bought those a long time ago when everything needed to be built like I was going racing, and foot pegs/bars would be set to that style to clear the big carbs. I'm now planning on a more original sport touring position (but still with a modified motor) and I've been thinking that converting to EFI could free up some space for my knees/shins. A single large TB might help a little bit more with that, but I would not want to affect drivability if it results in something like "turbo lag".

thanks for the info,
Michael
 

DeadEye

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From my arm chair, I would guess that as long as the volume of air is the same as or greater through the single as it was through the smaller doubles It shouldn’t cause any issues.

Weren’t long runners supposed to be good for something ?

Same pressure wave principle applies on the exhaust side...

From googling :
Intake manifold runner length is the linear distance from the Inlet port (the face of the head) to a common point shared by all cylinders. Depending on the throttle position this could be the open atmosphere (if each cylinder has its own throttle butterfly) or a plenum (if they share a throttle body).

The intake runner volume is the volume of that section of the inlet system, ie the cross sectional area multiplied by the length.

The very short answer to the final question is that short, wide inlet runners move the torque curve up the rev range whereas long, narrow runners move the curve down the rev range. BUT - there is a whole lot more to it than that. The effect is caused by 'pulse waves', waves of relatively high and low pressure in the runner.

The opening of an inlet valve causes low pressure at the engine end of the runner, as the engine sucks air out of the runner. Air starts to flow down the runner into the cylinder until the valve shuts, at which point all that air (travelling at high speed) crashes into the shut valve and creates a relatively high pressure 'slug' of air. This is reflected and starts to move back up the runner until the valve opens again, at which point it heads back towards the port.

If the dimensions of the runner are calculated well, the high pressure 'slug' or 'pressure wave' will make its way through the port before the valve shuts again, when the next 'slug' is created. The dimensions are critical to the operating speed of the engine. If the runner is short, the reflected pulse might fall out of the end before the valve opens to suck it back in. If the runner is too long, the larger mass of air may react too slowly to perform well at high engine speeds.

This is the reason car manufacturers have invented variable or twin length inlet manifolds, which change the dimensions of the inlet runners at a certain RPM to give improved torque across the engine's rev range.

There is an excellent feature written by Dave Walker, an engine tuning guru from the UK. He performs A-B-A tests comparing inlet runner lengths on a 1600cc engine in an amateur racing car
 

DeadEye

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Would there be mass airflow sensors in the TB ? And throttle position sensors ( plus more things I can’t imagine) feeding back to the ÉCU , controlling the injectors , timing etc...
All so complicated !
 

scottmastrocinque

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Honestly, I'm unsure. It all depends upon the ECU configuration used and what inputs it is looking for, Seems like an overly complex path if you ask me.
 

Michael Moore

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The intake, like the exhaust, can be tuned to different lengths for different harmonics depending on what is desired in the engine. If the ideal length doesn't fit on the vehicle as desired, a different harmonic and length can be used though there will be a variance in the strength of the wave in the system and so power may vary a bit. Just because the engine likes one length, it doesn't mean it can fit in the vehicle; after dyno tuning my 750 Laverda race engine I had to build a new frame because the optimum length intake tract was too long to fit in the stock one!

A single/central TB will need a longer length to reach from the head to the TB and then to the airbox, where with individual TB mounted near the head the length of the runner between the head and airbox can be shorter because it doesn't have to mate with the inlet of the opposite cylinder. As mentioned above, generally long lengths tune to lower RPM, shorter lengths to higher RPM. But that doesn't necessarily affect throttle response.

Do the large single TBs have a cam operation to control opening so N degrees of throttle twist may give less movement of the butterfly just off the idle position so there's better control of the TB opening? That kind of problem is often the cause of complaints of poor throttle response on large carbs, people forget that for a given throttle (say 90 degree close to open) a small carb or TB will allow smaller movements for a specified number of degrees of twist in the throttle. Some throttles have different profile cams that the cable wrap around to give a non-linear change at the carb/TB.

I suppose it might be easier if I'd asked the question "does anyone ever notice a difference in throttle response between a Guzzi with individual TB or one with a single TB?" If not, then it probably doesn't make much difference (on a street bike) which is used as long as the question is about throttle response and not power or packaging of the system in the vehicle.

As I mentioned above, if I decided to try an EFI conversion, the first concerns would be throttle response (smooth, linear) and minimal intrusion on the rider's legs. I don't have a very sensitive seat of the pants dyno, I can't detect a +/- 5% (or probably more) difference, so as long as my knees aren't sore and the throttle response is smooth and crisp, a horsepower (or 3 or 4) here or there is probably not a big concern.
 

GTM®

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I was looking at some of the posts on the newer (and new to me) EFI bikes and noticed that some are running two throttle bodies up near the head, while others are running a single throttle body (controlling air to both cylinders) located under the seat.
I'm curious if there's any significant difference in throttle response between the two methods. With injectors at the head/port there should be no concerns about fuel drop out with a long and curvaceous inlet runner between the head and single TB (which can be a problem in some automotive applications).
Throttle response with either is good. The 2013+ V7/V9, 1400 & new V85TT all use a single throttle body. All have very good throttle response. I think it comes down to power requirements. All higher HP motors use one throttle body per cylinder normally. If it's a street bike, and you don't care about power, then it should be a non-issue.
Our old friend and land speed legend Sonny Angel (San Diego) designed his own manifold pictured below, but consensus was that no one ever got it to run well using a carb. Hope that helps.

Single carb sonny angel
 

Michael Moore

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Thanks Todd. If throttle response is a non-issue, then I can pick whatever works best for packaging/etc. Running a big twin at high RPM on the street seems contrary to why I want a big twin on the street in the first place -- lots of low end and mid-range power. And if a 1L 2V can make 75-80 RWHP like you expect from the V85TT, that would make it the most powerful street bike I've owned.

On the Sonny Angel manifold it seems like no doubt poorly atomized fuel from a period carburetor would tend to centrifuge out on the left side of the inlet and then keep on going to the left head. Problems getting it to work would not be a surprise.
 

GTM®

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Thanks Todd. If throttle response is a non-issue, then I can pick whatever works best for packaging/etc. Running a big twin at high RPM on the street seems contrary to why I want a big twin on the street in the first place -- lots of low end and mid-range power. And if a 1L 2V can make 75-80 RWHP like you expect from the V85TT, that would make it the most powerful street bike I've owned.
Welcome. My '00 1064cc 2V engine made 72 rwhp with open lid air box and free-flow mufflers, once the injection was corrected; https://archive.guzzitech.com/EVPower-Todd_E.html -- 61.36 was stock:

Dyno TE Max
 

GTM®

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Moto-Uno

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Well for what it's worth I had a cobbled together 950 kit in a T-3 engine I used for a 3 wheeler a few years ago . It originally had
29mm square bodies on it , ran well enough , thought why not put the 30mm round bodies I had with the better accelerator pumps
on it , ran well enough , but lost interest in syncing the carbs . So I made an intake to take a single 38mm CV carb I had from working
on HD's , installed a dyno jet kit and it ran even better over the rev range I used . It was a heavy beast and the single carb just made
the over all running more satisfactory . Your mileage may vary as they say , but larger carbs ( on MG's ) seldom improve power at street legal running . Peter
 

Michael Moore

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Hi Peter, this motor is fairly heavily modified and should be able to use the extra air. Once I get the rest of the bike home (I brought home a small SUV of tubs of parts today, but I hadn't realized the bike was a roller now so I need to arrange to borrow a truck) I'll pull molds of the ports on the modified heads and see how they look. They were supposed to be modified with the goal of a torquey street bike, not a BOTT racer. I can always get another set of stock heads and start over if needed.

A friend had a single Mikuni on his Notrun Commando and he liked that. I remember that the single carb Triumph twins (500 and 650), while lacking the pose value of a Daytona or Bonneville, were often said to have "sweeter" motors for day to day use.

cheers,
Michael
 

Moto-Uno

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Interesting you bring up a British bike analogy here , in my youth I bought a new 1969 BSA Lightening and a
friend bought the single carb Thunderbolt . Knowing I had the more powerful bike , it never failed to piss me off
that every time we rolled on at highway speeds , he would walk away from me . Unless I geared down and ran
it to redline through the gears I never kept up with him . Learned then the difference between max power and
real world usable power . Peter
 

Michael Moore

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I used to vintage MX a BSA B50MX, and while I was pretty fair at getting good starts, my lower power thumper could often beat the more powerful 2T singles to the first corner because it put the power to the ground. Once past the first corner, faster riders showed why I was not a fast rider. :)
 
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