MPG with throttle bodies ? Your experiences

cant wait to get my mk2 back together running on 45's, I was hoping for 20mpg on road and around 10mpg on track, thinking I might have been hoping for too much.

 

Anyone drive a car with ITB's as a daily?

Hey

Just wondering does anyone drive a car with ITBs/Jenveys as a daily?

Whats it like noise and fuel wise?

Cheers
Craig

 

if you put an airbox on, noise can be controlled. Fuel use is all in the mapping.

 

Sounds good, would you drive yours daily? if it was in standard trim?

 

nope - mine is horrible on the road - now very much a track toy - but a few years ago it was very nice and driveable and could have been a daily if you were keen enough

 

Anyone economically running bike carbs on a valver as a daily?

As the title really. I've been reading the guide on bike carbing a valver, and I really fancy it. However I'm concerned about the MPG being impractical. I'd be quite happy with mid to high 20's, low 30's when I'm not hammering the throttle - is this a realistic goal on a KR engined valver?

Anyone who drives a valver on a daily basis with carbs, I'd really appreciate your comments!

 

i ran my mk1 16v for a year as a daily on twin 45 webers and averaged 12-15 mpg so youre probably looking at doubling the fuel bill you currently pay on kjet

 

this is madness, if the carbs are properly balanced and set up, then it should be comparable to k-jet, and over 30mpg should be easy.

LS7 500bhp corvettes get 28mpg on a run...

 

Corvette has a number of enhancements over a 16V on bike carbs, which help fuel economy. For example, displacement on demand, etc.

The best fuel injection will still need to burn the required amount of fuel, somewhere in the lean condition, so its AFR will be something like 17:1. A 16V set up on bike carbs might be running rich and achieving 12:1. So, given the 2 extremes, if the efficient car does 30mpg then the inefficient tune up would give 21mpg. But these are extreme ends of the scale.

 

modern bike carbs are pretty sophisticated, however i bet in a lot of cases they aren't balanced, or remotely set up properly, or use badly conceived filters

 

AFRs over 17:1 on port fuel injection will actullay hurt FE due to lean misfire and the corresponding rich excursion that would be required to keep the engine running.
A 16v on carbs that are sync'd and jetting properly for the condtion it is subjected to, should have comparable FE to a STD Kjet system. However it will take more maintainace to keep the carb's vehicle within these targets. I do not see however, what benefit there is in running such a system on an otherwise stock engine unless one is after the brrrrappp noise it produces.

 

I thought the main advantage of bike carbs was their simplicity?

 

I don't know the first thing about bike carbs, or the (bike) engines they go on, but I'd've thought that in the sort of set-up I see on here - with one carb per cylinder - there would be a problem with the 'stop-startiness' of the airflow?

 

the old tech terms again pete, the bike applications are the same, however I imagine you'd need some decent length trumpets to contain potential fuel standoff

 

What do you mean by stop-startiness of airflow? Surely one carb per cylinder is the best possible method of airflow into the engine ala individual throtte bodies producing a great deal more power than a single throttle body feeding a number of cylinders? My carbs are webers which are designed specifically for car engines, bike carbs on the other hand are designed for an engine which revs significantly higher, therefore will never be set as accurately as webers or dellortos (although these are still a lot more crude than injection).

 

a fair bit of misinformation in here.

My angle is there is no reason for it to get 15mpg, unless the carbs are not tuned to the current applicaton, or you have some poor filter/manifold design going on [swanneck etc]

however if you drive it round at 6k all the time then expect 15mpg on k-jet or carbs, and airflow moves in waves, it's not all one way

 

Good luck finding someone with a 16v on twin carbs who averages significantly more than 15mpg!

People don't install carbs to improve their fuel economy, they install them because they're simpler than injection, the throttle response is noticably quicker and they sound good. They're also found to help produce more torque on some applications but I'm not starting that argument

 

as a note, dont judge mpg by the mfa, it calculates the fuel consumption using manifold pressure and engine rpm, distance covered ( I think!) individual runner installations have a lower vacuum than plenum FI. so mpg calculation will be lower. also if using the k=jet ignition ecu, part throttle ignition timing is also affected. Personally i've forund with carbs you can achieve any fuel ratio chosen at any rpm, if the ignition map is sorted, air is cool, mild cams with max torque/efficiency at appropriate rpm then fuel economy can improve on k-jet. But really carbs give awesome throttle response, k-jet fuel injection feels lazy like a turbo.

 

I mean that when the intake valves open, airflow through the carb will start. When the valves closes it will stop. Actually it's a whole world more complex than that because air has momentum and elasticity, so as prof says, there will be flow in both directions, and complex pressure waves (different at different rpms and different runner/trumpet lengths). But to keep it simple, say that you have airflow through the carb in the 'right' direction for approximately a third of the time and zero flow, or flow in the other direction, for the rest of the time. That's OK-ish 'cos of course you only want fuel flow when the valves are open, but...

Carbs rely on the airflow to 'pull' the fuel through the jets, and you really only want the fuel to flow when the air is moving in the direction toward the intake valves. The trouble is that fuel is relatively dense and viscous so it will lag relative to the airflow. Max fuel flow will be significantly later than max air velocity. So some of your fueling will end up 'bouncing' off the back of shut valves and back through the carbs the wrong way. I imagine this is what prof refers to as fuel standoff? Short runners and trumpets could end up with a fair amount of fuel wafting around in the engine bay, perhaps.

As for the comparison of carb per cylinder to ITBs, the big difference is that carbs are required to meter fuel, ITBs aren't. The idea that you get a 'great deal more power' with carb-per-cylinder would seem to be based on the misunderstanding that the cylinders are all drawing air at once. Single carb for four cylinders is only really a disadvantage w.r.t. distribution of the mixture (flow direction changes within the manifold and differing distances to the destination cylinders). Much better for evenness of airflow velocity through the carb because as one cylinder finishes its induction stroke another starts (with a bit of overlap). Twin carbs on a suitable manifold address the distribution to a good extent, presumably at a slight cost to flow velocity evenness through the carbs.

That's my understanding anyway.

 

So what factors dictate the eventual MPG with such a setup? Would be down to the initial choice of carbs or the size of the jets?

Could reasonable MPG (say mid 20's) be achieved with some patience, but on a relatively cheap setup, or is a third party ECU needed?