Outer Limits 4x4 Board

Does anyone have figures for the amount of air consumed by the G13A at certain RPM?

A 32/36 weber has a flow rate of 300cfm and apparently a well set up 1300 can flow that much.
Not sure how much that will help but its all Ive got.
Nik

THanks Nik, Just after a ball park figure. If anyone else has more detailed volumes consumed at say 3000 and 5000 that would be great.

I don't have heaps of knowledge in this area, but I'd think you'd be able to work it out fairly accurately based on volumetric efficiency and engine displacement.

As I understand it, a naturally aspirated engine has a volumetric air intake efficiency at Wide-Open-Throttle (WOT) of about 70% (ie when the piston moves down on the intake stroke, it creates vaccuum which sucks air/fuel into the cylinder. The vaccuum and intake valve timing isn't sufficient for the entire cylinder to fill with air at atmospheric pressure, however, and the cylinder fills only to approximately 70% of its capacity before starting the compression stroke.)

The volume of each cylinder is 1.3/4 = 0.325L, thus 70% efficiency gives 0.7*0.325 = .2275L.

For a four stroke engine, the intake stroke occurs once every two revolutions of the main crankshaft. Thus we have 0.2275L of air intake per cylinder every 2 revolutions. Fiddle the maths to get air intake for four cylinders every revolution, V = 0.2275*4/2 = 0.455L/rev.

Thus at 3000RPM V = 3000rev/m*0.455L/rev = 1365L/m = 1.365 cu.m/min = 48.2cfm

At 5000RPM V = 2275L/m = 2.275cu.m/min = 80.4cfm

Even if you assume 100% volumetric efficiency, which ain't going to happen AFAIK, you'd still only be sucking 115cfm which is far less than the 32/36 Weber flow rate given by Nik. So I'm not sure if I've stuffed up somewhere ... I'm working on logic rather than actual knowledge ... so let me know if I'm wrong!!!

Moph wrote:Even if you assume 100% volumetric efficiency, which ain't going to happen AFAIK, you'd still only be sucking 115cfm which is far less than the 32/36 Weber flow rate given by Nik. So I'm not sure if I've stuffed up somewhere ... I'm working on logic rather than actual knowledge ... so let me know if I'm wrong!!!

Those calcs look pretty reasonable to me.

Carbs are rated in CFM @ 3"Hg. So using a 300 CFM carb doesn't mean the engine should be pumping 300cfm, it means the carb will flow 300CFM when tested on a flowbench drawing a 3"Hg vacuum.

Ben

Im only being a parrot repeating what I read
Sarge the weber guru in the states said somthing along the lines of a 32/36 flows 300cfm and a 1300 if set up right use all of it.
But I have no sats or maths to back it up.
Maybe somthing to do with cams, porting, valves??
Nik

Hi Nik,

Can't imagine that cams, porting, valves, extractors etc would have that sort of effect. A quick calc shows that for the engine revving to 10,000rpm (poor Zook! ), to suck in 300cfm you'd have to turbocharge/supercharge sufficient to fit 0.424L of air (measured at atmospheric pressure) into each 0.325L cylinder.

Might happen, but not likely in anything wearing mud terrains and lockers

Cheers,
Tim

PS Gwagensteve, you got any thoughts on this? Question seems right up your alley

Great work Moph.

That info is gold. I had been thinking about the maths for it and pretty much hadn't got around to actually working it out. Its seems perfectly logical the way you describe it in theory so in reality its most likely pretty accurate only considering other variables are constant.

My G13A has recently been blowing a bit of smoke and oil out of exhaust. THe last 9000kms have been 98% offroad, and It hasn't had it easy either. I'm thinking the pistons rings are gone/going (the oil ones) and my plugs are slightly oily. Was interested to know how much this thing sucked standard.

I've got a spare head and torquier cam sitting at home at the moment. THe head and cam are matched but I'm reluctant to put it in as a couple of the cams have slight nicks in them. I'm thinking of maybe sending it in to get cleaned up and also upping the intake and exhaust valves to a 1.6 carby size to maybe get a bit more go out of it. I like the thought of a conversion but I'm keen to have a play around with this one....

No worries Moph, the missus proves me wrong all the time
10000 rpm in a zook , Sarge said he melted a piston at 9000rpm in his 1300!
Nik

NIK wrote:No worries Moph, the missus proves me wrong all the time

You've got it all wrong mate. You're giving the right answers, she's asking the wrong questions

Gold
So whats the plan dank?
Nik