This is great... a writeup along the lines of the 8v post at some point in the future would, as previously stated, be most welcome.
Excellent stuff quantifying and documenting this. Those bolt on losses really do add up. You will lose more cfm when the carbs are on, plus more still if you fit an air filter. It just goes to show that you need to pay close attention to the whole inlet and exhaust tracts, not just single aspects of it such as the head, valves, or manifolds.
Cant believe moded head had bigger losses! Loss of air velocity? This may explain why some people dont achieve figures with performance bolt on parts on flowed heads and why some standard cars arent far behind with figures on rollers
This equates to a 5.5% bolt-on loss with the standard head and 8.1% on the modified head. You would expect these to be in proportion. It's likely to be caused by a break up (turbulence)in the laminar airflow in the transition phase from a round shape (at the carb end) to oval shape (head end). The increased velocity of the airflow on a correctly modified head will increase the losses created by this turbulence in the manifold in a greater proportion to the total airflow. A good engine builder would work on the manifold wherever possible to reduce these losses to as close to Zero as possible.
one of the things that struck me at the recent autosport show is why do DTH bodies go from oval to round? why do they need to be round? why not oval to oval avoiding a shape transition at all...you can have oval butterflies....the more expensive works units are not round and not all even use a butterfly
Interesting Rob, have you got any pictures of these TB's? I guess carb manifolds have to be round, I have never seen an oval carb...
All carbs I have seen are round but I dont know why they have to be round - anyone know? I suppose its a universal shape that can be used for any engine which makes sense but this logic goes out of the window when considering DTH units as they can only fit the engine they are made for. I suppose if you make a special shape for your engine, you also need to make airhorms etc etc etc all special shaped so maybe thats why 'customer' units are not like this. I saw some roller throttle bodies like what prof posted, oval ports, but with twin opposing half rollers.........so the opening started in the middle of the port rather than at the edge like in a conventional roller body.......VERY trick, lots of engineering gone into them (milled from solid billet) and I would think big money. They had oval ports, oval rampipes etc etc, not a round section anywhere. And they were 8-injector.......and had been fitted to factory cars in the past the dogs bits - will post some pics later
taken to the extreme are the roller barrels on millington diamond heads. they're integrated into the head and the divider actually starts in the roller barrel itself. considering some of the power outputs claimed(well over 300 bhp, albeit from 2.5+litres) they must work fairly well. I can't find any pics anywhere though.
there are oval shaped butterfly units, saw some at autosports.. very thin ones to so unimaginable to think they impeded airflow in any way shape or form. now.. the rub... Airflow does not equate to power, and word is the "gains" between round to oval to, both ignoring many other aspects as throttle plate in relation to valves, injector position etc etc, makes the airflow off a bench, just an interesting number. slap it on a dyno and know for sure, but it will onyl be relevant for that "exact" spec engine under those "exact" test conditions.
Just for you Mike... And yes I did flip the top pic so as to make it easier to compair to a standard port. It's a long way from the finished article, the seats need cutting so there will be a little more material to come out in the throat area when thats done, and I want a better finish to the port runners.
There is a theoretical equation that 1 cfm = 0.43hp per cylinder. So, an engine that has a head flowing 120 cfm works out like... 0.43 x 120 = 51.60 x 4 = 206.4hp. A cometition spec Ford Pinto producing around 200hp normally has around 118-123 cfm air flow through the head... so not a million miles off the mark. I saw a flow test many moons ago with a 5cfm gain on a ported 1500 Golf head, so 5 x 0.43 (=2.15) x 4 (number of cylinders) = a potential gain of 8.6 hp. The engine in question gained 9hp. Our pc program can work out theoretical hp based on as many factors as you feed into it, airflow, bore/stroke, cam spec, exhaust size, c.r etc, the more info you put in the closer it gets to the result (obvious enough I spose). When my flowbench partner fed in all the parimeters for a given engine spec he knows inside out, the "suggested" potential hp came out at around 10-12hp (320-330hp Jag straight 6) less than he's seen on the engine dyno. So as a good "guide" as to what is possible, knowing the airflow helps but an engines potential in the ball park. I can hear the calculators clicking away as I type this ! Along with head shaking from the doubters.
Thats an interesting calc - we know that factory valvers made at least 270hp which equates to 157cfm - CNC claim 142 cfm for theirs which is on std valves (valves fitted for test) so I reckon this isn't far off the mark at all My 45mm DTH jenvey bodies flowed 168 CFM (ta Neal H for getting em tested) so a resonable performance intake tract will flow these numbers also.
