That is true, but as per Slipngrip's thread, Honda vtec engines are weird like that. The name brand sports exhaust currently on my friend's car already has 44mm ports with no lip, while the standard manifold has the same size port, but with a 4mm lip a couple of mm down the throat. So at worst, he goes back to the same as whats already fitted to the car.
But as per this thread, if it were measurable, comparable or quantifiable then a flowbench would be able to see the difference if any.
In regard to exhausts here are some of my favourite tests from Youtube
Yeah, Engine Masters is good, because its the same air conditioned engine dyno and usually tested back-to-back, so a lot of variables are taken out of the equation, compared to a open air chassis dyno.
That sounds very interesting Robert. I started racing with an "A" series engine, initially a 1098cc then a race 948cc with 1275 head and 649 cam. A 45 DCOE with three branch exhaust and open collector topped it off.
Looks like with a bit of fettling I should be able to get the Honda head on the old Rover fixture may even be able to sleeve the liner down a bit further to replicate the bore as well.
The old Rover head fixture does leak a bit - about 12CFM at 28" but I will try and rectify that or compensate for it - If I can organise some supports I may be able to put the full induction on the bench so get the complete picture.
Darkspeed, thats a lot of leakage, have you tried using closed cell foam as a gasket on both ends of you bore fixture.
I use it and have zero leakage. I cut up a 6mm foam camping mat and glued it to the bottom end of my fixture, the same at the head end but without gluing.
It is - Its a POS plywood jig that served it's purpose long ago and I doubt that sealing it to the bench with silicon sealer would stop the bench dragging air through the ply itself. I will get around to building myself a new one but for now it will do - What would be nice is a 20mm alloy bench top and then have 20mm alloy precision jigs with o'ring seals - but this is just for hobby work and the bench build was a bit OTT for just satisfying curiosity. For seals I use 3mm rubber and use a smear of rubber grease to position them on the heads. I will try and hot glue seals to the fixture and PVA and pant the jig again to see if I can get it leak free
Dropped into the DIY shed and collected some 18mm MDF cut it up and made an adapter for the IDF and fired the bench up.
Ran some tests at 28"
So for those wondering
40 IDF with:
32mm home (others) straight bored choke - 185CFM 34.5mm home (Me) taper bored choke but not 100% finished - 200CFM 34mm New Weber choke - 201CFM
I then did a number of tests with the trumpets - not a great fit as they leave some small steps
However the trumpets picked up 3 CFM 204CFM
Then I fitted the K&N top plate as it was pretty close (20mm) to the trumpet - 204CFM not a bit of difference
I then fitted the very dirty air filter - I had to block off the other choke to prevent the air bypassing - 197CFM so lost 7CFM with only one choke pulling through the filter - I will give it a good clean and test it again.
I pulled all of the chokes out to run as close as I can to a clean bore - 220CFM
Next the new 44IDF with 36mm chokes
This made 253 CFM
So from what was originally fitted 185 CFM which probably gains 3CFM with the trumpets - 188 CFM we have a 35% increase in flow with the new carbs at 253 CFM.
Did a calibration run at the start and finish and the bench was spot on.
Better get the manifold and the head on the bench and see where that goes
Interesting that the Triumph Daytona throttle body flows about the same as the 44IDF with a 36mm choke - what are the odds !
So cracking on whilst we had some momentum I made an adapter to simulate the head so that the carb and manifold could now be tested
This is the IDF 40 with the 34mm genuine choke and the trumpet that managed to flow 204CFM
The manifold pulled the flow down - to 196CFM
Next set up for the 44IDF and see what happens
A small gain with the 44 IDF making 206 CFM - but the manifold creates quite a step down so I expect to see some further gains after its modified.
However - what should be noted is that the original home brew 32mmchoke only managed 185 CFM - I will test that on the combination tomorrow and see if we loose any flow - we lost 8 CFM on the 40 IDF with the 34 choke and we lost 45 CFM on the 44 IDF with the 36mm choke.
When my latest head (Avenger race head) was done by Petteri he supplied me with a flow against valve lift set of figures that were all very interesting, especially the low lift figures but also included were a set of included theoretical power figure for each lift.
So for example at 11mm lift the head flows enough air for a theoretical 190hp.
Is that a valid figure do you think? After all the valve is only ever at max lift very briefly but the cylinder has has time to fill during the ramp up and down again??
So, have you seen any power back to back figures with flow increases on anything you have done? So for example if the carb on an engine was the restricting element in the system and you increased the flow by, say 20%, would there be a 20% increase in the engine? Be it power or torque or volumetric efficiency or some other metric? (all other factors constant).
I have always promised my self that when i retire i will build an engine dyno....... not sure the neighbours will be impressed.
One of the calculations for estimating HP from an engine is [email protected] x 0.257 x number of cylinders
Working back from your 190HP /4 /0.257 = 184 CFM that is going some for a push rod 2 valve motor.
I think Vizard worked on the flow at 25" being equivalent to the possible output of a 4 cylinder motor - so if the bench said 130CFM peak flow at 25" that's the HP capability of the 4 cylinder motor. From his own figures and at the standard cam lift the flow of a Pinto was 130CFM @ 25"
Do I think they are valid figures? - yes and no - I have never seen any stack up in back to back tests as there are far too many variables in the combustion process - but are they theoretically possible - who can argue? However on the side of flow improvements resulting in power improvements I firmly believe so, but not in so far as x% = x% as again there are to many other variables. Getting the air fuel mixture in is one factor but if the combustion chamber can't burn it well as there is a disparity of AFR in the chamber giving odd flame characteristics - prone to detonation etc.