JMR Development blog

Hopefully before Curby Come and see it... and bring a car!

I didn't get a chance to digest those messages yet.

EDIT - will take a pic of the manifold-injector boss fit.

 

I would come along but we are on holiday that weekend down in the SW, plan to drop in on Jason and see how he's getting along with my build while I'm down there......

Cheers Mike, I have been chatting to ratfink about the flanges and I have created a CAD drawing of a one piece flange....just need to check that it has correct clearancing to allow access to the sparkplug for cyls 2 & 3 before I send the file off to get it cut, I will also be getting some DCOE type flanges cut to mount the ITBs on

 

8v exhaust flow

I've been meaning to do an exhaust flow article for ages as I've never been able to understand the "big exhaust valve" phenomena on the 8v's when no other 8v I can think of prioritizes the exhaust over the inlet...normally for good reason.

Exhaust flow only needs to be in proportion to the inlet flow. Making the exhaust flow more than is needed "sometimes" helps further reduce pumping looses in theory, but in practice it's not always the case, and certainly not what I've found in testing to date.

For an engine to make power it needs to be fed as much air as possible...the more the better. Fueling is easy as we can get as much in as needed via correctly set up carbs or fuel injection...then we need to light it up to make it go bang...so a nice strong ignition system is needed...again easy enough.

The most pressure possible helping to get air in is atmospheric pressure...forgetting turbo & supercharging for the moment.

To help get the exhaust gasses out there is a pressure build up after combustion of anywhere between 80 & well over 100 psi ready to burst out of the cylinder. So as the exhaust valve opens the actual gas velocity can exceed the speed of sound for an instant...reducing as the valve opens further but still expelling fairly rapidly....it certainly doesnt get that much help being forced into the cylinder!

When a performance cam is thrown into the mix (as many will run with a ported head) which opens the exhaust valve earlier, to a higher lift, and closes it later, the exhaust flow, or I should say exhaust blowdown time, of the cylinder has even more of a helping hand.

It was said may years ago that the exhaust flow needed to be at least 75% of the inlet flow for best power. On a good 40mm inlet valve ported head that makes around 86-88cfm, that would need around 65-66cfm from the exhaust port...which from a std 33mm ex valve is asking a bit much. Latterly though, through many more years of testing it has been determined that in fact 65-70% will do the job well enough...in fact some race engines make do very well on 60-65% exhaust flow...though the more radical the engine spec, in particular the higher the compression ratio, the less measured exhaust flow is needed...but thats another story for another day.

So in our case, with a std 40mm GTi inlet valve making 86-88cfm when ported, the 65-70% ratio sees us needing around 55-60 cfm on the exhaust side...which is perfectly possible from the std size 33mm exhaust valve when ported correctly. When a 40.5mm inlet is used the std exhaust valve capacity is being stretched a bit @ 65% but can get close. So how much power do you think your really loosing by having an exhaust port that flows a couple of cfm less than maybe ideal?

I can answer that...not a lot. I've run race spec engines (100hp per litre) with between mid 50cfm exhaust flow (60% of the 94cfm inlet flow) to almost 70cfm exhaust flow (70% of 97cfm inlet flow) with no real world power difference in 185-190hp power band 1800's. In fact tony b's original head I re-ported had between 65-70% exhaust flow from it's 94cfm inlet flow to make over 200hp from his 2ltr 8v and win the Welsh hillclimb championship outright...so not really lacking much.

So...anyone think their road engine "really" needs a bigger exhaust valve with a std inlet valve to make 140hp now?...make inlet flow a priority every time.

Once you get into the bigger inlet valve territory...41-42mm inlets...then yes, the 33mm std size exhaust valve is being stretched, so a 34-35mm exhaust valve starts to become a help in keeping the balance.

I have however seen some 35mm exhaust valve heads lacking in potential....fitting a bigger valve and just blending the seats in...exhaust "or" inlet for that matter...wont automatically improve flow...the port and throat area needs increasing to make it worth the effort & expense...a correctly ported 33mm exhaust port can outflow a below par 35mm port...and cost less money in the process.

For the record...a std 40mm GTi inlet port makes around 72-73cfm, a std 33mm exhaust port makes 44-45cfm....that's 62-63%...are VW really that stupid or was it the luck of the draw...err...no...I doubt it very much.