Cvh 1600 cylinder head flow verses air speed

[Shippers555]

Gentleman

Is there any head porters that have experienced this ?

Background, I spent many years in the motor trade before studying refrigeration. Spent many a dirty weekend building A series / crossflow etc engines for buddys porting cylinder heads using a homemade flow bench using DV’s rules of thumb. All good fun drag racing at 2am around Brum.
Many years on a friend has asked me to build him a 1600 cvh for his 1989 Xr2 restoration. He has asked for a little more HP than standard. A supply of copious JD is supplied for my efforts. Everbodies a winner. It’s Looking real good so far, the shell was stripped back and rebuilt without any rust. Engine was low mileage and good for a rebuild all be it it’s a lean burn with kidney shaped combustion chamber. Keeping it cheap I’ve built another basic flow bench, sealed properly using 3x 1500 Watts vacuum motors and using accurate refrigeration vacuum gauge to measure vacuum pressure through the spark plug hole and a air velocity tool. Simple as it comes.
The 1600 rebuild will consist of minor blue printing, I can’t be arsed for the full monty. Kent cam 22 ,standard sized valves with angle cuts on a modified kidney cylinder head with possibly the orgianal carb or twin Webber’s . Using a 4-2-1 exhaust manifold. Lighter flywheel.

Using all the porting technics I can remember and reminding my self of David vizards rules and the written advise of others like puma racing and other cvh specialists. Ive never ported a cvh before but I always modify to achieve a balance between maximum air speed and least flow restistance for a street application. Nothing worse than a street engine that picks up poorly from low rpm to only enrage it’s self topend. I had a cracking 1100 mini that would beat many 1275 / 1300 with big everything off the line to 70!
After many hours I’ve porting, blending , cut different angled valve seats etc etc I’ve ended up improving all areas of valve lift using 3mm , 5mm and 11.5mm valve lifts as starting points. I use a coloured dye mix, flow balls etc to test how ports flow. I have no meaning full cfm numbers but my vacuum results follow in Kpa pressure.
Before any modifications 3mm lift = 21 kpa vac , 5mm lift= 17 -18 Kpa vac, 11.5mm lift = 14 Kpa pressure.
After all modifications I’ve ended up with
3mm lift = 12kpa , 5mm lift= 12kpa , 11.5mm lift = 12 Kpa vacuum pressure. Air speeds increased initially especially around 3 to 5mm lift but have decreased slightly since my thinking was I had more room to allow a better flow at 11.5mm lift and took a little more out the busy sides of the port. The vacuum restance of 12kpa stayed the same but it meant the air speed was decreasing. I know I have no meaningful cfms

My question is do I take the port further and try and increase the high lift flow volume or is a result of flowing as much at 3mm lift with good air volicites a good point to stop.

Shippers

[PeterBurgess]

We have always found one reaches a crossover point whereby low lift flow starts to fall as high lift flow increases. The point one decides 'enough is enough' will depend on your interpretation of bulk flow requirements and your cam lift. For road use we prefer more flow at lower to mid lifts rather than mid to high. I suppose it is ' a piece of string' question and only experiment will provide a known path forward if you wish to develop using your flow bench..
Peter

[Mignon]

Unfortunately what you're doing is wrong on so many levels it's impossible to say what flow results you're achieving or if you're even making the head flow better. Taking pressure measurements down a plug hole is not the same as measuring the pressure drop across the whole head and in any case that number is meaningless without a pressure drop across a known resistance like an orifice plate to compare it with. If you had both of those numbers then you can calculate flow as being approximately the sq root of (pressure drop across orifice plate / pressure drop across head) x Flowbench CFM Coefficient. (The latter item needs to be experimentally determined for each flowbench)

In truth even that is only an approximation due to Reynolds number changes but it gets you in the ballpark. Measuring just one of those pressure drops tells you next to nothing though. I've got flow sheets on actual CVH heads where the pressure drop across the head at 500 thou lift is the same on a stock head as on a heavily modified big valve one - about 130mm of water on my manometers. However the CFM figure went up from 135 ish to over 180 because the pressure drop across the orifice plate had risen so much.

I suggest you finish building the flow bench properly, measure the required pressure drops in the correct places and then calibrate it all to either reference orifice plates or someone else's Superflow bench. Until then you're wasting your time I'm afraid.

[Shippers555]

cheers gents
ive taken on board both opinons. realistically im only aiming for a mildly tuned engine for street. Been a novice and The fact id never seen those type results before intrigued me to investigate further. i might see if i can upgrade my bench or there is a local guy in bromsgrove that apparantly is good, just to check my work if hes willing to be paid .
shippers

[GreenV8S]

Shippers555 said:

The 1600 rebuild will consist of minor blue printing

Do you mean you'll try to get the clearances right? Unless you're obliged to do it to comply with motorsport class regs, actually blueprinting the engine would be a huge waste of time and money.

[Shippers555]

true blue print is quite extreme attempting to get to the original spec of the designers drawings. i should nt of used blue printing term really. what i really mean is getting all sizes, weights, tolerances the same. when i rebored the block we checked and adjusted the angle of the bores in the block as one example. a balanced engine is better than not as it can waste usable energy. In one sense your right as you'l be suprised what you can get with. manufacturers use tolerances all the time to keep costs down. lets face it a cvh wasnt the best but it was cheerful