tubbylad
Odd question perhaps.
Anyone used these in a 4V head?
I'm recutting my 33mm seats and was wondering whether going 1mm oversize with these would viable.
Thanks
Martin
griff851
Don't know the valve personally, but as long as it's the right length why not.
How an oversized valve works:
When modifying things quite often you here I put bigger valves in it. It sounds like a cool thing to do. But very few understand what it actually does. Well what is to do with is standing waves. If you have even looked a a river rapid and seen the wave formed that just sits there, that one. Water at a fixed velocity flowing over a rock usually. Now imagine if you took that rock and flatten it then put it back. The wave wouldn't be as high. Back to valves. When air moves towards supersonic velocity I've a flat surface a standing wave begins to form. For a valve if you go over 0.6mach the hight of the standing wave starts to choke your flow. In a jet engine over 0.5mach will cause the compressor to stall due to standing wave on the blades.
So you do some port work and maybe muck around with different cams and some how you manage to increase flow into your cylinder increasing dynamic cylinder pressure and giving you more torque, but your rpm runs out of puff lower than before your work. The reason is you've gone closer to 0.6mach or over it and choked the valve curtain area with the standing wave or shock wave because we're talking air. Effectively cutting off air/fuel flow into the cylinder. By putting bigger valves in, for want of an analogy, is like putting a flatter rock in the water.
Is there anything to gain by putting bigger valves in and nothing else? Few more rpm up the top, and a negligible torque increase, but then engine fatigue and the inevitable destructive consequences come into play. These 851/888 series engines for long case life are best kept under 10k. Hang about at 11-12.k a lot, best you have another set of cases in the shed or attic. ST4 cases are stronger a way better suited to rpm increases.
The Exh valve increase in size has a small net gain, but in cost effective terms negligible, better off spending cash in something like an exhaust system.
My Bevel is beautiful proof. I pissed around with flow so much that i'm Down to 108 main jets, normally they are 165's there abouts. Pulls like a god damn train and accelerates like a cut snake, but runs out of puff at 7k rpm. And there is no more room to put bigger valves in. My 888 has 748 drop cam heads just cleaned up ports and chamber and ST4 cams 34 inlets and it gets to 12.5k rather quickly from 6k, with one mechanic comparing the drive to a 996. Both were fun to build, but not really for road riding, they are more for track stuff, which I don't do. 😁. Still got kids at school.
Mr.R
I glazed over reading what Griff has posted (SORRY MATE), I quite sure he was scientifically on the nose with what he's wrote, but it was getting a bit on the deep side for me...and sometimes the results don't reflect the science IME.
When I boosted my '92 851 Strada (originally fitted with 33/29mm valves) to 916cc, I later fitted it with 916SP heads with the inlets taken out to 35mm with the standard 916SP exhausts of 30mm, using 9*6SPS T1 cams with the inlets advanced by 12.5 degrees it hit 128rwbhp at 9,300rpm.
With the benefit of hindsight...
If I'd skimmed the heads by 1mm to boost the comp' ratio and fitted 36mm inlet valves, I've NO doubt what so ever that it would have made 130+rwbhp and still under 10,000rpm.
Since in a earlier post you were talking about fitting new valve seats, why not fit some seats big enough to take 36/30 valves or bigger?
My 916SP heads are now fitted with 36/31 valves which I've now skimmed by 1mm and my NOS 888 Customer Corse "G" heads have 37/32's, the "G" heads are on my 955cc motor. The 32mm exhaust valves are to gain some extra radius on the floor of the "G" ports which are pretty poor in that department.
I heard of 888 heads being fitted with even bigger 38mm inlet valves with good results.
Apart from the stud spacing, combustion chamber shape and valve sizes, the 888 heads are pretty much the same as the 996 motors.
Steve R
P.S. There's top quality valve makers here in the UK and they normally cost less than genuine Ducati prices, I haven't used genuine Ducati valves for decades now.
griff851
Sorry Steve,
Just saying if you increase engine suck you get an air lump and a bigger valve fixes it. 😁
Mr.R
Hi again Griff,
What size valves are in your Bevel heads?
Steve R
griff851
I all honesty i've forgotten, they're stainless wasted stem (can't even remember where I got them)and small enough so clipping at crossover wasn't a problem. I did have it written down somewhere, it was 10 years ago I was on that part of of the project. Pity as the paper has all the calculations for port size diameters and angles. When the paint work gets done I have to pull the JB weld out and put proper glue in. When I do that i'll Let you know. I might get keen and take latex moulds. 😄
