Hey gentlemen, I've started some development work for a blokes speedway engine and thought I would share it along the way and the things I've discovered with the new flow bench.
I've only got a couple of weeks until the season starts so this first lot of development is reasonably basic just to get some improved results to go racing with and develop some more as time goes on. The bloke that the engine is for has asked me to do the inlet, head and carb for him, he wants to build the bottom end himself to save some time as the timeframe for development work is very short.
First of all the biggest restriction in the category is the use of the stock intake manifold, I started with an 84da manifold as they seem to have the most meat in the runners to work with, the stock manifold bolted to a stock head flows between 135 and 138 cfm, the best runners were number 1 and 6, the worst were 3 and 4.
The first thing that I noticed is the abrupt turn from the plenum into the runners, the air can't make the sharp turn so becomes very turbulent and noisy before it even starts to make it's way to the runners themselves, I found big gains in just reworking the plenum area for a smoother and more contoured entrance into the runners, flow jumped to 150cfm. This is a worthwhile effort for anyone that wants to gain a bit of power just by reworking the plenum of there stock manifold without having to do anything else, a cheap improvement that will just cost you some time and some new gaskets.
I've taken it a bit further and cut the manifold up to get access further down to rework the short turns of the runners themselves to improve the flow path further down the manifold.
The engine has to run a 350 holley, so an adaptor is used, the taller the adaptor the better I found the stability of the airflow into the runners, this also gives the plenum more volume which is what I wanted. All of the speedway guys I have spoken to told me that they swap a 500 holley baseplate onto the 350 holley, so I did this and run some comparisons to a stock 350. First of all a 500 base doesn't just bolt on, you have to clearance the venturis for the butterflys to clear, I blended the base plate and reworked the venturis, with just the carbs on the bench the stock 350 pulled 325cfm at 28in depression, my modified 350 pulled 360cfm which is a good gain for topend power. The thing that surprised me was that when I flow tested the carbs on the manifold and head the modified carb dropped airflow compared to the stock 350. After probing the velocities to find out why I discovered that the discharge point of the bigger butterflies was closer to the walls of the plenum and put airflow in a position that made it more difficult for the air to make the turn into the runners, the smaller stock 350 discharged the air more central to the plenum so made the change in direction into the runners less aggressive.
So after reworking the shape of the plenum again to make the walls of the plenum further away from the carb butterflies and change the contour of the curve into the runners I got to a stage the modified carb is no longer a restriction to flow.
After that I finished the rough shapes of the inner runners and there short turn areas which need to be made taller and a wider radius, to aid in airflow making it around the corner into the main runner which leads to the head port, then I finished up the texture on the walls in 80 grit and welded it back together and try to make it look like it hadn't been played with hahahaha
The finished product now flows between 162 and 165 cfm across all the runners so that's around a 20% gain in usable flow, as a comparison a stock 2brl redline manifold flowed 170cfm on the same head so my modified 1 is not too bad for my first prototype, the next 1 will have a bit more time put into it and needs a larger plenum so I will share that when I start it.
The head is next, it is just being reconditioned at the moment then I can start on those ports, with the restriction of the manifold the aim for the head is all lower lift flow so I will show the development of that as I go as well, again this will be on a very tight time frame so just the basics will be done but will show very good results.
Here's some pics
manifold I started with
After I cut it up
Pics of the rough burr work showing the shape of the plenum and runners, I forgot to take pics of the finished texture before I welded it back together, I always forget to take pics hahahaha
Welded back together and cleaned up
The 350 holley with blended 500 base
Some pics of the roughing out of a port getting ready for some port shape changes, more on this in the coming weeks
C2 Head chamber with laid back inlet swirl. (D,C2A are the same)
Note how the chamber shaping goes all the way to the head gasket face.
C1(C1A) Head chamber with laid back inlet swirl and reduced sharp point where exhaust and inlet meet.
Similar to C2 but note how the chamber shaping stops short of the head gasket face.
I believe that these heads have the best inlet flow potential and best initial chamber shape. With additional shaping on the opposite side to raised inlet swirl area and a bigger valve I reckon low lift flow figures would go through the roof. Cam and I are playing with this idea on my new speedway head.
I'll get an E1 snap and then an unmarked head and we have pretty much got them all covered.
I've got an E1 head in my shed in town. I'll grab a chamber shot when I'm in there next.
I have a cylinder here at home that has a chamber like I have never seen before. It came off a HM Gem engine and the casting number had been ground. It sort of looks like a C1 but with less of a rise. I'll clean up the chamber when I have time and post a pic. It is truly weird.
Since I have all the head castings I should really take pictures of each chamber and post them up. Hmm time.....
Piston to valve is around 86th inlet and 104th exhaust from memory? I'd have to check my thread but 80th is the suggested minimum on a street based engine.The exhaust gets hotter and stretches more than the inlet.The inlet cops a cooling charge from the fuel it lets in so it always seems to suffer less.
My cam is a Clive Cams roller which has 709th lift and has custom valve reliefs in the Wiseco forged pistons and sunken valve seats in the head to help.This is why cam timing HAS TO BE spot on and dialed in and never just dot to dot or what you think.It has to be checked via atleast lift @ TDC and the centreline method to be extra sure if you want.I got to within .5th on TDC and spot on in centreline measurements.Yes you HAVE to be this anal when setting up a big RPM big lift camshaft as you cannot trust anyone.It took me hrs to get the cam timing exactly spot on and 2 timing kits (which I used a cam gear off 1 set and crank gear and chain off another) and then I moved to plasticine to check piston to valve.I do this on cast piston engines too,you just have too.
Race engines which DO NOT use engine braking (drag cars) can and do run alot tighter like around 40-60th but I wouldn't suggest that on any street engine.
P.S on checking my notes I got 103th inlet and over 120th exhaust...
