# Chapter 10: 1/16/01 to 2/22/01 Some updates and conjecture on calculations of HP, ET, MPH, piston speed. Some rumination on manifold design, and my attempt at a variable plenum/runner manifold.

1/15/01:  I’ve been working on the air injection. I’m rebuilding the water injection plate which was made out of wood, this time out of aluminum, since I now have access to a milling machine and can do a proper job of it.  This will have provision for both the water injection nozzles and the air passages to route boost pressure to the throttle shaft “air bearings” which keep the A/F mixture inside the carb.

I have also been doing some calculations based on two sites on the net which together give me some concept of where to concentrate my efforts if max output and min damage are to occur.  The first is the Turbo Power Calculator and the second is CSG car math site.  To get an idea about the output requires a few assumptions which I may or may not have made correctly.  One is the Volumetric Efficiency of the engine (VE) and the other is the compressor efficiency of the turbo.  The VE changes with rpm and with the engine’s ability to actually get the entire swept volume of a given piston into the cylinder.  This is a function of cam, intake restrictions and exhaust restriction.  To figure out what a stock Champion motor had for VE, I worked backward, plugging the bore, stroke, and factory info on HP and rpm measured at that HP.   Turns out that the Champ’s 85 HP at 4000rpm is a VE of 50%.  That’s with box stock carb and exhaust.  TurboStude has a 4 barrel carb, huge, short, fat intake runners and a 2 1/2″ exhaust pipe.  I expect that one can assume at least a 10-15% improvement in VE from this (if not more).  4000 rpm is pretty conservative as a redline calc, and I then determined the piston speed for my 4″ stroke motor at various rpm.  A stock motor should not be asked to spin faster than when any given piston is traveling at 3500 feet per minute.  3750 to 4000 fpm can be run if an engine has certain features including forged crank, heavy duty rods and substantial end caps.  My little engine does have a forged crank.  3500 fpm translates to 5250 rpm and 4000 fpm translates to 6000 rpm.  Using the calculator above, I determined that at 5250, and at 13# boost, and assuming VE is 60 with a 65% efficient turbo (60% to 70% is usual), my engine will crank out one HP per cubic inch, or twice stock.  The HP and Torque curves cross at this point and are equal.  Calculating ET and speed in the quarter mile can be done at the site as well, and my 2750 pound car will give 14.75 at 92.43 mph.  This beats stock performance which calculates to a believable 18.54 at 73.52 mph.  If I want to get more aggressive, than with boost of 20 pounds at 6000 rpm, the HP will  be  13.41    at 101 mph!  If the VE is 70 (could be?!) than the engine is making 263 hp and I’d be running 12.73 at 107.09 mph.   One begins to see the importance of the VE and the effect of boost. One might expect the air injection to somehow improve the VE and probably the ET.  Of course these formulas assume that one hooks up well and that the driveline is adequately matched to the motor in terms of ratios etc.  In the spring, I’ll have to determine how close my calcs are to reality….(Actually, I later determined that a flathead has a V.E. of 75%).

Manifolding:  The VE is partly effected by intake manifold design.  I will be adding a way to adjust the resonance of the manifold with rpm. This will effect the harmonics by changing runner length.  I will build in a “swinging wall” to create a maze within the manifold which can internally lengthen the runners for low rpm and swing back to effectively decrease plenum size at higher rpm. It will pivot on a rod which will attach to a bell-crank connected to a bellows (like the Hobbs switch or waste-gate can) or to the throttle linkage.  Getting this just right may be difficult, but if it doesn’t work, I’ll just swing it out of the way.  With the “Wall” open, the gasses travel over a runner length of 7.5″ from valve edge to where the runner meets the plenum.  I figure that the effective runner becomes  2″ longer (and narrower) when the “Wall” is down.  When open, the wall leans against the turbo side of the manifold, and somewhat decreases the active volume of the plenum, though a slide-whistle affair will be welded to the side of the plenum (at the circle) for further volume adjustment. My “wall” when down, doesn’t separate the runners from one another, and I need some guidance on whether I should weld in small partitions on the runner side of the wall between the runners to isolate them from one another. Any comments?

Some say that making the manifold a little longer just past the end runners is a good idea one way or another for better filling.  Maybe I’ll add one on both sides….  These changes may do nothing except give fuel more of an opportunity for pooling, or act as an additional throttle plate, but now is the time to add it.   That’s what hot-rodding’s really about isn’t it? Trying something different on your old thing pirated from new technology, but done in a backyard sort of way?