Hi Doc,
Some of you guys following Milwaukee Midget's build diary might have read some of my posts. Short version of the long story is: I'm an engine guy for 40+ years. Started out in drags, went to road racing, and have thence spent some time with Imsa, Indy & Nascar teams. Lots of dyno experience, lots of engineering experience. Just read through your thread today & I have some thoughts & experience that can confirm some of what is being said.
1) When the vehicle's total drag hp = drive wheel hp/traction available, then that's your top end speed. Overall gear ratio changes
(trans ratio * diff ratio) usually only matter if you are traction limited. (This only applies to wheel driven vehicles.) Trans gear splits
will not matter if the engine "falls out" of the "power band". If the hp required is not available where you are in the rpm band when
you shift, you can no longer accellerate. This is where wide trans ratio spacing kills your speed.
2) For all high speed racing vehicles, the need for progressively lower ratio steps between gears increases as speeds increase. This is
because aero drag increases as a function of the cube of velocity.
3) For LS Racing your needs are: drag/F-1 style accelleration + narrow focus on high top end speed (such as Indy/Daytona) + reliability
to provide both (more than drag, less than Indy/Daytona) without an engine maintenance nightmare.
Your comments about your engine leave gaps in the information & specs that I need to know to make an intelligent assessment of where you are at with your engine. But in general:
A) "Long" duration race cams are typically a poor choice for low (static) compression engines. The duration of your cam should be
based on a number of variables, starting with octane of the fuel being used. It doesn't do any good to engineer a combination for
114 RON, and then use 90 MON, or vice versa. What is your 'effective' C/R? (when does your inlet valve close?) If you have 10/1
static C/R, and a "long" cam, your effective C/R might only be 6/1, depending on build specs. An additional complication is "dynamic
C/R" (effective C/R * volumetric efficiency) which may again, lower or raise the effective C/R.
B) Engines can "stop pulling" for a variety of reasons. But let's assume that it is valve train related. It could be, among others:
1) Inadequate valve spring pressure. For the rpm being run, for the mass of the valve train, for the "negative" accellerations of
the cam grind, etc.
2) Valve spring harmonics ("surge") Same basic reasons above. If the springs natural frequency is in the rpm range you use, you
need a different spring/spring combination.
3) Valve train motion uncontrolled. Same reasons again. Parts "flexing"/bending/binding, etc. Some of this is simple, most is
more complex to analyse.
C) When running your engine on a dyno, it is absolutely vital to use components (such as exhaust headers/pipes) exactly the same as
used in the car. It misses the point, to tune for a different combination on the dyno, than what you use in the car................
D) Whether your engine is "high winder" or a "stump puller" you are best served and get the best results by tuning for the "largest area
under the hp curve". I'll usually trade peak power for a "flatter/fatter" hp curve with better hp 'area', IF, I benefit from it.
E) You choose the rpm range your engine will run in, by making choices about the build specs/geometry. Some engineering and good
sense may be required here. The components chosen for reliability (bottom end) must co-exist with components chosen for power.
The power range produced must match with what is required for the vehicle.
If I know more specifics about the build, I can make a more specific comment.
Fordboy
