Blower Overdrive Change

How much ET change should I see at 2800 pounds going from 44% overdrive to 35% overdrive on a 14-71 Littlefield High Helix

 

Every motor would be different

 

I realise that... But is a 14-71 littlefield going to be losing efficiency at 44%

I am hoping it wont be that much of a change.

 

with a 10% drop in overdrive you might see about a 4lb drop in boost, all other things staying the same.

 

Other things come into play also like top opening length , and end clearances .

 

some times lower overdrive may make the power more managable and some times run faster but agian every application is different speaking from experience i had a blown 514 chevy at 47 over with a powerglide hit the throttle tons of power but i didnt know which way it was going to go it would run 4.50s easy coasting went 4.03 with a crower glide one time finely sold it to a friend of mine and he spun it 8 over and it went 4.30 all day long thats right 8 over go figure i was pissed but way back then i didnt know much about a blower other than they looked cool and i wanted one

 

more stuff in the technical pie

A trend now is gearing the blower to the racetrack. One blower source said their 14-71s work good to about 12000 blower RPM. So if you are spinning yours over 12K and it works like some other blowers, if you slow it down to 12K, your car may be faster. In addition, there is a lot going on at the inlet. The size of your inlet opening and injector scoop approach affect the mass air flow through the blower. Again slowing it down may help in that area also. In effect, you may slow down, stay the same, or even speed up with less blower overdrive. I know of several racers last year who went faster when they slowed down their blowers. An increase in boost reading from higher blower speeds is not necessarily an indication of more air. It may be from more heating, and it may also be boost backing up behind the intake port or valve from reaching a sonic speed limit. Likewise, slowing down the blower may produce a lower boost reading but not necessarily a slower car. What happens through that blower is pretty goofy. Mostly trial and error.
Probably all you can do is try it and see. It may go in one direction for you and another direction for another car. At the overdrives you are talking about, it may be unpredictable from one car to another.
Bob Szabo
author of "Fuel Injection Racing Secrets" and "5000 Horsepower on Methanol"
www.racecarbook.com

 

Sonic

Bob
Using head CFM @ 28" and a real life boost number could this sonic stuff be calculated. I'm assuming not as the airflow characterstics would seem to change. EX. When flowing a head at 10" vs 28", turbulent air can show up at 28" and screw up the CFM.

 

sonic

Interesting you said that 28" can produce some turbulence not seen at 10". Wet flow is even more interesting in the cylinder head department. Yes the port speed can be estimated, but it looks like most blown 2 valve engines go sonic in the port when boost goes much beyond 2 atmospheres. Then it does back up in the manifold, increase density, then flows through the head at sonic speed. Sonic speed at STP is about 1100 FPS if my units are correct. It is different at higher temperatures and pressures. An optimum port velocity in a optimum computer designed normally aspirated racing engine is around 300 FPS (a lot less than 1100 FPS). Higher port speeds in these NA engines cause pumping losses. Lower port speeds cause air to fuel separation. There is some argument from combustion engineers about the optimum speed, so don't hold me to that exact figure, but it is in the ballpark, and a lot less than high boost, blown engines. So the blown engines are in a world of their own.
I think you cannot put a large enough valve and port in a high boost blown 2 valve engine. You end up with the trade-off of RPM limit from the larger valve vs. the benefit of reduced manifold boost from good head flow. Also you end up with a trade off on the short side radius where if you increase the port volume cutting into the short side radius and decreasing it, you get some weird separations. We wrote about the fuel mixture in our methanol book from some heavy duty technical sources. It is an interesting goal to get (a) vaporization for combustion without flow separation and port collision, and (b) without overheating the mixture causing excess boost (blower drive losses) and (c) chemical dissociation causing other problems. A lot of racers developed winning combinations from a lot of trial and error and a lot of experience. Yet I found that science explains a lot of the fuel behavior and engine flow characteristics and can provide short cuts to power making. This port velocity issue is a whopper of a limit. Add to the the air flow through the blower from the inlet design and the outlet and you have a real challenge. Changing the blower overdrive, searching for better performance, becomes a solution to a big mystery. And as said previously by others, different from one setup to another.
bob szabo author of 5000 Horsepower on Methanol and Fuel Injection Racing Secrets
www.racecarbook.com