cardo0 wrote: ↑Mon May 25, 2020 12:07 am
So lets look at a 350 ci chevy at only 2000 rpm. That's 350 cubic inches of air times 1000 per minute at 0.8 efficiency = 280,000 cubic inches of air per minute and divide by 60 = 4,700 cubic feet per second. Well a 55 gallon drum has only 7.4 cubic feet of air so the engine would empty that 55 gallon drum plenum in 1.6 milliseconds. I don't think you can have too much plenum volume to delay throttle response.
There are 1728 cubic inches in a cubic foot. The small block in your example has an air flow of 162 cubic feet per minute, or 2.7 cu ft per second.
We tried a lot of tunnel rams back in the day. The dyno always seemed to want more volume, but at the track we were gluing wooden blocks into the plenum and the car went faster down the track for the same reasons outlined above. It bogged down on launch.
Yes I made a math error but there's no way to edit a post to correct that. It should read 4,700 cubic inches per second. Good catch. But using 4700 ci/sec will empty a 350ci plenum in 0.7sec. Or FYI a 434ci motor will empty a 377ci plenum in 0.065sec at 2000rpm.
i remember reading about the effect of a new honing process on vac , this was a fully build flowed engine until this they though it was perfect , but the new process it now ran vac at wot instead of atmosphere , showed they needed to relook at everything , but vac readings would also be an indicator
What are the advantages and disadvantage, regarding IR and plenums? In particular, IR's with throttles in the runner and not drawing from a common plenum.
How close would sizing the individual intake runner ( injected sprint car set up for example) to a percentage formula of the intake valve diameter??? Mark H
ITB/IR intakes are usually sized for either cylinder displacement or hp per cylinder.
Advantages: Instant throttle response. Less sensitive to idle issues when running big cams. Easier to tune to specific power/torque range because stacks/trumpets are easily changed or adjusted (Ferrari took it even further with a continuously variable intake runner). When properly set up can have broader power range than common plenum. Airflow to each cylinder easier to balance than common plenum.
Disadvantages: Initial cost. Throttles and linkage can fall out of adjustment. Not legal in all classes. Mechanical injection was often "continuous" or "constant" flow as opposed to sequential EFI. Harder to tune than intake with common plenum and one air/fuel source. Air filtration more difficult.
If you compare two properly prepared engines, peak power will likely be the same. IR will often have a broader torque curve UNLESS compared to Variable Length Intake Manifold, which could have a torque even more broad than either a "normal" common plenum or IR.
Good friend of mine runs s old Hilborn gas injection on bbc with velocity stacks. That thing is dead on each run and flat out hauls ass. They set on dyno and never touch again. Seems to never change.
cardo0 wrote: ↑Mon May 25, 2020 1:06 pm
Yes I made a math error but there's no way to edit a post to correct that. It should read 4,700 cubic inches per second. Good catch. But using 4700 ci/sec will empty a 350ci plenum in 0.7sec. Or FYI a 434ci motor will empty a 377ci plenum in 0.065sec at 2000rpm.
You got it backwards. As throttle is closed plenum is empty, as how much vacuum engine could make. After throttle is opened first plenum has to fill up and only after that can plenum fill cylinders.
If plenum is big, like multiple times of cylinder volume that void in plenum will affect significantly throttle response. For good throttle response ITB with blades close to intake valves are used, so there's not much volume in vacuum when throttle is closed.
