February 16, 20179 yr 13 minutes ago, oni nou said: It is particularly effective in the 4t engine at creating strong vacuum on the piston down stroke. I.e. Induction stroke with throttle closed. But curious to understand how does a valve in crankcase effect induction tract?
February 16, 20179 yr Author 49 minutes ago, oni nou said: It does not..... it releases engine braking effect of pressure in the crankcase that pushes against the bottom of the piston [on the down stroke] that would cause a braking effect If the engine produced anywhere near the amount of crankcase pressure required to provide engine braking then surely the oil seals would be under huge pressure too ? Longer duration cam lobes seem much more likely to be responsible for the reduction on later engines , Well to my mind anyway
February 16, 20179 yr Most 4ts vent crankcase to atmosphere so presssure on down stroke is negligible ?? Std 4rt goes into bottle under headstock and then air box for emissions but most dump this and vent to atmosphere.
February 16, 20179 yr 6 minutes ago, chris200 said: If the engine produced anywhere near the amount of crankcase pressure required to provide engine braking then surely the oil seals would be under huge pressure too ? Longer duration cam lobes seem much more likely to be responsible for the reduction on later engines , Well to my mind anyway My thinking exactly. std set up on 4t is no crank seal. 4 rt has separate oil for engine an gearbox so must be a seal between gearbox and crank on off side ( there are posts about this blowing out if to much engine oil is used), if crank pressure is the holy grail then if you take the dip stick out engine braking would increase?? project for the weekend??? Edited February 16, 20179 yr by jimmyl Error
February 16, 20179 yr Author Just now, jimmyl said: My thinking exactly. std set up on 4t is no crank seal. 4 rt has separate oil for engine an gearbox so must be a seal between gearbox and crank on off side ( there are posts about this blowing out if to much gearbox oil is used), if crank pressure is the holy grail then if you take the dip stick out engine braking would increase?? project for the weekend??? I was thinking of measuring mine
February 16, 20179 yr 2 minutes ago, chris200 said: I was thinking of measuring mine I like the all or nothing approach !!
February 16, 20179 yr Author 9 minutes ago, jimmyl said: I like the all or nothing approach !! Yeah I'd like to measure it all but have a strong feeling I will get nothing
February 16, 20179 yr Quite a few cars run the crankcase a less than atmospheric pressure to reduce oil consumption and the chance of oil leaks.This is done in combination with bleeding crankcase air into the inlet tract before and after the throttle. Engine braking is by vacuum on the top of the piston when the throttle is closed. Altering the crankcase pressure alone would probably not have that much effect on engine braking as the pressure acting on the piston underside on the downstroke would be similar to the pressure on the upstroke, thereby cancelling each other out and giving little or no net braking effect. The piston moves at a leisurely 10 to 15 m/sec so you would not think air resistance would have much effect either. Some cars are fitted with a throttle body damper that slows the rate of final throttle closure, this was done to remove jerkiness which was a problem on early EFI engines. This significantly reduces feeling of too much or sudden engine braking. Another way to reduce engine braking / pumping losses is to have variable valve timing and lift but 4Rts don't have this. I would think the reduced 4RT engine braking is by a combination of reducing mass of air in crankcase, combustion chamber and subtle valve timing and throttle body actuation but its just a guess.
February 16, 20179 yr BSA B40 engines have negative crankcase pressure FYI . Still plenty engine braking however.
February 16, 20179 yr Author Just had a thought , Most fuel injection systems cut the fuel completely on the overrun for the sake of economy but what if they altered the ecu to inject a small amount of fuel, Just enough to provide a push down on the piston on the firing stroke ,That would definitely reduce engine braking . I think
February 17, 20179 yr Wow: you boys are really technical. I simply increase 'tick-over' ....... and lower the gearing to compensate......
February 17, 20179 yr My 2005 has no noticeable engine braking ,it was achieved with a modified map !! has worked very well for many years !!
February 17, 20179 yr Author 1 hour ago, geeman said: My 2005 has no noticeable engine braking ,it was achieved with a modified map !! has worked very well for many years !! In which way was the map modified ?
February 19, 20179 yr On 2/17/2017 at 11:24 PM, oni nou said: The first part of the video shows the location of the reed valve at the bottom of the crankcase for those that are interested https://youtu.be/9raMMmxmJ8U The reed valve appears to allow the crankcase to pressurise, but relieves a depression. Could the reed valve be something to do with the oil system. If you allow the crankcase to build up pressure, that pressure could be used to scavenge oil back to the tank (gearbox?) without needing a scavenge pump. I'm not familiar with that Honda engine but I think I remember someone saying the same oil lubricates engine and gearbox.
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