alnukem

I know this is probably a Stupid Question........I know in racing, if you're not on the brake, you should be on the gas, but, in spirited street driving, does a higher compression ratio slow you down quicker if you let off the accelerator (without using brakes) than the same engine would with less compression? And also, what does numerically higher gear ratio do in this instance? Thanks!

Kevova

An engine with a higher dynamic compression ratio will have more coast down braking than one with less. The camshaft profile can play in how much cylinder pressure is developed. Some long duration high overlap cams can develop lower cylinder pressure in a engine with a high static compression ratio, than a lower static compression ratio and a tighter quick action cam. Quick or Fast action cams are typically stock class "cheater" cams. They have the the stock lift and duration, but the lobes are made in such away the to maximize the amount of time the valve is open at full lift, which can boost cylinder pressure. When coasting down in gear you are allowing the rear tires in conjunction with the differential are turning engine over. Depending on fuel system there will minimal fuel for spark plugs to ignite. The energy required for compression is being dissipated during coast downs. A numerically higher ratio will allow for more braking because of the increased compression strokes (rpm). Engine braking will not stop you quicker than using brakes. Unless the brake system has overheated ie... brake fluid is boiling or pads failed. Engine braking can help prevent brake fade particularly on twisty mountain roads, by reducing dependence on hydraulic brake system.

alnukem

Thank you very much for your reply. I assumed that this was correct & explains why I like certain cars over others in curvy road driving, they have steeper gears, higher compression & modest cams, these make really nice cars to drive spirited. Not everything has to be maxed out!

outhouse

Not always true.


Throttle control is much more important then a simple on or off.


Yes, but not enough on a V8 to make enough difference, the weight of the car and multiple cylinders means you will use your brakes heavily. Not enough back pressure on these cars to mess with, or they would have electronics letting us adjust the amount of backpressure.


Where you notice your questions come into play is on motorcycles. Example on a high compression twin like a Ducati you really notice engine braking and can use it in the canyons. On a 4cyl despite high compression, you are on the binders much much more.


Last example, I have a spot on my bike where I hit 100mph in a very short straight section with curves going in and on exit, I do this almost daily. On the car the first time I tried to engine brake though like the bike, and I flew around the corner a little to hot for my liking. Lucky our cars out corner bikes Best ive done on the car yet is 93.

alnukem

Thanks....the last part sounds fun! What compression is the Ducati?

outhouse

12.5 to 1. 200+ HP, high 9 sec bike.