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Originally Posted by boneheaddoctor
Diesels are not throttled....thats why they really don't have any engine braking you will have in a gasser.
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SI engines, which ARE throttled have significantly less engine braking than diesel engines of the same capacity, which are NOT throttled.
The reasons for the greater engine braking on diesels are that the higher compression of diesels (20 - 25:1) Vs SI engines (8 - 10:1) and that the throttling of the air charge reduces the inlet pressure (manifold vacuum) and there is much less air to compress in SI engines.
It takes more energy to compress the intake air to 1/22nd of it's volume than it takes to compress a throttled air charge in the SI engine to 1/10th of it's volume.
And when you restrict the exhaust flow significantly (exhause brake), there is little expansion of the air, which could otherwise recover some of that energy.