CR versus HP

[ponyboy88]

Question: I was wondering how much HP id lose building an engine to run in the low 9:1 comp ratio over one built to run in the 10.5 range. Im thinking of building a 427w off a world or dart block (hence the other topic) for my street car, and for now it will be relativly well manored but if i decide to bolt a blower down the road i'd like to have the motor ready at least comp wise. I know a new cam would be important for the motors performance, but was wondering how much the two were related. Is it possible to get 450-500 HP to the wheels on a 9.5:1 comp 427w. Thanks N.A

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[Adam90LX]

With the right combination of parts properly tuned, yes it is very possible to get 500 HP to the rear wheels with a 427W engine at 9.5 to 1 CR or even at 9.0 to 1 CR. As for how much power you will loose building the engine at 9.0 CR compared to 10.5 CR, I can tell you the old rule of thumb for CR ratio is about 4 percent total power loss for each compression point lowered. I can also tell you that rule of thumb as almost all other rules of thumb are false in real life. There are simply far too many variables to consider in order to determine the actual power loss of dropping the CR by 1.5 points on different combinations to have a general rule of thumb be accurate. Proper camshaft selection and installation is as always critical in how either the na or sc combination will perform. Are you considering changing the cam when you add the sc later? The intended purpose and or goals you have in mind for the vehicle are also very important as optimizing a na vehicle as compared to optimizing a sc vehicle will require more than engine differences.

[ponyboy88]

if i decided to run a blower later on i would pull the cam and replace it for one better suited to run boost. For now the hopes is a simple and well built street motor, but i figured if i built it in the relativley low compression range, the heads and pistons could be kept if indeed i do swap to a boosted set up.

Right now i am planning on doing an aftermarket block, 427CI stroker, afr 225 or tfs high port heads, and a tfs R efi intake, with all the fuel trimmings needed to keep up. (42lb injectors enough?)

i found a compression calc. that said about the same as you, so thank you very much for the information.