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Old 05-14-2003, 02:59 PM
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Post 9 to 1 compression or 11 to 1 compression. What is the best?

I have read the engine specs on several new performance cars such as BMW Z3, porche boxter, and others. I'm not into these types of cars but many of them have upwards of 11 to 1 compression on pump gas. I'm getting ready to build a 302 ford engine and have been told not to go over 9.5 to 1 compression because of detonation. Why would I be so limited in this area while other cars have no problem? Can I go 11 to 1? What would a person need to do to acomplish this?
Just wondering how this is done.


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Old 05-14-2003, 03:07 PM
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Sorry about the misleading question. My real questions were a bit different.
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Old 05-14-2003, 03:26 PM
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Do a search on the board. Been covered in depth many times.


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Old 05-14-2003, 03:36 PM
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Chevy LS1`s and LT1`s had 10:1 compression also on pump gas, they can do it as can the engines you metioned due to more efficient combustion chambers that need less spark advance to make the same power, and total control of the air fuel mixture and spark advance via computer. on the older engines as we hot rodders love, don`t have these luxuries on shoe string budgets. you can run high compression on the older engines with longer rods being that it`ll "dwell" on top dead center longer or just with regular rods depending on the camshaft size, the bigger the cam the more the overlap figure which bleeds off cylinder pressure at lower RPM and builds it as the engine revs being there is less time for it bleed off.but as I`ve learned and read about, there`s a limit here also, in the upper RPM ranges with the bigger cam it still can detonate and you won`t be able to hear it. another example is the Quench distance, which is the distance between the flat surface of the head and the top of the piston, a tighter quench of around .040 will enable a engine to be less sensitive on pump gas with higher compression, but also you have to be careful on how tight the quench is set, if it`s too close and the piston has a lot of cylinder to wall clearance, there`s a possiblity of the piston smacking the head. most street rod guys don`t check quench clearance and don`t know that the basic cast "rebuilder" pistons are cut .020 off the tops, the piston company advertises them as "10:1" compression pistons with s 64cc head when it`s really about 8.75:1. so as you see from what I posted, there are ways to run higher compression, but a few varibles enter the picture, another way to run higher compression on pump gas is to add water injection.
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Old 05-14-2003, 05:21 PM
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Most of these engines probably have alunimium heads don't they? That makes a difference.
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Old 09-22-2003, 10:12 PM
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ya double vision is competely right. They have done many things with the late model cars to make it possible to run high compression from the factory on todays pump gas how ever its generally not a good idea to try running that kind of compression on the street. To run it with out pingin on pump gas you would probably have to retard the timing so much it would deminish any power gain you would have got from the compression increase from 9.5 to 11:1 besides the amount of power you would gain even with optimal tuning on both would be very minimal, alot of people would be suprized how little power gain there is to be had by increasing compression.
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