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Old 06-08-2018, 04:09 PM   #1
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Optimal CR for boosted street truck?

What is considered the optimal compression for a street truck running 15psi @ 6000 rpm on pump gas? My guess is 8.5 cr with a big stroker. With a standard water/meth injection setup this IMO will keep the engine away from detonation under the varying load conditions found while drive on the street year round.

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Old 06-08-2018, 04:15 PM   #2
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That’s an old school thing new stuff can take a lot more compression (IE: my truck) the common thing back in the day was around 8-9 and high boost now today’s with new head designs and what not and vvt higher compression is the normal.

I am running 10.5+ easily likely around 11:1 I think is what eddy the guy that built my motor assumes from the specs we have and the times we have turned the heads down now lol!

Tuning also plays a big role is all of this as well. If the tune is junk the motor will not handle it well at all. Methanol is highly suggested on 93 as it will allow you to keep timing in it without it timing should be pulled down a good bit for engine safety.
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Old 06-08-2018, 06:53 PM   #3
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Yep, your truck is a mystery for sure. I have no idea how a 11:1 static cr can run with 10psi on pump gas. Even with w/m injection giving you a fuel octane of 100 it is still looking at very high compression ratio. The cam of course has a lot to do with regulating cylinder pressure. Red has seen 15psi on static cr of 9:1. It's cam has very little overlap so it also has a unreasonably high cylinder pressure. Of course it uses w/m for 100 octane pump gas. These engines seem to do very well with 15+psi. My next engine will be a lower cr, not higher.
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Old 06-08-2018, 09:25 PM   #4
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If I build another it will be the same way likely around 10.0:1 with correct pistons this time evn though mine are yet to let go! I run 15-17 psi now well as soon as I get the trans issue solved lol.
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Old 06-09-2018, 12:14 PM   #5
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With the modern engine management systems, and the advances in Cam designs, 9-12 CR is not out of the question .
Some of these "boosted cams" are closing the Intake valve 10-15 Degrees later in the cycle than they would of 10 years ago, which has dropped dynamic compression to areas that are more suited for boost but allows a higher static compression to help with quench, and the more efficient the combustion cycle is the less there is chance for detonation
9.-10 seems to be the edge with 93 add meth and you can get to 11. , the E85 guys can get away with a little higher.,
More Boost = the need more cooling, think about how far that area of a Boosted build has come just in the last 10 years.
Most 4 bangers in the 600HP range are doing it with 11-12 to 1 and 20 Lbs of boost on 93, it comes down to tuning and right part selection. keep in mind that a smaller bore(3.88-3.97) are easier to control all the variables.
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Old 06-09-2018, 12:35 PM   #6
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The hellcat is 9:1cr... I may have to re-evaluate what I think I know.... lol
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Old 06-09-2018, 03:35 PM   #7
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Quote:
Originally Posted by Redtruck-VA View Post
The hellcat is 9:1cr... I may have to re-evaluate what I think I know.... lol

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