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Join Date: Oct 2005
Location: New Orleans, Louisiana
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2.4 S upgrade

Does anyone have any experience/data on approximate power increase in 2.4S when going from 8.5:1 compression to 9.5:1? (Assume nothing else is changed.)
Old 05-24-2006, 05:20 AM
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Common rule of thumb is 10 HP for that increase. A bigger improvement would be to go to 95mm P/C set. Your engine would become a clone of a Carrera RS engine.

-Andy
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Old 05-24-2006, 05:57 AM
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JGalt, it's not really going to do to much for the HP, although it will help a little. It will make a noticable difference in the torque both on- and off-cam.

- John
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Old 05-24-2006, 07:57 AM
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Hello, John.

I'm really surprised to learn that John..I know you will have the data to prove it!

The differnce subjectively is very significant IMHO..but as I say I know you will have the dyno plot to show different..

Kind regards
David
Old 05-24-2006, 02:19 PM
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I suspect that what you're noticing is the increased torque across the rev range. Driving my 2.0E with S pistons compared to a 2.2E with E pistons, they're actually pretty close. The peak HP number doesn't change much since that is really a function of peak air flow. Increasing the CR does help to prop up the combustion chamber pressure above the peak torque RPM, but the percentage benefit is pretty small when you look at the peak HP number, not to mention how many people can feel a few % improvement at 6500 RPM?

But the engine's performance at lower RPM's (off-cam) is much easier to sense, especially since most of us spend most of our time at those rev's. Here's one piece of data, note that I've plotted BMEP rather then HP. BMEP is not dependant on the engine size like HP is.

BMEP
RPM 70S 72S
CR> 9.8:1 8.5:1
2000 133 125
2500 133 133
3000 138 143
3500 144 151
4000 153 160
4500 162 167
5000 166 172
5500 168 171
6000 167 169
6500 164 162
7000 150 148

Note that the '70S had better performance below 2500 RPM, and above 6500 RPM. The '72S did better in between? Why? I suspect that the lower dome in the pistons improved the combustion chamber shape (always a weakness of Hemi heads) which made up for the reduction in CR. Now getting 9.8:1 in a 2.4 liter 72S doesn't require as large of a dome as in 2.2 70S because the 2.4 liter 72 has a longer stroke, which allows you get the same CR with a smaller dome. So I would expect the 2.4 with 9.8:1 to have a BMEP level more like the best of both of those columns of data.
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Last edited by jluetjen; 05-24-2006 at 02:57 PM..
Old 05-24-2006, 02:51 PM
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I hate to slip in like this but this begs the question of will 2.2Spistons on the 2.4L crank have a compression ratio of higher than 9.8? You said "getting 9.8 in a 2.4 '72S doesn't require as large a dome as in the 2.2S due to the lengthened stroke. I ask as I'm now concerned as I'm putting 2.2S pistons on that crank and I don't want more than 9.8!!
Thanks
Nabil
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Old 05-24-2006, 08:33 PM
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Hello Nabil.

No worry..this is the normal replacement piston for the 2.4S now..few use the low CR..Bruce Anderson notes this results in about 9.8, ie less than expected..the factory must have wrongly quoted the CR of the 2.2S..

Kind regards
David
Old 05-24-2006, 10:53 PM
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thanks brother, Had me sweating.
nabil
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Old 05-25-2006, 04:56 PM
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Best way to know for sure would be to "cc" your heads, cylinder volume, and compute the cr. So many things can change it, from surfacing the heads and cylinders, etc. You never ,know what has happened to these parts before you aquired them. Just another HO
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Old 05-25-2006, 07:03 PM
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If I were wanting more power out of the 2.4, I would take the case, and put it in my garage, and find a 2.7 7R case and build that up into a 2.7 with S cams, and have the fuel injection pump recalibrate to 2.7RS specs. As I say in the book, it will cost you a lot of money to rebuild a 2.0 or a 3.2 - you might as well rebuild the highest horsepower engine you can create.

-Wayne
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Old 05-26-2006, 01:08 AM
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Hi

Moving on from Waynes suggestion, put your 2.4S in the corner of your garage and dont touch it! Build or buy a totally new 2.7/2.8 engine, someday putting the 2.4S back in the car (if the numbers match) will double its value

My 2c worth
Neven
Old 05-27-2006, 03:17 PM
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