SCR / DCR general question

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Bubblhead
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SCR / DCR general question

Post by Bubblhead »

I am in the beginning planning stages of a build. I have a 99 Xj motor that is going to be my donor and eventually dropped into a 2000 XJ. Pulling the distributor and going to the dodge coil pack ignition. the vehicle has a 4.5 lift with 4.10 gearing and auto trans. I am going to use it as a DD and mild/moderate off road and over landing. I am looking for good low to mid power and torque or just a solid boost throughout. I have a good general idea of the build I want to do, but stuck with the whole dcr / scr questions. all things being equal, it seems to me that dcr is highly cam dependent. Is the goal to create the highest scr possible with a more moderate dcr? are there ideal ranges that each should remain in? Obviously want to remain on pump fuel even if it means 92/93, but would like to stay lower. Appreciate any help!
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Re: SCR / DCR general question

Post by Tolemar »

General consensus is 9.5:1 static with iron heads and 10.5:1 with aluminum heads is the limit on 91 pump gas. Ive seen first hand 12:1 on a 600hp small block 408 with aluminum heads with a pretty big cam. Granted the dynamic compression of that engine was around 8.5:1 with the intake valve closing later on that cam. From my recent research ive found any dynamic around 8.25:1 with iron heads needs race gas and 9:1 with aluminum heads. Now you can probably get away with a bit more if you back the timing down a bit. My 4.6 build I ended up with 8.5:1 dynamic with a .036 quench which is exactly where I wanted to be.
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Re: SCR / DCR general question

Post by Cheromaniac »

There are other variables to consider such as air:fuel ratio, ignition timing advance, cam timing, spark plugs heat range, and cooling system efficiency but Tolemar is in the ballpark.
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Tolemar
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Re: SCR / DCR general question

Post by Tolemar »

I meant to say 8.25:1 with iron head and 8.5:1 dynamic with aluminum head should be safe range with pump gas. Anything over would be pushing higher octane needs. Of course this is all with good cooling system and optimum fuel and timing which Dino pointed out. Of course you could possibly get away with more but why push it. My buddy did say his block 408 mentioned above did have very slight ping in overdrive but it could be manage with a slight timing adjustment but a bit harder to do with our computer controlled vehicles without a tuner. That 408 was also tuned on a dyno but on road driving with overdrive trans can change things a bit.
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Re: SCR / DCR general question

Post by Tolemar »

Bubblhead wrote: May 22nd, 2020, 5:34 pm I am in the beginning planning stages of a build. I have a 99 Xj motor that is going to be my donor and eventually dropped into a 2000 XJ. Pulling the distributor and going to the dodge coil pack ignition. the vehicle has a 4.5 lift with 4.10 gearing and auto trans. I am going to use it as a DD and mild/moderate off road and over landing. I am looking for good low to mid power and torque or just a solid boost throughout. I have a good general idea of the build I want to do, but stuck with the whole dcr / scr questions. all things being equal, it seems to me that dcr is highly cam dependent. Is the goal to create the highest scr possible with a more moderate dcr? are there ideal ranges that each should remain in? Obviously want to remain on pump fuel even if it means 92/93, but would like to stay lower. Appreciate any help!

I just want to be clear about something; static compression ratio and dynamic compression ratio are directly related. If static goes up so does dynamic. Dynamic will always be lower than static also. Dynamic is calculated using the intake valve closing angle which in my opinion is a better way to figure out what will work and what wont with pump gas. Old school thinking is 10.5:1 static is highest you can run with aluminum heads and pump gas. This is simply not true considering my buddies 12:1 pump gas 408 build I posted in thread above and the many other examples especially with the super efficient combustion chambers of todays engines. Also realize its better to run more compression with a good quench then less compression without a good quench. Ive been building engines that way since the 90s imo its truly the best way to build any engine.
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Re: SCR / DCR general question

Post by Bubblhead »

Thanks for the input! I understand the differences between SCR/DCR, i may have interchanged them in my first post. More directly, A combo of 0 Deck, .043 Quench, .030 over / 21 cc pistons, and a comp cam, I come up with 9.5:1 SCR and 8.016 DCR. was trying to figure out if that is acceptable. I am guessing there is a point of "to low" as well as "to high" that will affect performance in general. I don't want to build up a motor that is a dog in comparison to stock because I chose wrong.
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Re: SCR / DCR general question

Post by Tolemar »

Sounds good to me. Should work perfect.
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Re: SCR / DCR general question

Post by Cheromaniac »

Yup. Looks good to me too.
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Re: SCR / DCR general question

Post by Bubblhead »

Thanks!
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Re: SCR / DCR general question

Post by Shamusdick »

Hey Bubblhead, thanks for the post. I'm in the planning phase for my 02 TJ and had similar questions, so this thread was helpful. What did you mean by comp cams? Stock 4.0 cams?
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