4.7 Build DCR Question?

[Cdn96XJ]

Hey everyone first post here.
I've searched high and low on the whole DCR issue for the past 8 hours. I've been doing a stroker build for the past seven months while deployed in Afghanistan. All the parts and calculations are almost done on paper. I'm worried about pinging but want to run in the area of 10:1 SCR. I'd like to go with either Comp Cams 68-235-4 retarded 4 degrees or 68-239-4 straight up. RPM range will be from 1000-5200ish

This is a higher dollar stroker built from the ground up, but heres everything related to the DCR.

-Iron Heads - Port, polished, 2.02 intake, 1.60 exhaust, 57cc chambers (smaller chambers handle higher CR's better?)
- Block Decked to Zero
- Head Gasket and quench 0.043"
- Pistons KB944 21.73cc bowl +0.060" (3.935" bore)
- 6.125 rods
- 4.2 3727 4CW Crank (3.895" Stroke)
- Intake Closes 36 Deg @ 0.050" (ABDC) + 15 = 51 (used in the KB compression calculator, http://www.kb-silvolite.com/calc.php?action=comp)

That being said, I also intend to run a good cooling system (Flowkooler pump) cold thermostat, Griffin 3 core aluminum rad and cooler spark plugs. Am I going to run into ping problems if my DCR is somewhere around 8.5-8.8? I can run up to and expect to run 91 octane as its available everywhere.

I hope I provided enough for an informed reply, and thanks for looking!

[banipal19]

First of all, I thank you for your service!

I am in the same boat as you are. I am by no means expert on this subject. This is my first engine build. With that said ... here is my take on it:

My SCR is 10.46 and DCR comes out to 8.61 which is considered beyond the limits of pump gas (8.5 is max on 93). To be on the safe side I am going to retard the cam couple of degrees which will bring down my DCR to 8.36 with 0 deck and .043 - .044 quench. I did a lot of research on this subject and there is no set DCR that determines which octane you will end up using. It depends on a lot variables (long vs short rod, altitude, air temp, spark plugs, pistons, heads, etc ...). I am going for 8.36 because I have seen few people running 8.3 - 8.4 DCR and their engines run ping free on 91 octane.

I am at sea level so prone to ping a lot more than some with higher altitude. Going to start putting together my engine today. When I am done, will let you know how it runs on which gas.

I am sure experts will chime in shortly and help us both on this subject.

[Cdn96XJ]

Thanks Banipal!

I'm curious as to which cam you have in mind, thats a good drop from 10.4 to 8.6 and 8.36. It seem's I can't get the DCR to drop more than 1.2 cr from the SCR. I hope I'm using that calculator correctly, I just think the Comps have too low an intake close at .050".

[banipal19]

I am using crane cam pn: 753901 and grind no: H-260-2. Here is my data from the calculator.

Calculated Numbers.JPG

[Cheromaniac]

Hey everyone first post here.
I've searched high and low on the whole DCR issue for the past 8 hours. I've been doing a stroker build for the past seven months while deployed in Afghanistan. All the parts and calculations are almost done on paper. I'm worried about pinging but want to run in the area of 10:1 SCR. I'd like to go with either Comp Cams 68-235-4 retarded 4 degrees or 68-239-4 straight up. RPM range will be from 1000-5200ish

This is a higher dollar stroker built from the ground up, but heres everything related to the DCR.

-Iron Heads - Port, polished, 2.02 intake, 1.60 exhaust, 57cc chambers (smaller chambers handle higher CR's better?)
- Block Decked to Zero
- Head Gasket and quench 0.043"
- Pistons KB944 21.73cc bowl +0.060" (3.935" bore)
- 6.125 rods
- 4.2 3727 4CW Crank (3.895" Stroke)
- Intake Closes 36 Deg @ 0.050" (ABDC) + 15 = 51 (used in the KB compression calculator, http://www.kb-silvolite.com/calc.php?action=comp)

That being said, I also intend to run a good cooling system (Flowkooler pump) cold thermostat, Griffin 3 core aluminum rad and cooler spark plugs. Am I going to run into ping problems if my DCR is somewhere around 8.5-8.8? I can run up to and expect to run 91 octane as its available everywhere.

I hope I provided enough for an informed reply, and thanks for looking!

The engine specs you've quoted are almost identical to those of the 4.7L medium buck stroker recipe on my site, so I think you'll be just fine.
I assume that stroker is going into a '96 XJ so make sure you get oversize injectors to prevent the engine from running lean.

[Cdn96XJ]

Great, thanks Dino!

Your site has helped out so much especially with all the stock vs aftermarket specs. It's really sped up the process of selecting parts.
All this morning I've been crunching numbers some more and I think the KB Calculator isn't that good. I managed to get 9.98:1 SCR and 8.39:1 DCR with the Comp 235-4 Retarded 4 degrees at the crank (identical to 239 but with under 1/2" lift) with two other calculators. I'll be safe there I believe. Deciding to go with 62.5cc chambers instead to unshroud the larger valves and the KB IC945's dished to 15cc's. I'll probably go with FoMoCo 26lb injectors with stock FPR and Split Second PSC1 unit to adjust fuel maps. Not sure yet. Thanks again!

[banipal19]

I just noticed that you said 6.125 rods. Just want to confirm that it is correct. Stock 4.0 rods are 6.123 and eagle rods are 6.125.

[amcinstaller]

6.123 is entered in the calculator but i see the 6.125 in the first post. kb945 pistons are mentioned so im thinking he means jeep long rods, not eagles

[Cdn96XJ]

Sorry yes its 6.123 OEM rods polished and shot peened.

[Cheromaniac]

I just noticed that you said 6.125 rods. Just want to confirm that it is correct. Stock 4.0 rods are 6.123 and eagle rods are 6.125.

Eagle rods are 6.150". Stock rods are 6.123-6.127".

[banipal19]

I just noticed that you said 6.125 rods. Just want to confirm that it is correct. Stock 4.0 rods are 6.123 and eagle rods are 6.125.

Eagle rods are 6.150". Stock rods are 6.123-6.127".

You are absolutely correct sir! Thanks for correcting me.

[Cheromaniac]

Yeah, it's a shame that Eagle stopped making them and the ones that do remain in circulation (mostly from 505 Performance) cost an arm and a leg.

[4.whoa]

Be carefull with unshrouding the valves. There is damn little room for it. Be sure to put the HG in place & scribe the bore onto the head 1st.
I see you plan to use 2.02 int valves. Are you planning on moving them in? It would prevent making the shrouding worse. If you haven't already, check the thread on head porting. There's a pic/ drawing of that

[gradon]

Eagle does make a 6.125" rod, but it's not originally for a Jeep app(you can see the big end is smaller for the offset ground crank). Here's the 6.125" on custom Ross 34cc dish pistons w/ ceramic coated tops and nitrated skirts(just got them and block from the machinist yesterday). The 6150 is in the bag next to it(some future build).

[SilverXJ]

505 performance or some other company also sell an aftermarket rod that is the 4.0l rod length with a press fit pin.

I second the caution on unshrouding the valves. On my 97 6120 head I scribed the head gasket line and there was like .020" at most between the fire ring and the combustion chamber. I opted to not attempt unshrouding the valves. Also, I have never seen and documentation on and gains by using a larger valve.