Using Pat Kelly's DCR calculator:

http://www.empirenet.com/pkelley2/DynamicCR.html

I am considering someone's setup that would require the Mahle pistons to be 11cc domed with 6.0 rods. This bumps the DCR to 9.4.

Now granted I am running a current setup that is giving 9.2:1 in Phoenix and have NEVER had a problem with predenotation. However, I keep my coolant in the 190 F range when racing.

Just seem all thoughts are to never go above 9 on the DCR and with this guy's rods at 6.0, I have limited choices on domed pistons to get me to 9ish.

On another note, are domed pistons an issue? I hear stuff about limiting flame fronts, etc.

Suggestions on what to do are appreciated.

Ben

 

Big block Chevys use huge domes and make good power*. So, while all else being equal it is better to have a dish, with flattop second best in terms of combustion efficiency I wouldn't worry about a small dome. The DCR concept has more to do with low speed operation when under high load, which you shouldn't be doing with a high performance car anyway. Still, with all the SBC pistons available I am unclear on why you think your only choice is the piston you mentioned. What is the rest of your combo? What is your cam (which will largely determine the CR you need).

Example of BBC dome piston that works fine:



A SBC piston with an 11cc dome should pose no problem. However, I have no idea if that is really the right CR for your combo.

*of course, the chamber is larger and shaped differently, but that's just an example.

Rich

 

Rich, as always, thanks for the input. If this is my real worry: low rpm high load, I am not going to be doing this so it "sounds" like this issue goes away.

I thought there was not many choices since I went to both Summit and Thunder Racing to see what was available for 6" 30 over domed hyper pistons. 11cc was all that came up. Going N/A in a 383.

As for the rest of the combo, they have modified Vic jr, 23*, 70cc heads. I have not seen the cam card yet so the only spec's I have are the 0.050 numbers: 236/246. I had to "estimate" the advertised numbers to punch into the DCR calculator.

The engine is not together yet so now is when I can make changes.

What would determine the right CR (DCR or SCR?) for me?

 

I don't think you would lose much with a flat top, but it would be better with a little more compression, I agree.

SRP 140348 might be a good choice.
bore: 4.030"
stroke: 3.750"
rod length: 6.000"
7cc dome
So, there are other choices. Who designed the cam? They are the best source for CR information. What is the piston to deck clearance? Have you thought about adjusting your CR with gasket choice?

Rich

 

Rich,

Now we are getting somewhere with that selection. Thank you!

Schneider Racing did the cam. I will get the complete cam spec's before this thing moves forward.

I don't have the deck information yet either but asked for it. My intent would be to keep the quench in the 0.040 area.

 

Domed pistons are not a problem. There are a ton of choices out there for off the shelf pistons. If you put your combo together right a dome will work fine.

I'm running a JE domed piston with 6.0 rods. With a 63cc head, .039 gasket I came out with STR of 12.4 and DCR of 9.0. Cam is in sig, I dont have the exact specs off hand to do the math again but you get the idea.

With 70cc heads you will probably need a rather large dome anyway. Have you looked at other manufactures?

 

To do that it should be zero decked, or pretty close. The best all round LT1 gasket is the Fel Pro 0.039" (#1074). If it's zero decked you are going to be at 11.95 static with the 7cc piston and the 0.039 gasket. Pretty high but your DCR is going to be in the 8.9:1 range so that may be a real good choice. Don't be "lugging" that motor though, or pingpingpingpingping.

Rich

 

This is a guy's setup with cam custom ground for his heads that have been worked. I would prefer to use his cam but we will really see when I get the cam spec's.

 

OK Rich, one final question:

What is the SCR that I should not go above? I intend to rev to probably 6800 or so.

 

12:1 is going to be pushing things. You may have to compromise timing so much that you lose more than you gain. I guess I'd like to see you around 11.5:1. The only way to know for sure is to actually try combos out. Since 99.99% of us don't have that luxury I tend to be pretty conservative when advising others to avoid mistakes and maybe lose a few hp in the process. Maybe that 11.95 combo will be good, I think it would be right on the limit and you had better have a good tune and have a good cooling system, not run it hard when it's real hot ambient and so on. I'd be tempted to try it, and then I would only have myself to blame if it didn't work.

Sounds like I told you two separate things, but I didn't. I can't decide for you if the "rsk" is worth it. I think the cam manufacturer might help by giving you a range of CR the cam is supposed to work with.

Rich