Is there any rule of thumb for figuring out what DCR will detonate for a given octane rating? I'm building a 381 stroker and my DCR will be 9.17:1 and the best pump gas I can get here is 92 octane (it has ethanol in it too if it matters) so will I be ok or should I lower compression?

Thanks

 

PGM it is a little on the high side. We are going to retard the cam some. looks like a lose of 1Peak HP but a gain in the mid range.

I will shoot JimBob the link so he can come give up some of his research

BTW I am in Kirkland during the week and live in Monroe

 

Yeah that could be bad....

First question is what is the IVC point (and not at .050) ?

What calculator did you use?

Gotta make sure you are really there, been there done that before.

Bret

 

Bret: I'm using an LT4 hotcam so I think intake closes @68* ABDC (not sure this is exactly right, but I think close) and my SCR is 11.76:1. I'm using the RB Racing calculator.

I haven't actually ordered the bottom end parts yet, so luckily I can still make some changes

This isn't a really wild buildup, I don't plan to spray and I'll just be using KB hyper pistons, I beam rods and cast crank so I'd really like to avoid detonation. Should I shoot for about 9.0 or less?

Thanks again guys

 

Way to high for that cam and pump gas.

 

You need something around 16cc dish piston depending upon fel-pro or gm gasket, and I assume you are using 58cc lt-1 heads? 5.7 or 6.0 rod?

 

wow... not a chance of that staying together.

First thing is hyper pistons aren't gonna take the detonation. The rest of the parts aren't going to like it either.

With a 11.7:1 SCR I would want a cam over 230degs @ .050....

You should look more at another cam and a SCR 11:1-11.5:1 and a DCR of 8.3-8.5 range to be safe.... With your HotCam you really have to get that SCR down below 11:1.

Bret

 

Hey Bret...

Just for info here....would you favor a DCR of 8.82 and quench of .04 or DCR of 8.58 and quench of .05. That is what my engine comes out to using Kelly's calc. The same goes for Ellis's engine. We are just trying to skim the line but not go over...especially using air conditioning in the summer here, although it doesn't get over 85* in the summer... We still have time to change the head gaskets out.

4.06 bore
3.875 stroke
5.85 rods
4 cc dish
.011 piston to deck
combustion chamber of 59 CC's
gaskets of .039 and 4.125 ring (felpro) or .029 with 4.1 ring (impala)
cam Intake 289 base at 114 Lsa and Icl "78.5 ABDC by Kelly calc"

Thoughts?

Thanks Jimbob

 

You sure it's that big of a change in DCR? Most times the DCR changes pretty close to the same percentage as the SCR....

I like a tight quench to keep detonation away but even a .050 is ok, you can still do the racerdude trick and run a MSD timing retard box and dial it back if it's too hot for the motor.

Bret

 

Thanks for the info...

Yeah it works out to...

SCR 12.128 DCR 8.58 .039 gasket

OR

SCR 12.480 DCR 8.82 .029 gasket


Jimbob

 

I would be very very worried about doing those 396 motors, especially on a .060" block. It's not worth the extra grinding and a potentially scrapped block for the extra 10 cubes. Make sure at a minimum you have the block sonic tested competently.

 

MachinistOne...

Man no love.......Actually, this is the engine I'v been running and racing on since 1997. Just changing heads, cam and compression (hense the DCR question). But, yes that is good info for sure....and, I would strongly agree you should have any block sonic tested if going that big....otherwise followed by


Jimbob (man of low post count due to this site erasing mine a long time ago)

 

I am running 9.1DCR on 92 octane on the street and so are several others on the board.
A 9.1 with good cooling and tune will be OK on the street on pump gas.
If she wants to detonate put a boost timing **** on the console and turn the timing down a couple of degrees during hot days but cool nights at the track it will be right.
If you build it with 8.2/3 ya will be leaving a lot on the table and it will always be like that,if you go high ya got ****, cooling,and an upgrade to fuel if ya want.
Try to keep your quench as close to .040 as possible,but it ain't chiseled in stone.

 

I used 56cc chambers in my scr calc., because it seems like noone agrees what the stock chamber volume is--I've read everything between 54 and 58cc on this board, so I plan on measuring my chambers before I actually buy parts just to be sure. Here's what I was going to use:

Eagle cast 3.75" crank
KB .020" over -7cc dish hyper pistons, 1.433" CH
Scat 4340 5.7" I-beams
piston-deck clearance is .013" (block decked .004")
SCE Pro 0.032" gasket
stock chamber size

So if I go with a bigger cam, like the gm847, DCR is 8.77 instead of 9.17, maybe a bit on the high side still but the chambers probably really are 58cc in which case my DCR would actually be 8.56 with this cam. My only concern then is emissions. Do I have any chance of passing the sniffer with the 847?

And if I opt to lower the SCR, I don't see how I can do that without hurting quench. Doesn't a big dish piston hurt quench too?

 

The quench area is between the flat of the head and the flat of the piston, good aftermarket pistons have the dish located so it matches the placement of the chamber...so it doesnt matter how big the dish is, just the piston crown surface to the head surface.