I am building a 383 LT1 engine for my Corvette, seeing as nitrous and stock pistons didn't get along to well together. I am stuck between what pistons to use and what ultimate compression ratio to go with. I already have a camshaft and ported heads:

Cam - Comp
Grind# LT1 3442 / 3443 HR112+3
Duration 233° / 239° @ .050"
.5696" I / .5776" E

Heads stock LT1 castings with 2.00" / 1.56"
Combustion chamber: 52.5 cc
Flow #
.100" 75 54
.200" 126 108
.300" 180 145
.400" 219 170
.500" 251 186
.600" 265 188
.700" 270 190

I am building a 383cu.in either a 5.7" or 6.0" rod depending on piston choice, I really want a shelf piece of price and fast availability, cause most places are 3-4 weeks for a custom set

Gasket: Felpro .039" COT / 4.125" bore


Pistons option 1:
Mahle Powerpak Inverted dome -26cc volume comes either 5.7" or 6.0" rod
With a zero deck 9.000" block this will give me 10.008 SCR & 7.615 DCR (5.7 rod) or 7.579 DCR (6.0" rod)

Piston option 2:
Wiseco Pro Tru Reverse dome -20cc volume comes in both 5.7" and 6.0" rod. With a zero deck block this will give me 10.675 SCR & 8.104 DCR (5.7 rod) or 8.066 DCR (6.0" rod)

Piston option 3:
Mahle Powerpak Inverted dome -16cc volume with 6.0" rod. This piston with a zero deck will give me 11.177 SCR and a 8.433 DCR

I have a CSR electric water pump, and tuning software along with a nitrous system that I plan on using. I really like the Mahle pistons because they are priced right, are coated, and come with rings. Is the 11.177 compression ones too much to run nitrous on pump 93 octane. Will I lose much power going with the 10.0 Mahle pistons, or will this give me a safety margin when using the nitrous?

Let me here your thoughts

 

I'd probably use the Wisecos. the 11 to one Mahle deal wouldn't be too mad if the cam isn't run too far advanced but it still might be more detonation prone.

 

I just wouldn't run that cam.... I'm not a fan of those cam lobes and trying to fight physics to run them.

BTW you DCR calcs are WAY OFF.... you have to give the actuall IVC not the IVC at .050"

Bret

 

I think I am leaning toward option 3:

Mahle PowerPak -16 dish
Zero deck:
SCR 11.17
DCR 8.433
Quench .039"

.005" deck
SCR 11.04
DCR 8.333
Quench .044"

.010" deck
SCR 10.91
DCR 8.236
Quench .049"

 

Except for very unusual situations, if you use N2O the way most people do (when at the track) you should pick the cam (and CR) using the same criteria as you would for an NA application. If you want to make the combo a bit more "nitrous specific" you can add ~5 degrees to the exhaust duration ans open up the LSA 1-2 degrees to avoid excessive overlap with the larger exhaust duration.

Rich

 

BTW as I said..... DCR is not even close...

That 233 lobe on a 109 ICL and a 11.1 SCR = 8.2:1 DCR.

Try 73 for you IVC and you might get closer.

I wouldn't give up quench area to lower the compression ratio in ANY situation.

Bret

 

The calculator I am using has been giving me 70.5 for the IVC, is the Pat Kelley DCR calculator wrong?

 

yeah and that's why the DCR is a tad high.

 

BTW, option #3 looks fine. 11.2 static is not all that much with aluminum heads and a moderately stout camshaft. Heck, Iron Headed Caprices are in the low-mid 10's with a mild cam. Just going to aluminum heads neccessitates a point jump in compression just to maintain power. (Overgeneralized, but true.) Keep in mind that LT1 heads are much more compression tolerant than Gen I heads because of the reverse flow cooling.

Speaking of which...
Is there a rule of thumb there, as for the iron to alum switch?

 

I just lower the DCR number down, usually High 7's will be safe for IRON... which I think is what you were asking.

Bret

 

Iron?

He said "stock LT1 castings", which I took to mean aluminum, since this is an F-Body board. Maybe I missed something...

 

I am talking about Aluminum LT1 heads

Is there anything wrong with having the block surfaced and leaving the pistons .005" in the hole just for future builds. It still gives a .044" quench area, will I see that much gain with a .039" quench

In regards to the Pat Kelley DCR calculator, is the program written wrong or am I getting a wrong answer from it because I am using the advertised duration which is rated at .006" tappet lift instead of actual seat to seat measurements