Okay, here's the deal. I'm in the process of adding some (DIY)home ported and polished stock LT1 aluminum cylinder heads to my current set up (see signiture). After unshrouding both the intake and exhaust valves in the combustion chamber I cc'd the heads to have an average combustion chamber volume of 55.8cc. The heads are now at the machine shop for a competition (multi-angle)valve job and decking. I'm having the heads decked .020" to insure flatness, and to bring valvetrain geometry somewhat back into place because I'm using Ferrea 2.00" and 1.56" valves that are .050" taller than stock. Using a well worn "Rule of Thumb", I'm assuming that having these heads decked .020" will lower the actual combustion chamber volume to some where around 53.25cc(???). Now here's the problem:

When I use the Dynamic Compression Ratio Calculator located at www.kb-silvolite.com/calc.php. and enter my data, I get a calculated DYNAMIC compression ratio, (with the Comp Cam XE 230/236 cam I currently have), of approximately 11.5:1. And the big question is: Is a DYNAMIC compression ratio of 11.5:1 TOO high to run in our LT1's on 93 octane premimum unleaded pump gas???

Any reply's from those of you who have "been there and done that", would be greatly appreciated.

 

You messed up calculating. Around the 9.0:1 DCR range is the max you want to have on an every day street car. ESPECIALLY with a fairly good size cam like the 230/236, there's no way in hell your DCR is 11.5:1

 

Loadre, okay WHERE did I "mess up". Here are the data inputs I put into that calculator:

Cyl Head Vol. = 53.25cc
Piston Head Vol. = 0 (Does anyone KNOW the actual valve relief volume on STOCK LT1 pistons?).
Gasket Thk. = .026" (Compressed)
Gasket Bore = 4.010" (I'm using Mr. Gasket P/N 5716 head gaskets).
Cylinder Bore = 4.000"
Deck Clearance = .025" (A "stockish" average number for a stock shortblock???).
Stroke = 3.48"
Rod Lenght = 5.7"
Intake closing point @ 0.050" lift = 39 ABDC (The cam card for the Comp XE 230/236 says 43 ABDC installed "straight up", BUT mine's ground +4 degrees ADVANCED, so I used 39 ABDC).

Now when I put this info into that calculator I get a STATIC CR of 12.242, and a DYNAMIC CR of 11.331

Sooooo... where's my error/errors???

 

The piston relief volume is ~7cc.

Your gasket bore is going to be larger than 4.010. Probably wouldn't hurt to just call it good with 4.090. The gasket manufacturers know people will use the gasket on blocks bored to the max, so they size them to cover that.

The real problem is in your intake valve closing point. Should be somewhere in the neighborhood of 70º with that cam. Ignore the advance that's ground in, all cams have that, you want to use actual card spec for intake valve closure ABDC.
I couldn't find that cam on Comp's website. If you can give the part number, I'll look it up and see what the actual spec is. Either way, it's not 39º ABDC.

Two threads on how much is too much...

http://web.camaross.com/forums/showthread.php?t=282275
http://web.camaross.com/forums/showthread.php?t=46128

-Mindgame

 

My forged SRPs have an advertised valve relief of -5ccs, they have only two reliefs as opposed to LT1s four, stock. They also APPEARED to be exactly the same size. Put in -10ccs, it changes things a bit. I'm still looking over it.

Edit - Mindgame to the rescue while I'm typing my post.

 

Mindgame, you're a gem. Regarding your comments: 1) You were "right on" about the head gasket bore diameter. I just measured the actual gaskets, and the bore diameter should have been 4.100" not 4.010". 2) I will also use the 7cc figure you supplied for the valve relief volume for stock LT1 pistons in my next calculation. 3) As far as the actual XE 230/236 intake valve closing specs, I do have the cam card right here in front of me and it definately says, "Intake Valve Closing at 0.050" 43 degrees ABDC. I purchased this cam from Nutek Motorsports and their P/N is 07-000-8, (which I think just indicates a Comp "custom" grind?). Their "grind" number is LT1 3315/3316 HR112.0. Remember, the calculator I used asks for Intake Closing at .050" as an input not actual intake closing (i.e. 0.00" lift), which I guess would be around 70 degrees ABDC???

At any rate, your comments have been VERY helpful. I'll now redo my DCR calculations using your newly supplied inputs. Thanks AGAIN!

 

Loadre & Mindgame, just re-did my DCR calculation using 7cc instead of 0cc for "Piston Head Volume"; and 4.100" instead of 4.010" for "Gasket Bore", and got a new calculated DCR of 10.100:1.

Mindgame, if you come up with any more info regarding the intake closing point @ 0.050" on my cam, do let me know, and thanks again to ALL who replied!

 

The dynamic compression ratio calculator HAS to use the seat (or advertised) valve timing. If you use the 0.050 lift numbers, the valves are already open, and the engine is not making any compression. The only time the engine is compressing the mixture in the cylinder is when both valves are fully closed (not really, but close enough). The advertised duration on the 230/236 is 282/288. The LSA is 112 and the ICL is 108 (at least mine is). Try plugging those numbers into the calculator.

Don't worry about your combo. I have a ported heads that were shaved 0.030". I am using LT1 Impala head gaskets that are a 0.029 compressed thickness. That computes to 11.8:1 static compression. With the 230/236, it comes out to 8.9:1 dynamic, I think. I can't find my DCR calculator program now. I run it on California 91 octane MTBE gas (we can't get anything else here without going to racing fuel) with no problem, even in 120 degree heat. 6,000 miles and counting.

 

llafo, first off, thanks for your reply. The best part of your comment is, of course, the affirmation that my proposed setup will indeed run well on 93 octane premium unleaded fuel. In the final analysis, that is precisely the question I wanted answered.

You are, of course, also "right on" about the advertised duration of the Comp XE 230/236 being around 281/288, but the confusion about calculating the DCR figure is not lessened by the fact that the DCR Calculator I referenced, (and used!), definately calls for the Intake Closing Point at 0.050" One assumes that since these cam numbers are usually easier to get at and/or find for any particular cam, that the "Calculator" itself applies some sort of internal correction based on the intake closing specs at 0.050", which it definately asks for as and input.

One final question for you. I see from your signiture that you are curerently running LE ported heads with a Comp XE 230/236 cam and 4.11 gears which is real close to what my new set up will be. You got any 1/4 mile times for your car? (I saw your dyno numbers there in the sig, but what about track times?).

Many thanks AGAIN to ALL who replied.

 

I have been to the track exactly once with the car. It's more just a cruiser for the weekends, and I don't race much. When I went, I managed a best of 14.1 at 103. The car was definitely not running right. It had a huge leak in the driver's side header collector and was running on seven cylinders due to a burnt plug wire. I was also driving on 245 tires, and got NO traction. I was spinning throughout second gear and into third, and that was with the old 3.43 gears. Add that to the fact that I suck at drag racing and have done it only a handful of times. My BEST 60' time was 2.6 seconds.

The conditions that night were awful. The track is at about 3000 feet. There was a 40 mph headwind and blowing dust with temperatures in the 40s.

Despite that, I still managed to beat a brand new 04 Viper heads up. We both ran about the same time and MPH. The Viper was racing the same ZO6 Corvette all night long, and neither of them was even getting into the 13s. Go figure.

 

llafro, what a nightmare scenario, your only time to the track and you encounter those conditions. The fact that you still managed to beat a newer 2004 (505cid, 505hp) Viper speaks well as to how your car would run on another day.

For anyone else viewing this thread, I ASSUME that after adding my ported and polished heads and getting it re-tuned, that eleven(11) second passes are possible with a car set up like mine???