Dynamic compression ratio

is 8.4:1 safe on 93 octane?

for the sake of arguement.

3.75" stroke
4.03 bore
5cc relief pistons
64cc heads
288/300(236/248) .54/.54 lift 112 LSA 4 degress of advance

.04 gasket thickness
4.06 gasket bore
0 piston to deck clearence

If this is going to work all I need is a rotating assembly.

 

You can go up to 10:1 if you have 93

 

To answer the question...without knowing all the details....I would say..yes that will work.

Variables-

What heads-chamber design?
Assuming hyd cam? Hyd roller?
Intake/type?
Exhaust type?
Trans type?
Gearing?
Heat? A/C???


I'm not a fan of all the cool dynamic compression ratio calculators. They are a neat toy to play with and can sorta give you an idea...maybe.

Last time I ran mine through one it predicted I could push just under 13.0 compression and do fine with my cam. I'm not that bright...but I've been doing this a while......that's just not going to work. I could ride around all day on 87 octane and never hurt a thing....but the first time I kicked into it beside a Honda it would be embarrasing.

Here's the problem....as much as we would all like it to be...engine dynamics and design aren't that simple. Yes...a big cam will bleed off some cylinder pressure at low speeds....but eventually all those neat hot rod parts are going to get into a good harmony (hopefully!) and those cylinders are going to start getting very full. Exactly when that happens is determined by lots of things...the actual cam events..which are a lot more than adv and .050 number can tell you, the stroke and rod length, the intake design and runner size, head runner size and shape, the valvejob, the head chamber, the carb, the ex ports, the headers, the exhaust system all the way to the bumper..and even things as simple as do you have a fresh air system or hood scoop to name a few.

You can push the envelope a lot....but it's usually best to stay safe with that many variables. Just making that a solid cam..or a hyd flat tappet with the same overall numbers would change the picture drastically. Having a stick trans and some deep gears allows you to get by with more vs an auto with a tight converter and 3.08's.


JIM

 

Wow, Jim that is comprehensive. Well, I will try to answer your questions.

I dont have anywhere to host pictures at the moment.

1.)This is pretty close

2.) hyd. roller cam
3.) Weiand Xcelerator

4.) Some form of header. Finding a header that fits angle plug heads from what I understand an entertaining and fun filled adventure. I am hoping a hooker super comp(1 3/4) will work. It not I am told the dynomax blackjack(1 5/8) works

5.) M21 transmission

6.) 3.36/3.55 I dont know......... its changable

My heads have been ported and a valve job was done.

Thanks

 

May i suggest you take a look at this concering your 8.4-1 dcr,
http://www.empirenet.com/pkelley2/DynamicCR.html

 

Good info there. ...and a comprehensive calcuator that uses seat events instead of extrapolating them from .050". However, I'm aiming my 496 build at no more than 8.25:1 DCR, because when it gets spinning good things do change.

btw, we're talking dynamic, not static CR here...