Hey all,
I just turned 18 and I'm not that experienced with small blocks yet. I don't know that much about calculating the CR and was wondering if any of you more experienced people could help me out.
I have a 67 327 i believe with a stock bottom end, flat top pistons, 64cc aluminum heads (https://www.summitracing.com/parts/mll-h8002k), 1.6 roller rockers, comp cams XE274h flat tappet cam, and an edelbrock performer intake. Does anyone have a ballpark CR estimation? maybe a horsepower estimation as well? I am looking into putting a roots style weiand 142 series mini blower (5-7psi) and am worried my CR is too high to begin with.

thanks all,
Wesley

 

Do a search for online compression ratio calculators. With flat top pistons two valve reliefs us -7cc. 327 and flat tops is generally around 9.8

 

summit has a CR calculator. you need to look up some info on the pistons to bet it right. i get 10.13:1. so your going to be high. using 76cc heads and leaving the rest drops you to 8.85:1

 

If you don't have a forged rotating assembly [pistons, rods, crank], that is what the blower will blow. I'm running flat top pistons 74cc heads 1.5 rockers with 8lbs of boost.

This is what a blower engine looks if expected to last. T

 

What altitude are you at Wesley? You looking to make up for density altitude or just pump up the HP?

A 327 has a 4.0" bore with a 3.25" stroke and a 5.7" conrod. So very similar to the 350 just shorter stroke.

To get HP out of a 327 most guys resort to RPM to do that. Solid lifters, good springs and 7500 RPM. For that you will also need a solid bottom end.

I use this DCR/CR calculator
https://www.jeepstrokers.com/forum/calculator/

It can incorporate the altitude as well as the cam specs and the head gasket spec.
Putting in yours and assuming a .025" deck clearance with a .015" (giving the ideal .040" squish band) It comes up with 9.43 CR with a 7.61 DCR.

The DCR is affected by the cam specs and in this case is lower than could be tolerated with the aluminum heads you are running.
So it could tolerate a little boost in that department. The CR could be boosted slightly, it would be purely experimental to avoid detonation and manipulating the timing to avoid such.

atmospheric pressure at sea level is 14.7 psi. You loose about .5 (1/2) psi per 1000 feet gain in altitude. So if you boost the engine even 2 psi you just decreased the engines perceived (actual running) altitude by 4000 feet.
7 psi and you have decreased it's altitude by a wopping 14000 feet, that's a lot of boost.

The calculator does incorporate negative altitudes and will calculate a DCR based on the negative altitude.
So if you put -4000 in the "altitude" field it comes up with a DCR of 8.41. A difference of .8 CR. It's safe to assume that you CR will be affected likewise. so now you're at 10.23 CR. Still safe on both fronts for decently built engine.
Play with it and see what you get.

11.0:1 CR is pushing it on pump gas, especially if you go shoving in compressed air (which sill be heated some from compression) without an intercooler, probably too much.

A guy is better off with a purpose built boosted engine if he wants it to live very long.

 

Thanks for all the replies guys.
I live in centeral georgia, that's about as much as I know about what altitude I'm at. I'm just looking to gain more power. The motor pulls hard as it is, but I've gotten used to it pretty fast. Does anyone have any estimate of my motors horsepower? What's the best way to get more power over what I have now?

 

A blower is a good way to get more power but I think you should do a lot of research first. I ran a B&M 144 on a built L-82 for many years but the education was paid for with a number of pistons. I wouldn't recommend a blower with your 64 cc heads and flat top pistons.

 

Do the 142 youll love it, you can throw heads cams etc at a 327 all day and bet the mild blower motor will still be better.
You have to wind those motors up to make any power, too many 4 door cars that will blow these old cars in the weeds with the typical 327 with a cam/headers

x2 on forged. Lower your compresion to 8.5 no more than 9-1 ..even a cheap forged piston, decent rod bolts youre good to go .
Get an 02 sensor AEM kit for tuning so you know you arent going lean and have fun it will last a long time. 4-6 lbs boost isnt much but sure feels like it

Ran a 383 yrs ago with and without it difference was unbelievable. Torque was everywhere, passing on the fwy was fun
Its a small blower but can still make good power with them;2 bolt main fine

edit:supercharged how are your pistons that far down in the hole?

 

Im using a 327 with 350 pistons and 64cc heads with my 6v71 blower no toy blowers for me. 350 pistons are down the hole .125 just right for a street motor.