compression ratio


http://209.208.111.198/Topic41972.aspx 		Print Topic | Close Window

By jepito - 15 Years Ago
in the planning stages of rebuilding my 312 and was wanting to know how much compression i could get away with on 91 octane?  Going for a zero compression height, a healthy cam. heads  have 73cc chambers. Individual Runner mechanical fuel injection. The car is light (around 2000 lbs) which should also help. Is 9.5-10.0 realistic?
By charliemccraney - 15 Years Ago
In most cases, 9.5 - 10:1 is not realistic on 91 octane. Anything above about 8.8:1 will not be possible unless you use domed pistons or get more displacement.

To get 9.5:1 with the heads you are using, with flat tops, you will need to displace about 345ci. Or, assuming a .020 overbore, you will need about a 7cc dome.

For 10:1, you're looking at about 365 ci or a 12 cc dome.



Despite Ford's advertising,most Ys were never actually above 9:1. 8.8 won't be bad.
By aussiebill - 15 Years Ago
jepito (4/4/2010)
in the planning stages of rebuilding my 312 and was wanting to know how much compression i could get away with on 91 octane?  Going for a zero compression height, a healthy cam. heads  have 73cc chambers. Individual Runner mechanical fuel injection. The car is light (around 2000 lbs) which should also help. Is 9.5-10.0 realistic?

CURIOUS AS TO WHAT CAR AND INJECTION YOU ARE REFERRING TO,

By Ted - 15 Years Ago

Although 9.5:1 static compression ratio is the theoretical limit for 93 octane fuel when using iron heads, performing the calculations for a dynamic compression ratio check would give a better feel for what the engine combination can tolerate.  For 91 octane fuel, I limit the dynamic compression ratio to 8.0:1 regardless of the static compression ratio.  Cranking compression can also be used as an indicator at which point I’d use 165 lbs. cranking compression as the upper limit for 91 octane fuel.

 

And the age old question about the fuel quality at the pump continues to crop up.  There’s no guarantee that the fuel you’re getting out of the pump is as advertised.  If the engine is built on the ragged edge and barely tolerates using 91 octane fuel, then the first load of 87 or 89 fuel that goes into the engine either by accident or mislabeled fuel will damage the engine.

By jepito - 15 Years Ago
3.820 bore, 3.69 stroke.  hilborn injection. Chopped and channeled model t.  My static compression would be around 9.4 :1 if my math is right.   was hope that the agressive cam would also help to get by with this compression, by giving it a lower running ratio at lower rpm. 
By Ted - 15 Years Ago
If you can provide the ‘real’ intake valve closing value (ABDC) and the length of the connecting rod, the dynamic compression ratio can be quickly calculated based on what you’ve already provided.  The 'real' value is where the valve actually seats during closing.  This value varies with the lash that’s being used.  If the ‘real’ intake valve closing value isn’t known, then supply the advertised intake closing value and the intake value at 0.050”.  From those two numbers, the intake valve closing value can be estimated.
By jepito - 15 Years Ago
6.125 rod. honestly haven't gotten to deep into cam selection but maybe Isky with 260 advertised, 216 @ .050. 108 lobe sep. Says .425 lift with stock rockers, however I have a set of 1.6 alum roller rockers. I was looking to be shifting @ 6000 - 6500rpm. Any thoughts on cam choice would be welcomed too!
By charliemccraney - 15 Years Ago
I think you'll need a more aggressive cam than that. You'll be running out of power long before you shift. For your light car, I think something like the 505 T will be alright.
By Ted - 15 Years Ago

I’ll agree with Charlie in that the E4 Isky is on the mild side for your particular combination and especially with your rpm expectations.

 

Your 9.4:1 static compression ratio comes in pretty close to what I get in checking your numbers.  I get 9.36:1 using 9.9cc’s for the head gasket volume.  With the 6.125” rod, the dynamic compression ratio values for your particular combination are as follows when using the following intake closing event numbers.

53.0° ABDC = 8.1:1 dynamic cr

55.1° ABDC = 8.0:1 dynamic cr

57.1° ABDC = 7.9:1 dynamic cr

59.0° ABDC = 7.8:1 dynamic cr

61.0° ABDC = 7.7:1 dynamic cr

62.8° ABDC = 7.6:1 dynamic cr

64.5° ABDC = 7.5:1 dynamic cr

66.2° ABDC = 7.4:1 dynamic cr

Also remember that by advancing or retarding the camshaft, the dynamic compression ratio can be moved around.  Altering the valve lash will also help to manipulate the dynamic compression ratio so there are several options.

By DANIEL TINDER - 15 Years Ago
Sounds like those running 9.5:1 static compression, when traveling to areas where 93 octane cannot be assured, should consider (aside from retarding the ignition & carrying along a case of octane booster) tightening up the valve lash a bit?
By jepito - 15 Years Ago
After some research this is the cam I'm thinking about. What do you guys think?



Comp cam



PART NO. 37-601-5



DUR @.050 i 239 e 253



LOBE LIFT .3260 .3200



NET VALVE LIFT .490 .480



DUR @ .020 TAPPET LIFT 268 285



LOBE SEPARATI 107.0



ADVANCE 5



VALVE TIMING OPEN CLOSE



@ .020

INT 31 57



EXH 71 34



THESE SPECS ARE FOR CAM(S) INSTALLED



@ 102.0 INTAKE CENTERLINE



Also after talking with John Mummert it looks like i will be going with a 3.62 stroke. This should bring static compression to 9.3:1
By Philo - 15 Years Ago
jepito (4/6/2010)
6.125 rod. honestly haven't gotten to deep into cam selection but maybe Isky with 260 advertised, 216 @ .050. 108 lobe sep. Says .425 lift with stock rockers, however I have a set of 1.6 alum roller rockers. I was looking to be shifting @ 6000 - 6500rpm. Any thoughts on cam choice would be welcomed too!

Stock 312 rods are 6.254" long...Are you building a stroker?

By jepito - 15 Years Ago
Yes. Its a 312 that will be offset ground to 3.62 and bored .030 over.
By bloodyknucklehill - 15 Years Ago
Chopped and Channeled T with a gnarly Y?  I gotta see pics man.. i'm drooling just thinking about it..
By jepito - 15 Years Ago
Been sitting like this for the last 4 years. finally have time to get back to it.