High C/R With Big Cams
Thread Starter
Advanced
Joined: Nov 2020
Posts: 55
Likes: 8
From: Vacaville, CA
High C/R With Big Cams
Hey all!
I’ve been following this site around for a while and finally signed up to clear up some stuff about compression ratios and big cams.
I’ve read all over big cams and big compression are fine on pump gas. But when the cam starts to spin up faster, wouldn’t my cylinder pressure skyrocket?
Regardless of DCR, wouldn’t this pressure require more octane?
Here’s the setup:
454 Mark IV stroked to 489
6.385” rods
43cc domed piston
121cc chamber, 335 runners
270*/276* @50 (advertised 304*/310*)
.055” quench
DCR is ~8.2, which I’ve heard is fine for pump gas, especially for aluminum heads. I’m just worried that as the RPM’s climb and cylinder pressure increases, I’ll run into problems. Anyone out there with high compression and a bigger cam that can give some insight for higher RPM’s?
Thanks
I’ve been following this site around for a while and finally signed up to clear up some stuff about compression ratios and big cams.
I’ve read all over big cams and big compression are fine on pump gas. But when the cam starts to spin up faster, wouldn’t my cylinder pressure skyrocket?
Regardless of DCR, wouldn’t this pressure require more octane?
Here’s the setup:
454 Mark IV stroked to 489
6.385” rods
43cc domed piston
121cc chamber, 335 runners
270*/276* @50 (advertised 304*/310*)
.055” quench
DCR is ~8.2, which I’ve heard is fine for pump gas, especially for aluminum heads. I’m just worried that as the RPM’s climb and cylinder pressure increases, I’ll run into problems. Anyone out there with high compression and a bigger cam that can give some insight for higher RPM’s?
Thanks
Race Director
Joined: Oct 2004
Posts: 12,864
Likes: 4,670
1 thing about high rpm is that usually you are at full throttle, with max velocity air/fuel rushing into the combustion chamber, which does help cool the piston tops, etc. Your dcr would increase when when the engine is "on the cam". But unless you are at WOT at 40mph in high gear, I dont think you'll have any problems with a bigger cam and aluminum heads. BUT-Domed pistons do slow the flame travel, requiring more ignition advance at higher rpm, all things equal.....
Drifting
Joined: Nov 2017
Posts: 1,394
Likes: 303
It seems to me that SCR is more meaningful in general, and DCR just tells you how much you're going to lose at low RPM, where the port velocity isn't high enough to make the engine "act like" it really has the SCR. And with sane spark timing and a sane SCR, knock is generally a bigger problem at low/mid RPM than it is at high RPM. But I have no idea what the SCR will be with your engine, or what's reasonable for that engine type.
Also, as long as you're not knocking, high combustion chamber pressure is a good thing, it's what creates torque. More is better.
Also, as long as you're not knocking, high combustion chamber pressure is a good thing, it's what creates torque. More is better.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Agree! Dont get too wrapped up over dcr, I go by static and never have issues.
I love compression as said you feel it at all levels....even if you had to mix a little Torco or? With a tank why risk your engine?
If you got the grind # run it past Mark at Vortecpro.com hes a BB expert.
Are those import heads you listed?
I love compression as said you feel it at all levels....even if you had to mix a little Torco or? With a tank why risk your engine?
If you got the grind # run it past Mark at Vortecpro.com hes a BB expert.
Are those import heads you listed?
Burning Brakes
Joined: Sep 2018
Posts: 1,023
Likes: 174
From: Auburn Al
I'm running 11.65 to 1 Cr my cam 231/239 .617/.624 113+4 109 CL Comp Cam for cathedral port heads, I had this cam and heads combo put together and was disappointed with the dyno #'s these are Trick Flow 220's fast as cast. chewed the fat with a bunch of guys on LS1 tech. And they talked me into pulling the heads and milling .025 65cc chamber to 61 cc might be 60cc and I went with a .044 head gasket, I'm glad I did! pump gas 93 octane, no problems!
