WHAT IS THE HIGHEST COMPRESSION RATIO YOU CAN RUN ON PUMP GAS? DOES HEAD PORTING AND POLISHING AND CAMSHAFT PROFILE LESSON THE CHANCE OF PROBLEMS?
between 9-1/2 and 10-1/4. Yes! Cam design has a big affect on cylinder pressure (how much overlap, etc.), which all affect how much compression you can run. -Bob Cunningham
MY CAM WILL BE A KB 107, THE SPECS ARE DURATION 240*INTAKE, 262 * EXHAUST @ .050" LIFT .499 INTAKE AND EXHAUST ,INTAKE OPENS 17* BTDC. WILL THIS CAM ALLOW ME TO RUN A HIGHER C/RATIO?
Yes, you can look at Pat Kelly's DCR program on team Chevelle as a starting point for CR/cam interactions.
A lot of factors........DEPENDS!!!! Head porting is not one of the factors, IMO. I've heard as high as 11:1, or BETTER! But those are EXTREME cases! 10:1 (I ran 10.4 with no detonation o No: ) is the general rule, with iron heads, the right cam, polishing the rough edges on the pistons and in the heads, AND having very good quench! DEEP subject!:eek2: :Smarty: Unless you REALLY know what you're doing, better to stick to the "general" rules.:Smarty:
I've been trying to get a straight answer to this question too. I was thinking about running a similar type of cam (poston 112- same as 107 on 112 center). If you use the DCR calculator, you can theoretically get away with 12:1 compression with this cam. Everybody here said it wasn't a good idea. I would think you could get away with 10.5:1 with iron heads on pump gas with the 107, if you run it straight up, with everything else just right- quench, good cooling system, timing and jetting dead on. That is only a guess on my part, but the DCR would be around 7, which is on the low side. You have to consider what your combination is. Are your heads going to be ported? The 107 seems to work well with unported heads with a good valve job- it works by keeping the velocity up. That means that you aren't going to have much of a top end, so maybe it would be better to drop the compression back a little more and advance the cam to fatten up the midrange. My dad's car runs a kb107 at a true 8.95:1, and it works well. He says the cam is about 4 degrees advanced. The DCR is way low on that motor, but it still works well enough. It does fall off on the top end though. I think he shifts it around 5400-5600.
Dave, my 10.5:1 with TA-413 runs fine on pump gas. The pistons and chambers are polished nicely. Look at the IVC of the cam to help determine cylinder pressure (with everything being equal later closing allows higher static compression without problems). This is one reason why I picked the 413.
Aluminum heads can run higher compression than iron heads I believe. I think it all depends on the total engine combo.
Ouch Rob! 40 deg? Don't you mean 30? I had 10.6 comp with my iron heads and the SP4 and it rattled like a mutha at 32 deg timing and 93 octane. Maybe you had your helmet on and couldn't hear the detonation? I can drive my Stage 2 with 12.2 comp on the street but I wouldn't nail it. (for a number of reasons, including not being fond of jail cells) Bruce
nope.........i mean 40 believe it or not. I just pulled the heads off last week and everything looks sweet...
If you plan to drive this thing much as in an out of town cruise, power tour, car shows or whatever I would not go more than 10.20-1. What about the fuel in your area? 94 octane Sunoco Ultra vs 91 octane stale generic premium can make a big difference. All the tricks listed above will help. I am just thinking of Murphys law and that some people live with mild detonation and don't know it. Also what is that last couple tenths going to buy you anyway? Is it worth the risk to you?
Head porting itself will have no effect on whether you have detonation problesm or not. Chamber work will have a huge effect. Getting rid of all sharp edges in the chamber and on top of the piston will help a bunch. Cam profiles will also have an effect, a bigger , more aggressive cam will allow you to get by with more compression , but you will lose vacuum at idle which may be a concern for you. Another thing to consider is your altitude, higher altitude will allow you run more compression. I have easily run over 11to 1 at my altitude (4000 feet on a good cold day) and used pump gas, in fact , in the cooler weather , I was able to even run regualr gas on this engine. I was running a healthy cam though, I don't use power brakes so I was'nt worrie about idle vacuum. Jim Burek P.A.E. ENTERPRISES
The chambers are fully polished and the valve notchs in the pistons are also polished, no sharp edges anywhere. the KB 107 with a 107 centerline is fairly radical, this car does not have power brakes. Is their other things to do so I can run close to 11 to 1 C/ratio. I do go to alot of rod runs, dragstrips and car shows so I put on about 4000 miles during the summer. keep in mind I am one of those guys that there is no such thing as TO MUCH HORSEPOWER!!!!!!!! Thanks for all your feedback!!!!!
Well you've done most of what youcan there sounds like. Another thing that does help is ceramic coating the piston tops and cumbustion chambers, this will allow you to run more compression on less octane. The ceramic spreads the heat out across the entire surface, much lie the tiles on the shuttle do. Jim Burek
