?CR and pump gas?
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Post Fiend
Joined: Mar 2005
Posts: 8,542
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From: Louisiana
?CR and pump gas?
Ok I was wondering what's the highest CR you can have while still running pump gas.
I was wanting around 9.5:1 with pistons and not new aussie heads or somthing of that sort, while running a good low end cam(265 or so) and about a 650cfm carb along with everything else for a complete rebuild.
Thanks for any help.
I was wanting around 9.5:1 with pistons and not new aussie heads or somthing of that sort, while running a good low end cam(265 or so) and about a 650cfm carb along with everything else for a complete rebuild.
Thanks for any help.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
I am running 10.2:1 with flattop pistons and alloy heads. I run 93 Octane without any problem. Sometimes I mix 89 into the tank of 93. Still no problem. My cam however is 281 degree intake duration with an Intake Valve Closing at 66 degrrees ABDC.
I think that you can get by with 9.5:1 with a Comp Cams 265DEH. I would cc the heads and check the deck clearance to be sure that you do not exceed 9.5:1. A shorter duration cam may be riskier, but an XE262 may be doable.
Retarding the cam 4 degrees will make the IVC later and reduce the Dynamic Compression Ratio.
I think that you can get by with 9.5:1 with a Comp Cams 265DEH. I would cc the heads and check the deck clearance to be sure that you do not exceed 9.5:1. A shorter duration cam may be riskier, but an XE262 may be doable.
Retarding the cam 4 degrees will make the IVC later and reduce the Dynamic Compression Ratio.
Laughing Gas
Joined: Feb 2003
Posts: 1,234
Likes: 3
From: In the woods of Oregon
I have either 9.555 to one or 9.8 to one. I have earlier 71 400 heads and some sources say they are 75.4cc not 78.4cc. I have never cc'ed them to be sure. BUT with a comp cams 268H with an intake closing of 60 I have to run 92 with octane booster or I ping with 15deg adv. 15deg adv is where she runs strongest at. This would lead me to believe I have 9.8 to 1 not 9.5. Either way go to the low side of compression when building, cause the open chamber heads aren't the best at staving off the pings. I built thinking the heads where 78.4cc's and I later found out that they could be 75.4cc's and than it all fell into place why she likes the premium fuel so much. My 2cents.
Laughing Gas
Joined: Feb 2003
Posts: 1,234
Likes: 3
From: In the woods of Oregon
I'm running a rebuilt points dizzy with a pertronix Ignitor II and coil. Full vacume adv. total adv is 30deg. I have read that most performance rebuilds with this engine run about 15 deg adv. What was the adv with Tim Meyers 400 build? I think it was around 15 deg. Anyway thats where it runs best.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
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Post Fiend
Joined: Mar 2005
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From: Louisiana
Thanks for all the help and comments so far.
I really need to keep reading on what I need and what to do with my engine, but I get discouraged when I know I don't have the money to do it right now.
I really need to keep reading on what I need and what to do with my engine, but I get discouraged when I know I don't have the money to do it right now.
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Posting Guru
Joined: Mar 2005
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From: western montana
you can run any compression ratio you want (kinda)...it all depends on how much compression your cam bleeds off- a rv type cam may bleed off 2 full points while an aggressive cam may bleed of 4.
Posting Guru
Joined: Mar 2002
Posts: 2,070
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From: Boise, Idaho
The question of how much compression to run is interesting. The answer is not so cut-and-dried. With open chamber heads you need to run less static CR for a given octane. But if you have a nicely tuned closed-chamber engine, you can boost the static CR and gain torque and power.
The Oz 302 cylinder heads are closed chamber. If you run pistons with about .035-.040" of clearance, you can run lower octane, or boost the compression ratio. But the problem with Oz CC heads are their relatively low size.
I've read that the guys down under will grind on the chamber walls of the cylinder heads to increase their size. They grind in some very specific areas. IIRC, it was on the wall next to the spark plug and intake valve. BTW, do not 'unshroud' the intake valve, that will hurt combustion performance.
Another way to go is to get Tim Meyer's pistons with the dish milled into them.
I'd like to run with 89 octane fuel at 3000' above sea level. I think that's equivalent to 91 octane at sea level. I think that it's a trial an error thing, figuring out what static CR you can run, but I'm going to design for about 9.2 with a relatively 'short' cam (low overlap, wide LSA).
The Oz 302 cylinder heads are closed chamber. If you run pistons with about .035-.040" of clearance, you can run lower octane, or boost the compression ratio. But the problem with Oz CC heads are their relatively low size.
I've read that the guys down under will grind on the chamber walls of the cylinder heads to increase their size. They grind in some very specific areas. IIRC, it was on the wall next to the spark plug and intake valve. BTW, do not 'unshroud' the intake valve, that will hurt combustion performance.
Another way to go is to get Tim Meyer's pistons with the dish milled into them.
I'd like to run with 89 octane fuel at 3000' above sea level. I think that's equivalent to 91 octane at sea level. I think that it's a trial an error thing, figuring out what static CR you can run, but I'm going to design for about 9.2 with a relatively 'short' cam (low overlap, wide LSA).
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
Alloy heads will allow higher dynamic compressions. Also the new alloy heads have fast burn chambers which allow you to use less spark advance.
An alloy headed motor with zero deck pistons should run on pump fuel with a Dynamic Compression Ratio of 9.0:1 or higher.
An alloy headed motor with zero deck pistons should run on pump fuel with a Dynamic Compression Ratio of 9.0:1 or higher.