11/1 Compression Ratio for a cruiser?

I am the final stages of putting the 350 SBC together. The specs (59cc heads 4 in bore) I was told that would make it 11/1 compression ratio. It is built to be a cruiser 700 r4 325 ford 9 in rear. My question is should I put a larger head gasket (Felpro 1044?) to drop the ratio down to 10/1? What issues if any will I run into.

3x2 holley 94 will be feeding it with an RV cam.

Thanks for you valued help!!

 

Hi,
You're asking for trouble, the only way to run that much compression on the street is with aluminum heads and a big cam. Open the combustion chambers up around the valves out to the gasket line to help reduce compression, or find some heads with a bigger chamber.
Rick

 

What he said!!!!

Tony

 

you need to drop about 2 points to about 9:1....i doubt any head gasket can do that

 

i've ran 12:1 on the street, not a problem but you better buy a drum of 110octane.

 

How do you know your compression ratio? Did you measure it, or just go by what the piston manufacturer said?

 

ouch...thats a little high for the street. i think like the other guys said 9.5:1 is a good target for a street motor in a "cruiser". it just needs to get you from one place to another and sound good.

 

Yep, it needs to be lower, especially with an "RV" cam which is typically ground to build pretty high compression at lower RPM's. High compression pressures at low RPM's don't get along too well with today's available pump gas.

 

11:1 can work on the street but not with 3.25 gears and an RV cam. You're gonna lug that motor too hard at low RPM and get into ping. That kind of compression works much better with 4.10s (or deeper) and long duration cams to bleed off cylinder pressure at low RPMs. Go for the thicker gasket.

Good luck

 

I say run it. just post some pics after you blow it apart.

 

What kind of pistons are you running? Because from the info you gave your no where near that high of compression unless there's been more work done. You running 305 heads? I see alot of people now running these heads and thinking they have 12:1 or 11:1 compression. Has the block been decked? Has anything been measured to see if you really are in that kind of ballpark figure?

I've had alot of so called high compression 350's that in reality were only 9:1 compression or 9.5:1 at the most. You got flat top pistons or dome style pistons?

I see alot of people building 350's around my neck of the woods who put a set of flat top pistons into a block thats been bored then slap a set of double hump heads on it and think they automatically have 11:1 compression or higher when in reality its closer to 9:1 maybe. Most flattops that rebuilders get have big valve reliefs in them which make for less compression.

Now I'm not saying you can't or don't have that high of compression because I have seen some engines that were built with flat tops that ran real high compression but they had alot of machine work to the block and heads to get there. They also ran like stink when they were put into a car setup correctly also.

 

You could always leave it at 11:1, Jet up the carb, advance the timing and run E-85 only.

But 11:1 does not work well with regular unleaded in a street engine.

 

Without doing any measurements, I'd think you'd need at least a 72cc chamber with flat tops to use "pump" gas without issues. If you have dished you may be able to get away with 64cc versions.

 

It's funny how many people think they know what they have...because the piston box has it written it's label or was stated in the Summit description.......too many factors come into play......their is some work to be done to figure out what you really have........anything else is an educated guess.....I really thing 11 to 1 or higher compression is a waste of compression if not raced unless it came stock w/ it....in my opinion..you can make most anything work w/ some sacrifice....Littleman

 

i did it for a summer and part of a fall. 11 to 1 402 bbc....closed ovals . came out of a k / stock chevelle wagon. i ran it with a 623 roller duration wasn't too bad on the street. i'd say get the timing right , and you might get away with it .....to a degree and use a good ignition setup.

 

This link will help you get close to the correct number.

http://www.rbracing-rsr.com/compstaticcalc.html

 

Here's another compression calculator http://www.kb-silvolite.com/calc.php?action=comp

 

Id get a bigger cc head or differnt pistons. 11.1 is to high for the street. You can do it if you only fuel where they sale 103 octane gas but its going to kill your wallet. My 383 has just under 9.1 and i only run 93 octane. Anything less and i get pinging when i floor it. I also have aluminum AFR heads.

 

Why not run a pair of Edelbrock Performer RPM 64CC Heads?

 

11:1 can definitely be made to work in a light car with a big cam and low gears...works really good for me but I'm at 6500ft above sea level which makes a difference too. I would have to say that if your really making 11:1 then the rv cam will have to go.

 

Do the numbers and find out what the comp ratio really is. What is the deck clearance?, flat tops? and if so how many CC's are the valve reliefs? 59cc heads are going to up the final numbers a bit, but a thicker head gasket may help. The quench area may be affected by the thicker head gasket / gasket bore and you may end up with ping anyway even if the final compression ratio is 10:1 or less.

 

I run 12:1 comp. in my Caddy mill, but I polished the combustion chambers have a long duration cam to bleed off comp. at low end and a Jacobs electronic ignition system. I run only 4 degrees initial timing and total of 28 degrees. If I run it hard have to blend some juice with the fuel. But with 93 Sunoco gas it only has a slight ping when you really get into it. The right cam makes the difference and how its degreed in. With the Sunoco race blend I can advance inital timing to 10 and this is a nite and day difference. This will work on a small block Chev. also cause I tried it.

 

Exactly. So many people think they have 11 to 1 when they really have 8.5 to 1 at best. Detroit lied about stock compression, piston manufacturers have no idea what your setup is, and head volumes are not consistant.

Measure it, then make an informed choice. It is highly likely that you don't even have a problem.

 

I will, my infor was based on a stock 350 SBC with flat top pistons. The only thing that was measured was the cylinder head cc's. I did not realize that the bocks info could not be relied on. The heads are aftermarket World heads. They were actually measured while I watched.

Once again, I gained a ton of great feedback, back to drawing board!!

 

Good info and links..Thanks..
Not to highjack, but just figured i'd ask. Any of you guy's by chance know the piston top volume for a TRW forged 12.5-1 piston, or are there different designs of that model?
I can't pull my motor apart to check it and would like to get a little better idea of what my actual comp ratio is in my truck.

Again, thanks for the links etc..

Tony

 

In my Max Wedge I run 11:1 compression and I have to run 110 Leaded in it, yeah they're original Maxie Heads without hardened seats..Not much fun because I can't afford to drive it much, thats why its up for sale.Race gas is like $7 a Gallon now around here..
I would personally make sure you can run pump gas, there are alot of cars out there that run on 93 Octane and are very, very quick.

 

I have an O/T car with factory 11:1 compression SBC with a carb.. It has 4:11 gears and it still pings. It is a pain in the ass really. Do yourself a favor and lower the compression.

 

#$#$ Yeah that's really funny as it goes both ways !! My favorites are the cats that installed the vortec heads on older engines and didn't measure anything !!! Measure everything including the thickness of the head gaskets !!! >>>>.

 

What you are really talking about is cranking pressure. Some guys get away with running a larger lift cam with alot of overlap. What happens is the overlap in the cam bleeds off cranking pressure, resulting in a lower overall actual compression ratio. The centerline that the cam is ground on has alot to do with it also, as well as whether the cam timing is altered. More advance timing on the cam helps a little as well, so when you pick your bumpstick, you need to degree it in. Getting the ignition lead right is also critical. You may have to run super unleaded, but you can get away with the small chambers if you you spend a little time and plan everything out. Getting the tune up right in the end is what keeps it all together. I'm running 11.5 to one on an aircooled Buell with no pings or problems, YMMV...

 

i got high compression with aluminum heads and an rv cam, i pollised the chambers and run pump gas and im always adjustiong the timing, seems like the gas gets worse every year. i always run premium, I used to have stock heads on it, the new ones made a huge difference. but it is a little much for cruisein. runnin a 700r4 and 3.55 out back.