I was browsin around one day and long story short decided to build a 383 stroker. When I got my rotating assembly, I decided to look if I can use my heads from my 350. Together, the compression would turn out to be 14.0:1. I read somewhere that above 12.7:1 is dangerous. I know it all depends on what crank, rods, and pistons you have, but in general would it be bad? I have a cast eagle crank, sir i beam eagle rods, and srp forged pistons. Everythings good up to 550 hp, bvut could it handle to 14:1 compression. Yeah I know all engines and engine assemblies are different, but could it work without problems, probably not but just checkin. And what's the highest YOU would go? Thanks.

 

Another thing I didn't bring up. Yeah octane is a factor with detonation, but octane booster could be added reguarly so that's not a problem. This is a street/strip engine.

 

Domed pistons I assume? Thats way more compression than you can run with octane booster. Would have to run a mix of race gas straight race gas. I think that unless you have domed pistons that your calculations are off.

 

yeah thats pretty high for most kits that are sold, i would check your calculations again, whats your head cc?

my friend runs 13.5 compression in is 427 bbc, and uses 50/50 race gas and 91 octane and says it still runs like **** and occaisionally pings, and so he very rarely drives it on the street (uses 100% race gas for racing)
i would say 11:1 to 11.5:1 is the highest id run on the street and thats only if your still willing to run 91/92 octane with octane booster

 

that's in the range of pro stock or comp engines. you'd have to be running race gas, which wouldn't be legal for the street. domed pistons also aren't the most efficent design as far as flame travel goes.

 

I can buy 93 octane around here and I can get 100 race octane with like 10 miles of driving. But anyways, you think that 12 - 12.5 is good with 93 and octane? or do you think i could run it with plain 93, obviously some testing would be done on my part but I just wanna hear what you guys think. Thanks.

 

Personally I would have to say that 12.5 would be the highest I would go with any type of fuel. 14.0 is in the catagory of a diesel engine if you ask me!

 

Are you sure that you are at 14.0:1 AFR?

What type of pistons do you have? Do you have a part number?
If they are domed pistons, what volume does the dome take up?

If you are ar 14.0:1, then you will have some big problem with detonation. Send back the pistons and get other ones,,, possibly with a recess,, inverted pistons.

 

The highest I would go would be 11-1 comp. with aluminun
heads. Iron heads 10-1 and thats pushing it with
93 octane pump gas. Anything above 11-1 would require some kind of race gas mix to stay out of detonation.

 

why do you even need a compression ratio that high? if you're asking the questions here then you need to do a LOT more research on compression ratios. 9.5:1 on iron heads, 10.5:1 on aluminum heads...93 octane. any questions?

and no buying 110 octane gas at 7.99 a gallon around here is not fun at all especially at 8miles per gallon. it's a dollar a mile if you wanna cruise.

 

The only reason why I was asking was because everyone's opinions are so mixed. No one opinion is fluent throughout forums or opinions, even with technitions from heads companies. It's very confusing trying to research something and finding 50 different answers. Anyways I'm just trying to get the most out of what I can. I think I'm gonna shoot for 11:1, seems safe with 93 and maybe some booster.

Funny story, friend filled up a whole tank and 2 2 gallon handheld tanks on the way to a cruise on race gas. Thought he was gonna race a lot of people there. Ended up getting lost, wasted half his tank, then when he got there no one wanted to race lol. Then the cruise went up to Chicago, 30-40 miles away, to eat. Ended up wasting all his gas . Well worth the $76 he spent .

 

with iron heads i run 10.5 1 on 93 with some boosters, with enough tuning i can get it to run good, when it gets hot i normally have to back the timing off a few degrees to keep the percussion section at bay.. as soon as i get money im gonna get some aluminum heads.....

 

Generally I shoot for 11:1 with aluminum heads, 10:1 with iron heads and that's with a decent sized cam. Anything above that would require race gas (or alky), which as ede said isn't legal to run on the street, not to mention it's not very nice to the pocketbook either...

Also, octane booster doesn't raise the octane very much, a couple points at most. You're better off buying some toulene from a paint shop than buying the stuff from an auto store.

 

that is the thing that sux with compression ratio

being that static compression ratio isn't what kills the car

dynamic compression ratio is where problems can come into play as well s tuning and cooling

you might see someone running 9:1 compression
yet he can ONLY run 91 ocatane or higher
yet might see someone else running 12:1 on 87 and it works just fine


just to run things down in a nutshell without going into too much depth

longer duraiont cams
large port heads
alum heads
retarded timing
good combustion chamber design to increase burn rates
higher amount of squish
piston coatins
all allow for higher compression ratio
as far as how much???
who knows depends on when else is going on
and the list goes on further then that


but really most would say 11:1 seems to be about as high as you want on alum heads
little lower for iron

but again mileage may vary

 

this may help :lala:

 

this may help :lala:

 

i have a question about my CR as well. i have a '90 L98 and im doing a rebuild on it right now with ported iron heads, 10:5:1 cr, headers, and a CC502 cam (218 / 224, .495 / .502 112 LSA). is that too much CR for me on 93 octane gas and that cam? or do you think i'd be fine?

 

probabily too high... Just do some usefull deshrouding on the chambers to increase the cc's.

get it down to 10:1 no more than 10.3:1.

The deshrouding will be a win win overall cure for ya.

 

Domed pistons or flat?

 

****...my buddy has a 16:1 ratio in his 350. Runs like a raped ape, but thats because of the pistons he is using. He hates it though. Even though it gives great power, he needs to use 110 race fuel. I guess he will never make that mistake again.

 

If he's getting away using 110octane in a 16:1 motor?!! something is not right at all. At a minimum he should be running B37 118 octane with that kinda compression. Block filled too I take it? Sounds fishy to me.

 

88tbi4x4: i think the pistons are flat ... why do you ask?

 

A couple other guys asked the same ?? and there was no response.
It never hurts do double check that there isn't a miscalculation.

dj haf: The cam you're using is similar to the one I'll be getting for my stroker. You're using flattops on yours with 64cc iron heads?

I'm trying to decide what pistons to get for mine.

 

im not exactly sure. when i took the iroc down to my friends shop, he asked if he should put a performance rebuild kit with 10:3:1 cr and i said go for it. dunno what type pistons they are though but i assume they're flat topped because the cr isnt that high. im hoping not to have kr or detonation problems with 93 octane and that high of a cr, but damn... if i do im going to go crazy! grr lol

 

 

I do remember seeing a 14.5:1 motor, 400 CID small block running on 93 in a hot rod magazine. It all has to do with what rx7 said. You can totally run that safe if you use one hell of a cam, some huge headers, a big intake, big carb, and some head work. Your bottem end will probly hold up. I am currectly running a 11:1 motor on iron heads with no problem. I just have a big cam with a low lsa keeping the cranking pressures down to a minimum along with a victor jr. intake and a decent set of heads. IT can be done, but you will have to know how to do it. Loose converter, big cam, high gears, nice heads, big intake, big tubes, nice carb, etc. The machine shop who built my engine builds all of his with around 11:1 becasue he usues big cams and the right combination of parts. I hear little pinging if any and is one powerful engine.

 

To run on 93 octane with iron heads you need to shoot for a maximum DCR 0f 8.0 or 150-155 cranking psi. This is with a perfect tune and proper quench! Any more and your going to run into major detonation problems.

 

Kinda reminded me of when one off these internet wizards off this board actually came to the track and I met him. All this big hp aluminum heads, race fuel, etc... he kept telling eveybody here on the boards, then the fool made his forst pass at 14.89. Once he got back I asked what happened then the grab bag of excuses. Had to pull the timing back since I did'nt have any race gas, transmission is slipping, could'nt hook up. Alot of guys here know what they're talking about but when that kind of bs starts flying then of 16:1 motors...you get the picture, another internet wizard that has a buddy whos bigger and faster than anyone else.