Comp Ratio's

**kineda** · 05-14-2006, 08:24 PM

i remember being told with iron heads the highest you can go is 10.5:1 for pump gas...

**RustyLX** · 05-14-2006, 08:39 PM

Here is a better ?
What are you looking to do with the car.
FYI 10-10.5 iron heads 10.5-11.0 AL heads.
But the is only a general rule.
Static compresion is one thing but Dynamic compression is what really determines fuel needs. If you run a cam with a ton of overlap that bleds off cylinder pressure you can run a 12.0 static and still run pump gas. Cam choice is the key.
Pick your goals and then ASK YOUR BUILDER what will work.

**2 beer** · 05-14-2006, 08:43 PM

I am stopping by to talk to him tommorow so I will be sure to bring this all up. But if i plan to run nitrous I dont want a lot of overlap correct?

Also if I have a lot of overlap that is bleeding off compression why run the high compression. I'm sure its an easy and obvious answer but i dont feel like thinking right now

**RustyLX** · 05-14-2006, 08:51 PM

you answered your own ??

**2 beer** · 05-14-2006, 08:56 PM

Originally posted by RustyLX

you answered your own ??

thats how smart I am

**Osiris22** · 05-14-2006, 09:25 PM

Compression ratio change running gas isn't responsible for that much power increase.

The summit techs claim that on the same motor increasing comp ratio to change from 87 octane compatible to 110 octane compativel comp ratio gains around 8% increase in power. which seems rather small. What is the motor for. If it's an all out drag motor it's worth running 110. If it's a steet use motor 110 is not a good idea. on a 400 hp motor you should gain less than 50 horse on comp alone. NOS adds 125HP or more. If running a super huge cam you'll need higher comp. If cam is not made for running extreme R's you'll be able to get away with lower comp ratio.

On a BB motor 10:1 is borderline to use pump gas. On a small block you can get away with 10:1 on pump.

If it's a motor you plan on using often on the street anything above 10:1 is redic. just build a bad 9.5:1 motor and spray it.

**Meegs** · 05-14-2006, 09:36 PM

this cuda is at 13:1 with iron heads but runs 110. i know you said you did not want to run 110...but it sure is fun

hell with gas the way it is 110 will be cheaper than 92 in another few days

don't fear the Pb....(lead)

**mnstang** · 05-14-2006, 09:42 PM

Originally posted by 2 beer

I know this is a really general question but with an iron headed engine about how much compression can you run on pump gas. Would i be right in saying that 11.1 is about the max? I am looking at pistons right now and I dont want to get something that is gonna make me have to run 110 all the time. I am happy to mix up to about 50/50 though. I just dont wan to run all 110.

Also I know that you can get rid of a LITTLE detonation buy pulling the timing, correct? Whats gonna give me better power though pulling a little timing with higher compression or a litte lower compression with more timing?

I need help

1st off, don't listen to orisis.

chances are if you're n/a, you're not going to need to run straight 110. you'd have to have really really high compression to need that. even 11:1 isn't that extreme and probably wouldn't require a 50/50 mix, but maybe just a gallon or 2 per tank mixed with premium fuel. you'll find out just by testing when you get the combo together.

as far as your pulling timing question. it seems that the more compression you have, the more critical timing is. on my 11:1 motor, for every 5 degrees i bump timing, i gain 10-11 rwhp. and that's n/a! so if you're between 11:1 with pulled timing because you're scared, or 10.5:1 and you'll run full timing, i'd probably do the lower compression.
but what i'd really do is just run whatever compression you want, and run the fuel you need to run.
on my engine, if i pull the timing back about 3-4 degrees, i can run pump gas all day. but it starts to ping a little when i put it up to 34 degrees total. and i hate driving my car when i know there is more hp in it... so all i do is splash a little 110 in it every tank and it's good to go at whatever timing i want.

your camshaft question is a little tricky.. basically bigger camshafts bleed off compression, that's why you need a bigger compression ratio to run a big cam. ..basically

oh and my car is 95% street driven with as good of manners as stock. ridiculous i know

**Osiris22** · 05-15-2006, 01:02 AM

Originally posted by mnstang

1st off, don't listen to orisis.

whatever, I speak the truth. compression isn't that important. Y would anyone run 110 when they don't have to. anyone running race gas under 650horse is a friggen idiot.

peeps like to act like internal combustion engines are so complicated. The truth is they are not at all.

the fact so many peeps ask comp ratio q's proves that they haven't done research.

for some reason people think running race gas is so cool. If the car don't run 10s or lower it looks stupid.

**tim** · 05-15-2006, 01:13 AM

Originally posted by Osiris22

whatever, I speak the truth. compression isn't that important. Y would anyone run 110 when they don't have to. anyone running race gas under 650horse is a friggen idiot.

peeps like to act like internal combustion engines are so complicated. The truth is they are not at all.

the fact so many peeps ask comp ratio q's proves that they haven't done research.

for some reason people think running race gas is so cool. If the car don't run 10s or lower it looks stupid.

How doesnt your compression ratio determine what octane fuel you need to use? Higher compression=more heat=chance of detonation if you are running too low octane fuel. Cylinder pressure/compression ratio definitely has a play in what octane fuel you need to use.

And you comment about running race fuel if you arent running 10s or lower is just plain retarded.

**Osiris22** · 05-15-2006, 01:25 AM

Originally posted by tim

How doesnt your compression ratio determine what octane fuel you need to use? Higher compression=more heat=chance of detonation if you are running too low octane fuel. Cylinder pressure/compression ratio definitely has a play in what octane fuel you need to use.

And you comment about running race fuel if you arent running 10s or lower is just plain retarded.

Of course octane fuel depends on comp ratio. the dynamic compression is what really matters in what fuel is needed for the engine. I'm saying that a 10.5:1 comp motor doesn't make that much more HP than a 9.5"1 comp motor.

I will stand by the claim that higher than pump octane fuel in a street car is retarded. hey, I have a steet car but I can't meet you there cause I need fuel.

take the same 2 motors with diff pistons. 1 is 9.5:1 and one is 10.5:1. with a 125 shot NOS on the 9.5:1 motor would smoke the other one. Y are we talking compression and fuel. we have 93 octane fuel available so we build 93 octane compatible motors.

**Fast One** · 05-15-2006, 01:59 AM

Put some coated pistons in it -- they'll raise your knock-limited HP potential significantly. Which means you can run high comp. on lower octane fuel & get the same HP as an otherwise identical engine with regular pistons would get on higher octane fuel. In other words -- you get more HP per dollar of fuel with coated pistons.

Bottom line: Whatever you build -- you can make more power on any given octane fuel with coated pistons than you can with regular ones,

**LES** · 05-15-2006, 08:46 AM

If yo uare running a street car most of the time I would run 9.5 to 1 with a good flowing head and cam. Then spray the hell out of it. Nos makes huge torque, and you can have a good solid 400HP motor with around 400ft# of torque and 9.5 -1 on pump gas. GO for CUBIC INCHES. Stroke it (331 OR 347) good strong internals. Then when you go to the track SPRAY about 150 shot with good fuel and timing. This will make over 500 hp and well over 600ft# torque with right cam and heads. When looking at cam for spray or blower look for good lobe seperation, 114 or higher. This is my 2 cents.

I know you guys think PINKS is a gay show but if anyone saw the one with the JPC performance mustang, I have seen that car in person. It is a 331 carb stroker with 150HP shot. We all seen the way that thing set it on the bumper! And it only had AFR 185 heads. Was a 400HP N/A motor. That car runs 9'S on the juice.

**mnstang** · 05-15-2006, 01:53 PM

Originally posted by Osiris22

Of course octane fuel depends on comp ratio. the dynamic compression is what really matters in what fuel is needed for the engine. I'm saying that a 10.5:1 comp motor doesn't make that much more HP than a 9.5"1 comp motor.

I will stand by the claim that higher than pump octane fuel in a street car is retarded. hey, I have a steet car but I can't meet you there cause I need fuel.

take the same 2 motors with diff pistons. 1 is 9.5:1 and one is 10.5:1. with a 125 shot NOS on the 9.5:1 motor would smoke the other one. Y are we talking compression and fuel. we have 93 octane fuel available so we build 93 octane compatible motors.

yeah the 10.5:1 motor might not make a whole lot more power, if you're still using the same stuff.
just like anything else with a motor, if you change 1 thing you might want to look at changing others so everything works together.
did summit tech tell you that???
not everyone likes NOS either

Comp Ratio's

Comp Ratio's

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