compression for street?

[HAM1974]

is 11-1 to much compression for street motor, 93 octane? motor was 10.2-1 had a bad pin need new pistons,thinking of uping compression. what kind of hp increase should i expect?

[stroker1]

Conditions have to be perfect to run more than about 10.5:1 on the street. Engine Masters Challenge has allot of guys exceeding this but they have controlled the variables. Street driving is much different. You cant control the outside temps and the humidity, etc. And cooling, air intake temp, and stop and go traffic make it a NO_GO in my book for 11:1 compression on the street in a normal driver. You can pull some timing and do some other tricks but if I where going to plan on driving it allot I wouldn't exceed 10.5:1. You can make plenty of power for the street on 10:1 or 10.5:1 compression and 93 octane.

[stevo67]

Unless you have a Sunoco local that sells 100 octane "Purple" race gas, 11:1 is too much for the street(pump gas).

Keep in mind also, that Gas rated at 93 octane is not 93 octane by the time you pump it into your tank. 93 is what it was rated when refined, and everyday after that it slowly drops octane points, little by little.

So, if it sat in the tank underground for a week, after sitting at the refinery for a month, and it takes you a week to use a whole tank, because you don't drive it every day....your down to like 87~89 octane for the last few gallons in the tank after a month and a half of sitting.

[1badblazer]

Yea keep it around 10.5:1. I just built my motor and it is around 11.5:1 and I have to mix 110 & 93 to keep it happy. It runs great but is costly to drive, but I don't drive it to much.

[chris1972]

i keep it around 9.5 because of gas these days

[spacecadet]

Any more than 10-1 on the street isn't going to be very good for your motor or your wallet.
In my roundy round I run a mix of 93 and 114 giving me an estimated ratio of @ 100 or so. I have flat tops with 10.25 -1 I can notice the difference between 93, mixed and 114. On the 93 it runs too hot @ 220degrees. The 114 @ 180. and @ 180 for the mix. The thing I notice is between the 114 and the mix is that the motor is stronger with the mix. I have tried 60/40 mix also with no noticeable differnace. ( I stick w/ 50/50 cause its easier to make the same again and again.)

My 2 cents... Space.

[Rinkrat456]

5 posts after the OP, and not a single mention of any other variable other than static compression ratio and gas quality. The static compression ratio of a motor is only 1 variable in a string of half a dozen or more things that need to be calculated in order to determine what's really going on. You know we all have that one friend with an 12:1 compression motor and donkey d!ck cam that says he's fine on 87 pump gas. It's because other factors within the motor allow him too, mostly his cam.

Things to consider:
Bore
Stroke
Rod Length
Static Compression Ratio
Intake Centerline of the cam
Advertised Duration
Duration @ .050"
Volumetric Efficiency of the motor
Altitude
Boost pressure (if applicable)

HAM1974, try to answer as many of the things above as you can and we'll go from there.

[Scotty_S-15]

Another factor is the heads. Generally, you can get away with an additional 1/4 or 1/2 point with Aluminum heads. Look at some of the factory alum. head CRs for instance.
But of course they've got the computer to pull back the spark advance at the first sign of a knock when using crappy gas.
.
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[Rinkrat456]

Another factor is the heads. Generally, you can get away with an additional 1/4 or 1/2 point with Aluminum heads. Look at some of the factory alum. head CRs for instance.
But of course they've got the computer to pull back the spark advance at the first sign of a knock when using crappy gas.
.
.

This is very true. Dollars and technology play a big role. Take for example my personal DD truck. 10.3:1 compression on 87, yet I designed it to accept any gasoline from 85-92 in case I'm out of gas near a farm and the only gas available has been sitting in a 5 gallon for a year in the back shed. New motors such as mine have the ability to tune multiple octane tables so that when I fill on 87 all the time as I usually do it's running at the peak of its ability for spark advance and knock retard, yet if I were to put year old shit gas in there it will automatically detect that and move down to the low octane spark table. Sure I lose a few hp/tq everywhere, but the tuning ability to save the motor from detonation is wonderful.

[hillbillymike]

For a good street motor you want your DYNAMIC COMPRESSION between 7.5 to 8.5 There are calculators online for determining dynamic compression. This is good reading on this subject http://www.empirenet.com/pkelley2/DynamicCR.html

[Ced]

11:1 compression is OK for the street as long as you plan out your vehicle. I have an 11:1 383sbc in my 94 S15 Jimmy. It uses 93 octane and nothing else. It also has a vacuum advance didtributor with 14 degrees of vacuum advance on top of the the initial (18) and the mechanical (16). The DCR is 8.19, cranking compression is between 200-220 on all eight. The cam is a 242/248@.050 hydraulic roller and it has aluminum heads.

I also have an 11.2:1 396 sbc in my 75 Vette. The timing is very close to what I run in the Jimmy. It also has vacuum advance. The DCR is 8.30, cranking compression is 215-230. The cam is a single pattern 244@.050 hydrauic roller. It also has aluminum heads and also uses 93 octane exclusively.

I have had the motor in the Jimmy for 4 years and the Vette for 1 year and experienced no noticeable pinging/detonation, even during the hot/humid NW Florida Summer. I do have good cooling systems and cowl induction on both, so that helps too. I do race them both in the 1/8 mile, but they are 90% street use. So it can be done but you should have a plan.

[OTHCUSTOMS]

i have 11.5-1 compression, i do not do daily driving, if im lucky i take it out once or twice in a week n thats on nice days. and its a 3-5 mile round trip. def too much for street but then again my whole truck became too much for the street. but def a higher octane is needed. i use 93 most of the time on the street and when i go to the track 100-110. and i always notice a big diff.

[OTHCUSTOMS]

sorry i meant 11.2.1, not 11.5.1