Compression ratios

**98redconv** · 12-01-2008, 12:57 PM

I am looking at a set of wiseco pistons that would bring my compression to somewhere around 12.5:1 is that fine for 93 octane? Would i have to retard the timing?

**Mr. Luos** · 12-01-2008, 01:01 PM

I wouldn't want to run 12.5:1 on pump.

It can be done, but it takes a very good tune.

**Johns00Z28** · 12-01-2008, 02:35 PM

I see a lot of guys going for a max effort engines making 11:5-12:0 compression so where's the cutoff for being able to safely run on 93 octane?

**Mr. Luos** · 12-01-2008, 02:36 PM

Well...there are a LOT of variables.
Cam selection plays a large role.

I would stop at around 12:1 personally. But again, that depends on cam.
Mine is just a pussy 11:1.

**Johns00Z28** · 12-01-2008, 02:38 PM

How much more power can be had when moving from 11:1 to 12:1?

**Mr. Luos** · 12-01-2008, 02:40 PM

Originally Posted by Johns00Z28

How much more power can be had when moving from 11:1 to 12:1?

That again depends on the dynamic compression more than the static.

10-12 RWHP on average though would be my guess.

**Bottesini** · 12-01-2008, 04:20 PM

It also depends on where you live as well, some climates will let you get away with higher compression. With my build I am going for right around 11:1 maybe 11.5:1 (depends on quench distance and heads I decide on).

**Hi-Po** · 12-01-2008, 05:42 PM

Static doesn't really mean nothing, Find out dynamic ball park figures.

Upping the CR to the 12's and above just doesn't seem worth it to me, for the HP increase that you receive.

**Firebirdjones** · 12-01-2008, 05:53 PM

Originally Posted by Bottesini

It also depends on where you live as well, some climates will let you get away with higher compression. With my build I am going for right around 11:1 maybe 11.5:1 (depends on quench distance and heads I decide on).

Generally the way that works is the higher in elevation you go the more compression you can get away with,,,and/or the more spark lead you can dial in.
Camshaft timing has alot to do with it as mentioned. It's not so much what static compression you can run,,,,but how much dynamic you can get away with,,,and camshaft plays a roll in this.

A good general rule of thumb is to keep your cranking PSI around 200 for 91-93 octane pump gas with a proper tune. Give or take 10-20 PSI depending on conditions such as elevation, cylinder head material, outside ambient temps, engine cooling temps, how much the car weighs, how much gear you run,,,,etc....

For a full weight street car that sees diverse conditions I would lean towards convservative personally.
For the original poster,,,I wouldn't run 12.5:1 on pump gas no matter what the conditions, thats really pushing the envelope. If a max effort motor is what you're after then why not just go for 14:1 or more and just run the good gas and be done with it.

**Mr. Luos** · 12-01-2008, 06:26 PM

Originally Posted by Firebirdjones

Generally the way that works is the higher in elevation you go the more compression you can get away with,,,and/or the more spark lead you can dial in.

We ran Slowass to 34 degrees on pump gas at 11:1 without any knock.

Backed it off though, as it didn't really show any gains over 31 degrees.

**mark21742** · 12-01-2008, 08:41 PM

Originally Posted by Mr. Luos

We ran Slowass to 34 degrees on pump gas at 11:1 without any knock.

Backed it off though, as it didn't really show any gains over 31 degrees.

are by any chance running hptuners? if you are, any chance you could post or pm a copy of your hi/low octane spark tables.
I'm here in southern PA so I'm a lot lower than you are, but we have been playing with my tables and set up to 27 ° with no knock retard on 93 octane in my 04 goat......how does the car know what octane your running anyway?

**Mr. Luos** · 12-01-2008, 08:44 PM

Originally Posted by mark21742

are by any chance running hptuners? if you are, any chance you could post or pm a copy of your hi/low octane spark tables.
I'm here in southern PA so I'm a lot lower than you are, but we have been playing with my tables and set up to 27 ° with no knock retard on 93 octane in my 04 goat......how does the car know what octane your running anyway?

It judges by how the motor reacts to the fuel it gets.

Mine is tuned via HPTuners, but we have both timing charts the same, as it never gets a low octane fuel.
I can see if I can get my tuner to send me the .bin file if you want to see it. Your car will be completely different though.

**mark21742** · 12-01-2008, 09:06 PM

Originally Posted by Mr. Luos

It judges by how the motor reacts to the fuel it gets.

Mine is tuned via HPTuners, but we have both timing charts the same, as it never gets a low octane fuel.
I can see if I can get my tuner to send me the .bin file if you want to see it. Your car will be completely different though.

true, just curious....lol tried swapping my friends primary VE into mine and goat would start, lope like it had a big cam then stall.... so we put mine back in.
we looked at the stock VE, spark, and PE tables in his 06 vette, 98ta, suburban,and my 04 goat, and its amazing how different they all are

**mark21742** · 12-01-2008, 09:11 PM

also looking at my primary VE the whole table has big spikes and dips in it where in his tuned/ built 98ta it very smooth. any ideas to why my stock table would look like the rockie mountains?

**Firebirdjones** · 12-02-2008, 12:27 PM

My VE table was also somewhat rocky when it was stock.

You'll have to log VE and drive. I used fuel trims to dial mine in and it works fine. There is a smoothing feature after you are finished to take out the rough spots. You can even use a wideband to do it,,,and some say it's the only way to dial in the VE and MAF tables. I simply used fuel trims and gave the car what it asked for then used the wideband to dial in WOT
You won't be able to simply install someone elses VE table as you have found out. Every car is completely different in the amount of fuel it requires.

**mark21742** · 12-02-2008, 02:01 PM

Originally Posted by Firebirdjones

My VE table was also somewhat rocky when it was stock.

You'll have to log VE and drive. I used fuel trims to dial mine in and it works fine. There is a smoothing feature after you are finished to take out the rough spots. You can even use a wideband to do it,,,and some say it's the only way to dial in the VE and MAF tables. I simply used fuel trims and gave the car what it asked for then used the wideband to dial in WOT
You won't be able to simply install someone elses VE table as you have found out. Every car is completely different in the amount of fuel it requires.

lol yeah we learned REALLY quick

**Firebirdjones** · 12-02-2008, 06:35 PM

Originally Posted by mark21742

lol yeah we learned REALLY quick

LOL,,,ya but it sure is fun to play with. I'm still experimenting with things myself and learning as I go.
Having tuning software for these cars is a must have in my opinion. Once you realize what you can do you wonder how you ever got along without it.

**mark21742** · 12-02-2008, 07:59 PM

Originally Posted by Firebirdjones

LOL,,,ya but it sure is fun to play with. I'm still experimenting with things myself and learning as I go.
Having tuning software for these cars is a must have in my opinion. Once you realize what you can do you wonder how you ever got along without it.

true,very true.....and its even better when a friend has it and let's you tune your car for free....if I learn how to do something I just have to show him how to do it.....we are kinda learning together but he is way ahead of me....
if you read this, thank you Bill!

**Bezerk** · 12-03-2008, 02:17 AM

hey guys, its interesting to read about your comp ratios and octane levels, i dare say the fuels must be different here in Australia as 91 octane is our base unleaded, 95 is our high octane and 98 is our premium. we would not dare run much more than 10.5 on pump 98, im running 11.1 but using e85 which is an ethanol based fuel. basically part alcahol. i guess altitudes would also play a huge part in it. but anything like 12s is definately methanol territory here.

**mark21742** · 12-03-2008, 04:20 AM

Originally Posted by Bezerk

hey guys, its interesting to read about your comp ratios and octane levels, i dare say the fuels must be different here in Australia as 91 octane is our base unleaded, 95 is our high octane and 98 is our premium. we would not dare run much more than 10.5 on pump 98, im running 11.1 but using e85 which is an ethanol based fuel. basically part alcahol. i guess altitudes would also play a huge part in it. but anything like 12s is definately methanol territory here.

we must use different measurements to get the octane ratings from you guys......i always read about your higher octane fuel and assumed you had a better grade of fuel.....you got me curious now