nubs4ck

I have 2 options for combustion chamber size on my 408 heads. Should I go with a 72cc, putting my static comp ratio around 8.4:1, or should I go with the 62cc chamber putting my CR around 9:1?

The engine builder is recommending the larger chambers for a "safety" factor. I'm wondering if the lower compression ratio will kill the spool time on my turbo which is a rear mount in a T56 car.

Does anyone have experience with dropping or raising compression ratio and power output/detonation safety?

Please give me as much feedback as you can because if I want to go with the 62cc chamber I need to let them know ASAP.

GM PHASST

Heres an example.
On 93 oct with the 9.1:1 compression ur engine should make more naturally aspirated hp than the 8.4:1. With the 8.4:1 u can safely run more boost and possibly make more power than the 9.1:1 on less boost.
More compression will spin the turbo faster. It really just depends on wut ur using the car for.
For mostly racing, I say lower compression with more boost. For more street, I say more compression less boost. This is assuming u will be running on pump gas tho.
If using meth, e85 or alky I say more compression with more boost for street and race use.

dschmittie1

iTrader: (64)

How much power/ boost are you running and what fuel.


All of my motors have been in the 9.5-9.9:1 CR range on my pump gas and Meth combos.

I have been running 15# of boost and near 1000 crank HP


I personally would never build a boosted motor with anything less than 9.5:1 even on just pump gas

02blackbeauty19

iTrader: (22)

I agree. To go a little further, I would ABSOLUTELY never go below 9.0:1.

nubs4ck

Sorry I didn't post this info before. The car will run on 93 octane with meth injection. It will see quite a bit of street use, much more than track use. I am looking for a 1000whp dyno sheet.

My previous combo, with the same cam on a 347 hit 15psi without meth injection. The compressor was too small and it peaked at 556hp/618tq.

Thanks for the feedback guys, keep it coming.

NicD

iTrader: (7)

For the best pump gas reliability and power you GENERALLY run a lower compression ratio. That being said it varies heavily how low with what you are trying to accomplish and what the rest of the combo is. I personally built mine to run hard on pump gas and am running 8.5:1 compression on my little 347 and it's doing pretty well with the small 76gts and it's in no way laggy or soggy down low. Ultimately you are limited by your turbo anyways and since turbos tend to favor high pressure ratios lower compression can be better.

nubs4ck

I know a lot of older cars that came with turbos had low compression ratios. I believe the old DSMs had around 7.9:1, the grand national had 8:1... the SRT4 is 8.1:1, STI is 8.8:1, lancer EVO is 8.8:1 as well... I'm thinking that 8.4 shouldn't be too bad.

dschmittie1

iTrader: (64)

my biggest issues it that you are rear mount. That will be harder to spool. That and a 8.4CR motor will be a dog out of boost.

slow67

iTrader: (2)

For pump gas, go with absolutely as many cubes as you can + don't go real high on compression.

NicD

iTrader: (7)

Yes there is a reason that a majority of all OEM boosted engines are in the 8s and it's because of their ability to stave off detonation with normal pump gas and make more power in the process. Besides, a full point in compression is only about a 4% change in power. Going from 9.0:1 to 8.5:1 on a typical 347 is going to be a whopping 10 rwhp when out of boost but it will certainly get you more head room in the process. GM runs the LSA and LS9 right around 9:1 because they have to contend with efficiency and gas mileage ratings and they are "big" motors.

qqwqeqwrqwqtq

Generally I prefer 9.5 to 10.0-1 for a street car and 11.0-1 for a race car.

I like to run a bit more compression for nice fast response. I don't mind if that limit's pump gas HP as I have no interest in tuning aggressively on pump fuel anyway.

nubs4ck

Thank for the feedback guys. I think I'm probably going to stick with the lower compression ratio.

02blackbeauty19

iTrader: (22)

I really think you should reconsider. With pump 93 + meth, there is no reason to go below 9.0:1. Listen to what INTMD8 is saying.

Ryans99ls1

iTrader: (16)

i agree. there is no reason to drop below 9:1. hell at 9:1 you could take plain 93 pretty far..

99SS-T

iTrader: (5)

funny when people ask for opinions and get 90% in one direction just to decide to go the other direction!

tripblackls1

iTrader: (11)

With a rear mount I would go with as high of compression as possible

In my build my motor is 8.5 comp but everything has been matched to work with the low comp and I know if I had a rear mount instead of the front mount ,that the car most likely be very painful to drive

98Z28CobraKiller

iTrader: (17)

With a rear mount T56 combo, you'll need all the spooling help you can get. 9:1

nubs4ck

I think you guys are right. I previously had a 9:1 setup with the rear mount with 15psi of boost on just 93 pump gas. The only thing that is different is now I have a 408 instead of a 347 and a much larger turbo, 8079 from comp. I'm going to try and get the AFR230s in 62cc instead of 72cc.

I really want my car back and this will add another 3 weeks or so to what i have to wait. This makes me sad.

Just so you guys know, I bought the car, drove it for 2 weeks and spun a bearing. It has been down for over a year now... I really want it back, it's no fun driving a stock pickup truck around.

b4cz2896

Low compression = sucks...you'll make less boost, less power, less fun

FastKat

iTrader: (10)

My current setup is at about 8.5:1 w/dished pistons. One of these days I hope to get a set of 243 or 799 heads to bring it up to about 9.0:1.

From what I understand, there's a lot more to it than just CR - the shape/volume of the combustion chamber is also important, and changes (relatively) when you start messing with different chamber sizes, piston shapes, etc.