Help choosing compression ratio???
Help choosing compression ratio??? So after speaking to many different tuners and engine builders I still don't have a good feeling of what compression ratio to run for my current build. Basically it will be a 427 on pump gas with methanol injection running about 18lbs of boost. I have gotten answers everywhere from 9.2 to 10.5. I was getting ready to build the motor for 10.0:1 but the guy I am probably going to have tune it thought I was crazy for running 10:1. I know the classic answer is to run lower compression and make up for it with boost but I don't want to have a car that has to be in boost all of the time to make good power. I want something that is responsive as well when not in boost. I guess the best was to ask this is how high of a compression ratio could you run with a 427 and around 18lbs on pump gas with methanol injection?
I vote 10:1 because Ive had 2 different setups running twin turbos running 8:5:1-8.7:1CR
1 was a 6 speed & the other was 4L80e and both were sluggish out of boost making them not much fun. This was with a 347cid. You got alot more cubes so thats going to help you out ALOT. Id still go 10:1 though. Im installing another TT setup on my current WS6. Im leaving it 10:1 and running small turbo's. Im hoping I will like this setup more than the previous two.
1 was a 6 speed & the other was 4L80e and both were sluggish out of boost making them not much fun. This was with a 347cid. You got alot more cubes so thats going to help you out ALOT. Id still go 10:1 though. Im installing another TT setup on my current WS6. Im leaving it 10:1 and running small turbo's. Im hoping I will like this setup more than the previous two.
I am building the LS3 for my WS6 with a D1SC as close to 10-1 as i can get it. I do not intend on running much more than 10-11lbs however. I am going to run an Alky Control Meth kit for protection but also have a large front mount. I was in the same boat as you....coming from the import side most everyone runs 8.0-9.5 and thinks 10-1 is nuts. In the end its going to be down to how good your tuner is and if you put it together with some decent parts.
I am building the LS3 for my WS6 with a D1SC as close to 10-1 as i can get it. I do not intend on running much more than 10-11lbs however. I am going to run an Alky Control Meth kit for protection but also have a large front mount. I was in the same boat as you....coming from the import side most everyone runs 8.0-9.5 and thinks 10-1 is nuts. In the end its going to be down to how good your tuner is and if you put it together with some decent parts.
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Alot depends on the tuner, his experience level and what he's comfortable with. Personally, I would prefer a little less compression and make up for it in the tune. It will afford you the ability to run a little more timing and it will have a wider tuning "window".
Ed makes a good point, but its worth noting also that most new OEM motors are in the 10.5:1 or higher range, and seem to work really well with FI. The numbers the new 5.0 makes with a little boost are crazy.
In the end, it does indeed come down to tuning, and intended fuel use. I see you are in NV, so I am assuming crappy 91 octane, you may want to err on the lower side.
In the end, it does indeed come down to tuning, and intended fuel use. I see you are in NV, so I am assuming crappy 91 octane, you may want to err on the lower side.
Launching!
Joined: Jul 2006
Posts: 214
Likes: 0
From: Tampa
I'm running 10.6 to 1 and 13lbs of boost on pump gas, some meth 15 deg timing. I have the APS kit thats part of the reason I went so high I knew I was limited to a certain size turbo. 741rwhp/689rwtq. Last engine was about 9 to 1 noticeable difference in driveability I like it much more than I did before. "427 on pump gas with methanol injection running about 18lbs of boost" Plan on driving it on the street alot? If you do I would turn it down on the street I'm only at 750 and you'll have quite a bit more than that.
Last edited by Ironmancan; Feb 23, 2012 at 06:30 PM.
FormerVendor
Joined: Dec 2001
Posts: 2,444
Likes: 2
From: WWW.SPEEDINC.COM
So many variables.... but generally I like around 10.0-1 for a street car.
My preference for tuning/fuel is to run a safe boost/timing level on 93 (maybe 10-12psi max if it has a decent cam and not crazy high DCR) and add race fuel for higher boost levels.
I just have more trust in running good fuel through the primary fuel system/engine management than trusting a secondary system (meth) to spray liquid through a manifold designed for dry flow.
For cars requiring 1000+hp on the street dual fuel systems and staged injection is nice but I generally find that the times I need more power (a situation out of the ordinary/everyday driving) adding race fuel is not an inconvenience.
My preference for tuning/fuel is to run a safe boost/timing level on 93 (maybe 10-12psi max if it has a decent cam and not crazy high DCR) and add race fuel for higher boost levels.
I just have more trust in running good fuel through the primary fuel system/engine management than trusting a secondary system (meth) to spray liquid through a manifold designed for dry flow.
For cars requiring 1000+hp on the street dual fuel systems and staged injection is nice but I generally find that the times I need more power (a situation out of the ordinary/everyday driving) adding race fuel is not an inconvenience.
Not taking away from this post, but speaking of OEM engines, the ZR1 Corvette and the Cadillac CTS-V are both supercharged engines and both have a CR at about 9.1:1 and 9.0:1. I have never driven a ZR1, but I bet it's not exactly sluggish... ever!
With that being said, this is aftermarket and not OEM. You don't need to make good on a 100,000 mile powertrain warranty. People put FI kits on their stock 10.5:1 engines all the time!
With that being said, this is aftermarket and not OEM. You don't need to make good on a 100,000 mile powertrain warranty. People put FI kits on their stock 10.5:1 engines all the time!
Ed makes a good point, but its worth noting also that most new OEM motors are in the 10.5:1 or higher range, and seem to work really well with FI. The numbers the new 5.0 makes with a little boost are crazy.
In the end, it does indeed come down to tuning, and intended fuel use. I see you are in NV, so I am assuming crappy 91 octane, you may want to err on the lower side.
In the end, it does indeed come down to tuning, and intended fuel use. I see you are in NV, so I am assuming crappy 91 octane, you may want to err on the lower side.
I definitely want my tuner to be comfortable with whatever he is doing but at the same time I don't believe that any one tuner is by an means magic, and can tune a car when one can't. As far as meth goes I will be fabricating up a direct port injection system with 8 nozzles to easy the concern with the meth flowing threw a manifold designed for dry air. When I comes to the ZR1, while I personally don't own one I have driven a friends of mine and I personally could feel a difference. The boost does come on very fast making it feel like very peppy but if you watch the boost gauge and see what it feels like when outside of boost you can feel the lower throttle response. Now when it comes to why the ZR1 was designed with such a low CR ratio I would say it was because with it being a factory can that may very well get a bad tank of gas from time to time and even be run on lower octane and yet still be expected to go all out for a mile without any downtime and at the end still be fine with a 100k mile warranty, there is a low of safety margin that has to be built in.
Well the pistons are ordered so there is no turning back. They are specked out to be at right about 10:1 but can go a little bit lower by playing with the heads and gasket thickness. I recently decided to go with a different blower and am probably going to up the boost a little to around 22psi or so. Think 10:1 will still be ok or should it be lower?
INTMD8 is the tuner for Speed Inc. i'd definitely pay attention to that dudes post! lol
i also vote for around 10:1 and run pump gas on a street tune to about 12 psi and run race fuel above that. or, E85 if you have it readily available in your area.
These numbers just don't make any since to me. My previous build was a stock LS2 @ 10.9:1 CR and ran 13lbs of boost on pump gas with no sign of nock even with a PD blower and its associated high AIT issues. So for someone to say that I should run less boost with almost a full point of compression less confuses me. Race gas is out of the question and I am asking for input on pump gas. I know for a fact that it can be done at 9.1:1 CR with 20lbs of boost and no methanol injection. That build produced over 1100rwhp on 91 octane pump. The shop who I will probably have tune it suggested 10.0:1 for 91 pump gas and methanol injection with about 20lbs on a 427. Just wanted to get an idea of what other people think about this.