The Compression Ratio (CR) seems to be somewhat of a mystery when building an engine for a Forced Induction application. I have heard 8.5:1, 9.0:1. 9.5:1 up to 10.0:1.

The project car being C5, building the engine from the ground up, starting off with Naturally Aspirated, then looking to add power with different power adders. Long Term, looking for 15 to 30lbs of boost (or so). Looking for five answers from the experts.

Starting off Naturally Aspirated, the bottom end build is a 427 (LS7X, Callies, Diamond, Clevite, AllPro Heads, etc… (all the good stuff ) and 10.0:1 CR..

First question:
What is the optimal Compression Ratio for FI system?

Second question:
Having a 10.0:1 CR what would the estimated Naturally Aspirated HP be?

Third question:
Can a Supercharger (PC, Novi2000) or TT, be added to an engine with 10.0:1 CR without having to redo the bottom end (other than cam change)?

Forth question:
Would the handle 15 to 30lbs of boost?

Fifth question:
Would it make 900rwhp +?

Thanks Guys

 

I used to tell people but I won't anymore-let your engine builder decide but when you see people making more power and torque then "you" should of told your engine builder what it should of been.

I've been building centri motors since 91-back then we didn't have nearly the same technology we do now-but we have alot more opinions on how to do it!

I'll tell you this right now though-you can either build the motor for turbo/twinscrew or a centri-if you build it one way then you can't flip flop between power adders because the cam and compression will not be optimized.

 

The challenge is that if you ask 5 builders you get 5 answers!

I don't even want anything THAT exotic. I just want a nice streetable engine that is daily driven and makes 650 to the tires with my A and A P-1SC kit that is already making 587 RWHP.

I'm not trying to steal the post, but it gets frustrating! Now a days there are Soooo many options!

Brian-

 

10:1 is too high for 15 psi. I wouldnt go over 9:1 with 15 psi.

 

 

Opinion#1

 

If you build it low compression for future power adder later you will be probably mid 300 rwhp NA. Of course when you add boost you can go much higher.

I built mine NA at first with 10.9 compression and now push 5.8 psi with my Maggie. The nice thing was that I ran 407 rwhp NA. Now with the Maggie and the way the by pass valve works I still run good HP even when I'm not in boost. Obviously I can not push it to 15 psi with this compression and a stock lower end but I could pulley for an additional 2 psi and run meth anr run approx. 550 rwhp.

 

Do you have any idea what 30lbs of boost would get you on a 427CI engine?

 

Not necessarily looking to have 30lbs of boost, but would like a strong, flexibile lower end. Would like to try SC (Paxton or ProCharger) and then TT. The TT kits that are out, even APS, I don’t really care for, but I have seen some TT top mount kits that look great.

 

This is what ATI Procharger recommends on their website. I built a 8.9:1 416 CuI motor for 18 PSI of intercooled boost.

Hope this helps some.

Sean

 

Interesting graph.

I sold my Z06 about a year ago to my dad to finance my new business, but hopefully by the end of the year things will be on track that I can buy it back from him and send it to EnglandGreen to do a Novi2k. My motor is 11:1 due to milled LS6 heads (makes about 425 rwhp). I plan on just limiting the boost to 5-6 psi and running Meth.

Several years ago I ran a centrifugal supercharger on a naturally aspirated "high" compression rotary motor. I was very paranoid when tuning the car and ran it very rich to start (10.5:1 AFR). On the dyno, I gradually leaned the car out. I ended up running it 13.5:1 AFR with no pinging on the dyno but made no more power than 13:1 AFR so I put it back there for safety on the street. I was running about 6 psi then too.

Moral of the story...if you're running around 6 psi, I don't think your compression ratio matters all that much.

Opinion # ?

 

13:1? Safe for the street?!!?

 

A few points to your post:

1. Assume you intend to use the car either as street machine or 1/4 mile. If you intend to HPDE often or RR, stay away from FI and pursue NA options. I'll assume you aren't an RR guy.
2. Next question: How much hp is your target and how imp is bottom end torque at low rpms to you. I take it by your post that you are looking for big hp #s and to run a good deal of boost!
3. That said, if you in fact want to run over 900 hp and run 20+ lbs of boost, that narrows your choices. I suspect the best blower for you will be an F series Procharger. Novi makes a great product, but at the type of insane boost figures your quoting (> 20 lbs, the F series will likely be the most efficient set up). Otherwise you will be running the Novi or the Procharger out of it's intended range and will be using ridiculously small SC pulleys (belt slip issues).
4. Now that I've recommend a blower, next we turn to other required items. You will absolutely need a forged bottom for boost levels over 8-10 lbs. You will also need to run meth injection and should get a separate oil cooler and a Ron Davis or Dewitts radiator (don't go with combined radiator/oil cooler). If you don't address heat soak issue, you will fry your engine.
5. You will need a clutch, trans and output shaft upgrades. Also assume you will be needed a trans and diff cooler.

Conclusion: You may be deciding at this point to run less boost and settle for 700+hp? Good luck!

If your still serious about running that amount of boost, suggest you speak with Bob at EPP. He's doing a F series SC project right now at 20+ lbs. He does great work.

 

Air fuel not compression, I forget whats stoich for normal premium gas?

 

Check out our build on my Z28. We started with an iron block and an 9.5/1 compression ratio, and we always shoot for a 11.5/1 air/fuel ratio when using forced induction.
http://www.exoticperformanceplus.com...Car.php?car=12

We started with a new LS2 block on this CTS-V build, and we're shooting for 800 to 850 rwhp.
http://www.exoticperformanceplus.com...Car.php?car=29 Bob

 

No expert but have owned, installed, participated in design, and operated 3 different power adders and H&C combos with both of my C5's. Plus on my 2 Mustangs, 2 Camaros, Bikes (street and dirt) and Trucks.
Depends on your use, type, fuel, and or intercooling/alky injection. You have to be able to control detonation for long term and stock parts don't allow for too much give when things are not optimal. There are some parts that hold up well...for example I am still using my stock crank, but the pistons on LSx have proven to be the weakest link and are very succeptable to cracking under any detonation with higher cylinder pressures associated with boost. The hyperuetetic alloys do not expand and are brittle...so they crack like saltine crackers under detonating conditions. But it looks like you have forged pistons...so you have a higher degree of reliability with your motor...it still doesn't make it safe from damage due to detonation.

Not enough information to determine without knowing fuel type, power adder, and characteristics of the setup (intercooling, alky, both). Your range of 15-30#s is too large without knowing all that is involved with your kit. If I had to take a stab at it it would be 750-2500 rear hp....but long term with 25-30#s would not be long term and nothing short of back halfing the car would support that kind of power.The highest LSx single Turbo setup that I have seen was a 408cid Iron Block with a Single Turbo that made 1800rwhp and ran the 1/4 @ 200mph. It was pushing 25#s of boost if I recall correctly....and it wasn't in a Vette.

Based on your build you will have a redone bottom end, but to answer this question yes...but it too depends on boost and fuel (octane or alky) to control detonation.
Are you referring to the 427cid in your build...what are you applying this boost number too. 15#s on a 427cid at 10:1 could be done with 100+ octane all the time. I think you should look at Nick Yoskin's setup as an example of a great 427 setup. If you Google search you can probably find a website that has suggested Octane requirements for Compression Ration and Boost #...I have seen plenty of charts that can be used as references...but often they do not account for intercooling or alky injection.
I'm sure that it could be made to make 900rwhp...my 347cid TT setup could make 900rwhp...it just depends on the whole setup. It is going to take a higher octane fuel or additive like alky being mixed to control detonation more with a 10:1 CR motor running 15#s boost then it would a 9:1 CR motor and 15#s boost. If you have to cut timing to control detonation ont he 10:1 setup...you would be defeating the purpose. It would be better to run more timing, with more air and more fuel..then to run less timing with more air and fuel due to the higher CR.

It's a general rule to run less compression with more boost, then it is to run higher CR and more boost. The higher the boost pressure the more molecules of air that can be packed into the cylinder and mixed with fuel..thus making a large combustable charge. Having a higher compression ratio and running 15# boost will have a higher demand to control detonation then a lower compression ratio and 15# boost.

This is a blanket statement because you used a very large range for boost, but most LSx that I have seen have used Compression Ratios between 9-9.8 for Centri SC and 8.5-9.2 for Turbo setups.


I don't know that you will find any big bores or small bore LSx that have pushed 25-30# successfully (could be that I missed it).