I am building a forged motor for an MN6 intercooled ATI set-up with a projected 10 PSI and plan on street use with 91 octane gas. What would be the most desireable final compression ratio to shoot for?
Those that have already built them--what CR do you have and are there any issues?

 

I have ready access to 93 octane here so I'm not familiar with the diff, with the 93 I would keep my CR (forged motor of course) around 10, I drive my car all the time and seldom use the boost and its nice to have some comp for 99percent of the driving, I have had no probs for at least a year and a half with 10.7 to 10.8 running 6lbs and have had the best of both worlds but If I forged I would not blink an eye at running at least 8lbs even with the high cr-- so at 10cr I would run 10 or 12 and feel very comf about doing so---

 

I have those specs (that you are talking about) and I am running a 9.6 to 1 ratio.

VR :cheers:


[Modified by vetterdstr, 1:51 PM 10/24/2003]

 

These motors seem to take more CR than many (due to head chamber design, for the most part), but the FACT is that the gains are exponential the more boost you can safely run. A 10:1 motor at 6psi, will not make more power than a 9:1 motor at 10psi...even if you could have gotten away with 9.5:1 and 10psi. The extra .5:1 CR is not going to make a big difference on a properly tuned blower/turbo motor running substantial boost. Now, if you were only running 5-6psi... losing the 1/2 pt CR would hurt more.

All it takes is one pi$$-poor tank of gas, if you're running on the edge.

Is there a reason you're limiting yourself to 10psi? I would consider a combo at ~8.8-9.0:1 CR and 13-14psi. Should make insane power, and be tough as nails, and run on pump gas with a good conservative tune. :D

 

I was only estimating 10PSI as a reference point. Actually, I have a set of heads and cam in hand but upon number crunching by the assembler a comp ratio of 9.5-9.8 was showing up and I was concerned as I thought that 9.2 was ideal.
I definitely want good part throttle and low end pull for comming out of hard turns but also want great top end when requested. Estimated 600+rwhp with a linear hp/tq curve line is the goal which should be attainable and realistic.

So, my origional question is; is 9.5-9.8 CR OK or should I re-group for 9.2/lower? :steering:

 

vetterdstr specs are where you want to be, I'm not sure how much boost he is running now but he's in the 650rwhp range and running about 9.5cr so you should be fine for 12+lbs it if takes that to get there--

 

I would PM someone like the gentelman above who's got a like combo. Find out what kind of gas he runs, how hard he drives the car, what type of driving he does, what kind of gas he uses when he races, etc.

Since you are running 91 octane, I would choose to err on the side of caution. The difference b/w 9.2 and 9.6:1 is not going to be much at all as far as power goes, and will give you more flexibility down the road should you choose to swap pullies and go for more power.

Poor throttle response and low-end TQ from a centrifugally blown combo is the result of poor camming more than low CR. I'm sure your engine builder has chosen a good blower cam for you. Been there, done that.

Also, realize that timing in a blown/turbo motor is a much more important factor than a N/A motor. I've seen cases where 4-5^ timing can make a 30-40rwhp difference on a smallblock. If you have to retard the timing excessively in order to alleviate detonation concerns, then your extra base CR advantage went out the window in a big way.

I'm just trying to point out some theoretical and practical things. I am NOT trying to say you wouldn't be ok at 9.6:1 on 91 octane. I don't know b/c I personally have not tried it on my C5. I can tell you from tuning blown apps for over a decade that the beforementioned issues are real, and can crop up and bite you in the butt.

That being said, you can certainly get 600rwhp on pump gas with the right combo, so best of luck! :cheers:

 

I would suggest no lower than 9.5 CR optimal being about 9.7 CR with a D1 blower and 13lbs of boost like I was running. The trick to overcoming "getting out of the hole" or sluggish feeling down at low rpms is to make sure you have some 4:10 gears M6 or 3:73's M12. With a Centrifugal Blower it takes power to make power, when this is combined with a lower compression motor you have got to have the gears to compensate...the gears make up the difference. You can still have a cam that is designed for mid to hi rpm also. I made 610/545 rwhp/rwtq and needed bigger injectors 42lbs were too small...so my redline/revlimiter was set to 6200 rpms...had I had 48-50lb injectors I could have gone up to 6700 rpms and picked up probably another 25-35rwhp...just some food for thought..you don't want the hi cylinder pressure of running boost on a 10.5 CR motor....forged piston are still breakable...they are just a whole lot better than the stockers! Good Luck. :thumbs:

 

Thanks guys, I appreciate all the input and confirmations. Looks like I will be able to use the heads I had in mind after all. And with the 48# injectors that I'm sitting on all should be OK. As for the cam, it is my own spec and have had it in hand for over 1 year (I designed it to compliment the heads I want to use) and am very happy to be able to use it also.
Thanks :cheers: The assembly begins... :D

 

Hey xtreme1! :cheers:

Your biggest problem with a low-CR motor is getting enough boost with the pulleys and belts available. I'm getting 8psi with a 3.5" pulley and MMS brackets, and I had to groove the pulley to stop slippage.

With the same 8psi boost, a low CR with stage2 heads/cam feels about the same as the stock block/heads/cam motor did. Difference is that there is less low-end torque and tirespin, but more in the midrange. Downshift 'break-loose' speed rose from 50mph to 65mph. If I could raise the boost it would be a kick-azz car.

I did the -8.6cc .010 deck pistons, .060 cometic gaskets, and 71cc heads. With a bore of 3.905" and stroke of 3.622", http://www.rosspistons.com says my CR is 8.62:1.

 

blu00rdstr,

You need a D1 blower with that compression. Might even want to consider F1, but I think a little D1 will do ya.

It's a plug and play replacement (besides tuning).

You could possibly even trade with someone. There is one guy upset that his D1 is making too much boost.

 

I think that's a good point. Don't try low CR motor with P-1SC.

I am talking to the member in FL. Hard to believe that someone would dislike high boost...

I think I have the lowest CR C5 on the forum.

 

I build all my street driven blower motors in the mid to high 9s. That way they're not sluggish off boost. We still make over 600 RWHP on boost. Sort of a best of both worlds compromise. It has workewd very well so far.
I'm building my 409 CID stroker at 9.7 and plan on 10 PSI boost.

 

My comp ratio is apprx 10.4:1 and it runs like a champ..Awesome low end with zero delay pre-boost.

Car hasn't burned a drop of oil in 5000 Miles and she purrrr's like a putty cat ;)

:cheers:

Jeff


P.S I almost forgot to mention , on my last dyno pull I was pushing 12 PSI Boost :auto: .


[Modified by BLUTHUNDER, 6:57 PM 10/25/2003]

 

Dayum! I build 'em good, huh? Seriously, I build them all at fairly high compression and still use a lot of boost. This one was a little higher because his heads were already milled quite a bit when I did the short block.
I'm doing one at 9 even now that I have my 8 rib working. Before, I felt that in order to make enough boost to overcome the low compression, we would run into belt slip and negate any gains. I'll use a D1 witha 3.8 ultragrip pulley and my new bracket.

 

Dayum! :eek: 12 PSI AND 10.4/1 CR on pump gas :eek: I guess there's something to be said for computer tuning! :cheers:
Remind me to never challenge YOU on the road.

Andy, I guess that I should've just called you before posting. :banghead: You'll be seeing it soon enough though...better make room. :lol: