XX:1 ?????

[SuperCoupe007]

Hello guys/gals. If i remember correctly the 89-93 SC's have an 8.2:1 compressoin ratio and the 94-95 SC's have an 8.6:1 ratio.
I am trying to make a sketch on paper to see which way to go, if 8.6 is good enough or to bump it up more. question is what would be an ideal ratio. If the ratio would be bumped up, would that affect the way the engine works ??? Lets say forged everything, ported heads and ported super charger.. I'd like the idea of keeping the redline at about 6500, 7000 TOPS ! ... would the stock rocker arms be safe for this, i think i read that our stock set-up is 1.73's... any and all help is greatly appreciated... Thanks

[TBone95]

Youre gonna need some serious heads and cam and a real good blower to still make decent power at 6500 or 7000rpm. The M90 just isnt efficient enough. A cam setup for that probably would not have very good street manners. I think most of the top SCs still top out not much over 6000.

The flat hp and torque curves of the sc engine with decent heads and/or a good measure of bolts ons will give you plenty of acceleration without the need of humungous hp or redline numbers. If you want to shift at 7000rpm go buy a honda :) .

-Travis

[SuperCoupe007]

Hey Tbone, uumm i guess i didn't explain myself clearly.. I don't wanna shift around 7000 RPM.. i could explain it as insurrance.. just wouldn't want 5000 RPM redling and have to rev to 4500 then ~~~~f. but 6000 RPM is good.. I never thought about the street manners.. :( you are right.. now that i think about it 6000 RPM sounds very nice... :p oh by the way HONDA ?!?! HEEEEELLLL NOOOOOO !!!!!!!!! heheheh :D just my .02 cents..

[Aaron Pedroza]

Well if you are trying to make optimum power then it is better to keep the compression down and increase the boost. Find something that calculates the effective compression ratio with added boost. It would probably be best to be at around 7.8:1 compression and run about 18+ lbs of boost. You will need to have the blower reworked to handle the rpm it is going to take to get those kind of numbers though. You will also need the new jackshaft pulley 10 rib setup. As far as the rockers go, it is best to just get a new set of rollers, the stamped steel ones aren't real good for a performance engine.

[TBone95]

Ahh thats different. Most redlines are marked where you run out of usable power, not where its gonna be close to breaking something. Otherwise youd have the fast and the furious kids running it past the redline pretending they have nos and blowing up engines left and right.

Id recomend the Steigemeier head package, best flowing out there and looks like it can handle plenty of rpm.

[Rob Noth]

If the ratio would be bumped up, would that affect the way the engine works ??? I'd recommend getting an experienced engine builder to help blueprint this engine for you, such as a local performance shop or any of the SC specialist shops that are well known in this club. On the internet you'll get ten different answers depending who you ask, and I'm sure it will mostly be valid advice, but it's hard to know which advice to take without already knowing a lot about engine design in general.

[SuperCoupe007]

Hey Aaron 18+ sure sounds like serious BOOST.. Whats the maximum BOOST our M90's can provide ?? What kind of rocker arms are good for out car ? and are 1.73's good enough ??
Hey TBone, oh no i am not looking to be like the guys in fast and the furious. Hhhmmmm i remember reading something about the Steigemeier heads, i am gonna look into that...
Thanks Rob, i am gonna go talk to some shops and see what options i have..

HHmmm hey Aaron, or anyone, i just thought of something, woudl it be possible to run 18# of BOOST w/ 91 octane ???

[Slysc]

We had a thread going last winter about this very topic.
It sounds like there is no number that we can pick and say that that is the best compression ratio. I guess it depends on your strategy.
I went with higher compression. I realized the limitations of the M90 and knew that I wasn't going to be creating the kind of boost that turbo cars or cars with more efficient blowers do, so I went with about 9.3:1 when I built my motor.
I wondered if I would be able to handle much boost with that compression but it turns out I am running a 10% OD pulley creating about 14-15 PSI on the dyno and about 28-30 degrees of timing at WOT depending on the weather. So, I'm pretty happy with the higher compression.
If you were gonna run some other blower or an M90 which I'm not aware of which can create gobs of boost efficiently without turning into a flamethrower, then lower compression would be the way to go.

[David Neibert]

I'm pretty much doing what Aaron described but my compression ratio is stock at 8.2:1. The engine likes lots of boost and lots of fuel. I'm also able to run 28 degrees of timing with 18 pounds of boost on 93 octane pump gas.

I know I'm using a bunch of power off the crankshaft overdriving the blower 23%, but that's what works best on my engine.

David

[Aaron Pedroza]

The biggest thing you will need to consider is the blower you intend on using. If you want to just use the basic M90 then go with the stock compression, maybe a little higher, and keep boost numbers down to about 15lbs. If you want to spend the cash on the reworking then go with the lower compression. The blower will need a new bearing and have a new shaft installed for the higher rpm and be ported to try to keep outlet temps down and allow it to flow. You will also need to have a good IC to keep the intake charge at a bearable temperature. That is quite a bit of money but it is the way to go for max power. If you're not into spending that much cash then go with the stock compression ratio and 15lbs of boost because the stock type blower can't handle those rpms for long.

[SuperCoupe007]

Slysc, well my strategy is to be able to use it as an everyday driver. No i don't plan on using any other type of blower. I'd rather stick with the M90. 14-15 PSI sounds good.. that way i could still use the stock gauge...
David, here in CA super unleaded is 91 octane :( wow !! 23% overdrive, is your M90 stock or is it an MPII?
Aaron, Well i was considering getting the blower ported to an MPII, raised top 3/4", 70MM TB, MP inlet plenum, so far thats a tab over $2K... i plan on using a double IC and the plan right now is getting forged pistons/rods. but not sure if i would need them... :confused:

[David Neibert]

is your M90 stock or is it an MPII?

It's an MPII case with late model teflon coated rotors, ESM titianium input shaft, ESM upgraded bearings, ESM nylon coupler, viton nose seal and an MP heavy duty seal retaining ring.

Other than plastic coating the inside of the case, I've done pretty much everything possible to improve effeciency and air flow. It sounds like a coffee grinder at idle and a skillsaw rippin plywood when your into the throttle hard.

David

[Jimmy 2Coupes]

Just so you know the stock static compressoin ratio on 94/5 SC's is 8.5:1 :)

[SuperCoupe007]

David, :eek: it sure sounds like it cost u a fortune to do all that work. must be nice when you are hitting 18 PSI and people turn to look at your car... :) ....
Jimmy, i didn't know that.. now i know.. thanks...

[David Neibert]

must be nice when you are hitting 18 PSI and people turn to look at your car...

It's okay at the track...but on the street it tends to get me in trouble.

David

[SuperCoupe007]

Hey David, I was wondering, could it be possible to run lets say a 9.3:1 ratio and make about 18 LBS of BOOST with 91 octane ??? or am i dreaming ??

[David Neibert]

I don't think so...most of the guys I've seen go to higher compression (over 9:1) have problems with detionation if they add more than 14 pounds of boost, and that's running 93 octane. I'm sure you would need 110 octane race gas to run that much compression and boost.

Keep in mind, that more boost doesn't always mean more power. More airflow with less heat allows you to burn more fuel. Burning more fuel is where the power comes from.

David

[Slysc]

I'll let you know the answer on the 9.3:1 with 18 psi of boost question. I've got 9.3:1 compression and running about 14-15 psi of boost right now. I'm planning on picking up a few psi when I upgrade the IC and reduce some pressure drop. Right now, I'm running a ported 94 model blower and raised top which produced 18psi on my stock longblock. After I swapped to a bigger cam and ported the heads, added big valves, etc. I dropped to 14 psi with the same blower. I got some detonation at 30 BTDC at 4500 rpm @ WOT and had to drop down to 28 DBTDC with 93 octane on the dyno. But it was a really hot day. Now I have an IC fan and it doesn't seem to do it. But just to be safe, I usually run 28 on the street and only go to 30 DBTDC at the track when running a 110 octane mixture with pump gas. Couple of things that may help me reduce the detonation that others may not have is that I'm running some real cold denso irridium plugs and my rod to stroke ratio is 1.829.

[SuperCoupe007]

David:
hhhmmmm very good point, never thought about it like that. and yeah i do wanna keep using 91 octane pump gas...

Slysc:
what kind of porting work did you have done to your 94 blower ? How can u program the timming ?? u know (30 BTDC @ 4500 RPM w/ WOT) :confused: how does that work. Hhmmmm "rod to stroke ratio is 1.829" :confused: gonna have to find out exactly what that means.... sorry guys, not very very knowledgeable about engies but want to and will learn. :)

Is there much of a difference between the 7.8:1 ratio, and 9.3:1 ratio while the SC is not kicking in (N/A version) ? what would be advantages disadvantages ??? thanks guys...