compression ratios 4 a super charger

[slammed4v]

im looking into buying a 97 cobra vert here is what the car has It has 41k miles, Rio Red/Black/Black, Vortech S-Trim Supercharger with stock 9-10psi pulley, PowerPipe, BBK 255lph fuel pump, C&L Mass Air, SVO Shorties, Bassani X-Pipe, 2-chamber Flowmasters, 3.73 gears, Nitto Extreme ZR Drag Radials, KB Boost-a-Spark, KB Boost-a-Pump, and Kenny Brown AGS 3 suspension. It has less than 10k miles (about 6k) on the engine rebuild that included Manley rods, JE flat tops (10.5:1), balanced and blueprinted, JMS chip. The raised compression makes the car great around town. No detonation with 93 octane. I had JMS make the first program very conservative, but it can still run mid 12s on a crappy track.

now my ? is is the 10.5:1 a good set up for a daily driver it will only be drivin until about nov when it will be put away until may if its nto a good set up what is the best????

thanx for all your help in advance

 

[GR8WHITE]

That is a high compression ratio for boost. Under boost the dynamic compression will be over 15 to 1. Granted the forged internals are great, but the upper ringlands are notoriously weak on the 4.6 pistons, even the forged ones. It has alot to do with the pin location. That being said, if you're not getting any detonation that is very good. Keep an ear open for any signs of detonation and you should be fine. Provided you don't boost it 24/7 , which is very hard not to do when it is there. ;-)

 

[shakylx]

I would lower the compression to 8.5 for an everyday driver

 

[shakylx]

and turn the boost up

 

[mnypit]

Originally posted by shakylx
I would lower the compression to 8.5 for an everyday driver

Somewhat Ditto. I went with a 9.1:1 CR on mine (2001 Cobra). The plan is to be able to run 13-15psi on 93 octane pump gas, everyday, all day. Allthough the car is not a daily driver, I wanted daily driver reliability.

Steve

 

[2F2F]

What shakylx said...
Ask the Civic and Integra boys...
They know all know what happens with 10:1 and 18+ PSI...Ka-BOOM!

 

[slammed4v]

its only gunna run 9-10psi

 

[Zate]

i am asking the same questions around.. looking to bolt a stock Eaton from an 03 Cobra onto the mach1.. 10.5:1 compression.. thinking stock boost ( 8psi) is all i will go.

 

[Booyah]

Originally posted by slammed4v
its only gunna run 9-10psi

Then leave it be, if it's running fine now then don't mess with it. Personally I wouldn't have dropped in higher ratio pistons on the rebuild but if it's tuned right and running safe on 93 octane then leave it be.

 

[Jay@Lamotta]

The problem is you can't run much timing with the high CR. The closer you can stay to stock timing the better. Starting the fire late will give the piston more time "to run down the bore" away from the peak pressure in the cly 12-14 after tdc. But I think camming will have to be addressed if you pull that much static out and it will make it lazy down low. Most tuners will tell you that 2 degrees it worth 30-50whp(with FI). Static compression is worth about 10whp per point. Look at static on any high FI car factory or not. When formula one had boost the CR were 6.7 to 7.2. I know this is extreme example. I'm sure the skyline.supra.911turbo and other OE turbo cars have a low CR prolly 8.1 .

 

[Booyah]

Originally posted by Jay@Lamotta
The problem is you can't run much timing with the high CR. The closer you can stay to stock timing the better. Starting the fire late will give the piston more time "to run down the bore" away from the peak pressure in the cly 12-14 after tdc. But I think camming will have to be addressed if you pull that much static out and it will make it lazy down low. Most tuners will tell you that 2 degrees it worth 30-50whp(with FI). Static compression is worth about 10whp per point. Look at static on any high FI car factory or not. When formula one had boost the CR were 6.7 to 7.2. I know this is extreme example. I'm sure the skyline.supra.911turbo and other OE turbo cars have a low CR prolly 8.1 .

Awesome info Jay, thanks. :thumbsup:

ON EDIT: Question, when you say per point of static compression do you mean a full point(ie - 7.0 vice 6.0) or a tenth(ie 7.0 vice 6.9)?

 

[Wildcard]

Your fine. Sean Hylands Cobra blocks come w/ 10.5:1 compression ratio pistons. And they are made knowing that your going to be running either nitrous or a blower.

Just get a good tune, and you'll be fine

 

[Jay@Lamotta]

I know that Sean runs alot of staic on the 4.6, But the margin for error ie (heat soak, climate,octane) is alot smaller with high CR. The only reason he does that must be to keep them from being a complete pig down low. I was just saying lower CR more boost/timing at same octane. We have a car with a SHM 8.5 4.6 at the shop and it is a PIG down low. I think we could learn alot from just looking at what has allready been done years ago by other car companys. O and I was talking about full point = 8.1 to 9.1.

 

[Wildcard]

I respect that, but I wasn't answering back to any other post or questions, only to the immedate[slammed4v], so that's what I was talking about. Futhermore, I was just passing along the info that is fact of which Sean Hyland builds their engines under. Yes everything that you say is true, esp. the reason for why they use 10.5:1 being they want more torque down low, and the error for margin being smaller. , but then again- I'm stating a fact. 10.5:1 compression can be done.

But hey your from Lamotta, so your the expert:rolling:

 

[Jay@Lamotta]

Yes I know it can be done I have done more than a few. But you can't really run much timing in them. If your going to build a engine I would pull some CR out of it. Good luck!

 

[Wildcard]

Good words. I'll be looking out for it. I know were running stock cams in mine, so the timing is probably not that radical. Though still I take it into advise. Thanks:beer:

 

[Route666]

Originally posted by mnypit
Somewhat Ditto. I went with a 9.1:1 CR on mine (2001 Cobra). The plan is to be able to run 13-15psi on 93 octane pump gas, everyday, all day. Allthough the car is not a daily driver, I wanted daily driver reliability.

Steve

The premium fuel we have in Australia is 96 octane, I plan on using that, in an engine with 9.1:1 CR and max 10psi boost from a centrifugal blower. From what You want to do, it sounds like what I want to do is very feasible. I was thinking also, that since the engine I'm going to use has a N/A static CR of 11:1 that boosting it on 9:1 would be the same as boosting another engine with static N/A CR of 10:1 at 8.5:1 or even easier.

I believe intercooling helps a TREMENDOUS amount in acheiving higher boosts too. Keep that motor as coooool as possible.