[jbspeed86]

I am building a 347 stroker and I am planning to put a blower on it in the future so I am thinking about getting a kit setup for 9.6:1 compression. The place I am getting it from has it for 8.6:1, 9.0:1, 9.6:1, and 10.0:1. And it goes higher than that but those are obviously not good for blown applications. So im thinking of going with the 9.6:1. Reason being because it is not definite that I will be getting the blower, so this way if I don't get it, it will still run good. Ok, now for the main question i even posted this, How much boost can I put on this motor with this compression? All the internals, crank, rods(H beam) and pistons are all forged. Anyone know??

[GTRaptor]

Hard to tell, depends also on piston design, but on the "safe" side i would not go above 20psi..but PSI is not everything...just a measure of air pressure before the combustion chambers. The tuning is the one that will make the difference.

[engineer76]

There are so many factors, I could not just give you a number and being honest.

1. Static CR
2. Material of cylinder heads (heat transfer of Aluminum > cast iron)
3. Fuel Type (alcohol / gas)
4. Fuel quality (If gas 87 octane all the way up to 108 octane)
5. Ammount of excess gas to "cool" the combustion due to evaporation
7. Additional systems to cool the comustion (water/alcohol injection)
8. Temperature of the air when entering the combustion chamber (depends on thermal efficiency of the charging system; T2-T1 @ P2-P1)
9. Temperature of the air when entering the combustion chamber (depends on what kind of an intercooling system you are using)
10. Timing and t. curve

To give you an idea:
I have a '79 IHC Scout with a 392 cui. CR is 8.0 : 1 the motor runs N/A.
Having the wrong jets in the carb, and timing @ 6° BTDC was enough to make it start pinging under WOT and that was with 93 octane fuel
On the other had side my Saab has a CR of 9.2 : 1 and runs a solid 21 psi of boost with 93 Octane fuel without pining. (without water/alcohol injection)

[jbspeed86]

Can anyone give me an idea if i give you this info??:

Aluminum Twisted wedge heads
93 octane gas
42# injectors
255lph fuel pump
Intercooler
with 9.6:1 compression (unless another would be better, but keep in mind the blower is not definite so it needs to also be able to run good without a blower)
and of course a complete dyno tune
thanx

[engineer76]

Originally Posted by jbspeed86

Can anyone give me an idea if i give you this info??:

Aluminum Twisted wedge heads
93 octane gas
42# injectors
255lph fuel pump
Intercooler
with 9.6:1 compression (unless another would be better, but keep in mind the blower is not definite so it needs to also be able to run good without a blower)
and of course a complete dyno tune
thanx

I assume you use a stock cobra intercooler
Your fuel pump has to deliver not only enough liters per hour but also enough pressure (if stock pump delivers x psi you need at least x psi + y psi; y would be max boost)

If you run a roots style eaton m90 you will max out somewhere around 8psi of boost.
if you run a roots style eaton m112 (cobra) you will max out at around 10-12 psi
If you run a twin screw (whipple, kenne bell) you will max out at around 14 psi

If you run a large water cooled turbo (water & oil) you should be possible to run even a little higher 14-16 psi
- But that would require a diffrent style intercooloer
- Additional engine oil cooler

As soon as you go above 8-10 psi with a stock motor, you might be blowing it up any time!
The oly way how you can safely run 14 psi of boost is with an "all forged" rotating assembly (crank shaft, conecting rods, pistons).
Then if you already have to get pistons you would probabely get them in a lower CR such as 8.5 : 1 to maitain streetability.

[jbspeed86]

Well, now lets go the other way. What if I don't get the blower and I end up going N/A for whatever reason. Will a 9.6:1 be good? and if i went with the 8.5:1, would that be good as a N/A motor? that would be too low i would think

[engineer76]

Originally Posted by jbspeed86

Well, now lets go the other way. What if I don't get the blower and I end up going N/A for whatever reason. Will a 9.6:1 be good? and if i went with the 8.5:1, would that be good as a N/A motor? that would be too low i would think

An N/A 9.6 CR allows you to run 87 octane fuel depending on the tune you get done.
If you dont care going with 93 Octane fuel you can increase the CR to 10.5 : 1 but that's not going to work with forced induction any more.
8.5 : 1 will be about the worst N/A appliction you could imagine! This setup would be very complicated to make decent N/A HP!!!
How ever, a motor with a 8.5 CR allows you to run high boost levels in a forced induction setup. but only as long as the rotaring assemby can handle the HP and RPM's.

Roule of thumb says:
For every Point increase in CR (for example 8.5 to 9.5) you get an increase of 5% in HP.
This effect can be a lot more effective (more than 5%) when you have a hot set of cams.

[jbspeed86]

So, basically in your opinion, what would be a happy medium I could use no matter which way I go (blown or N/A)??

[engineer76]

Exactly!

9.6:1 is not really bad for a blown, and it's not really bad for an N/A aplication.
If you know for sure that you ARE goig with a blown setup I would recommend you to go with a lower CR.
If you go N/A for SURE, go a little higher.

[jbspeed86]

Cool, so my guess was right. I figured the 9.6:1 would work, thanx