Compression, dynamic/static, question...

[Slowhawk]

You also have to figure in boost vs airflow. Since boost is measured by restriction, different boost levels can have more/less thermal efficiency.

[Bo White]

The way I look at is if you have a target cylinder pressure it takes less boost with high compression to meet compared to low compression taking more boost to recreate the same amount of target cylinder pressure. With more compression and less boost the engine will come up on the cam sooner than the lower compression higher boost engine with the same cylinder pressure. Also, cam selection will be different to optimize each engine. It will take a milder cam in the high compression/low boost engine to create the same cylinder pressure as a low compression/ higher boost engine with a healthier cam. Then what comes into question is streetability(if that's a concern).
Also, unless your comparing the same setup to create the boost then that's going to be much harder to compare between the two engines. Optimal cam profile for a turbo is different to a top mount is different to a front mount and so on. Then on each will need different profiles to optimize high compression/lower boost compared to low compression/higher boost. All of course shooting for the same cylinder pressure target.
One thing though is the higher compression you go the less forgiving it will be on timing-A/F-octane compared to less compression just like in a N/A build but turned up a couple of notches.

[PQ]

This is so freaking cool. It's like pandoras box for us dummies. hahaha

I did not do the math but I think I'm following the physics. A lot of other things I've heard in the past are making more sense now too. Plus the methanol stuff is making sense.

I do not have my head wrapped all the way around it just yet though. It seems like figuring the perfect set up is complex. Change x to maximize y might change Z too much.

I might have some questions.

What I think I'm hearing is that on a regular 376 (meaning stock quality pistons and rods), if I run 7 psi boost on an 11:1 engine to make 550 rwhp then 7 pounds of boost on a 9:1 engine would be the same or most likely less power. BUT I can add more boost to it and make MORE power because I can actually add the boost more safely. ..... ie. 12 psi of boost on an 11:1 engine not a good idea but 12 psi boost on a 9:1 engine ok.

Ya?

*Obviously nobody is going to change pistons and rods and not put forged ones in but just for the example.

[JANNETTYRACING]

Originally Posted by PQ

This is so freaking cool. It's like pandoras box for us dummies. hahaha

I did not do the math but I think I'm following the physics. A lot of other things I've heard in the past are making more sense now too. Plus the methanol stuff is making sense.

I do not have my head wrapped all the way around it just yet though. It seems like figuring the perfect set up is complex. Change x to maximize y might change Z too much.

I might have some questions.

What I think I'm hearing is that on a regular 376 (meaning stock quality pistons and rods), if I run 7 psi boost on an 11:1 engine to make 550 rwhp then 7 pounds of boost on a 9:1 engine would be the same or most likely less power. BUT I can add more boost to it and make MORE power because I can actually add the boost more safely. ..... ie. 12 psi of boost on an 11:1 engine not a good idea but 12 psi boost on a 9:1 engine ok.

Ya?

*Obviously nobody is going to change pistons and rods and not put forged ones in but just for the example.

Yes there is a lot to absorb but this is still only the tip of the Iceberg.

I have been doing this full time for over 35 years and I don't know everything and never will, I learn every day.

Here, I will throw another Wrench in the Gears;

Think about how we went 9s on just over 700 RWHP at 4400 lbs.

[Bo White]

I would say that a low compression 376 with 9lbs of boost(same power adder/same cam) would make slightly less power than a high compression 376 with 9lbs of boost because of the cylinder pressure difference. But you can up the boost safely on the lower compression 376 and will make slightly more to bring the cylinder pressure up to a competitive level.

[PQ]

I had argued that the lower compression would make more power because it had more volume at the same boost. But what I realize now would happen is the peak cylinder pressure would be lower with the same boost because of the extra volume so be same or most likely less power.

Good stuff.

running 9s is something I'll never do. I just do not have the money for the supporting mods needed. haha

[PQ]

So if I just hone my block and keep essentially the stock displacement with my 2300 I could drop to 10:1 CR and run it up to 12-14 psi boost safely enough? Or keep 11:1 and run it up?

Forged internals of course.

[Bo White]

Keep stock compression. If I had a forged piston/H beam setup I woukd throw the boost to it and let it eat.

[JANNETTYRACING]

Originally Posted by PQ

So if I just hone my block and keep essentially the stock displacement with my 2300 I could drop to 10:1 CR and run it up to 12-14 psi boost safely enough? Or keep 11:1 and run it up?

Forged internals of course.

I would suggest 9.6:1, forged rods and pistons, 12-14 psi on pump gas.

More important adequate fuel system and injectors to support it.

Ted.

[TinyToasta]

Yikes...

I've planned to move towards 15-16psi on a cr of 10.3 soon...

Well... we'll see where this ends up

[Slowhawk]

Really depends on the Cam used too. I haven't built a motor under 10.5-1 for a blower car in over 6 years.
Factors
Cam
blower
fuel used

[PQ]

Originally Posted by Bo White

Keep stock compression. If I had a forged piston/H beam setup I woukd throw the boost to it and let it eat.

I'd like to run it up to 15-16 psi on drag strip days. Toss a smaller pulley on for everything else. So I think 10:1 would be a good fit to be safe.

Originally Posted by JANNETTYRACING

I would suggest 9.6:1, forged rods and pistons, 12-14 psi on pump gas.

More important adequate fuel system and injectors to support it.

Ted.

Ya, fuel pump and injectors is first no doubt.

Originally Posted by Slowhawk

Really depends on the Cam used too. I haven't built a motor under 10.5-1 for a blower car in over 6 years.
Factors
Cam
blower
fuel used

Longer duration cam would keep the compression down a bit?

[Supercharged SS]

Originally Posted by PQ

I'd like to run it up to 15-16 psi on drag strip days. Toss a smaller pulley on for everything else. So I think 10:1 would be a good fit to be safe.

How do you think the trans is going to like that? Start saving. You won't make 16 psi without major changes to the blower. It's just not as simple as changing the pulley.

[BullF-16]

Excellent thread here! I know i laid awake at night when planning my current engine. I knew i would eventually be twin turbo (happening as i type this)

One of my concerns was dish volume on my pistons. I knew the general consensus here was keep CR in the 10s Vs going low at 9:1. I basically chose middle of the road with -23cc Wisecos. I calculate this puts me right at 9.5 CR.

No E85 or race gas for me, just 93 octane blended with Torco for an in tank octane of about 96. Plus the car will be tuned with the meth. Planning on 20psi so nothing crazy. The PT6766s should feed this motor nicely.

Again, great read. Now should I have Pat G spec me another cam......Nah, this should be fine for a while!