Compression question

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FomocoReformed

FomocoReformed

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Now being a relative novice to the whole forced induction scene I'm working on doing my research so I can understand everything before I finish selecting all the parts for my motor. Of course it doesn't help that this is my first from the bottom up engine build and I chose to construct a blow through carb'd turbo 408 stroker for this first project. I'm really trying to not grenade the thing and really hoping I can run it on pump gas (which my static CR is fine for, but the boosted CR... well I'll get to that)

One thing I'm trying to understand is the compression ratios I'm looking at, I've calculated it with a few different combos and it looks like I'll be running a static CR of about 8.2-8.6 depending on what I choose for a head gasket and accounting for possible variation in combustion chamber volume with my 596 heads. Here's where I run into question's I don't have answers to. Crunching the numbers on the dynamic compression ratio comes out in the mid 7s before boost and with 20lbs of boost (roughly what my turbo would be at at 3500) it appears I'm looking at 17:1 boosted CR, even plugging in 8lbs of boost it's pushing almost 13:1 boosted. Now I'm wondering in my head, is that going to prevent me from running pump gas and turn my block into a frag grenade? I know higher than 11:1 compression will make feeding it unleaded iffy but I have no idea which of these compression ratios I should be looking at for that.

Unless I'm just doing something horribly wrong in my calculations/choices. It's worth noting that I picked a cam with an intake valve closure ABDC of like 52* since I'm also not sure on what cam I should use, I know this affects the dynamic CR greatly and using a cam at 70* drops my CR @ 20lbs max to 14.

What the hell am I getting myself into... but I'm pot committed :burnout:
 
On thing you need to do is get the Naturally Aspirated thinking out of your head. It took me awhile to kinda comprehend what needs to happen when force feeding an engine.

My build is same as yours "Sorta" with different heads, Cam and EFI instead of a Carb. Mine will be running on Pump Fuel 91 Octane.

Here is my Compression on a Calculator.... Remember to ADD 15* to your ABDC #, mine was 48* + 15*= 63* ABDC

Link..... http://www.rbracing-rsr.com/comprAdvHD.htm

View attachment Screen shot 2012-03-20 at 7.53.37 AM.jpg

One thing you need to remember is like NOS you will need to back the Timing roughly about 1*-1.5* of timing out for each Pound of Boost to avoid detonation . So lets say your engine likes 32*, as you build boost you will be progressivley pulling timing out to your full boost at 20psi, SO, your timing will be retarded to 12* at a minimum lets say. Doing this allows you to feed boost to the engine and helps keep you away from detonation and run the cheaper fuel. Remember you are not trying to get Max Effort with Timing, you are getting the Max Effort using the BOOST. Unlike an NA Engine.

In addition to the above you will need to keep the Intake Air Temperatures as cool as possible. On my build I am running 2 Air/Water Intercoolers a Heat Exchanger with a pump to circulate the water and I will also be using Meth Injection to cool the cylinders. Cool Intake Air helps keep detonation away.......

I will be putting 15 PSI Max to mine. This is something you may want to rethink with your heads. 20 PSI without major modifications to O-ring the block and or Heads will probably lift the heads. Remember our SBM only have 4 bolts that go around the cylinders.

I used Cometic MLS Head Gaskets and ARP Studs on mine.

Here is some good reading about Meth Injection and its benefits. You can call Rodney and tell him what you are doing. He will design a nozzle system to take care of the job.

http://www.alcoholinjectionsystems.com/

This is only a Rough Explanation and I am sure more guys may have something to add to what I have said here.
 
For Higher boost applications you COULD have 2 fuel systems. Say Pump Fuel to run the engine to say 5lbs of Boost and a secondary pump that will add 110 Octane that kicks in above 5lbs to your max boost.

Just another thought that will add $$$ to your build.

Just so you know the Walbro GSL-392 Fuel Pump will act like a dead head if you were to go this route and they are good for 600HP each, very quiet and inexpensive at about $100.00 Each. I am running 3 of them that kick on all at the same time and you can barely hear them at all.
 
What are you running for a cam/heads? I sent in the specs on my motor to comp cams to get their recommendation, just waiting on it.
 
My plan was an ideal compression of 8.5:1. Didn't end up with that after completing my engine. I ended up with 9:1 after custom pistons, thermal barrier, 906 head cc and .065 mls gaskets. I ordered pistons that were suppose to be .040 in the hole but ended up being 0. I have no idea how that happened! Gave all my specs to Diamond. Is what it is!

I am using a custom grind Scott Brown cam. Just be sure to get a 112-116 lobe seperation and around 220-240 duration. It really all depends on what your ride will be used for. Mine is mostly a street setup with some strip use. Should be a real beast!

Good luck!
 
FomocoReformed said:
What are you running for a cam/heads? I sent in the specs on my motor to comp cams to get their recommendation, just waiting on it.
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Heads are Indy RHS X Heads that flow around 280ish CFM. Cam is a Solid Roller .634 Lift [email protected]

You should actually call Bullet Cams. They know their cams for dang sure. I called Tim over there and he gave me the specs on a camshaft specifically for my combination. I also had Brian at IMM Engines spec it. I did not tell either of them about the other. The cam was so close between the both of them it was actually weird.
 
Doing some more numbers based on the info here, taking timing and everything into account it looks I could get a dynamic CR under 15lbs of boost of about 10.75:1 with the right cam. I could live with that.

Are you running meth injection on yours mad dart? Theres still a lot of conflicting information about what compression ratio is the plateau for running pump gas. I've read some say 10:1 max and others say 12:1 is fine with modern gas and components. Then I've read if your dynamic CR is higher than 8.5 you'll be detonating. Don't know what to listen too and what's just blowing smoke...
 
Yes I have Meth Injection & Air to Water Intercoolers with a heat exchanger and circulating pump.

Guys run 15psi all the time with 9:1 Compression. The key is to back enough timing out of it and keep the Intake Air Temps as cool as possible. As close to 32* as you can get without freezing the throttle plates wide open.:D

You need to call AIS and chat with Rodney, he will set you straight on Pump Gas and Compression.

theturboforums.com
 
That's got to be one high dollar engine, IIRC you're running a factory block aren't you?

I'm not too keen on meth injection, E85, 50/50 race fuel/pump, or any of the other options I've read about. I'd prefer pump gas since it's going to have a lot of street/cruise time in between autoX-ing it, I realize this build will be a handful on the street but I like a challenge. :twisted: Though I have read about mixing toulene in with pump gas which I am interested in.
 
FomocoReformed said:
That's got to be one high dollar engine, IIRC you're running a factory block aren't you?

I'm not too keen on meth injection, E85, 50/50 race fuel/pump, or any of the other options I've read about. I'd prefer pump gas since it's going to have a lot of street/cruise time in between autoX-ing it, I realize this build will be a handful on the street but I like a challenge. :twisted: Though I have read about mixing toulene in with pump gas which I am interested in.
Click to expand...

Yes Factory Block. I am not relying on the meth injection that much if any. My Intake Tract cooling system is pretty efficient on its own. I am using the Meth to cool down the cylinders mostly. It is a 50/50 mix, 50% Meth & 50% Distilled water.

There is a guy on theturboforums.com that just boosted a 360 with the same heads you are going to use. He did not make much power on e85 at 13 PSI. Guys on that thread are thinking the Quench and using stock style open chamber heads are what is killing the power. He is 9.2:1 compression no intercooler just e85 and he could only run about 16* of timing MAX or it would detonate. You would think with the Higher Octane e85 he could get more aggressive with the timing but he couldn't. He is running 596 Castings....

Check it out here....... http://www.theturboforums.com/smf/mopar-turbo-tech-forum/got-my-sleeper-on-the-dyno!/
 
my 416 is 9.3 to 1... i run pump gas, air to water i/c, and drive it on the street every where.
 
forced 340 said:
my 416 is 9.3 to 1... i run pump gas, air to water i/c, and drive it on the street every where.
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I have seen your build and it is one of the cleanest installs I have seen. Can you answer a couple questions?

What is your max advanced timing?
How much boost?
How much timing are you pulling out?
How much Power to the tires?
 
I'm considering investing in the RHS heads, seen a lot of people recommend them, they flow good and are cheaper than going the aluminum edelbrock head route. I'm looking to run a solid flat tappet cam for my motor.
 
Mad Dart said:
I have seen your build and it is one of the cleanest installs I have seen. Can you answer a couple questions?

What is your max advanced timing?
How much boost?
How much timing are you pulling out?
How much Power to the tires?
Click to expand...

hey thanks a lot man!! my max timing is 36 and 18-20 at 17-18 #'s. she makes 935rwhp 890 rwt at 6700rpm on pump gas with no ice in the tank. just water. :burnout:
 
FomocoReformed said:
I'm considering investing in the RHS heads, seen a lot of people recommend them, they flow good and are cheaper than going the aluminum edelbrock head route. I'm looking to run a solid flat tappet cam for my motor.
Click to expand...

nothing wrong with a solid flat tappet.. that's what im running in mine.
 
forced 340 said:
hey thanks a lot man!! my max timing is 36 and 18-20 at 17-18 #'s. she makes 935rwhp 890 rwt at 6700rpm on pump gas with no ice in the tank. just water. :burnout:
Click to expand...

That is some serious ponies!! Maybe this will help the OP with his original question since yours is running and you drive it with real world experience.

I for got 1 question........ Are you running any Meth or Water Injection? If so at what point does it kick it?
 
forced 340 said:
nothing wrong with a solid flat tappet.. that's what im running in mine.
Click to expand...

Yup, I like solid flat tappets also. As long as its a solid whether flat tappet or roller I'll run it. I don't run hydraulic cams anymore, I like to rev my engines without worrying about plunger failure or floating the valves.
 
Mad Dart said:
That is some serious ponies!! Maybe this will help the OP with his original question since yours is running and you drive it with real world experience.

I for got 1 question........ Are you running any Meth or Water Injection? If so at what point does it kick it?
Click to expand...

no meth or water inj... jut a very efficient i/c setup.
 
That's actually great to know considering I really don't feel like springing for a meth injection setup, what's your CR with boost?

I like the solid flat tappet cams over hydraulic, they can scream a little louder :glasses7:
 
FomocoReformed said:
That's actually great to know considering I really don't feel like springing for a meth injection setup, what's your CR with boost?

I like the solid flat tappet cams over hydraulic, they can scream a little louder :glasses7:
Click to expand...

i haven't figured out my comp ratio with boost... haven't really thought about that to tell you the truth.
 
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