[arrowem]

As you guys may have noticed I have been asking a lot of questions lately. In responses, it brings up more questions (keep the responses coming-free learning is the best). Thanks for all your free flowing knowledge!

Now my question- I will already have KRE 85cc 290cfm heads and a hardblock to the lowest frost plug '68 400 block and a full roller valve train. I will be putting a D1SC Procharger on it and max out at around 10psi of boost.
With these heads, what would be the best way to keep SC down to 9.0 or less? Stroke, dish the pistons, have the heads opened up? My goal is mid 700's for HP and 91 octane.

The block is already .040 over before boring (someones previous build).

Again, thank you for all your help.

[Torment]

Leave the pistons down the hole a bit (.010-.020) and a .060 or .070 Cometic gasket will put you in the 8.5-8.8 or so range with a flat top SRP piston. You can do it with pistons but unless there's something off the shelf available you're into a custom set. This isn't necessarily that much more expensive, but it does add to the complexity and lead time to get replacements if you hurt one or more learning what boost the engine/fuel wants. I have no experience with Procharger boost and what you can run with what static, etc. Water/meth kits generally work well with pump gas setups so you might look into that for a bit more safety margin and/or hp.

[taff2]

You could buy a set of BRC custom pistons off PDude,you can get them made with virtually any CC dish you want,the top ring land set further down from the crown of the piston for improved detonation resistance and upgraded taper wall pins,all for around $600-$650. I had a set from him,decent pistons for the money, and you can still set them up down the bore a bit if you want to.

[69FIREBIRD76MM]

nice build man, i have ask plenty of questions here, but not verybody gives you a concrete answer, 9.1 cr---is fine i belive--on aluminum heads you can have a extra point...cause i am building a turbo set up for my car with 9.5 ---just do your own research on different forums...everybody isxpert here fron what i see....good luck on your motor...

[arrowem]

Help me understand how these engines work. Remember, these numbers are purely examples.
Say you want to make 700 hp. If you have 9.5 SCR and get 550hp na and use 6 lbs of boost to get to 700. Then you take an 8.5 SCR (which is for some reason more desirable for boosted engines), do you just use a little more boost to get to 700hp?
If that's the case then why do people want around 8.5 SCR? What's wrong with 9.5 and a little less boost?

[BruceWilkie]

Start here to better understand how a turbo works.
http://www.turbobygarrett.com/turbob...o_tech101.html

A Belt driven centrifugal supercharger basically works the same way but uses belt drive vs a turbine.

[Torment]

[QUOTE=arrowem; If that's the case then why do people want around 8.5 SCR? What's wrong with 9.5 and a little less boost?[/QUOTE]

Nothing is wrong with 9.5 and less boost. The more static you start with, the slower you'll spin your charger (less boost) before you run over your fuel, create too much heat and have engine damage. HP is airflow, not boost - boost is measuring resistance to airflow through the engine - slowing your charger reduces airflow. People are telling you 8.5:1 likely because experience has shown that is a 'good' place to start with the equipment you mention and the fuel (gas) you plan - if you start a bit higher it will run but you'll limit the expensive equipment you bought. Change the ci of the engine or charger, or the fuel, and you have a different scenario. This is why I mentioned in my original post that I have no experience with prochargers - I mostly mess with roots blowers that create more heat than a procharger or turbo. Heat creation then becomes a discussion of the efficiency of your charger of choice... Do some reading as Bruce mentions - the internet is a wonderful thing Good luck.

[bad69bird]

If it were me, I would set the compression at 8.5:1. It gives you room to grow and trust me you will want to add boost eventually. Plus, if you are off with your tune you will have a larger window for error before the motor eats a piston. With the slightly lower compression, you can give it a little bit more timing to make up for any loss of power because of the lower compression. My , FWIW

Oh yeah my compression is.......8.4:1

[BruceWilkie]

Fuel being used is the biggest factor in determining what compression to run. As Torment points out choice of compression ratio is really mostly about heat. The more you compress your air/fuel charge the hotter it gets.

Fuel octane is basically a measure of a fuels resistance to igniting. The higher the number the harder it is to ignite. NA and 93 octane 9.5 compression with iron heads is AVERAGE MAXIMUM PRACTICAL compression limit. With aluminum heads another 1/2 to 3/4 point higher is MAXIMUM AVERAGE PRACTICAL compression ratio.

Yes, I've seen 12:1 NA aluminum head combos run just fine on 93 octane but they are the EXCEPTION on the EXTREME!!! Its not really very street friendly, it's marginally difficult even in race application, and quite sensitive to variables that may be beyond the owners control. I'm talking best engine builders pushing their knowledge and skills to THEIR limits!

The guy running a full point higher compression than suggested is going to quickly find out how much he has limited himself if he intends to stay with 93 octane fuel(if it really is 93 at every fill up).
With turbo or centrifugal superchargers or roots type, a compression ratio of 8.2 for iron and 8.7 for aluminum heads has proven MOST PRACTICAL and variations up to a half point higher than suggested can usually be compensated for by a knowledgeable builder.
That compensation usually results in NO net hp gain over choosing a safer compression ratio. More likely to be a zero gain or even a slight loss OR a damaged motor should you get a slightly weaker load of fuel or an unusually higher than normal barometer.

I would IGNORE the advise of someone who asked a bonafide paid professional boost expert (plus others including myself) what compression to run and turned around and chose to ignore the advise and go with 9.5 compression anyway because he has "seen it done" on the internet!!!
I'm sure the day comes something goes wrong for that guy and he will NEVER tell us the failure was his fault or related to not following advise. He'll tell you its ok go for it. He's smarter than the guy with near 40 years experience to his first attempt!

[BruceWilkie]

Originally Posted by bad69bird

If it were me, I would set the compression at 8.5:1. It gives you room to grow and trust me you will want to add boost eventually. Plus, if you are off with your tune you will have a larger window for error before the motor eats a piston. With the slightly lower compression, you can give it a little bit more timing to make up for any loss of power because of the lower compression. My , FWIW

Oh yeah my compression is.......8.4:1

What better an example than you! Thanks, hopefully that guy at 9.5 sees this and changes his mind. (or builds a chevy motor to blow up) Compression is not a place to see what you can get away with.

[bad69bird]

Thanks Bruce! I might not be the most knowledgable on the tech side of things like you and T.V., but I have plenty of track time and tuning experience. LOL

[69GTO2]

But isn't the dynamic compression ratio what counts with either a N/A or boosted application?

[misterp266]

I ended up at 7.75:1 on my 433 build with a 6:71. Using Ross 30cc dished pistons. It's a street only build and I sure don't trust the gas you get these days so I went a bit conservative. Figure I can always turn up the boost!

[bankbook]

I am at 8.5. My opinion is that is the best to be. I do know guys that run up to 10 psi with aluminum heads, but they are burning race gas in their engine.
The reason to keep it lower is simple-compression ratio. The lower end is better because there is more room for mistakes and engine damage. Also, NO ONE will be happy with 6 pounds of boost after a while. Take it from me.

If you are in the planning stages then do it. If you are working around other parts of an already bought/built combo than that is different.

[bad69bird]

Originally Posted by bankbook

Also, NO ONE will be happy with 6 pounds of boost after a while. Take it from me.

I've been saying this for years!!!!!!!!!! LOL

[PONTIAC DUDE]

A lot has to do with a persons goals and useage.
A turbo likes a 'slighly' different set of rules compared to centri***ul superchargers for the street.
Spool up rate can determine engine compression you use and engine build.
Remember that the P & D1SC ATI units are made for a lot of factory add on deals that come with 9-1 to 10-1 compression at the factory.
Having runa 8-5-1 455 with a mild mild #068 cam. I was totally surprised and the power difference with on 6/7 lbs of bost and a full second ET gain with a P1SC ATI unit. And that was the 2nd time out. Sold it before I had a chance to work on it, but was the same as a 175 nitrous shot on her car. Only changes were carb reworking, regulator and boost timing.

Just a thought.
I think topics have to be looked at as to Street.......... Street/Strip.......... Race.

I would work on power management rather then a set boost reading for the street.

[turbo69bird]

for a turbo set up,

You will want 8.2 to 8.5 compression with Iron heads,

Add 1 point for FAST burn aluminum heads

Only .5 point for regular old wedge style aluminum heads.

Thats a basic rule of thumb. you can fudge those numbers all over the place based on tune, if you want to run it wicked rich and pull crazy timing out etc.

No need to et exotic in the short block just a good set of aftermarket H beams, nice radius on the crank, and Just about any forged piston you want to run will be fine I never run a 4 bolt block even though I own many of them. 2 bolt set ups have been very reliable for me.


The KEY is always in the TUNE. set it up fat and retarded (LOL) and sneek up on the tune.

Always err on the rich side never the lean side, load the hell out of the enegine with Highway type gears and enjoy!


Of course the proper size turbo is KEY as well cause the wrong size turbo can create un needed excesive heat and that causes detonation.

Oil backing up in the return line can cause the same.
Make sure your fuel system is up to the task at hand too. Cause lean is mean but not for very long.LOL

[dutch injun]

I agree on the lower CR. If you start on 8.5/ 8.7-1 you'll have some room for error. Also if you opt for dished pistons to get there, it might be good to stick to regular head gaskets. If you are out of tune, you'll only have some gaskets to replace.

[Tom Vaught]

Originally Posted by PONTIAC DUDE

Remember that the P & D1SC ATI units are made for a lot of factory add on deals that come with 9-1 to 10-1 compression at the factory.

Lots of Mustangs and Chebbys have had EFI and timing control for years on the 9 to 1 or 10 to 1 engines. A "Soft" P or D1SC Supercharger

Originally Posted by PONTIAC DUDE

I would work on power management rather then a set boost reading for the street.

X2

Tom Vaught