Compression vs boost question

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skycuda67

skycuda67

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ok im building a 360 right now and while im choosing the internals im taking into account the idea of twin turbocharging it in the near future. Ive been told to go with low compression pistons (8:1) if i want to go higher boost 8-12+ psi. So im wondering if im getting higher compression pistons, like 9:1-9.5:1, will it not perform as well with boost around 7psi and higher compression as compared to lower compression with higher boost? And using higher compression pistons with lower boost, will it give me more power from the engine off the line before the boost kicks in?? inputs much appreciated!
 
It's a catch 22. Your better off listening to who ever said get a 8.0-1 piston since it will help you out bigger later. It will allow you to adjust your boost as needed and nearly as much as possible without getting into other issues.

Starting off with a higer ratio piston limits you in boost as well as possible valve lift abilty's. Not to many pistons come fordged at med. ratio's like 9.0 - 9.5-1. There mostly Hyperutecic's and will break. Don't get me wrong, there strong, but will not survive a decent amount of boost and a Lord forbid ping/knock.

A properly done turbo system, these days will not have any, if much at all, turbo lag and will spool up the boost and power from a dead stop like mad.

Get intouch with Prine. Click here; http://www.forabodiesonly.com/mopar/member.php?u=6131
He'll guide you since this is exactly what he loves to do.
 
You are going about it *** backwards.... If you are going to do a twin turbo, you build the engine around that....

You don't build a normally aspirated engine around the idea of putting a twin turbo on it....

If you need an engine now, just build a runner or get a good used engine for cheap....

Then.....


Build the right engine to fit your plan..... Any other way is pointless, a waste of time and a waste of money.
 
thanks a lot thats what i was hoping. i found a really nice built 318 for cheap so i think im gonna do what they said and take time building "the real" engine for it the right way~
 
ok so when you said i should get a temp engine for it i went on craigslist an started searching! i found a 318 rebuilt by keith hershberger in woodburn (hershberger motors) with mopar purple cam 284/484 and .030 over forged pistons. less than 1000 miles, but he sold the swirl port heads off it :/ oh well im stickin my 340 heads on it and he's taking 300 for it! now i'll take my time building the tt 360 set up thanks for the advice~
 
Well....

In my opinion... Just get a low buck runner. The Idea is not to spend much on something you plan to replace...

The less you spend on this engine the more you will have for your build...

Really the best advice from my years of playing with cars is to get your self a driver. The down time of your project wouldn't matter then...

A Miata is fun and gets 30+mpg... you can get an early 90s for as low as $2000... Or go get a 90s Honda or even a 90s Toyota 2 wheel drive truck... The Toyota 4 bangers run forever and are good for running parts around.
 
well i think 300's about the cheapest possible thing i can do, i can hardly get the gaskets for the other engine for that! I have another car to commute with but im buying this engine because i dont have the funds yet to build the other one completly how it should be done and i would like to have the car driving again soon, ive really missed it. so while im happy havin my cuda back with a decent engine i can save my money an spend time building the 360 without the car gathering dust and taking up shop space. and even if i put 20k miles on the 318 and pull for the it for the 360 and resell it im not gonna lose any money on it, no worries~
 
AnotherA said:
You can hit ###hp with 8.5:1 at 15psi or you can do it with 10:1 at 8psi.

This is a widely debated topic on many forums.

Increasing static CR vs boost pressure
http://www.modularfords.com/forums/showthread.php?t=51059
Click to expand...

That's interesting because when reading "Chrysler Engines: 1922-1998" by Bill Weertman he has a chapter about the old V-16 WWII aircraft engine. The engine ended up having only around 6:1 compression but ran over 33 lbs. of boost from the in-line turbocharger + supercharger. I think you'll overall make more HP doing it that way but you definitely won't make much power below the speed where the boost kicks in.
 
Keith Hershberger built the tranny in our charger and ported a set of 906s for the same car great work. he moved out of here a while ago you should find out when it was built.
good luck,
Marvin Jr
 
MOPEkidD-3 said:
That's interesting because when reading "Chrysler Engines: 1922-1998" by Bill Weertman he has a chapter about the old V-16 WWII aircraft engine. The engine ended up having only around 6:1 compression but ran over 33 lbs. of boost from the in-line turbocharger + supercharger. I think you'll overall make more HP doing it that way but you definitely won't make much power below the speed where the boost kicks in.
Click to expand...

They had to, they were made to fly at high elevations. I don't know about you, but i'm pretty close to sea level!

The aircraft engines were always revving so there was no issue of turbo lag. This is a concern on the street. A motor with decent bottom will spool the turbos quick.
 
Is this going in a street driven vehicle? Pump Gas?
I'm running 10psi on 8.3:1 pistons and runs great on pump gas...I have water/methanol injection on it to lower the intake air temp once the boost gets past about 5psi.

There's a million and one considerations to think about..but the bottom line is detonation is not your friend. Build the engine for your application or you're going to be doing it again. I'm not sure what power level you're going for but any mildly well thought out 360 combination with 10+ psi boost is going to give you more than enough power for the street.

A couple of my recommendations:
Keep the Compression Ratio low...around 8:1
Use a quench dome piston design
Use commetic head gaskets
Use an intercooler or water/meth injection
Pick a Cam that's designed for boost, LSA around 114

If you have specific questions...ask away.
 
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