compression ratio & boost

[Schweppe23]

Is there a formula for finding a car's compression ratio with boost. I've heard that adding 3psi is like adding 1 to the cr. is this true? Thanks!

Pat Lavanty

[AaronGTR]

I don't have the exact numbers, but I know it's not that simple. Adding boost does raise the compression because it's compressing more fuel and air in the same space. NA can make more power by raising compression, but doesn't add more air and fuel like forced induction, so it doesn't make as much power. That's also why high boost engines use lower compression pistons. It makes more room for more air and fuel while keeping the final compression ratio at a manageable level. In other words... forced induction is always better.

[bszopi]

Most people, when they talk about CRs, talk about static compression ratio. Static compression ratio will be the exact same on a NA or a boosted car. Now if you want to know the effects of dynamic CR, then there will be a difference. I'll see if I can't find any info on it.

[phantom505]

Well I'm not an expert on this, but this is what I was told:

To get an effective compression ratio for every three pounds of boost add "1" to the static ratio.

This was just a rule of thumb. The idea was that if you knew the block should hold up to say a 14.5:1 compression ratio and your car started out at 9.5:1 then you should be able to handle ABOUT 15 psi of boost.

I don't think this is very precise, but it seems to work as a guideline. I was also told if I should get pistons in the future to get dished ones that will bring it down to about 9.0:1 ish. The strange part is they claim you can boost much higher by doing this than with the above rule of thumb. So I can't give you the equation, but that's not a bad place to start.

[phantom505]

Just another question....

How much HP (aka cylinder pressure) do you guys think you can put on the stock 3400? I'm sort of afraid to put my heads and cam on top of the turbo at 10 psi boost...

That depends on the mileage and the condition of your motor. I've seen stock bottom ends take insane anmount of boost. Not every case however. If you are going to add large amounts of boost, you may want to do a little bit of bottom end work. With that much boost. It is also important NOT to detonate. If you do, your car will become a very large, turbocharged, paperweight.

[phantom505]

Originally posted by Phantomus
That depends on the mileage and the condition of your motor. I've seen stock bottom ends take insane anmount of boost. Not every case however. If you are going to add large amounts of boost, you may want to do a little bit of bottom end work. With that much boost. It is also important NOT to detonate. If you do, your car will become a very large, turbocharged, paperweight.

I wasn't asking about boost. I know how much people have been boosting. Personally I wouldn't go over 10 psi on stock block.

If I put 450HP to the crank, something that I don't think would be very hard with my heads, cam, and 10 pounds of boost.... will it blow up?

I would say so.

Here is where I get that number. I gained 65 whp going to 5 pounds of boost. Assumeing that I get another 50 whp going to 10 lbs puts me right at 280 whp. Add 25% higher flow rate with the heads (I won't even give gains for the TB, UIM, LIM, etc as the heads will be the largest restriction). That puts me at absurd power at the crank.

The tranny I will be getting (hopefully) will handle that kind of power.

The question is.... will the bottom end?

I really don't think so, I'm willing to bet 350 crank HP is about the max, but I was hoping that someone on these boards knew a little bit more about these blocks.

[bszopi]

I've seen a stock bottom end 3.4 (in a Camaro... same block and bottom end basically though) put out close to 400hp at the wheels, so I'm sure you will be fine.

Just because the heads flow 25% more doesn't mean it'll be a huge gain in power. I've seen heads that flow gobs more than stock on a flow bench, but sucked ass once they were put on the engine. So until you get the stuff installed, I wouldn't start making assumptions on power gains.

[AaronGTR]

I would be willing to bet the stock bottom end can handle at least 400hp without detonation. The ecotec that the GM racing program made took about 375hp on nitrous before the connecting rods gave, and the 3400 is more stout. The stock crank should be able to handle loads of power with only a little work. It's the rods and pistons you'd need to replace.

[phantom505]

That's good to hear that the bottom end will put up with the power. I went through some of the GP threads and they seem like they'll blow up if they get any knock at around 450. I didn't know if the 60*V6 would be the same, better, or worse.

I know that I can't guess how much power I have based on flow numbers, but I do want to be cautious. I have to be, right now this is my daily driver.

I'm finding out that the tranny really doesn't like the amount of power I'm putting through it right now. It's start to act strange. I guess I'll be moving the tranny project up.

[nocutt]

Mechanical limints are very hard to substantiate...one person might do 500 hp here and another 250 hp there...the engine with the lower power level might either lift a head or puke a rod thru the pan...the rule I seem to always go by is aviod detonation at all cost!! It is all in the nature of your tune, octane requirements and of course drivetrain...the engine will take some beating given some respect and common sense...

[Knappy1]

Be very very cautious. From a very recent experience I can tell you that the pistons in the 3400 are definately the weak point. Even without detonation, they won't take over 10psi. They are very weak around the edge between the dish and the top ring. And, if you happen to have a similar experience to mine, you can kiss your heads good-bye. I've got to say that I'm suprised that the piston went before the head gasket, but that's the way she rolled. I'll see about posting up some pics later.

[iceman]

I don't think Pat has to worry about his pistons..

[nocutt]

On GM's Natural Aspirated motors...top ring lands are usually culprits...Ultimately this is done for emission purposes, but trust me...detonation is the culprit. Sometimes your generic scanners pick them up too late depending on their refresh rates...10psi without Knock is nothing on the piston, remember you haven't even added combustion pressure!?!? So in reality, rods are "suppose" to be the weak point...head gasket technology on GMs new V6's and 8 cyl are really up to par, compared to 80's vintage engines. Aviod Knock, provide enough Octane and remove just enough timing where necessary...you will be limited by mechanical and drivetrain limits...IMO

[AaronGTR]

I've run 11psi on my engine and it's still running. It's all about keeping the fuel and ignition timing right to avoid detonation. Cold air helps too.

[Knappy1]

Knock on wood man...I think the 2.2" pulley was just a bit much on mine, even with the tuned PCM. No worries though. She's all back together and now a I have a project engine sitting in the garage just screaming for new goodies. I've just got to be nice to this one for a few weeks until it gets me to Mississippi.

[jaketuff]

Originally posted by Knappy1
Knock on wood man...I think the 2.2" pulley was just a bit much on mine, even with the tuned PCM. No worries though. She's all back together and now a I have a project engine sitting in the garage just screaming for new goodies. I've just got to be nice to this one for a few weeks until it gets me to Mississippi.

Moving to Mississip should increase your times... I think you would be closer to sea level there?

Anyhow... I am interested in this little project you speak of? Fire up a PM. You must be out for my title as fastest Pure PSI E/M Car, no?

JOUT

[Schweppe23]

I was using a compression ratio calculator from fbody.com/calc.htm and came up with these numbers:

CR/PSI....Effective CR
9.5/10=15.9
9.5/6 =13.37

I know there are a couple of boosted guys running at these kind of #s...so this is my question....what kind of fuel do you run, cause i thought that premium pump fuel can only support CR of 12.1 or am i confused. Thanks

Pat Lavanty

[AaronGTR]

I just use 93 octane most of the time. Maybe that calculator is setup just for LS1 engines? Is there any place on it to enter engine displacement? Not even sure if it matters. Seems to me though that psi of boost wouldn't matter that much though since it's just a function of air flow resistance and not always the actually amount of extra air squeezed into the cylinder before the compression stroke. That's just speculation of course. Wish I knew the answer. Congratulations Pat, you've got everyone stumped good.

I run 14 psig on my 3100. It was a brand new long block to which I added low compression Ross forged pistons along with a ring set that have steel top rings (Total Seal Extreme's) rather than the ductile iron junk.

I tested this motor as it was with its stock cr, and could not even obtain over 7 psig without encountering knock. I would have to assume that statements which contradict this on a "stock" GEN III 60V6 would either be false, or they simply measured the boost levels incorrectly.
10 psig is not even obtainable with stock injectors, and 14 psig certainly won’t be happening with your stock PCM programming no matter how well you build your motor. I developed my own engine management system based on the Turbo Grand Prix's code, which was co-developed by McLaren.

I also have my doubts as to whether the auto trans would hold up to this much torque. I run a Getrag 5 spd, and expensive aftermarket clutches have become little more than a commodity. I have gone through 3 to date, two kevlars and a centerforce. The stock trans mounts were also a joke. Would rip apart on even a conservative (for me) launch, so I had to employ some aftermarket poly mounts.

Mid 14 second quarter miles are simply not competitive today, as nearly any high end performance vehicle made now can surpass that in stock trim, so it would not have even been worth the money and time I invested in this project for boost levels less than 8 psi.