boost VS compression on turbo engine

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sspikey

sspikey

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this has always been something that intrigued me
most people lower the boost and boost the hell out of a turbo. their turbo choice is usually something not too big because of lag. why not raise compression (im talking about stock turbo engine and raising it say a point) raise the compression and go with a slightly bigger turbo but run it at lower psi. off boost power will be great and turbo should spool equally fast not having to boost 21 psi to make any power. less fuel would be needed if my theory is right also. so whats everyone's thoughts on this? i hate pressing on the gas and waiting. even on my pathfinder theres a small lag why i dont know its an auto and n/a but it drives me nuts.
 
The main reason to reduce the cylinder compression is to reduce the risks of a detonation (fuel-air mix burn before normal ignition). This could cause a lot of damage to the engine. The power output of an engine is not really dependent on the size of the turbo, but of the pressure. Let say you have a T3/t4 turbo at 15 psi and a T04 at 15 psi also, they are going to put the same power output on the engine. The thing is that the t04 will spool a bit slower than the t3/t4 would. But both would use the same amout of air an the same amout of fuel, its just the size of the housing that will be smaller. (sorry for the french following, but i can't think of it in english)

debit = volume d'air x temps
volume d'air est proportionel au volume du tube. So, to get the same pressure, you need the same amout of "debit". So the T04 will need to spin slower than the t3/t4 to get the same pressure.

Hope this answer some questions.
If anything else, just ask.
 
cest ce que jessayer de dire dans mon dernier post. mais ce que mon idee ete est ou lieux de rouler a haut psi avec un petit turbo a low compression. baisee le psi plus gros turbo et monte compression. 9.1:1 a l'air cool. cest ca ce que je pensais
first vs second choice.
 
Avec un plus gros turbo, tu vas avoir un plus gros lag. Ce qui peux être très irritant. Monter la compression est plus dangeureuse, pour les raisons expliquée. C'est ton choix, mais entre les 2 setup, j'irais avec le petit turbo, plus petite compression. Comme 8.0:1 à peu près !
 
sspikey if the cfm of the turbo is greater than the head = lag , thats where I come in .
A well ported head with lots of velocity will be able to spool a bigger turbo faster with little or no lag ,just ask Adam he's proof of this .
The reason you want 7.5-8.5 max comp is cause when boosting you are actually raising the comp cause of the added atmospheres entering your engine,a cyl can only take so much before detonation.
All top turbo engine builders never exceed 8.5 to 1 comp on a F/I application.
8.0 will make more power at 15psi (or what ever psi) than 9.0.

So for all the ones that thought you can make lots without a ported head cause of the turbo,sorry buy a WELL ported head will always make more power and all engines will benefit from ported heads.Whether its a Taxi or a daily driver.
Also ported heads do not and I repeat do not take any drivability or reliability away from the engine.
 
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the idea i had in my mind was not having to boost as high with a larger turbo. to help with the lag raise compression. my friend in vermont has done this to a civic and his sr20det. worked very well. thats why i was intrigued by this. its also the fact that reading some parts of corky bell's book maximum boost my idea of a compression ratio of 9.1:1 would go against his beliefs.

on a sidenote THIS SECTION ROCKS!
 
also, once you've decided to go FI route, don't you give up on th N/A high compression alternative?
 
Its impossible to have the best of both worlds.
Need to learn the word comprimise.
You always need to give a little to gain a little.

Its impossible to have a 10 sec 1/4 and pull 1.3g on a skid pad and get 40 mpg and be a daily driver.
Either you go 1/4 mile or handling .Not both (you can but will not have the best of both)
 
it;s simple,, the higher the compression ratio the less boost you can use , and vis-versa

and thats' what all turbos's want:: BOOST!!!! as long as you rebuilt yuor block and eventually your tranny, the boost shouldn't kill the engine,,

Now my question is : if you have a pretty high compretion ration,, how do u lower it for turbo use, i have seen it done but never did understant how, ( pistons, head gaskets etc. must be changed but if i had to take a guess i would say that this is how they do it with a whole losts of other stuff) it's the other stuff that i am interested in. What else is done to lower it ?
 
thats' what all turbos's want:: BOOST!!!!

not really. you can have a small turbo and run it at 20 psi itll make the same power as a bigger turbo at 7 psi.......not all turbos flow the same
 
There are several ways to lower comp but the proper way is changing pistons;)

The other ways are : Thicker head gasket , open up the combustion chamber.
 
Can someone confirm or deny my theory?

I think that let's say 15 psi of boost will be the same wether it comes from a small or a huge turbo. The boost is actually a positive pressure in the intake, right? If we suppose the size of the intake, throttle body, etc. remains the same, 15psi produced by a T28 would give as much power that 15 psi made by a T66, right? The only difference is that the T28 would be near maxed out and the T66 would have plenty left. That is the way I see it having the same diameter intake tube and throttle body.

Any thoughts on this?
 
Pressure is pressure but volume is what makes the power.

Think a McDonalds shake if you could suck 15psi through a straw would you have more in your mouth with a skinny straw or with the ones supplied at MC;)

CFM (cubic feet per minute) of a turbo is what counts.

On the same motor a too big turbo will never spool to 15psi (maybe way over the redline).

Porting is what will help the most (its the best bang for the $)
It actually gives power not those up to _ _ hp stuff;)

For example cold air kit 2-5hp @ $400
Exhaust 1-3hp @ $600-1500
Pullies 1-5hp @ $100-300
add them up and at the most you have 13hp for $1100-2200.
Porting is between 1200-2000 and gives at least 25+ hp thats with stock cams imagine with cams.

Awaiting a pesamistic reply.
 
Originally posted by tkm
Pressure is pressure but volume is what makes the power.

Think a McDonalds shake if you could suck 15psi through a straw would you have more in your mouth with a skinny straw or with the ones supplied at MC;)

CFM (cubic feet per minute) of a turbo is what counts.
Click to expand...

I understand that. So, am I right to think that for, let's say a 60mm throttle body, 15 psi of boost will let X CFM of air enter the engine, no matter the size of the turbo? If not can you explain me how to calculate it?

Thanks
 
yes, 15 psi is 15 psi in the engine, assuming theoretically that all turbo setups use the same diameter piping (which isn't the case in the real world) but the next question is the size of the turbo.

bigger the turbo more psi is necessary to spin the wheel of the turbine, therefore it will spool up a lot slower than a small turbo which needs less psi to spool up..
You can't jsut assume that because you have 15 psi running on on either small/big turbo that your quarter mile time will be same.. it doesn't work that way...

The Mcdonald example was pretty good...
 
Originally posted by sspikey
thats' what all turbos's want:: BOOST!!!!

not really. you can have a small turbo and run it at 20 psi itll make the same power as a bigger turbo at 7 psi.......not all turbos flow the same
Click to expand...

i know this, but if your buying a dual ball bearing turbo, please tell me that your not going to run 7 psi on it......it will never spool,

if your willing to pay 10 k for a turbo alone, it assume your gonna rebuild the whole block of the engine an run at 25 or more psi.

This is why i said that all turbo's want boost, wether its a small one or big: they all love boost :D
 
15psi of 660cfm or
15psi of 330cfm.
Which one is going to have more air (think of the McDonalds example)

As for spooling of turbos everyone forgets that its the complete setup(car) that goes down the 1/4 mile or what ever situation.
not just the turbo so with that in mind you need to build the motor to match the rest of the turbo used.

If you plan on running a serious turbo then you need to build the motor to handle this power.
Same for any turbo setup unless longivity is not 1 of your concerns!
When building any motor you need to build in a safty thresh hold so incase something goes wrong you don't melt or break something.

Build it right the first time not the 2nd,3rd,4th,5th...etc
 
Originally posted by tkm
15psi of 660cfm or
15psi of 330cfm.
Which one is going to have more air (think of the McDonalds example)
Click to expand...

Hmmmm... I'm really having a hard-time explaining my thoughts.If I'm correct the "boost" is in fact the positive pressure in the intake manifold, right? I just read that at sea-level atmospheric pressure is at 14.7 psi. So if your intake as a volume of 1 cubic foot (for simplicity), in order to have 15 psi of pressure you need to fit about twice as much air in it. So, in order to have 15 psi of positive pressure in your intake you need to fill it with about 2 cubic feet of air. If the reasonning is good, the size of the turbo does not have much to do except being able to provide 2 cubic feet of air in the intake. In that case, having a turbo capable of flowing 330 CFM or 660 CFM would not make a difference since the engine would need 2 cubic feet of air in the intake to have 15 psi of boost and the rest would be "bleeded off". Of course I don't take into account the spool time and the capability of the engine to spool the turbo etc....

Shit, all this stuff is much nore complicated than I thought...
 
a smaller turbo at 15psi will create more heat than a bigger turbo at the same psi. Hot air has less o2 in it than colder air. The more o2 in youe mixture the better/cleaner your explosion is, therfore you get more power from a larger turbo at the same psi as a small one.

sorry, kinda confusing.....
 
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