Why is low compression better for a Turbocharged Engine?

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bongskag

Member +
I was trying to figure out a compression raito for my new forged lump, it comes out at around 7.3:1 the block skimmed to clean and the head has been skimmed to clean is anyone else running these kind of ratio's ? looking to eventually run 1.5 bar through a td04 ,

Cylinder Diameter (mm) 74
Stroke Length (mm) 77.4
Number of cylinders 4
Chamber in cc's 40?
Piston dome in cc's -10
Gasket Thickness in mm 0.6mm (trd)

You make horsepower by how much air you move through the motor. A high compression 10:1 engine is more efficient than a 7:1 engine, so the 10:1 engine gives you more bang for the buck. However, because the lower compression is not as efficient, it will move more air through it. So, at 15 PSI of boost, the 7:1 engine will have an effective compression ratio of 14:1, will not be into detonation, and be moving more air, making more horsepower than the same conditions for the 10:1 engine. That engine will be in self-destruct mode, have detonation, and an effective compression ratio of 20:1!

This is why the racers only run 5:1 or even 6:1. All of this is great for a drag car, but because the static compression is lower, you will not have much bottom end torque either. So, since most of us don't drag race every place we go, a good compromise would be 8:1 or 8.5:1 compression. This way you don't loose too much bottom end for driveability, and if you don't run too much boost, say 10 to 15 PSI, you stay away from the gray effective compression area of 15:1 and up.
 
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bumblebee

Member +
i think a good comp ratio would be between 8.5 - 9.0, unless its a dedicated drag car, having a 7:3 comp for road driving is gonna be a pain. I think there are people on here running more than 1.5 bar on standard comp ratio if not, slightly lower...

The higher comp ratio should also help in turbo spool up
 
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AdamB

Member +
bumblebee said:
i think a good comp ratio would be between 8.5 - 9.0, unless its a dedicated drag car, having a 7:3 comp for road driving is gonna be a pain. I think there are people on here running more than 1.5 bar on standard comp ratio if not, slightly lower...

The higher comp ratio should also help in turbo spool up
Click to expand...

x2 running a low compression ratio makes the engine really lazy to drive off boost, which makes it a terrible daily drive.
Spuddy is running stock compression ratio on a 5e and running 2 bar boost.
 
spuddy

spuddy

Ulster Area Rep.
You only want low compression if you've got crap fuel only available to you. Stock compression should be ok for most fellas.
 
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mech5107

Fresh Recruit
Things are not that simple. Running 1 bar of boost is not directly doubling the CR.

First of all not all turbos flow the same, the mass of air getting in the engine is not the same.

Second the main effect comes from the valve timing and as a result the camshafts. Valve closing timing greatly affects the pressure in the chamber and as a result the effectiveness.

And real racing engines don't use 5:1 and 6:1. I've seen endurance engines running 1,5bar with 13,5:1, no direct injection, proper management, highly developed head and camshafts etc...
 
sx_turbo

sx_turbo

Lifer
8.2:1 is a great compression ratio for these engines, and will enable you to run 2 bar boost with proper management and built engine.

you could push things and go for 9:1 but that would probably be pushing things a bit far for road fuel
 
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bongskag

Member +
I thought 7.3 would be a bit low, so what are my options ? skim the shit out of the head ?

I sent it for a light skim to clean the other day and will have to find out how much was taken off. maybe they havnt done it yet and I could ask them to take a bit extra off ?!!?! like 1mm ? LOL does that sounds crazy .

still gonna go with the trd 0.6mm gasket, wish they didnt stop making the trd 0.5mm gasket I could have used that extra .1mm off !
 
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bongskag

Member +
yes , thanks guys. ill try send out rep.

sent an email to the engineers to see if they have skimmed yet and if not ask them to take about 1mm off ( does that sound right ? ) and if they have done it , find out how much they have taken off and pay them some more money to take more off ! i dont mind 40 bucks if it means the C/R is good
 
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AdamB

Member +
Have you had a port and polish on the head? You don't want to skim too much off the head because you might end up getting valve to piston contact.
 
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mech5107

Fresh Recruit
1mm sound a lot...

The best option is to ask him to measure the cc of the head and then calculate the current CR and then decide on how much he will skim
 
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AdamB

Member +
mech5107 said:
1mm sound a lot...

The best option is to ask him to measure the cc of the head and then calculate the current CR and then decide on how much he will skim
Click to expand...

It will depend on cam lift to how much the head can be skimmed. The best way is to use play-doh, measure the height, put it on top of the piston, dummy fit the head and turn it over, measure the height of the play-doh again and that will give you your valve-piston clearance.
 
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hardcoreep

Member +
Compression ratio isn't just based on airflow. It has to do with the combustive characteristics of the motor. The design of the chamber, the cam timing are two other major elements. The E-series motors are designed for torque so they have high in cylinder pressures, based on those two design parameters, that affect combustion rates. Ultimately, you can run whatever compression ratio you like as long as the head can take the pressure and you don't have detonation.
 
HYBRID

HYBRID

Super Moderator <a href="http://www.toyotagtturbo.
well tbh i dont agree with alot of the above comments... im running a cr of 7.34:1 ... now sure my car is a drag car, but also a daily drive on many days.. its not just about ur cr which affects driveabily but also how it is tuned... i can honoestly say if u drive my car you wouldnt notice its a low cr engine because it responds very well as well.. there is something called timing compensation which is very beneficial on a low cr engine.. but is something you cant do on a high cr engine.. sure u dont really need to....

now, a low cr engine is a much safer engine for drag racing, as the internal stress on many many components will be reduced greatly.. racers which a higher cr locally have to strenghten their main caps for example to steel units (supplied by us) as there have been many higher cr engines breaking the main caps at boost which to a low cr engine is no stress at all.. (my engine has been used for drag racing since 2009... shes been built and closed for over 62000kms)... not many who can say that about reliability as far as a racing engine goes :)

there are disadvantages as well to a low cr engine.. mainly boost... you need to apply alot more boost to get a low cr engine to run the same times as a higher cr engine.. but in all honesty it goes down to each individual setup.. its not just a case of whats the general rule.. look at your whole setup.. including turbo.. to even gear/tyre ratios.. and where you want the power down the strip.. remember theres ALOT to tune when considering a track is 400m long :)

hope this helps
kon
 
S

Skalabala

Member +
There is a difference between Compression Ratio an Combustion Compression.
High CR = not easy to play with timing = low Combustion Compression.
Low CR = can go nuts with timing = High Combustion Compression.

Nothing wrong with a high CR engine. Things just have to get a lot more detail.
Like fitting the best management out there, pistons specially machined to be more det friendly, more expensive fuel.
And a tuner that knows what he is doing
 
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