Compression ratio and tune

[sr20detconv]

Hi.
I have a small question regarding a change a compression ratio.
I had a stock shortblock with OEM 8.5:1 pistons before. My setup was 264 cams, greddy intake, precision 5557 at 18psi. The car made 373whp on a MD but the engine blew at the end of the season.
I want to rebuilt it with forged internals but I wanted wiseco pistons but they are 9.0:1. I wanted to know if I need to do a retune because of the higher compression ratio. If yes what other pistons brand do you recommend. I dont really want to spend another 650$ on tune .
thanx

[Chronicle]

Since you're replacing pistons, I'm guessing you cracked one when you said the engine "blew". Anyway, all other variables being the same, increasing your compression ratio especially in a boosted motor will need a tune.

[sr20detconv]

I was guessing that the compression was just slighty higher and will not need a tune. What brand of pistons do you suggest that produce 8.5:1 pistons?

[fliprayzin240sx]

CP Pistons with Eagle rods...get the rods too since you're gonna redo the bottom end anyways. Rods are pretty much the next weakness after the pistons. Slap some fresh bearings on it and you're good to go.

[Kingtal0n]

if you are using pump gas, at 18psi, and the engine blew- depending on what you were doing with it- it may have had a bit too much timing to begin with. Increasing the compression ratio might exasperate the issue.

My initial sentiment is that you should not require any "new tuning" with only a subtle .5 increase to compression.
My experience, on the other hand, is telling me to tell you to inspect the engine for signs that there might have been too much ignition advance to begin with- you might see small pits in the aluminum around the combustion chamber, and the cylinder that failed may have experienced a broken piston ring or land.


In general, no, additional tuning should not be required. But honestly, I have seen engines come apart, and go back together, with nothing changed- and somehow, someway, the tune was suddenly wrong. Especially MAP based tunes. I would watch the air/fuel and I hate to just shoot numbers here- around peak torque you wont need much more than 9* of ignition advance. You can verify by EGT of course.

[sr20detconv]

The car ran great for 5000km(3000miles) during the summer. Went to a few drift sessions and I was really hard on the car all summer long. The short block had a refresh but I was still using the 15years+ oem pistons in it. My taught was that the piston was cracked or something like that on the rod pin. Here some picture of it.
http://img.photobucket.com/albums/v607/sr20detconv/SR20blow.jpg
http://img.photobucket.com/albums/v607/sr20detconv/SR20blow1.jpg

The piston number 2 is inexistant. The rod smashed it on the head(head is trash also). I'm pretty sure the piston cracked because of fatigue on the rod pin but that just my opinion.

[s14unimog]

Considering the headache of rebuilding the motor and feeding more money into it, I would redo the tuning. In fact, that's exactly what I am doing. I shattered the a ring land on my SR (which was running stock pistons @~390whp) and have just now fitted 9:1 CP pistons. I will most definitely, without a second thought, be retunning my motor.

You could just risk it and see what happens..... b/c that would be smart

[jr_ss]

Theorectically, you should retune anytime something has been changed... Upping compression ratio would def be a huge change to your old tune. Spend the money right the first time and save yourself the headache.

[Kingtal0n]

The piston number 2 is inexistant. The rod smashed it on the head(head is trash also). I'm pretty sure the piston cracked because of fatigue on the rod pin but that just my opinion.

At the very least, Inspect the deck of the block around the cylinders for signs of detonation.
Look for small pitting and erosion that was eating up the engine- not necessarily the reason for the failure, but a sign that you need to adjust the timing/octane/boost.

[KAT-PWR]

Enthalpy changes tunes every .4 points is what he told me.
1/2 a compression point is enough to warrant tuning

[sr20detconv]

Thanx for the input!
I consider using 8.5:1 pistons because I don't really think that 9.0:1 really have a benifit except maybe a small change un spool up time. Why 8.5:1 or 9.0:1?

As for my old engine, everything looked great on the other cylinders(spark plug color, sign of detonation ,etc)

[jr_ss]

9:1 will give you better off boost response and aid in spool up, however, it'll be negligible. 9:1 would be a good choice for a street motor.

[AdamR]

Changing compression will not effect spool rpm.

Edit: correcting myself. It will effect time, but not rpm.

[Darren]

changing compression will help you make more power at a given boost level though, and torque...

there is a benefit

[AdamR]

changing compression will help you make more power at a given boost level though, and torque...

there is a benefit

I agree. With more off boost power, it won't take as long to get to the rpm where the Turbo spools. It won't change the rpm it spools at, though.