AutoX / Road Race Build-Up @ 9:1

[tsunari]

So after going through (3) three different motors from Polk Performance in the past (2) two years, I've finally decided to cut my losses and build my own motor.

I've got a new block lined up, rods from the old motor are fine, just need to get a new set of pistons (skirts on the old ones were marred up).

So the question I pose here is:

Do I stick with 9:1 compression or drop it down to like 8.8:1/8.5:1 ?

It's mainly a street driven car (needs to remain reliable) that will see as many Auto-Xs and Road course events that my wallet will allow. I originally went with the 9:1 setup to help with off-boost response and low/mid range power since I'm more concerned with the twisties over straight-line / top-end power.

Quick run-down of setup:

PTE GT3076r (internally gated)
6-bolt no BS
Eagle Rods
J/E Pistons (9:1 c/r)
DSMLink

Motor is a half-step away from being fully built (still have stock cams )

I understand that the higher the C/R means I'll not be able to run as much boost as a setup with lower C/R, but I'm more interested in what kind of tuning/performance issues I may run into while trying to achieve a reliable 400-450 WHP on pump.

As always- Thanks!

 

[tsunari]

Turns out (unless my head was seriously milled or some other crazy engine setup) that I never had 9:1 compression. For the past two years i've been running 8.5:1 JE pistons. I'm not currently aware of anything that could be done to take an 8.5:1 piston and bump the CR up to 9:1 except for maybe milling the head a good bit, but then I'm still learning.

Just the same, this goes to show you why not to just blindly trust someone no matter how 'reputable' they may appear to be.

If someone else knows how these motors may have actually had a 9:1 CR using 8.5:1 pistons, please enlighten me!

 

[Tony90TSI]

Was this on a 2L motor or a stroker motor? I am not sure on DSM's but every V8 i have seen with a stroker ended up with more compression then it should have. My older brothers truck we built a 383 for and put in 9:1 comp pistons in for the head we were using. Well once the motor was done and we checked it out it turned out to actually be 10.75:1

Higher compression means that you are going to have a smaller window for error before parts start getting hurt. That seems to be quite the fight on this site though. Everyone on here seems to think its a bad idea to run high comp and boost. You just have to remember that the tune needs to be perfect and also the more boost(air flow) you run the higher octane fuel you will have to run to keep from hurting the motor. The local supra shop is trying to get me to run 10.5:1 comp in my car like they do in their 1200 rwhp "street car". More power can be made all through the power band with higher comp on the same boost, you just need to find out what fuel you will be running and build the set up for that.

 

[delta448]

On most engines, if you lengthen stroke and keep the same deck height, you can use a piston with the wrist pin closer to the top, or one with a dished out top, or use a much thicker head gasket. Some engines can use shorter conrods, but on others, you can't use a shorter conrod because the stroke would make the bottom of the piston hit the counter weights at near BDC. I don't know about 4G63's though, as I've never done a stroker on one.

 

[tsunari]

Yeah this one is just a 6-bolt 2.0L motor . . . Eagle Rods and JE Pistons with an EVOIII head gasket. I've got the blueprints for the buildup so maybe I could just sit down and calculate up what the actual CR was on that motor . . . I'm pretty sure the blueprints have the bore,stroke,etc . . .

I just want to get this figured out before I order my new pistons, which hopefully will be sometime next week- I've been far too long without driving her . . .

 

[Tony90TSI]

Yeah this one is just a 6-bolt 2.0L motor . . . Eagle Rods and JE Pistons with an EVOIII head gasket. I've got the blueprints for the buildup so maybe I could just sit down and calculate up what the actual CR was on that motor . . . I'm pretty sure the blueprints have the bore,stroke,etc . . .

I just want to get this figured out before I order my new pistons, which hopefully will be sometime next week- I've been far too long without driving her . . .

Let me help you out. Here is a website that has all the stroke, rod length, and general info for the 4g motors.
http://www.ariaspistons.com/mitsubishi.html

Now with that and knowing what thickness HG you have and what dish on the piston then this site will tell you what you have.

http://car.race-cars.net/calculators/compression_calculator.html

 

[wishihadatalon]

This is always something I have wanted to try. A 2g head has much more efficient runners and will creat much better low - and mid range tq. Why not try 1g rods with 2g pistons and a 2g head? RRE has said before that the the 1g/2g combo is one of the best for reliablity and performance.

 

[exiletalon]

Consider a SMIM, and cams in your 2g head. That would increase your low end quite decently. You may also want to look at an external WG, as they tend to help spool up on turbos of that size. ARP studs on a fully internaly stock 4g can easily handle the power you are looking for, and it will be more reliable....
Have you considered methane injection, It allows you tune more aggressively/more boost on lower octane....

 

[tsunari]

Thanks for the links for the info! I checked around a few other websites and based on the data I have, it looks like my CR is BARELY 8:1 !!! Quite a ways off from the 9:1 I thought I was getting


wonder if I should call up Polk and ask WTF?!


Here's what I have if anyone would care to double-check:

Cylinder Bore :: 3.3705
Stroke :: 3.4646
Deck Clearance :: 0.022
HG Thickness :: .067 (EVO III MLS)

Head cc :: 47 (stock)
Piston cc :: -13.2 (dished)

And yeah thought about water/alchy/meth injection but ruled it out as that wouldn't work so well for road racing- drag strip sure . . . but not 30+ min of constant abuse.


***UPDATE***
Turns out I believe I was calculating the C/R with the deck clearance @ .022" BELOW the deck - that's what was giving me the 7.9xxx:1 C/R.
Flipping it around to be .022" Above the deck gives me 8.5x:1 . . . much more reasonable, but still not the 9:1 I ordered 2+ years ago . . .

 

[Tony90TSI]

Thanks for the links for the info! I checked around a few other websites and based on the data I have, it looks like my CR is BARELY 8:1 !!! Quite a ways off from the 9:1 I thought I was getting


wonder if I should call up Polk and ask WTF?!


Here's what I have if anyone would care to double-check:

Cylinder Bore :: 3.3705
Stroke :: 3.4646
Deck Clearance :: 0.022
HG Thickness :: .067 (EVO III MLS)

Head cc :: 47 (stock)
Piston cc :: -13.2 (dished)

And yeah thought about water/alchy/meth injection but ruled it out as that wouldn't work so well for road racing- drag strip sure . . . but not 30+ min of constant abuse.


***UPDATE***
Turns out I believe I was calculating the C/R with the deck clearance @ .022" BELOW the deck - that's what was giving me the 7.9xxx:1 C/R.
Flipping it around to be .022" Above the deck gives me 8.5x:1 . . . much more reasonable, but still not the 9:1 I ordered 2+ years ago . . .

Deck clearance? Are you refering to the deck of the block being clearanced(material taken away) or is that refering to the clearance of the piston from coming out of the deck(hitting the head)? With out that info in i came up with 8.25:1

 

[tsunari]

Deck clearance? Are you refering to the deck of the block being clearanced(material taken away) or is that refering to the clearance of the piston from coming out of the deck(hitting the head)? With out that info in i came up with 8.25:1

Deck clearance as in when the piston is at TDC, the distance between the top of the piston (not the dish) and the top of the block surface.

I just re-checked and the piston is indeed below the deck surface at TDC, so please disregard my previous "update". With that I get 7.92:1 at this site

http://www.torinocobra.com/Randys_tools.htm