Compression Ratios / Short Block
 

I know this was covered in an older thread, but neither the questions or answers gave me the info I was looking for, so here goes.

I'm currently running a Ptuning Stage 1 Turbo with water meth. At peak 15psi Boost I get 365hp to the wheel and 250 ftlbs torque.. That said, I'm not going there very often. Nonetheless, I'm fully aware the FA20 engine has high compression and the engine was designed for NA. I'm also hearing enough stories about bent or broken rods that it may be cheaper to be proactive...

So, in the interest of having a really solid setup I've started looking at short blocks.

Some shops that make their short blocks build them with 12:1 CR because they say it can make more horsepower, others say 10:1 gives you the ability for more boost, and more torque and more hp.

In a perfect world, if this engine had 400hp with 400 ftlbs torque and was robust and reliable that would be pretty awesome. Hell, I'd be happy with 350/350.

So- does anyone have sage advice about the pros/cons of 12:1 and 10:1?

The higher your CR the higher octane fuel you will require to run. If you are boosting 12:1 expect to run E85 99% of the time unless you have low boost. You can get more pump gas with a lower compression and not be stranded if no E85 pumps are near by.

The main issue with our engines and rods is not the power its the tuning. People have run 500 HP on a bone stock engine for over a year with the right tune, but if your timing is too advanced to make power and your octane is too low you will see knock and detonation. This bends and snaps rods.

Safety factor is lower compression and pump gas.

E85 is not reliable from a pump unless you have a flex fuel setup to automatically adjust timing based on ethanol content sampling.

At the end of the day.
Flex Fuel setup and ECUTek with a proper tune and you are all set.
Mix and match E85 and any pump gas with peace of mind.

JE Pistons sell 13.5:1 CR pistons ;D

If/when I build my engine, I'll be trying to stick as close to stock CR as possible because a lot of the tuners these days know what they are doing with this engine now to not blow it up as easy.

There is a tuner here in Aus that regularly does at least 300kw at the wheels (on his dyno that would be about 470hp on a dynojet) on stock internals. Unfortunately he doesn't (refuses) to use EcuTek.

IMHO going 10:1 would be a step backwards as this engine's fuel management strategy allows us to run quite high compression from stock etc. Going 10:1 would be like putting an EJ engine in. Good, but old tech ;D

In conclusion, higher CR is good if you are able to run E85 mostly, and/or have a very competent tuner. Otherwise 10:1 isn't terrible, but it's a backward step.

Oh I like this one. This one is good.

So, you make high peak HP, and much lower peak torque, and you don't get to high RPM's very often.

You want HP / TQ to be roughly square, and you want a lot more torque.

Horsepower is just a math equation. It comes from torque.
Specifically (Torque x RPM) / 5252 = HP
What that means for you, is you have to hit peak torque below 5252 RPM to have nice square TQ / HP numbers.

Your gears reverse the equation and turn horsepower back into torque at the wheels. (if the output is a constant, and you lower speed, you raise torque.) Torque at the wheels is what accelerates the car.

There are many ways to achieve this, and it depends on many variables.

I'll be honest, The GTX 3076 in that Ptuning kit is a little big for that goal.

You're going to want to retain as much compression as you can, because if you're looking for a "torquey" motor, I guarantee you're also looking for quick transient throttle response. Lower compression slows transient throttle response, both in and out of boost.

People say high compression spools a turbo faster, but that's an erroneous conclusion from a chicken-and-egg scenario. Whatever. To the driver, that statement is true, that's all you need to worry about.

I can continue, but in order to sort out the inputs, like compression, you need to refine your output goals.

:popcorn:

@cdrazic93, when you read Spartarus' post, if you can see how it relates to your remark about the math here, then you have it made.

Astute.

Ok, you're on. Keep eating, Mister 10.5:1, divided T4 6266.

You made 710 WHP "Around" 6600 RPM. For easy math's sake, let's say it's 6658. That allows us to have a 1.0 transmission, 4.10 gear, and 24" tire. Stock.

That means you're making 560 Ft. Lbs @ 6658 RPM. That's nowhere close to your peak torque though. By the time your power peaks, your torque is a fair way along on the downhill slide.

How am I doing so far?

Your torque peaks at.... Hrm... 5340 or so. Right at about 628 Ft. Lbs.

I could draw the curve if you like.

:cheers: /drunkmath

It takes a village.

Especially, if you have someone the village (@Spartarus ) that understands physics.

;)


humfrz

He's the leader of the study group. I'm just making sure Chris shows up.

damn it you figured out my secret...

I would rather not expend my saturday night crunching a few pages of math...




nah, who am I kidding, Im gunna investigate what this might take...

edit1: If I had the equation for the piston (my plane) , I'd be able to give a cross product of the torque, assuming all the combustion equations are equal for all 4 pistons. Then dip into some airflow equations because I would love to see what kind of airflow the stock units get (insert someone's reference here). My first assumption is that since its a turbo, the best way for it to make more power would be to make it breathe better. Exhaust would be fun to think about too.

...didnt think about it too long. saturday night saved :thumbsup:

BUT I'm running 12.5:1 compression:thumbsup: and a 24.8 tire just sayin lol
and you are correct my torque did fall off before then...... UNTIL we adjusted the cam timing a little then strange things happened.........

The problem with these cars is going to trying to maintain adequate oil pressure to even support 300+Hp with these FA20's, from what i've seen and read if you aren't in the 60+ PSI range your rod bearings will be the first thing to go out.

Also I would just stay stock compression if you have E85 readily available in your area, if not go with which ever compression your tuner recommends.

Since my tuner recommended me to stay with the stock compression on my built block I just did, since I only run high boost 16+ PSI on E85 and less then 11 PSI on 91 octane.