What compression ratio for turbo motors with forged pistons???

[PETEW]

Hey guys. I am wondering what compression ratio most guys are using for thier Turbo motors. I was wondering if most are staying with stock compression, or are bumping it up to a higher compression for better off boost response. I am looking at building a motor that will have around the 400RWHP mark and am unsure if it is ok to go up to 8 to 1 or if it is better to stay at stock. I have a rebuilt p90 head with an isky cam, SDS for fuel and ignition controls, large downpipe, and 3 inch exhaust. I just need to figure out what pistons and what turbo to use and I should be good to go. Thanks for the help guys.

[SHO-Z]

I got this off the internet a while back. It on a 5.0L but the information is real good.

 

 

Engine Compression

There are a couple of areas in which optimizing an engine for naturally aspirated power and forced induction power are at complete odds with one another. Static (engine) compression ratio is one of those. Most musclecar enthusiasts are readily aware of the fact that increasing the compression ratio of a naturally aspirated engine results in more power per unit of fuel combusted. The amount of engine compression that can be run is limited by the fuel octane that will be used.

 

Forced induction engines are fundamentally different in that two sources of compression are available: the compressor and the engine. Effective compression is the term often used to describe the impact of the external compressor on the combustion efficiency.

 

So how does a lower static compression benefit a forced induction engine? Let's look at an example. Compare these two situations:

 

1) On a stock 5.0 with ~9:1 static compression it is found that a non intercooled supercharger can run a maximum of 9psi of boost without detonation. The effective compression is ~14.5:1 and 320 peak rwhp is produced.

 

2) Same engine/supercharger combination but with low compression 8:1 pistons installed. It is found that a maximum of 13 psi can be run without detonation. Effective compression is again ~14.5:1 but significantly more air is allowed to enter the combustion chamber. Peak rwhp is now 380. In both cases the same octane fuel is used.

 

So a static compression change in a forced induction setup, when compensated for with additional boost, is analogous to adding displacement in a naturally aspirated engine. Sounds easy enough right? So what is to stop us from 7:1 or even 6:1 static compression? Well there are a couple of considerations.

 

First is the size of the compressor. As static compression decreases, the flow requirements of the compressor increase. You don't want to go so low on the static compression that the flow requirements exceed the capacity of the compressor. This is primarily a concern only if a compressor upgrade is not in the budget. In other words the static compression must be matched to the compressor being used and the fuel octane being run.

 

The more fundamental limitation comes about as a result of the boost threshold. Since the compressor is being relied upon to maintain the effective compression at it's optimum ratio, when you are below the rpm at which maximum boost is available then the effective compression will drop along with some power and throttle response.

 

With this in mind some compressor types are better suited to taking advantage of a low compression engine.

 

Perhaps the best is the 2.2L Kenne Bell (Blowzilla). With full boost available at 2000rpm and plenty of capacity there is almost no downside to running as low a static compression as the blower can support. Centrifugals are at the other end of the spectrum. With the boost threshold at redline there is a clear tradeoff of some low end for top end power.

 

Low compression/high boost combinations are a staple of the fast import cars. This is the primary reason the fastest turbo 4 and 6 cylinder cars typically put out a higher hp/displacement ratio than 5.0 liter Mustangs. I've seen a 4 cylinder car running as low as 6:1 compression and 45psi of boost.

[rags]

Pete,

 

What are you going to be using the car for? I chose to up my CR and use a lower boost number for better off boost throttle response. My reasoning for doing this was I use the car on the street and figured most of the time the car would be off boost. I really didn't like the amount of power a stock 280zx was making while off boost and found myself constantly planting my foot to get the car to move. It did get quite annoying. I thought a NA 280Z made ok power and seeing that it only had a CR of somthing like 8.3-1 decided to use that as a base for my 3 liter. On my car which has a cr of 8.5-1 I can run as high as 16lbs of boost without detonating. To be fair, I do have a large intercooler, programmable FI, a larger than stock turbo and injectors and I run on the rich side when boosting that high.

My config won't make the most HP but I think it gave me what I wanted. A fast car that isn't a dog out of boost.

 

Joe

[PETEW]

Joe,

THe car will be a multi purpose car. I am looking to use it on the street some (not a daily driver), drag race it, and use it for some track days. I just know by todays standards the 280ZX turbo motor compression is ridiculously low. I figured if I bump it up by a small amount it wouldn't hurt me from boosting too much, but also help a little with off boost response.

[nismopick]

Low compression/high boost combinations are a staple of the fast import cars. This is the primary reason the fastest turbo 4 and 6 cylinder cars typically put out a higher hp/displacement ratio than 5.0 liter Mustangs. I've seen a 4 cylinder car running as low as 6:1 compression and 45psi of boost.

 

I love it... someone that understands.

[rags]

Sounds like your going to use the car for exactly what I use mine for. I'm happy with the way my engine turned out. When I'm lazy and don't downshift, the engine still has enough grunt to get out of it's own way. And when I dump the gas pedal and the revs climb, I get that nice "put back in the seat" feeling at about 3k. For me, it's the best of both worlds.

 

Joe

[rztmartini]

so can a large i/c, megasquirt, and big injectors "safely" overcome poor quench?

[BRAAP]

First off, EXCELLENT write up SHO-Z.

 

 

Rztmartini, Hmmm.. I’m not sure I understand your question, i.e. possibly you are meaning something a little different than the terminology you used?

 

I’ll take your question at face value and try to answer it as best I can.

 

 

The short answer to your question is a resounding no. An intercooler is good and will help keep the intake charge cooler/ i.e. air charge will be more dense for a given boost level, but the oversized injectors and Mega Squirt? No.

 

Injector size and the controlling unit that opens and closes the injectors, (i.e. the EFI ecu), will not overcome lack of squish/quench/poor chamber design or even band-aid the ill effects caused by the lack of squish/quench. By using oversized injectors, this will allow you to run pig rich, which will reduce the combustion temps and help to keep the detonation demons away, but it only helps a little and induces other issues such as fouled spark plugs, cylinder wall and piston ring wash down, more frequent oil changes due to fuel contaminated oil, etc. In short, running an engine pig rich to keep detonation down is not the best route around a poor chamber shape or too high a static compression ratio. You should size your injectors to the amount of horsepower the engine will produce, not size them to compensate for poor chamber design by running the engine pig rich. Also,I have seen a few posts on this forum claiming that just by switching from the OE EFI to Mega Squirt, detonation was reduced or eliminated. Those are FALSE claims. There were other variables at play that the people making those claims just were not aware of. Mega squirt, in of itself, has no special abilities to remove or reduce detonation over the OE EFI. The job of the ECU is to send a timed electrical pulse to the injector, (the injector is nothing more than an electrical solenoid which unseats a pintle, disc, ball, etc to allow fuel to flow through it). The electrical signal that comes from the ECU, whether it be OE ECU or Mega Squirt, use the same electrons to open close the injectors. No magical detonation reducing electrons are produced by the MS controller. With Mega Squirt, it is easy to run the engine PIG rich as stated above, but the OE EFI can also be ”fooled” and made to run pig rich as well delivering the same result. The OE EFI computer and Mega Squirt are just controllers that are opening and closing the injectors which are just electrical solenoids. The ECU has NO magic in it taking detonation away.

 

 

Now the intercooler is a help on many levels and is good idea.

 

To wrap this up, the L-series is prone to detonation at lower static compression ratios than most other engines for a few reasons. Unfortunately, this is an issue that we are stuck with, especially if you use the OE style dished pistons even with a squish head. Our only realistic options in overcoming this short fall with the L-series is try to and achieve that ideal optimum squish and keep the static compression ratios a little on the soft side of average. To get the ideal squish “AND” a decent comp ratio with an L-series is costly as it usually involves custom pistons.

[rztmartini]

thanks and you answered my question within the first lines. i know what the ecu does and my question should have been is runnign rich an acceptable way of reducing detonation. and the answer is no...but...

 

what if im too poor to get the optimal head work/custom pistons will it work for now?

[PETEW]

Thanks for the write up guys. Braap I am actually looking to buy a set of forged pistons, so I figured I would spend the money once and get a set that work best for my type of application. I have seen a lot of guys buying flat tops and I was wondering what type of compression they were running and did they have to do anything special with the piston if they are running an aftermarket cam like the isky cam I will be running. I will also have the head ported some and have all new parts for it valve job, blah, blah, blah. I wish I could afford your work, but I think you are out of my league. I have been told that turbo performance cams are not like NA cams in requiring pistons to have large valve reliefs. Is this really true?

I have a chance to buy dished JE pistons (at stock compression level), but I know that off boost response is reather anemic. I also understand lower compression alows higher boost levels, but I am not after a car that only has great top end but nothing else. To me that is a worthless endevor. For some guys it's all about the big numbers, but that doesn't always make you faster. I am looking for a sleeper car that will be decent at the track, fun on the street, and make people say what the h-e- double hockey sticks was that! Simple right.

[OlderThanMe]

so for best squish what piston shape/milling would we want? I saw somewhere someone said that a flat top piston with a divot in the middle would be best. I think a domed piston could get expensive really fast. I am looking at doing an 8.3:1 or 8.5:1 but most likely the lower. I just can't get the Z engine calculator to work on my PC so I'l keep trying to figure it out... I am going to have my new motor built right no matter how long it takes to build.

[rags]

If I had to do it over I'd use a flat top that came up even with the deck and had a D shaped relief cut in it to bring the CR back down to 8.5-1.

 

Joe

[rztmartini]

If I had to do it over I'd use a flat top that came up even with the deck and had a D shaped relief cut in it to bring the CR back down to 8.5-1.

 

Joe

 

where would you get a set of those?

[OlderThanMe]

You could just get some JE or whatever flat-top pistons and then have them machined. There are a couple of venders for custom pistons.