11 to 1 Comp w/ Boost
 

I was looking to raise the comp, then I thought of the F20c and its 11/1 comp ratio. I was thinking of running no more than 14 psi on on it, but just enough to make 250-270hp. What do you guys think?

When guys boost the f20c they usually add a block spacer to lower the compression. Your room for error on the tune is very small if you are running high compression. Yes, if you get it right you will make more power, but the chance of blowing your motor is high.

It's not a good idea.

If you can run E85 all the time, go for it. Pumpgas, a bit borderline.

Im also at 4300 ft alt and maybe add alcohol inj above 8psi

I haven't one S2000 turbo kit come with a spacer or a new head gasket

If you set up a failsafe boost/timing cut for when the alcohol runs out then sure. I personally don't like the idea of running out of something that prevents my car from blowing up. Do you have easy access to E85? My friends shop builds a lot of high comp 4g63's, but you can get ethanol on every other corner here so they build specifically for it. A couple of 12:1's have gone out the door there.

Okay here ya go:
http://www.s2ki.com/s2000/topic/9090...#entry21146790

Here's a power chart for FI s2k's. As you can see, when people start to get into higher power levels for that motor, they generally LOWER the compression ratio, with a head gasket spacer. Yes, there are some people still running stock compression ratio, but I bet when they blow their motors, the rebuild will be a lower comp motor.

Btw, thread jack, we should make a power chart on this site, it's a great way for noobs to see what they need to hit certain levels.

I wish it would be easy to install a F20c, it would be enough hp/torq that I would need in NA form. I am so used to higher hp engines. I will see if I can find some decent 10:1 comp pistons.
I also found a Honda Civic running 12.5:1 comp + 40-50 psi on E98

If I could start this all over, from scratch, I'd probably have a Honda engine in my Miata.

http://www.s2ki.com/s2000/topic/9090...t__p__21147325

------- 400whp out of that car on 12psi, I make 250 at 11psi. lol @ Mazda

s2000's heads flow more than a miata head can even imagine flowing. that helps them in more ways than one. tons of flow = less likely to det.

you throw compression like that on our truck motor and your "cushion" against det is non-existant

At 9.5:1 on crappy CA gas I have to pull timing WAY back, even at low boost levels. like 7-8* by 12psi

y8s hit 257rwhp on a dynapak at 9.5psi on his '01. that's 10:1?

omfg, Im hope theyre not doing something so damn stupid.


OP,

Of course you can run boost on 11.0:1, thats not even very high compression if youre a competent tuner.
The stock cam profile is going to limit the boost quite a bit due to the high dynamic compression created by having so little overlap, but with a more aggressive cam profile you can run more.

The F20C has a pretty radical cam profile compared to the BP, which is why you see people running so much boost on top of 11.0:1 compression.
Another example would be Fords new 5.0L modular motor which has VVT on both intake and exhaust camshafts. Tuners have been using the VVT to lower dynamic compression in boost by creating more overlap.


Heres what I suggest.
Dont get 11.0:1 pistons, and dont deck the block. Instead, mill the head to increase the area of the quench pads in the combustion chamber. With the right pistons you can get your desired compression ratio and have improved quench over stock. In other words you can run the same boost with more compression without detonation.
Id also suggest getting a good cam grind if you have the budget for it. I know Web has a good grind they offer but you may also want to check and see if Colt offers one.

10:1 here. 93 for the street(~350whp) and vp113(~500whp) for the track.

Keep in mind his location. He is at least 2psi short of most of us across the MAP. and that region is noted for having a higher effective elevation. A typical day could be 5000 to 7000 ft air the engine is breathing.

op - Will you be traveling much to lower elevations? 11 might be too much. I could see 10 working well, but no more. However, some of the fuel you get in that area is not the best from my experience. (Alb-Hobbs area) And if you do go lower elev, it will get into trouble real quick.

Plugged cat or tiny exhaust. On a 2554R@15psi I was able to run ~12-13* at peak torque and ~20* at redline on a stock BP4W motor and CA91. That was with a cast manifold and 3" catless exhaust.

The increased head flow of the BP4W means that a 99-00 9.5:1 longblock will actually take more timing than an identical setup on a '94-95 8.8:1 longblock.

Interesting Sav. I have a 99 motor now and noticed just as you say. boost is down until I get my clutch in, but at the lower numbers I am running more than the 95 motor liked.

too high

First off, what turbo were you planning on running?
Second, the F20c has V-tec.
3rd, you don't need that kind of psi or compression to make 250-270 on a miata engine. I'm positive you could make that easily by 10psi on a big turbo.
I said it a million times and I'll say it again: "It's not about the psi of air you can compress (a.k.a. boost)...it's about the volume of air that you can move."
I.E. Little turbo on 14psi may make 240whp, while big turbo on 8psi might make the same 240whp. If you can understand why that happens, then you can understand how to make power.

Ok I'll bite. I don't understand this. How does 8psi going into the same engine at the same RPM make the same power as 14psi. Assuming IAT is intercooled to the same in both cases, I don't see how this works.

You are too retarded to pull this off.

Because 8psi on a big turbo is not the same as 8 psi on a small turbo. PSI is compressed air measured in pounds per square inch. CFM is cubic feet per minute. Although the psi's are the same, the amount of cfm's with a bigger turbo will be greater.
The higher the psi's (compressed air), the higher your IAT's should also be. Hence, a big turbo will move a higher volume of air (CFM's) with less boost because of turbo size, plus IAT's should be lower.

I did 240 on york dyno at 10psi with fm base timing map. Five years ago. Dammmnnn

That was with the 01+ 10:1 pistons. I had no idea what i was doing with tuning back then. Motor never kabloodied.

I lost a lot of races at that psi :(

All this conjecture LOL.

A well-tuned motor will barely make 72 ft-lbs per L per atmo of MAP at torque peak, at the hubs, on a Dynapack. That may be 80-84 ft-lbs/L/atmo of MAP at the crank, and even F1 and NASCAR motors make 89. It's a number that can't be broken.

And then if the torque only drops 13% (which would be a very good flowing motor) at the power peak, at say, 6500 RPM, that means:

72 * 6500/5252 * .87 = 77 hp per L per atmo of MAP

At 15 psi that's 277 hp

y8s had to be making power peak at 7000 RPM with a 13% drop from peak torque to make 257 hp at 9.5 psi.

High revving Hondas make their hp/L by making their power peak at >7500 RPM.

Im looking at a GT2860 or a GT2871, but I dont know how laggy the 2871 would be on the 1.6

Im not drunk enough to do all the math, but yes I know what you mean. Alot of people say HP, HP, HP but actually all about torque

I've had a very hard time believing the 257 on only 9.5psi for the longest time.
And since he never re-dyno'd (to my knowledge) its still a mystery to me.

If he makes 72 ft-lbs per L per atmo, and makes peak hp at 7000, and his torque at 7000 RPM is only 13% down from torque peak...

or put another way ...

if he makes 72 minus 13% ( = 63 ), ft-lbs per L per atmo at 7000, then he will make 257 hp at 9.5 psi.

F1 dyno chart http://carpron.com/multisite/v/Uploa...+dyno.gif.html

FM's 2L stroker makes almost exactly 72 ft-lbs per L, and it loses ~12% of torque, at the power peak which is 7000 RPM:

https://www.miataturbo.net/attachmen...ine=1332349603

If that engine is 3.0L, it's making 75 ft-lbs per L.

Its BMEP, and yes that is very important, but revs are important too.
An engine at 300 psi BMEP at 4000 RPM and another engine making 300 psi BMEP at 8000 RPM, which one has more stress on it?
Its for this reason that power is typically used as the benchmark for where things like rods are going to fail.

Now if were strictly talking about knock limitations, then yes, between equal engines, the BMEP is going to indicate said limit. The problem is that were not talking about a simple comparison, such as different CR or boost with all else being equal.
Changing boost changes compressor efficiency, and changing compression changes quench.

well I know he made 246rwhp dynojet at the same boost (maybe less), but i know he was fooling with his tune and vvt stuff.

and he did make peak at 7000, or very close to it.


this all happened 4 years ago so everything is fuzzy.

Hey 250, crank up the boost :giggle:

I mean even 246 is pretty darn impressive at such low boost and pump gas.
And he does have a baller manifold and baller exhaust setup, with a turbo that is running about as cool as it can, so I guess it all contributes to it.

just a tubular manifold 2876 with vvt. nothing spectacular that anyone couldn't replicate easily.

By "baller" I meant he's not running a log with a 2.5" exhaust lol
And its funny you should say that, because somebody is soon going to try;)

If the 2 motors have the same displacement, the motor making 300 BMEP at 8000, will make twice the power of the other one.

V.E. is the limit. Around 210 psi BMEP for an n/a motor, or 75 ft-lbs per L per atmo, is 100% VE. You can very well have maxed out on your BMEP and be nowhere near the knock limit, by say, using E85.

No it doesn't.

Yes but quench affects knock resistance (which may not be an issue), and can increase burn speed (which can increase combustion efficiency).

Interesting read.....can anyone elaborate on this?


Combustion Chamber Dynamics
A cool charge may be the first step toward utilizing a higher CR, but what happens in the combustion chamber can make or break any such efforts. A prime factor here is never to loose sight of the fact that the faster the charge can be burned the higher the compression the cylinder will stand. Chamber cavities between the piston and the cylinder head between about .060-inch - .0120-inch appear most likely to be the site of detonation. Speeding up combustion mixture motion/agitation is vital. This means maximizing the quench action. On a small-block Chevy with a stock block height, a stock compression height piston is typically .025-inch down the bore. With a .040-inch gasket this makes the static quench clearance .065-inch, which is way too wide. By cutting the quench clearance the burn rate and quality improve to the point where the motor gains compression and is less likely to detonate even at the higher ratio involved.

So how closely can the pistons approach the head face? Although it comes under the heading of "don't do this at home" I have run the static piston/head clearance down to as little as .024-inch in a 350 with stock rods and close-fitting hypereutectic pistons. The pistons just kissed the head at about 7,000 rpm. As far as power is concerned, an associate of mine ran some tests in a nominally 450-horse 350 and found that each 10 thousandths of quench reduction was worth approximately 7hp. If you are building from scratch, make maximizing the quench your number one priority toward achieving compression and avoiding detonation.



Read more: http://www.popularhotrodding.com/tec...#ixzz1prkxGm41

Edit: Based on engine builder's specs my piston to head clearance should be .045"-.05". Hope I quench with 10:1

This answers a question I did not ask. So let me rephrase: Measured at the intake manifold, 10psi at +10 degrees F ambient is 10psi at +10 degrees F, regardless of what compressed the air. There may be differences in exhaust backpressure that will affect measured power output levels, but keeping all other hardware the same, the engine doesn't care what/how the PSI is created.

Note I am specifically not talking about compressor outlet temperatures, nor compressor wheel efficiencies.

The air is more dense, and there is more of it. Restriction (ie: boost) is just 1 part of the equation. The other is flow. Would you be able to push more air through a garden house or a coffee straw? Assuming same pressure?

...

I understand the difference between flow and pressure. 10psi through a garden hose != 10psi through a coffee straw. But, we are not discussing dissimilar flow rates - we are discussing a comparison of the same engine with different turbos.

Please note the initial conditions of the problem. 10psi measured at the intake manifold. IAT's equal. Same engine. Same RPM. Given those constraints, there are no variables left to change. Flow rate cannot be different unless different laws of physics are in effect. Any power differences between the two setups must be due to differences in the power lost driving the turbo.

I understand that a bigger turbo will flow more air at a lower boost pressure than a smaller turbo. I can read a compressor map. But that is not a part of this discussion. The claim is that for the same engine, with nothing else being changed, 8psi from a bigger turbo will generate more power than 14psi from a smaller turbo, because there is actually more CFM going through the motor. This reasoning is broken.

Edit: I understand that the exhaust backpressure caused by the different turbos will be different. There are two consequences for the turbo with higher backpressure - a) more power is required to drive it, to overcome the backpressure, and b) that higher backpressure will also reduce the CFM at any given boost pressure. So it's a double whammy to overdrive a smaller turbo.

Yes, it does. Anyone who's spent 30 seconds with a compressor map can see that the compressor efficiency varies with pressure ratio.

The asterisk to this is that the coffee straw is the turbine housing on the turbo. The reason that big turbos make more power at similar boost than small turbos is because the turbine inlet pressure is lower. Lower TIP = lower chamber pressure as the exhaust valve closes = more pressure delta between the IM and the chamber as the intake valve opens = more flow = more power.

I only read the first post. I deleted my nice long reply since I'm drunk an dp figured no one would understand.

Essentially, the F20c enigne has Vtec. I bet you're laughing at this statement. When Vtec engages, it greatly lowers the dynamic comrpession ratio by having crazy overlap that most engines wont have unless they have crazy aftermarket camshafts. In effect, the static 11:1 commpression means dick on a F20c. Add in a nice high octane fuel such as E85 with a nice free flowing tubular manifold, and your worries about an 11:1 compression ratio go out the door. Hell, I'm running 28psi on my stock F20c and it runs great with Vtec set at 4,800 rpm.

How this compares to a Miata engine, I do not know. I just wanted to clarify why a stock 11:1 compression ratio F20c is a capable engine and that it doesn't necessarily correlate to other engines.

Isn't this what I said? Although said better. My 11.5% Abyss Stout has had its way with me, and I'm going to bed.

Yes, you are correct. I was thinking along the lines of small changes and as a first approximation, efficiency doesn't change with small changes in boost.

Efficiency can go up or down with increased boost depending on where you are initially on the compressor map.

You are not using the concept of dynamic compression ratio properly.

What lots of overlap means for fixed-cam-timing motors means is that the cylinder pressures at torque peak are lower, reducing chances of detonation.

If you are comparing VTEC with non VTEC, a VTEC motor at 6000 RPM will be more detonation prone *at 6000 RPM* than a non VTEC motor, due to greater cylinder pressures from the greater VE.

There is also the factor of time (RPM). For the same cylinder pressure, a motor is less prone to detonation at higher RPM than at lower RPM.

So a motor with a higher peak torque RPM, for the same peak VE (and thus peak torque), will be more detonation resistant than a motor with a lower peak torque RPM. Ergo the former can run a higher CR.

I'll tell you how: it makes us sad.
Miata motors are in every way inferior to the f20c, so we either
1) be happy with sub 300
2) invest insane amounts of money to go higher
3) sell miata and buy s2k
4) swap f20c into miata
5) bitch about it on the forums and cry ourselves to sleep at night.

The equation is actually *only* flow. We are attempting to estimate flow from boost.

The first approximation is that flow is proportional to MAP (and to VE).

If VE goes up a lot, and boost goes down a little, you will have more flow (and thus power)