82 replies to this topic

[ENGINR]

14 pound in the E3 motor is 19 pound in a VR4/Early evo motor...

Huh? Why is 14psi different between motors?

[brisvr4]

Huh? Why is 14psi different between motors?

Jamie means that because of the higher compression in the Evo motor 14psi FEELS like 19 in a VR4 motor.

[Dean]

Im with John.
I think your problem is fuel related.
Got a dyno in Antigua?

[JAP63]

Huh? Why is 14psi different between motors?

Think of it like this
1 Bar of atmosphere, plus the one bar of boost your running is 2 Bar, there is 2 Bar of pressure entering your cylinder, so 29.4psi in total.

With a compression ratio of 9:1, this pressure is being compressed 9 times.
So....9x29.4=264.6psi
So, at full noise with your boost gauge reading 1 bar, each pot in your Evo 3 motor is flowing 264.6 psi of gas.

Now...

With a compression ratio of 8:1, this pressure is being compressed 8 times.(Assuming VR4 is 8:1, so Ive heard)
so....8x29.4=235.2 psi
So at full noise with your boost gauge reading 1 bar, each pot in your VR4 motor is flowing 235.2 psi of gas.

Now..Its obvious to see that the Evo motor pushes more Psi for the same given pressure.Ie. 2 Bar...
To determine the difference between the Evo motor, and the VR4 motor,we can take the pressure generated by the Evo motor, and divide it by that of 8 times, to see what residual pressure is left...

264.6/8=33.075psi
Now take away 1 atmosphere, ie. 1 bar (14.7 psi)
33.075-14.7=18.375 psi

There it is, at 1 bar of Pressure, the equivalent cylinder pressure in the 9:1 motor is 18.375 psi.
I rounded it up to 19.




A quick check just to see it holds true.
VR4
(18.375 psi on gauge+ 1 bar of atmosphere) x 8=264.6 psi

EVO 3
(14.7 psi on gauge+ 1 bar of atmosphere) x 9=264.6 psi

They are the experiencing the same cylinder pressures at different gauge pressures.

I hope this makes it clear enough and it's explained well enough..
Just because both motors read 1 Bar on the boost gauge doesnt mean the internal pressures are the same.
This is another reason why people should be cautious when changing the headgasket.
To people with built motors, it wouldnt matter at all, but as i said in a post earlier, its something to keep in mind when upping the boost on stock 7 bolt motors with higher Compression .

Edited by JAP63, 09 April 2009 - 12:42 AM.

[Liberoz]

Bahh who's running higher compression head gaskets anyways 8.5:1 Evo II comp ratio all the way

[JAP63]

Bahh who's running higher compression head gaskets anyways 8.5:1 Evo II comp ratio all the way

Thankyou for your insight.
Regardless of whether the compression goes up or down by changing the headgasket, the point is it can be different afterwards......
Personally, i think 9:1 feels great on the car, it brings the car on very well.

[Liberoz]

feels great

have you tried any of the other compression ratio's on offer? 8.5:1 is Fu@kin awesome

[JAP63]

have you tried any of the other compression ratio's on offer?

Yes, 9:1 makes the engine feel fatter across the whole rev range IMO, and espesh early, when coming on.

[Craig_CD9A]

Agreed with the 9.0:1 comp being pretty nice indeed! Works very well with the more aggresive E3 camshafts too...
My Evo 3 motor which is standard apart from a pod and exhaust with a split dump pipe, puts out 172.8kw at the wheels just below 7500rpm with 170kw at just below 6000rpm. This is running dead on 1 bar of boost, and these figures could no doubt be greatly improved with a chip or a aftermarket ECU. It is really the torque of these engines that impresses me most though!

[mrcyborg]

jap63, your explainations are very good to help understand what people are talking about with the compressions ratios and boost pressures, but psi is much different to flow.

i also agree with the 9.0:1 i think it feels better also, and in my experience with simalur mods to the late evo engine to the early engines, nets a better usable power. although funly enough doesnt always "feel quicker" as power is much smoother than then shove in the back of the lower compression.

[JAP63]

with the compressions ratios and boost pressures, but psi is much different to flow.
.

Yeah i wasnt really talking too much about flow at all, just presure and the consequences of different compression levels etc..

I myself, like you say, prefer the linear behaviour of the higer comp motor, prefer it more than the low boost "all hell breaks loose" type setups, i think its mored driveable, great through tight tracks etc too.

[WADADLIG_EVO]

Wow...great information guys.

Just to follow up.
I was right!!! The boost gauge had an inner ring that melted I guess because of the heat. I replaced it and went to test drive. Mind you as I said in my original post I had already turned down at least 15 half turns.

At WOT the car hit 18-19psi and bled back to 14 or so.

It was still hesitating at that boost so I dialed it back just a hair to 17psi.

Now the thing is. The night I originally started turning down was really cold for Antigua and the car felt Incredible. Now it feels BLAH on 17psi. I can only imagine what I was running that night.

As for fuel...I think and I mean I think we have 95RON which I believe is equivalent to 91 octane in the US. That is not supposed to be very conducive to high boost. The previous owner got the car from Japan running 23psi and if they have 101ron that explains why he blew the engine not too long after.

The engine is not stock any more. He rebuilt it with I think Wiseco pistons and Eagle rods for the 2G eclipse in the US. I cannot verify the exact make but I believe they are an 8.8 compression ratio. The engine is fully forged. Compared to the stock engine compression ratio and the new pistons rebuilt motor the car pulls alot harder in the higher rpms.

Its a miracle I did not blow my motor. I installed a walbro 255lph a while back and maybe thats the reason.

I'll throw in some fresh plugs and see if that helps with the little hesistation I get here and there.

To anyone that has bit Boost cut is it or does it sound like a Loud POP? Wondering if I could have blown my head gasket cause the car had to have been running at least 23-24psi.

WADAD

[WADADLIG_EVO]

No no 4wd dyno in Antigua. Forgot to say that.

WADAD

[JAP63]

To anyone that has bit Boost cut is it or does it sound like a Loud POP? Wondering if I could have blown my head gasket cause the car had to have been running at least 23-24psi.

WADAD

I reckon ud be close to poppin a gasket at these pressures....if u keep runnin that psi i caould almost gaurantee it.

[WADADLIG_EVO]

Just wondering if I did (pop something), but I dialed back to 17psi.

The car is not smoking but the feeling that I did something wrong is definitely there now knowing what I know.

Going to change the plugs in the morning hopefully. All is well for now.

WADAD

[WADADLIG_EVO]

Well, sorry if I hijacked any ones thread but it was on the same topic as we were discussing.

Just to finish off everything. I did a compression test yesterday morning before changing the spark plugs. Compression was great at 180 across all 4 cylinders so luckily I did not "pop" anything and what I experienced had to be the boost cut.

Car starts much better now and is not hesitating in the upper rpms any more.

Thanks for your help.

WADAD

[JAP63]

So what pressure do u think the boost cut is then from factory?

[WADADLIG_EVO]

Well not sure this is a factory spec ECU but on this I am going to guess 24-25PSI on this ECU. Until I knew what the gauge was actually reading I was not sure.

WADAD

[CLuTZ]

Boost cut does not kick it at any particular boost. Its calculated by the Air Flow Meter- when it see's too much air being drawn with not enough AFR's to keep the car happy, it will start cutting out. For example, on a very efficient setup, it might kick in on 16 psi with a 16g, or with an in-efficient setup with a 14b it might kick in at 20psi.

[JAP63]

Boost cut does not kick it at any particular boost. Its calculated by the Air Flow Meter- when it see's too much air being drawn with not enough AFR's to keep the car happy, it will start cutting out. For example, on a very efficient setup, it might kick in on 16 psi with a 16g, or with an in-efficient setup with a 14b it might kick in at 20psi.

Hence when you change the cooler piping and exhaust etc it comes in earlier.