Ok about to order some forged pistons,i'll be running 12psi max and my compression ratio options are 8.5,9.5 and 10.3 what should i order?

That is such an immensely complicated question. Talk to an engine builder. I was actually reading an article the other day about knocking and pre-ignition which went into incredible detail about how compression and boost affect the knocking process. If you get the compression wrong you can lose a lot of power just because the timing and leaning has to be backed off to prevent knocking. And since your car's probably going to be a rocket (ie: high power per litre capacity) it becomes even trickier.

I might be totally wrong and this is a popular set of choices and a popular set up though, in which case I'm sure I'll be corrected. Should the boost be picked after the compression?

I'll see if I can dig up the article. Was written by an engine designer for chev I think.

That is such an immensely complicated question. Talk to an engine builder. I was actually reading an article the other day about knocking and pre-ignition which went into incredible detail about how compression and boost affect the knocking process. If you get the compression wrong you can lose a lot of power just because the timing and leaning has to be backed off to prevent knocking. And since your car's probably going to be a rocket (ie: high power per litre capacity) it becomes even trickier.

I might be totally wrong and this is a popular set of choices and a popular set up though, in which case I'm sure I'll be corrected. Should the boost be picked after the compression?

I'll see if I can dig up the article. Was written by an engine designer for chev I think.

Interesting yer i 'd like to have a read of that if you manage to find it.

Think blackstar has been doing some research into this so he is probably the best to ask.

Found it.

http://www.contactmagazine.com/Issue54/EngineBasics.html

It's a bit of a mission read (well it was on my smartphone) but there's a lot of good, clear info in there. Even for those who understand detonation and pre-ignition, there's a lot of interesting bits and pieces in there

Think blackstar has been doing some research into this so he is probably the best to ask.

Blackstar has turned off his PM's.


Found it.

http://www.contactmagazine.com/Issue54/EngineBasics.html

It's a bit of a mission read (well it was on my smartphone) but there's a lot of good, clear info in there. Even for those who understand detonation and pre-ignition, there's a lot of interesting bits and pieces in there

Cheers for that.

Did a quick search, and boost isn't really mentioned :$. The point is that choosing compression for a particular engine is something much more complicated than "low compression for high boost and high compression for NA". That's why it's probably good to speak to an engine builder who knows these engines (MadMagna maybe, or he might know someone?) or an experienced tuner, just so you don't end up sacrificing like 10% of the torque due to detonation.

I've spoken with Mal he suggested 9.5 however my engine builder suggested 8.5 and i'm just waiting on a response from my tuner but going by this trend he'll say 10.3 lol

I would go the 8.5 also.... cbf explaining, but ill speak to you about it when I get back....

I've spoken with Mal he suggested 9.5 however my engine builder suggested 8.5 and i'm just waiting on a response from my tuner but going by this trend he'll say 10.3 lol

Ahh, that sucks. What have people done before?

another vote for 9.5 here

Ahh, that sucks. What have people done before?

There's not a lot of info out there that's close to my build so it'll just have to be educated guess work.

It's 8.5 from me.

Cheers Dave

Just going by the fact that the TMR 380's ran std engines with the sprintex, now I know that these are a totally different kettle her ebut lets face it, with the work being done to this engine I can not see why it would not easilly take it, will be a long way from a standard engine.

Is a hard one to be honest, you are nly going to 12 PSI and even that will not get there until the higher range so the lower range will be less boost and this is where you will be 90% of the time

What is the stock compression btw?

10:1.

10:1.

9:1 in a std 3.5 and 9.4:1 in the Ralliart engine.

Another epic read but might help?

http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html

The only real way to know which will give the best power would be to do back to back tests unfortunately. How much has been done to the engine? I assume a fair bit to accommodate the supercharger?

Mal's right (for a change :p) with what he said above though, if you go 8.5 you might end up wanting more boost, if you go 10.3 you might just end up knocking. At least 9.5 isn't an extreme increase in compression over stock, and means the margin for improvement if it's not the right call isn't as extreme, ie: power sacrifice is minimal, it could have been worse.

I get the feeling people don't pay huge attention to maximising the usable compression when boost and timing can be corrected anyway, the only real way to tell is by bench testing.

9:1 in a std 3.5 and 9.4:1 in the Ralliart engine.

It's a 6G75 so 10:1.

The only real way to know which will give the best power would be to do back to back tests unfortunately. How much has been done to the engine? I assume a fair bit to accommodate the supercharger?

Mal's right (for a change :p) with what he said above though, if you go 8.5 you might end up wanting more boost, if you go 10.3 you might just end up knocking. At least 9.5 isn't an extreme increase in compression over stock, and means the margin for improvement if it's not the right call isn't as extreme, ie: power sacrifice is minimal, it could have been worse.

I get the feeling people don't pay huge attention to maximising the usable compression when boost and timing can be corrected anyway, the only real way to tell is by bench testing.

This is exactly why I was voting for the 8.5 because you always want room to play. Between the 8.5 and 9.5 you are not going to overly notice too much performance loss, its still going to be a world of hell on full sing. But by having the 8.5 later down the road, this will allow tyson a greater tuning option if he wishes to go more boost and/or a different turbo setup. Trust me from experience that when the boost bug has bitten, you become like a crack addict and WANT MORE!


It's a G675 so 10:1.

As above comment dude, this is going to be fun :)

It's a 6G75 so 10:1.
Now thats clear. I thought you had a 3.5, my mistake.

I agree with dave tj. 8.5:1

If it's a 6G75 listen to DaveTJ - he races them, c'mon guys.

Some quick calculations. 3.8 with 8.5 comp and 12 psi boost; boosted comp = 11.10:1
3.8 with 9.5 comp and 12 psi boost; boosted comp = 12.46:1
3.8 with 10.3 comp and 12 psi boost; boosted comp= 13.55:1

These engines are very efficient so they are very knock limited around the peak torque area in N/A for already thats why I'd go 8.5 it will give you a bit of margine 9.5 or more will require a good intercooler to keep the charge temp down.

Hope this helps, Cheers Dave

Sorry I'm guessing you will be using 98 fuel? E85 is a totally different ball game.

Cheers

sr20det commonly run upto 9.5:1 @1.5bar these days for street carrs because going down from the stock 8.5:1 just makes it unneccesarily laggy. I would say the 6g75 being made 15years later would be technically more efficient then the old nissan design and thus 9.5:1 is not going to be a stretch for the engine and is also going to acheive more power with less boost at a start, and if they are 'raced'and not just 'dragged' then the chance of wanting more response over more peak hp is a better idea and suits the applicatoim better. In all honestly picking a middle ground is probably best, since if you down the track want more boost/power you can either raise CR by shaving the head or lower the CR with a .3mm thicker headgasket and add more boost.

The only real way to know which will give the best power would be to do back to back tests unfortunately. How much has been done to the engine? I assume a fair bit to accommodate the supercharger?Mal's right (for a change :p) with what he said above though, if you go 8.5 you might end up wanting more boost, if you go 10.3 you might just end up knocking. At least 9.5 isn't an extreme increase in compression over stock, and means the margin for improvement if it's not the right call isn't as extreme, ie: power sacrifice is minimal, it could have been worse.

I get the feeling people don't pay huge attention to maximising the usable compression when boost and timing can be corrected anyway, the only real way to tell is by bench testing.

Ports cleaned up,HD springs,272deg cams,pistons,rods,cam gears etc etc.



this is going to be fun :)

Certainly hope so :D


Now thats clear. I thought you had a 3.5, my mistake.

I agree with dave tj. 8.5:1

All good mate,i should have been clearer.


Some quick calculations. 3.8 with 8.5 comp and 12 psi boost; boosted comp = 11.10:1
3.8 with 9.5 comp and 12 psi boost; boosted comp = 12.46:1
3.8 with 10.3 comp and 12 psi boost; boosted comp= 13.55:1

These engines are very efficient so they are very knock limited around the peak torque area in N/A for already thats why I'd go 8.5 it will give you a bit of margine 9.5 or more will require a good intercooler to keep the charge temp down.

Hope this helps, Cheers Dave

Thanks Dave,will be using W2A and possibly water/meth injection if it needs it.


Sorry I'm guessing you will be using 98 fuel? E85 is a totally different ball game.

Cheers

I'm really keen on running E85 however there could LPG ECU issues because E85 uses around 30% more volume then petrol so if my maths are right the LPG ECU will be injecting 36% more gas then needed and there'd be issues getting the tune right for both fuels.

Ok engine builder reckons 8.5 but the tuner says 9.5,potentially will i be losing much power if i go 8.5 over 9.5?

Lower comp will give you more headroom. I would stick with the lower compression ratio. You'll loose a bit more power, but the advantage is you'll be able to run more boost

;1369219']Lower comp will give you more headroom. I would stick with the lower compression ratio. You'll loose a bit more power, but the advantage is you'll be able to run more boost

Go 10.3 and High boost, if engine breaks mal will sell you a new one. :)

boost wins over NA any day,, so you are better off goin lower compression and more boost than going high compression and being capped to a boost amount...

If i was going turbo then yer i'd go 8.5 no question but i can't really go past 12psi on the rotrex.

I would still go for 8.5 dude. We have 40+ deg days here and unless your going W2A straight up, you would want to play it safe... still go 8.5 then maybe you can go more than 12.5 later down the road with design refinements, ya never know, next year they bring out a 15psi pulley and you think SHIT which I went the 8.5....

I would still go for 8.5 dude. We have 40+ deg days here and unless your going W2A straight up, you would want to play it safe... still go 8.5 then maybe you can go more than 12.5 later down the road with design refinements, ya never know, next year they bring out a 15psi pulley and you think SHIT which I went the 8.5....

Hey Ben i'm talking about the compression ratio.

If i was going turbo then yer i'd go 8.5 no question but i can't really go past 12psi on the rotrex.

I'd go 9.5 - since the rotrex is centrifugal, you'll only hit that 12 psi sometimes at redline. 8.5 you'll lose low down power with no way to get it back with boost. Here are some numbers i datalogged with the rotrex:

3035 RPM 3.0 PSI
3163 RPM 3.2 PSI
3251 RPM 3.3 PSI
3367 RPM 3.6 PSI
3513 RPM 3.8 PSI
3684 RPM 4.1 PSI

I'd go 9.5 - since the rotrex is centrifugal, you'll only hit that 12 psi sometimes at redline. 8.5 you'll lose low down power with no way to get it back with boost. Here are some numbers i datalogged with the rotrex:

3035 RPM 3.0 PSI
3163 RPM 3.2 PSI
3251 RPM 3.3 PSI
3367 RPM 3.6 PSI
3513 RPM 3.8 PSI
3684 RPM 4.1 PSI

That's quite helpful thanks,do you have the numbers for higher up in the rpm?

By the way which model frozen boost W2A core do you have?

no worries. sorry i only did a log up to 3700 rpm, but it basically ramps up after that in a linear-ish fashion.

W2A intercooler is a Type-14, heat exchanger is a type-117 (which needed to be cut back to fit btw) + the bosch water pump :cool:

no worries. sorry i only did a log up to 3700 rpm, but it basically ramps up after that in a linear-ish fashion.

W2A intercooler is a Type-14, heat exchanger is a type-117 (which needed to be cut back to fit btw) + the bosch water pump :cool:

Cool thanks mate :D

Have you brought your rotrex kit yet? I have one four sale brand new charger and water air cooler setup.. everything you need..

Have you brought your rotrex kit yet? I have one four sale brand new charger and water air cooler setup.. everything you need..

No not yet,gotta keep the cash in reserve for the engine build as i don't wanna be caught short..could be interested in a few months though.

Why you selling?

I settled on 8.5CR and 12 psi boost.

The rebuilder said the motor would be indestructible for street use up to 15 psi and beyond.


What LPG kit are you using?

Chances are you will need to auto switch to ULP98 from about 2-3000 rpm.....no fuel pump on LPG so you can only use the delivery pressure.

Watch out when the LPG level starts getting low...pressure drops way down and will lean out above 15AFR.

Set the changeover to just under a 1/4 tank and you'll be sweet.

You'll ned the largest injectors they have....Please don't tell me you are using a mixer ring......

I settled on 8.5CR and 12 psi boost.

The rebuilder said the motor would be indestructible for street use up to 15 psi and beyond.


What LPG kit are you using?

Chances are you will need to auto switch to ULP98 from about 2-3000 rpm.....no fuel pump on LPG so you can only use the delivery pressure.

Watch out when the LPG level starts getting low...pressure drops way down and will lean out above 15AFR.

Set the changeover to just under a 1/4 tank and you'll be sweet.

You'll ned the largest injectors they have....Please don't tell me you are using a mixer ring......

As below.


August 08 i'd had enough of paying $80 for a tank of petrol so i decided to take advantage of the government rebates and had injected LPG fitted,after chucking it on the dyno i only lose 3.8hp@wheels going from petrol to gas.

Oh thanks...I missed that post....

I found that at certain points in the dyno curve LPG made more Kw than petrol....
( it was 100% propane though and only a few service stations stock it)

Good advice thanks,yer running a Prinns injected lpg system and the regulator is the forced induction model also at the time i had them fit Keihin yellow dot injectors.

Theres only one servive station in perth that sells 100% propane so having it tuned for 100% propane isn't an option.

Good advice thanks,yer running a Prinns injected lpg system and the regulator is the forced induction model also at the time i had them fit Keihin yellow dot injectors.

Theres only one servive station in perth that sells 100% propane so having it tuned for 100% propane isn't an option.


When your build is done try a tankfull anyway...it's 110 octane, colder than petrol and under pressure...win,win,win

Here's a vid of mine early in the mods program (no extractors etc)

http://www.youtube.com/watch?v=kw3fmaso1f4&feature=mfu_in_order&list=UL

When your build is done try a tankfull anyway...it's 110 octane, colder than petrol and under pressure...win,win,win

Here's a vid of mine early in the mods program (no extractors etc)

http://www.youtube.com/watch?v=kw3fmaso1f4&feature=mfu_in_order&list=UL

Will do,nice vid!