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Few Noob Qs about Compression & Forced Induction 

 

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 Post subject: Few Noob Qs about Compression & Forced Induction
Posted: Mon Sep 15, 2008 2:07 am 
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Just like to run through a few things re compression and turbos..

The higher the compression the closer the compression ratios? Eg. 1:1 VERY high compression?

High compression is bad for forced induction due to the possiblity of the turbo say, blowing a hole in the side of the head? Lower the compression (but not too low) the better?

But for an N/A setup the higher the compression the better so that none of the explosion and force is wasted right?

What's kind of compression is starting to get too high for a turbo setup?

Head & Cam packages usually raise the compression of an engine, which is bad? But there are specific head and cam packages avalible for turbo setups?

Are there bottom end upgrades packages avalible? Or is the only real thing out there getting your piston & conrods forged?



I know there are alot of questions but if anyone has time to even answer one of them I would be greatful.

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Posted: Mon Sep 15, 2008 2:59 am 
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Quote:
1:1 VERY high compression?


1:1 is no compression at all..

Look at it this way..

If your cylinder is at the bottom of it's stroke, and you measure the volume of the cylinder and head above the piston, this is the first number.

Then you do the same thing with the piston at top dead center, this is the second number..

Use those figures to work out the ratio of the two figures and you have your compression ratio, for example, say the BDC volume was 100 cubic CC's, and the TDC volume was 10cc's.

100:10

Divide both numbers by the lower number:
10:1

bobs your uncle.


This is not to be confused with compression pressure which is the actual compression caused by that ratio, minus whatever losses there are in your engine.


As for the blown part of it... look at it this way..

the more a fuel air mixture is compressed, the more chance there is of the mixture prematurely igniting.. which is a problem if your piston is only halfway up the bore going up because the expanding mixture will try to push it down again.

If you blow an engine, it's like you are already pushing compressed air into the engine (because you are) so the compressed mixture gets the same ratio of compression as before, except it was already compressed so the end result is higher. obviously this means there is much more likelihood of detonation.


looking at it in terms of pressure rather than ratio..

If your machine has a compression pressure of 185psi and you have already got 10psi of pressure going into the cylinders, then that 10psi will obviously be combined with the 185psi.... now if the normal compression ration increased normal air pressure up by 185psi, it would be considerably higher if the starting point was already 10psi higher to start with.


make sense?
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Posted: Mon Sep 15, 2008 11:36 am 
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You could turbo a std engines compression ... BUT you could possibly only run up to 10Lb or so..
Lowering comp to around 8.5 - 8.8 [std comp on early e series] You can run up to 18Lb with good tune..
Lower comp gives wider safety on tune, being that it mainly detonation that kills forced induction engines..
Keep in mind that std comp AND std pistons are not as strong as forged pistons. Which is what most people fit when lowering comp..
If you wanted a quick responsive throttle etc a forged 9- 9.2 to 1 with good tune and detonation sensor would work fine...
But total power would be limited. I guess to around 270 rwkw, as I said it would spool quick and very streetable with bags of low end torque..
A cam in this situation can bleed off compression, depending on cam specs..
Spool rods are one of the cheapest around..
http://www.spoolimports.com/content/SPO ... ONRODS.asp
They are B.A spec.. They have package for Spool BA rods and C.P - A.U spec pistons with the smaller B.A floating in pin size etc..
Another part that needs attention is the head gasket, AU MLS will do fine. Better still fit some head studs which clamp down harder due to finer thread and actually pulling from top of stud and not twisting whole bolt ....
Work out what power level you want ? Then work your build around that !!
Generally...
Going lower than 8.8 comp if you want power over 330 rwkw..

 

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Posted: Mon Oct 20, 2008 2:45 am 
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EBXR8380 wrote:
Spool rods are one of the cheapest around..
http://www.spoolimports.com/content/SPO ... ONRODS.asp
They are B.A spec.. They have package for Spool BA rods and C.P - A.U spec pistons with the smaller B.A floating in pin size etc..



Hey are u saying that spool rods will fit in an au :shock: :D sorry if i miss under stood something there

 

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Posted: Mon Oct 20, 2008 9:11 pm 
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horses4courses wrote:
EBXR8380 wrote:
Spool rods are one of the cheapest around..
http://www.spoolimports.com/content/SPO ... ONRODS.asp
They are B.A spec.. They have package for Spool BA rods and C.P - A.U spec pistons with the smaller B.A floating in pin size etc..



Hey are u saying that spool rods will fit in an au :shock: :D sorry if i miss under stood something there


yes, they are the same in size/dimension

 

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Posted: Mon Oct 20, 2008 11:01 pm 
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For the BA rods to fit you require CUSTOM pistons with the smaller gudgeon pin...I guess due to full floating pins?
In most cases a custom piston is required or NOT much more expensive than special order ..

 

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As in ZOOM 126 edition
331 Dart block,3.25/ 4340 steel crank, Oliver rods,TFS ported track heat heads, TFS track heat inlet Twin SC61 turbo's
Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

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Posted: Tue Oct 21, 2008 10:24 pm 
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There's some bloody good info there guys, thanks heaps! :wink: I certainly learned a thing or three.
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Posted: Tue Oct 21, 2008 11:58 pm 
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Spool and go through http://www.qri.biz/brands_cp.html
to get custom C.P turbo pistons.. They should have order number etc for this combo..

 

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As in ZOOM 126 edition
331 Dart block,3.25/ 4340 steel crank, Oliver rods,TFS ported track heat heads, TFS track heat inlet Twin SC61 turbo's
Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

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Posted: Wed Oct 22, 2008 5:56 am 
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sweet, thats something good to know. does any one know how good these
spool rods r, seriously thinkin about getin some. should i be able to get pistons to lower my compression down to say 8.5. lookin for a rliable
270 - 280rwkw motor.

Chers :D

 

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Posted: Wed Oct 22, 2008 10:07 am 
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They are used a lot in drift cars so I guess they are fine...
Just looking at them and comparing to std they look much stronger..Given that std rods can take 300 odd rwkw these rods should well and truly do the job... This brand is used on RB26 etc I don't hear any negative results given that they are fitted to modified engines..
Quantum have my AU piston up there now so if you order he'll know what you want...
8.8 compression will do fine with strong forged parts imo
Still make safe power and good low rpm throttle response off boost ...

 

 

Attachments:
AU CP pistons spool rods 001 (Medium).jpg
AU CP pistons spool rods 001 (Medium).jpg [ 67.83 KiB | Viewed 852 times ]

 

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As in ZOOM 126 edition
331 Dart block,3.25/ 4340 steel crank, Oliver rods,TFS ported track heat heads, TFS track heat inlet Twin SC61 turbo's
Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

Last edited by EBXR8380 on Thu Oct 23, 2008 10:37 am, edited 2 times in total.

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Posted: Wed Oct 22, 2008 11:03 am 
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They just arrived while I was replying...
You have to say they are stronger..
Sorry I didn't re size pic... Done...

 

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As in ZOOM 126 edition
331 Dart block,3.25/ 4340 steel crank, Oliver rods,TFS ported track heat heads, TFS track heat inlet Twin SC61 turbo's
Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

Last edited by EBXR8380 on Thu Oct 23, 2008 10:38 am, edited 1 time in total.

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Posted: Thu Oct 23, 2008 12:16 am 
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Cool they do look good a :D.

I've just pulled a motor out of a xg ute and was wondering if i can put the spool gear in this block,
if so what do i do with dizzy can i cap it off and still use coil packs and will the au head bolt on to this block without modin anything.
I know sumps are different but don't know if it will affect any thing in bolt up.

Cheers.

 

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AUIIIXR6, Ultra low king springs, monro shocks, 19" of Chrome XHP Jets ,Snort Stage 4 turbo mainafold, GT 42r turbo, 3" stainless cooler piping, Haltech E6X, 42lb injectors,10mm Topgun leads,rising fuel reg, Bosch 040 fuel pump15psi = 283rwkw 802nm of tyre shreding touqe and a 12.6@118mph.

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Posted: Thu Oct 23, 2008 10:29 am 
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You can buy AU engines cheaper than you think...
They fit easier than you think...
This engine we have was bought on e bay for $175...
With EMS 8860 and two ignition drivers ...
Yes you have no issues in E series
The 8860 was used in production / Aussie six racing so all the ECU stuff is sorted...
They re programed ECU for free...

 

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As in ZOOM 126 edition
331 Dart block,3.25/ 4340 steel crank, Oliver rods,TFS ported track heat heads, TFS track heat inlet Twin SC61 turbo's
Project 1UZ-EF has started.. S475 Turbo 4.0 V8 Mustang Celica.....

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