| Author | Topic |
|---|
Location:
Rocklea qld
Registered:
February 2003
|
|
compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 10:49
|
 |
|
been talking to the engine rebuilder who is oringing my block and he thinks my compression may be to low...but how low is to low. the engine is a 18rgeu. the black has been faced and so has the head. its oringed with a 2mm copper head gasket and 18rc pistons...anyone got any idea on thne rough compression id be expecting..should i go with rg pistons in place of the rc??? i want it to run to over 20 psi in the end.
|
|
|
Location:
Toronto, Downtown
Registered:
September 2004
|
|
Re: compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 10:53
|
|
|
the engine builder should be able to tell you the compression the engine has. I would say dont go any lower than 8.0:1 if you want some half decent boost response.
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 11:18
|
|
|
the engine rebuilder wants $50 and says the engine has to have bottum assembled to do it..i dont want to assemble it and then he says its way to low and i gotta pull it apart to swap pistons. as forf 8.0-1 i may as well run it stock as its about 8.3-1 i think stock...most people run about 7.0-1 but he thinks its gonna lower then that and not respond properly until its on boost. i may just run stock rg pistons i think unless anyone else can give feedback on what there running
|
|
|
Location:
Toronto, Downtown
Registered:
September 2004
|
|
Re: compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 11:37
|
|
i must say i have no exp with that engine but hell 7.0:1 will be a bit sluggish off boost wouldnt it considering the size of the turbo u will need to run 20psi.
|
|
|
Location:
Brisbane
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 12:43
|
|
|
my 18Rturbo is 7.4:1 - with the oldskool T04 it got on boost at just under 3k
|
|
|
I supported Toymods
Banned User
Location:
Brisbane
Registered:
May 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Wed, 02 March 2005 12:48
|
|
|
The engine doesn't need to be fully assembled to do a compression ratio check! You just need to fit the crank with one rod and piston. You don't even need the rings to be fitted. Same goes for the head, you just need the spark plug fitted and the valves just sitting in.
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Thu, 03 March 2005 07:41
|
|
|
sounds like a plan...im thinking off keeping the rg pistons now anyway as i think there stronger then the rc onesa to get me going for now
|
|
|
Location:
Toronto, Downtown
Registered:
September 2004
|
|
Re: compression...how low is to low (boosted engine)
|
Thu, 03 March 2005 08:14
|
|
| thechuckster wrote on Wed, 02 March 2005 23:43 |
my 18Rturbo is 7.4:1 - with the oldskool T04 it got on boost at just under 3k
|
not talking about when it spools up but how much power it will make off boost if you have good comp then it will seem less laggy although the turbo will spool up at the same time
|
|
|
Location:
Brisbane
Registered:
May 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Thu, 03 March 2005 11:21
|
|
Anything less than 8.5:1 is silly and unnecessary. Alloy head and hemispherical combustion chambers means you don't have to go ridiculously low compression like the old-school iron V8 boys have to.
|
|
|
Toymods Social Secretary
Location:
Sydney
Registered:
July 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Thu, 03 March 2005 23:24
|
|
teehee.... im running a 10.5:1 compression turbo engine... its a 4A though 
Im with norbie, anything below 8:1 is silly. 8.5-9.0:1 would be ideal... and so would forged pistons dude!!!
|
|
|
Location:
Brisbane
Registered:
October 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 06:22
|
|
|
just buy a burette and do it yourself, it'll always come in handy
|
|
|
Location:
Brisbane
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 06:31
|
|
| 4agte wrote on Thu, 03 March 2005 18:14 |
| thechuckster wrote on Wed, 02 March 2005 23:43 |
my 18Rturbo is 7.4:1 - with the oldskool T04 it got on boost at just under 3k
|
not talking about when it spools up but how much power it will make off boost if you have good comp then it will seem less laggy although the turbo will spool up at the same time
|
was reasonably perky - probably due to excessively high timing and the carbi feeding too much fuel 
the uber low compression was to let it deal with non-hemi chamber, steel, non-cross-flow head. It was also at the suggestion of the original supplier of the turbo. long story ... <grrr>
is being replaced by a smaller & newer turbo and some custom EFI gear to improve response and tunability (of which it had none of the later). It should spool up quicker but produce less boost.
|
|
|
Location:
Toronto, Downtown
Registered:
September 2004
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 07:16
|
|
| thechuckster wrote on Fri, 04 March 2005 17:31 |
| 4agte wrote on Thu, 03 March 2005 18:14 |
| thechuckster wrote on Wed, 02 March 2005 23:43 |
my 18Rturbo is 7.4:1 - with the oldskool T04 it got on boost at just under 3k
|
not talking about when it spools up but how much power it will make off boost if you have good comp then it will seem less laggy although the turbo will spool up at the same time
|
was reasonably perky - probably due to excessively high timing and the carbi feeding too much fuel
the uber low compression was to let it deal with non-hemi chamber, steel, non-cross-flow head. It was also at the suggestion of the original supplier of the turbo. long story ... <grrr>
is being replaced by a smaller & newer turbo and some custom EFI gear to improve response and tunability (of which it had none of the later). It should spool up quicker but produce less boost.
|
sounds cool wasnt trying to have a go at ya just clarifying what i was saying
|
|
|
Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 07:39
|
|
| Mr DOHC wrote on Fri, 04 March 2005 17:22 |
just buy a burette and do it yourself, it'll always come in handy
|
www.livingstone.com.au will deliver one (blass B glass, teflon stopcock) for about $50, excellent value. i've gotten a few from them so far
|
|
|
Location:
On your mum!
Registered:
May 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 08:51
|
|
I use a large syringe and a piece of 5mm acrylic to cc the head. It is not the quickest method but it is effective.
I've said it before and I will say it again - cylinder pressure is not entirely governed by the compression ratio, this is where cam timing comes - short duration cams minimal overlap for low comp engines (although this may hamper top end). The honda guys running in the states are reported to run 11:1 comp and 60 psi boost - if they did this with a cam developed for an 8.0:1 engine then the starter would never crank the engine over.....
Seriously how many people have driven a reasonably late model EFI car and have been able to pick whether the compression ratio was lowish or highish?
That said - I run about 8.4:1 and I will agree with what has been written with modern engines and the available fuels you don't need to go too low.
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 09:48
|
|
|
after speaking to the engine rebuilder he basically said the same thing...he said run standard pistons and just use my copper head gasket and leave it at that for now and do not go any lower...he also said i should advance the exhaust cam a touch to get less over lap or something but that will be something he does once i finish building the engine...now that its oringed the next step is to take the parts in for balancing and go from there....
|
|
|
Location:
sydney
Registered:
March 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 11:43
|
|
half the engines produced these days are close to 8
i had 7 to 1 in my fj20t and i didnt have too much lag
mind u that made me able to pump boost up to 30 with good fuel and octain booster
so on one had its good on the other its not
u have to compromise
just another thing
make sure u can use a copper head gasket on an o ringed block
i have heard some stories years ago bout them leaking on oringed blocks
maybe theyre a new way these days of doin things
but best to make sure
|
|
|
Location:
Brisbane
Registered:
May 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 15:19
|
|
Modern turbo engines are way past 8:1... the late-model 1JZ and 2JZ are 9:1 and a lot of Euro stuff is around 10:1. You won't find anything below 8 without looking at old gear like FJ20's and 3T's.
|
|
|
Location:
sydney
Registered:
November 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Fri, 04 March 2005 21:15
|
|
|
i run 8:1 and 20psi in my car. Its fine to drive around off boost.
|
|
|
Location:
Adelaide
Registered:
June 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Sat, 05 March 2005 00:08
|
|
Yeah, I was speaking to a well respected engine tuner about the same thing, he said that 7:1 is very seventies. The engine is pretty flat until the boost comes on really hard, then it's time to change gears.
You won't get anywhere near the linear curve that makes for the best drivability.
If you want a really good kick in the back, low compression will do it for you.
I have run some figures for you through the "Compression ration calculator".
Note, this quite theoretical, but takes into account head cc's (72ml), head gasket dimensions to suit pistons and deck clearance (piston distance below deck)
88.5 bore = 7.085:1 with 18RC pistons.
89 bore = 7.15:1 with 18RC pistons.
92 bore = 7.19:1 with $600 Forged VW 311 Bigbore (non stroker)
90.5 bore = 7.05:1 with $600 Forged VW 331 Bigbore (non stroker)
Note: the VW pistons come as a kit with rings etc, 22mm pin, Cylinders (aircooled). It is suggested that you get teflon gudgeon pin hole blockers.
Note 2: The VW pistons come in two compression heights:
- 39mm (non-stroker kit) will give you a .5mm deck
clearance
- 36mm (stroker kit) will give a 3.5mm deck clearance
and therefore shit all compression (and I mean shit!)
For US$600 plus ~US$60 freight (~$850AUS), Toysport will do you up some forgies that have an 18RGEU dome on a 92 bore (ends up being about 8.7:1) These will also have big valve releifs (2mm wider, 1.5mm deeper if I remember correctly.
I hope that helps.
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Sat, 05 March 2005 11:09
|
|
most people i have talked to suggest that if you are running a copperheadgasket you should oring the block. And run around every port with 3bond.
basically this is a seventies engine really so i should stick to what was done back then...no point really running it like a modern engine if itsa not a modern engine. but on the other hand its modern parts going in it...ill have another ring around for more info but at this stage id say stock internals are the go with the copper headgasket and oringing.
|
|
|
Location:
Adelaide
Registered:
June 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Sat, 05 March 2005 11:57
|
|
Yeah-I concur, 8.5 should be fine-any lower and she'll be a tad sluggish off boost. My 5M psycho motor was around 7.5, and off boost it was a tad casual.
If you are going to the CHG setup, make sure the builder pays really good attention to the head and block prep. Try to get the head surface almost mirror smooth, as well as the finish on the block. A really fresh cutter on the machine and slowing the feed right up is a good way to start, followed by an absolutely flat aluminium section and superfine wet&dry paper (and elbow grease...). This reduces the small passages for gas to get between the CHG and head and block, and also improves the sealing of water/oil galleries. You don't have to o-ring it, but it will make it even stronger. Head studs are also a VERY good investment as you can really give the setup a definite crush, instead of just stretching the stock head bolts...Careful prep can ensure set-and-forget-it, the alternative being ongoing headaches with a CHG..
And someone mentioned 3Bond-brilliant stuff.
A really good bloke to call and get advice from about prepping is the bloke at Copper Head Gaskets here in Adelaide (Somerton Park).
Anyway-take it easy
Sean
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Sat, 05 March 2005 12:06
|
|
[quote title=Steve M wrote on Sat, 05 March 2005 03:08]Yeah, I was speaking to a well respected engine tuner about the same thing, he said that 7:1 is very seventies. The engine is pretty flat until the boost comes on really hard, then it's time to change gears.
You won't get anywhere near the linear curve that makes for the best drivability.
If you want a really good kick in the back, low compression will do it for you.
I have run some figures for you through the "Compression ration calculator".
Note, this quite theoretical, but takes into account head cc's (72ml), head gasket dimensions to suit pistons and deck clearance (piston distance below deck)
88.5 bore = 7.085:1 with 18RC pistons.
89 bore = 7.15:1 with 18RC pistons.
92 bore = 7.19:1 with $600 Forged VW 311 Bigbore (non stroker)
90.5 bore = 7.05:1 with $600 Forged VW 331 Bigbore (non stroker)
so can u use the same calculater to figure what the compression will be with a 88.5mm bore and include a 2mm head gasket...
|
|
|
Location:
On your mum!
Registered:
May 2002
|
|
Re: compression...how low is to low (boosted engine)
|
Sun, 06 March 2005 06:54
|
|
There has been much written about using thick head gaskets as being one of the worst ways to achieve a lowering in comp ratio (and it looks like you will be trying to get it up a shade anyway).
When using O rings with a cast iron block and aluminum cylinder head, I suggest (based on study of lots of boosted engine builds inc top fuel) that the head be O ringed and a receiver groove cut into the deck - 3 bond around all water and oil passages.
|
|
|
Location:
Toronto, Downtown
Registered:
September 2004
|
|
Re: compression...how low is to low (boosted engine)
|
Sun, 06 March 2005 08:12
|
|
| fatmr2 wrote on Sat, 05 March 2005 08:15 |
i run 8:1 and 20psi in my car. Its fine to drive around off boost.
|
but your used to not making any power anyways fat 
|
|
|
Location:
Rocklea qld
Registered:
February 2003
|
|
Re: compression...how low is to low (boosted engine)
|
Sun, 06 March 2005 09:44
|
|
|
does anyone know the thickness of a standard headgasket??? im considerin on of them now im hearing other stories about the copper ones. but on the other hand clancey from the forum ran a copper one and had troubles. as soon as he oringed it he had no more problems with blowing gaskets.. other thing i can consider is run rc pistons with standard gasket. how strong are the rc pistons compared to the rg ones. does anyone know of a rg headgasket that has metal rings in the centre that are actually circles and not wierd out of shape circles..everytime i got one they were cheap and nasty. other i guess that has to come into it is how much has been taken from the block and head when getting machined.....but from what im hearing i should try to keep up the comp so i guess standard is gonna be better?
|
|
|
