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dezertdoug
Re: Lower compression ratio needed
#13
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2003/6/18 5:05
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thanks again for the replies more info
cam shaft is set at zero deg witch is no ! mark on aftermarket gear the timming of dist is 24 at full boost is that to much?do you think the carb is enoughfor such a motor?just tell me what pistons, head gasket combo and carb (year make & mode) and we'll go that way. we dont want to go back to carb only we want H.P.!but dependable H.P.what timming? all answers are appr.I know everybody has their own opinion but i will take what looks the most correct to my knowledge help we want to go off roading soon

Posted on: 2003/6/19 4:24
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naukkis
Re: Lower compression ratio needed
#12
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Quote:

dezertdoug wrote:
hello I'm deans brother thanks for all the replies.hear is the info we are working with;
bore 86.5 already over1.5mm or.060
stroke 86
head 36 cc's this a SSS head from nissan
deck hgt .018
stk gasket is 1.5mm
dish piston .120 deep 2.710 dia
CR looking for is 7.5-1\8.0-1


STD gasket is only 1.2mm when compressed. With those parts you got about 11:1 comp, just right for naturally aspirated engine. (min volume = chamber + piston dish + gasket - deck height as pistons come over deck height) is about 36cc + 12cc + 5 - 2 = 51cc

Comp ratio is (500+51)/51 = 10.8

Quote:

15 psi boost with intercooler max adv of timming of 24 deg.cam is 112 * special turbo grind


How much advance is retarted when on boost? With those comp and boost there should be very little advance in boost, probably under 10 degrees as mixture burns very rapidly at high pressure.

Quote:

this melted the edge of the pistons within a short time no other damage was done fuel sytem is a weber progesive 36\42 with 100 octane fuel holley blue fuel pump with
if you more info contact me please we are eager to get this running correct thanks doug


Change that SSS-head to standard A87 closed-chamber which has about 43cc chamber. That itself lowers comp ratio under 10: and chamber should be ported for better flow which also increases volume by few cc. And remember that with high comp ratio and high boost needed ignition advance under boost is minimal, with 24 degree advance engine probably won't detonate but as mixture burns too early pistons can't handle heat.

Posted on: 2003/6/18 12:00
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nutcase
Re: Lower compression ratio needed
#11
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L-20 crank and rods Z-20 pistons t4 garrett and a 750cfm holley 25psi boost at nearly 500hp through a gm power glide to a ford 9inch
11 sec 1/4 miles with this setup in Ron Folck's 1200 I'd say go with what others have proven on the track.

Posted on: 2003/6/18 10:58
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Phil (aka nutcase)
I have C.L.W.E.A.
Can't Leave Well Enough Alone
120y exelant clone gone :(
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2332owner
Re: Lower compression ratio needed
#10
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Says he's using 100 octane fuel. Sounds like rgrinder's on the right track but also sounds like it's not getting enough fuel and running too lean. And like he said it could be a combination of things.

Posted on: 2003/6/18 8:14
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rgrinder
Re: Lower compression ratio needed
#9
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what is your initial timing set at? how quick is your advance curve? what grade of fuel are you using? what plugs and heat range are you using? is the cam timing advanced?

it sounds to me that your spark plugs are the wrong heat range and retaining too much heat in the combustion chamber, or possably thhat your advance curve is too quick there by causing preignition, or maybe a combination of both.

Posted on: 2003/6/18 6:35
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Re: Lower compression ratio needed
#8
Guest_
could you not just, for cost and simpicitys sake, just make a spacer plate with a head gasket each side between the block and head. Rudimentary but effective in old style engines. It will get you that 4mm space you need.

Posted on: 2003/6/18 5:51
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dezertdoug
Re: Lower compression ratio needed
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hello I'm deans brother thanks for all the replies.hear is the info we are working with;
bore 86.5 already over1.5mm or.060
stroke 86
head 36 cc's this a SSS head from nissan
deck hgt .018
stk gasket is 1.5mm
dish piston .120 deep 2.710 dia
CR looking for is 7.5-1\8.0-1
15 psi boost with intercooler max adv of timming of 24 deg.cam is 112 * special turbo grind
this melted the edge of the pistons within a short time no other damage was done fuel sytem is a weber progesive 36\42 with 100 octane fuel holley blue fuel pump with
if you more info contact me please we are eager to get this running correct thanks doug

Posted on: 2003/6/18 5:26
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naukkis
Re: Lower compression ratio needed
#6
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Quote:

nutcase wrote:
if you are looking to lower the comp big time I would look into getting some z20 pistons and install them on your rods this will lower your comp with the open head to 6.5:1 and 7.5:1 with the closed head. 20 to 25psi is not out of the question with that comp ratio


Absolutely not. Low compression ratio is one goal but other goal is high turbulence wedge combustion chamber. L20B stock pistons are great with closed chamber A87 or W53 head with normal high flow modifications in combustion chamber, in normally aspirated engine problem is to get comp high enough without exensive high dome piston buty with turbo setup comp ratio would be just right 7,5-8:1.

If you had well made high compression head use it with naturally aspirated engine or sell it to somebody but with turbo setup head flow is secondary, good combustion chamber is primary goal and even stock closed-chamber head is superior compared to good flowing head with some 4mm head gasket or Z20 pistons which won't reach deck level.

Posted on: 2003/6/17 22:25
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naukkis
Re: Lower compression ratio needed
#5
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Quote:

rgrinder wrote:
SIMPLE, use L28 Turbo pistons from an 81-82 280ZX, std. size for them is 86.00 mm where as standard bore for an L20B is 85.00 mm. Obviously your block will need to be bored, but other than that it's a straight bolt in item, this should net you between 8.0 and 8.5 compression ratio.


L20B stock pistons have bigger dish than 280ZX turbo pistons.

Posted on: 2003/6/17 22:16
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nutcase
Re: Lower compression ratio needed
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if you are looking to lower the comp big time I would look into getting some z20 pistons and install them on your rods this will lower your comp with the open head to 6.5:1 and 7.5:1 with the closed head. 20 to 25psi is not out of the question with that comp ratio

Posted on: 2003/6/17 11:16
_________________
Phil (aka nutcase)
I have C.L.W.E.A.
Can't Leave Well Enough Alone
120y exelant clone gone :(
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