ok i am trying to decide on a piston for my engine build. I want a 10:1 piston, but not having much luck finding them. so was thinking about a different piston and a cometic head gasket or some machining to make up the difference.
I found a few compression calculators, but they dont seem to make sence...
my engine (02 LD9) is supposedly a 9.7:1 engine. with a 90mm bore and a 94mm stroke, flat top pistons, 60mm combustion chanbers, .40mm head gasket
when i punch in 90mm bore and 94mm stroke, 0 dome, 0 deck, 60cc, and a .40 gasket... it tells me my STOCK engine is at 10.5... not 9.7.
is the calculator screwed up? or did i enter something wrong?
http://www.csgnetwork.com/compcalc.html
anyway... i am running about 7psi normally... can peak in 8-9psi range. but not often.... I have HP tuners, so i can tune it, but i am not a HPTuner pro yet...
I just want to get the most power out of what i have... look at my profile for my list of mods... there are a few of them...
with 7psi... i doubt i would gain anything from going less than 9.7:1 but i also like to have the room to adjust my timing, and to high a static ratio will cut into that.
i thought 10:1 would be safe... but so hard to find a 10:1 piston... i have a good source for a 9.5:1 set... but i would need to use a thinner gasket, or shave the head or block to get my compression back with that.
i could be wrong, but i think the zero deck height is where your numbers are getting messed up. did you have the block zero decked?
JBO Stickers! Get yours today!
here is another calculator that shows the same figures (pretty close)
it claims 9.99:1 ratio on stock engine based on the specs.
http://www.rbracing-rsr.com/compstaticcalc.html
3.54"
3.7"
.040
0
0
60
=
9.99 CR...
but interestingly....
if you do the math for a 91mm bore (about 40 over) and a 9.5:1 piston (2cc dome) it comes out to 9.90
add a .030 gasket (cometic) and you get 10.12
a 2cc domed piston in an otherwise stock engine would give you 10.27. (i dont know what size dome what pistons have, i just heard a 9.5:1 piston has a 2cc dish, 9.7:1 is flat top, and so i assume a 10:1 would be about a 2cc dome)
ken soggs wrote:
my engine (02 LD9) is supposedly a 9.7:1 engine. with a 90mm bore and a 94mm stroke, flat top pistons, 60mm combustion chanbers, .40mm head gasket
when i punch in 90mm bore and 94mm stroke, 0 dome, 0 deck, 60cc, and a .40 gasket... it tells me my STOCK engine is at 10.5... not 9.7.
is the calculator screwed up? or did i enter something wrong?
http://www.csgnetwork.com/compcalc.html
I'm sure that's your problem. Stock thickness is .043" which is roughly 1mm. I'm also pretty sure the combustion chamber is 62 cc's and not 60. With those values, it came out to about 9.74.
Or if you don't want to be lazy, you can just calculate it yourself, unless you failed Junior high/Freshmen geometry...in which case, I can't help you then.
I am pretty sure i read that the LD9 had 60mm CC's and a 0 deck height...
i could be wrong... let me see if i can find the link....
I know it is somewhere between 60-62cc's, but I have forgotten over the years. And you are correct on the 0 deck height
http://www.j-body.org/forums/read.php?f=40&i=136684&t=136331#136684
that is the most useful and informative thread i have found on here so far... speedracer seemed pretty sure on the 0 deck height... i could double check on my own engine... but i already disconnected the rods and yanked the bearings out. and i have read several places that the stock gasket is .040....
but i have found no hard official figures anything beyond bore and stroke....
I know when my piston is TDC it is nearly flush with the deck... maybe even a hair higher. i recall that from scraping the head cleaning for the new gasket... was kinda shocked at how high the pistons came up.
lol. That red 086 head on the bottom looks awfully familiar...
keeping all else the same, changing the CC to 62 as opposed to 60 puts us at 9.73... which is pretty close to stock specs... wish i knew for sure.
but lets go with that for now... assuming a 62mm CC.....
all else stock... 9.73:1
2cc dish, 1mm (40 over) overbore... 9.65 (slight loss of compression from stock)
but then go with a .30 cometic gasket... and your 9.86
or stock gasket, and shave your head to 59cc to get 10.03
or .030 gasket and shaved (61cc) head for 9.99
or .030 gasket and shaved (59cc) head to get 10.26
So it sounds like these dished pistons and 1mm overbore might be just what i need....
any thoughts?
Whalesac wrote:lol. That red 086 head on the bottom looks awfully familiar...
I assume that is/was yours at one point?
what is the big advantage of the 086 head?
Shaving the head on an OHC engine is
NEVER a good idea. The only acceptable reason is if you are trying to clean up the surface of imperfections in order to reuse it. It's best to just get the right pistons from the start.
Also, a .040" overbore on these small engines is playing with fire. I would make sure the shop that does the overbore has your block sonic tested first.
I'm confused though, are you trying to lower or raise compression? An m45 powered LD9 really doesn't need lower compression.
ken soggs wrote:Whalesac wrote:lol. That red 086 head on the bottom looks awfully familiar...
I assume that is/was yours at one point?
what is the big advantage of the 086 head?
Yeah, that's my head. The 086 head is essentially the closest thing to a "race" head there is for a Quad4/Twin Cam, which means it really doesn't do anything more on my engine than raise static compression.
I would LIKE to rasie compression.... but that is really part of the reason for this post....
where will i gain the most power / tune-ability both in and out of boost....
MOST of the time the car is in a non-boosted cruise state (cruising down the freeway at 75mpg) So i dont want to loose power there... (as lower static compression would do)
but i also know that the higher compression, the harder it is to get the right spark advance and power under WOT at boost....
So it is kind of a trade off.... i know i dont need some crazy low 8:1 compression... i would rather not go less than 9.5:1
10:1 sounds good to me... but was hoping for some insight.
i am looking for the best compression to give me the most power at cruise, and the best WOT tune-ability.
also, i was under the impression that 1mm wasnt a big deal on these engines... I am trying to find a shop now that will do my block for me... i would have them check it all out before doing any work.
and if you read that post (with your red head in it) SpeedRacer says
SpeedRacerZ wrote:According to the Compression Ratio Calculators I've been using, my Wiseco .040"+ Pistons (9.5:1) + .074" gasket + head shaved .013 (58cc combustion chambers) = 9.3:1
When I plugged my stock numbers in (9.7:1 pistons, 60cc head) + the .074" gasket, I belive it was 9.3:1 also...... (my Wiseco pistons have just over a 2cc dish to them)
That is a guy who seems to know whats up, and he has 40 over, shaved head, AND a thick gasket... (all of which are appearantly no-no's)
So again, i am here putting my thoughts out there.... and seeing what the pros think about them.
i am looking fo your input.
