Chris Etemadi

I am looking to buy a set of JE Ultralight forged pistons they are rated at 10.8:1 compression with 64CC heads, and a standard thickness head gastket. I will be using my stock L98 heads. (they are going to unshroud the valves will this make any compression differences?) I was wondering If I could get away with this much compression on the street. (with 91 octane) If I were to go this much compression and be forced to put in 87 octane would the computer just retard the timing (sensing knock) and no harm would be done? Or could I just try to get a thicker gastket to lower the compression (I am looking for around 10.3:1)
Sorry so vauge, but I want this to be daily driven in the summer with the A/C on if it matters.

Thanks Chris

F-BIRD'88

10.8 to1 is way too much for a daily driver on 91 octane
gas. Get the right pistons to build for a true 9.6:1
for a carbed motor and 10.0:1 max for efi.

http://www.kb-silvolite.com/
There is an excellent online compression ratio calculator
for you to design your motor right.
things like deck height, piston pin compression height, valve reliefs, gasket volume will affect your true compression ratio.
Any thing over 9.5 will get you where you want to be.

jcb999

about 90+ degrees.
It will bleed off pressure ..but 87 octane is still unlikely.


Drop it down into the low 10s high 9s and you will be ok with an aluminum head (even on 87)

BenDoe91z28

is 10:5 okay with aluminum heads on the street?

Chris Etemadi

Its should be fine. LT1's run 10.5:1 comp.

So I should look into something more like 10:1 - 10.3:1????

burnoutrpm

JoelOl75

High compression here, with no ping as long as I fuel with 92 or better. 87??? Not a chance. Probably won't even work in a 9.5:1 iron head.

Rob P

iTrader: (1)

My ZZ4 is 10:1 compression and needs 92 or 93 to run properly, though it has not for about 3 months.

F-BIRD'88

Burnoutrpm:
"thats b.s. im running about that and have to run 93 octane ,but, im also running 14 degrees inital advace and 37 total with no "audible" detonation.with a carb. "

About 10:1 cr leaves a lot of room once you account for
all the variables
A advertized 10:1 cr piston could be 9.0:1 or 11:1
depending.
Your motor with a .045" deck height would be 9.43:1
or .025" deck height would be 9.86:1
These are very common piston deck heights for hi perf replacement pistons.
91 octane is not the same as 93 octane
"A daily driver" doesn't have a big racing cam in it
that bleeds off cylinder pressure. ( inefficency)

Running a rich mixture (over fueling) can hide a lot of demons
but sucks on a daily driver.
You can't compare a Lt-1 motors cr with aluminum heads, computer controlled EFI and knock retard and reverse cooling design with a conventional small blocks detonation tolerence.
By the way GM redesigned lt1 into the lt4 and lowered the cr
by about 1/2 a point BECAUSE IT WAS TOO HIGH!!!
for a daily driver.......


On those 100deg F Oklahoma summer days with the air on you'll be glad you went a little easy on the true cr in a daily driver.

jcb999

it depends on too many things. You can always make it work by creating a real slow and limited advance curve but your giving up hp there. The hp difference between 10 to 1 and 11 to 1 is less than about 2 percent. Pick a head that will keep it in the low tens and then the advance curve will not be as crucial.

ME Leigh

What are the "cc's' on an Camaro L89, i think they are smaller that 64 cc for some reason, but i could be wrong.

Vader

Right on. I bumped my older LT1 to 10.8:1 static with the head work. It tolerates the extra compression RATIO because the actual dynamic compression PRESSURE is lowered due to the aftermarket cam overlap. My vacuum suffers a bit, but the MAF compensates for any MAP discrepancies.

And yes, the OBD-II PCM controls the timing advance and mixture a lot more accurately and in a broader range than the OBD-I ECMs. This allows the LT1s to run without problems on 87 octane, albeit at significantly reduced peak power output.

If you can keep your heads stone cold, and liberally add EGR and retard, you can use 10:8:1 on the street. The 355 I had in my '50 truck was 11:1 static, and I had to use 94 octane and keep my foot out of it when it was hot. It would take the pounding for short runs, but once warmed it would ping like xylophone.

badgta

iTrader: (4)

so far all of you have forgot that 76later heads have 76cc chambers... this will lower his compression to 9.1 i'm going to have speedpro l2491f30 forged flattops with 10.7:1 advertised compression... with a 74cc chamber a 3.75 stroke,and a .038 gasket.. the compression comes to 9.52:1... also with a stock deck height even with aluminum heads it will still detonate with 10:1 compression because the quench is too big... if you zero deck the block to have a .040 quench distance you can run 11.1:1 with aluminum heads!!!! i used the book john lengenfelter on modifying small block chevy's.. for the formula i used to calculate mine... john really goes into detail on specifics on the 350 small block as well as other procedures for any motor... i'm not saying i'm icompatant buy reading this book but he really has explained every performance related aspect of the 350...

Grim Reaper

Not in L98s. They use 64cc.

ChrisFormula355

The old myth "yah can't run more than 9.5:1 on pump gas!" is just that, a myth. The ability to run high compression lies mainly in your CAM specs, your cooling system and your heads. If you have a cam with enough duration and some good fast burn combustion chambers you could run just about any amount of compression as long as the duration was long enough to bleed off the cylinder pressure. I'm running 10.7:1 on my firebird with a 246 duration @.050 cam, and I also live in Tucson where daily temps are well above 100 degrees, and I drive the car through traffic with 36 degrees total timing...........no detonation as long as I don't get past 4 grand.......and as soon as it cools down to around 80 (night time temps) I can race it full out without hearing any detonation. I know of one guy in Tucson running 12:1 on pump gas because he has aluminum heads and a 260 duration @.050 cam. Its possible, it just takes the right parts and a GOOD cooling system. My grand prix on the other hand is an example of what happens when you don't pick the right parts..........Its got 11:1 compression and a 218 duration cam, not NEARLY enough to bleed off the pressure, which is why I had to buy an MSD timing control unit to back off the timing to 26 degrees total for street driving unless its cold outside or I put race gas in......then I can kick it up to 37 degrees. One other thing to consider with high compression is your starter. Consider installing a ford style solenoid if you have a manual tranny car with a small starter and high compression, otherwise it will be very hard to start the car on a hot day.

ChrisFormula355

by the way, if you don't believe me, pick up the recent issue of Chevy Hi Performance (don't remember the exact date) I think Dec. 2001??...........the title says "FLat out Fast Street Thugs" and their is a 10 second naturally aspirated Nova running 13.5:1 on 91 octane gas because he's running a cam with like 268 duration @.050.

F-BIRD'88

"no detonation as long as I don't get past 4 grand.......and as soon as it cools down to around 80 (night time temps) I can race it full out without hearing any detonation. I know of one guy in Tucson running 12:1 on pump gas because he has aluminum heads and a 260 duration @.050 cam. "

Ooops can't go up this hill cause it's day time, what ever will I do.

" Its got 11:1 compression and a 218 duration cam, not NEARLY enough to bleed off the pressure, which is why I had to buy an MSD timing control unit to back off the timing to 26 degrees total for street driving unless its cold outside or I put race gas in......then I can kick it up to 37 degrees"

"Oh, just a minute while I change my fuel and reset my timing"


Ya Right!!!



Remember this is a street motor with a "street cam".
Read that short duration.
Yes you can build a motor with high compression and a big cam and putt-putt around without detonation because you have no cylinder pressure at low rpm but as soon as you get in it and
get to peak torque (high cylinder pressure) it will detonate.

You can mask it with retarded timing and over rich mixtures, but
you will have an inefficient gas guzzler.
What have you accomplished?
You 're fooling your self...!!!
Building a motor with exessive high compression and then
compromizing it with a huge cam to bleed off cylinder pressure
just to say you did it, makes no sense for a daily driver at all.

You read all the time about this guy with the pro-street
big block camaro with 12:1 cr that cruises around on 87 octane gas, in the Hot Rod magazines. Trust me he don't race it with 87 octane. And he don't drive it to work and back every day.
Unless he's got real deep pockets. Not even John Linkenfelter. Don't believe everthing you read in the Hot Rod Books.
This is "The Old Myth"
And it's "Junk Science" like Cold Fussion.....

ChrisFormula355

Jeez.....if it you have to rev it past 4 grand just to make it up a hill you must have ONE hell of a motor under the hood The point I was making, if you'd payed attention instead of acting like a smartass.......was that MOST people don't drive like John Force in the middle of traffic on the way to work in 100 degree heat, so why do you need to sacrifice your compression and power so that your motor will be capable of all out racing in high temps???? Makes absolutely no sense to me.

Man are you blind? I said MSD TIMING CONTROL UNIT! Takes less than a 1/2 a second to twist your wrist and back it off a FEW degrees. You call that a lot of work??!? And I always run 91 octane even when racing, except that when I'm racing at the TRACK I run race gas for insurance and so that I can run a few degrees extra timing if I want to. And who build a high compression motor just to say they did it? If you think thats why high compression is a quality in a hi performance motor, I just don't know what to say to you then....I think its pretty damn obvious. More compression = more power with everything else remaining the same. Thats free power in my book. And if you wanted a car to run on 87 why didnt' you just go out and by a Honda 1.3 liter 5 speed car and you'll have your dream setup?? Gets good mileage, runs on ****ty gas, and you can rev it past 4 grand in traffic without getting sideways causing trouble

EDIT: before you make some retarded response saying how my car are just race cars and a "normal" person's car has to be different.............I drive my Grand Prix to work every single day (5-6 times a week). I drive my Firebird to school at least 3-4 times a week. Hmmm, yep, high compression just WON'T work on the street

Grim Reaper

I guess it really depends on what a person is willing to live with and tolerate. Sure you can run 12:1 with a 268@.050 cam and a 3,600+ rpm stall...AND some guys will consider this a good street/daily driver.

However, the majority of people would not. But your point about cam selection, cooling systems and good control of the spark will allow a person to run much higher compression ratio than most would consider practical is correct.

But as for a good "street setup", I have to agree with FBird-88 - for MOST people. But I also know guys that enjoy engines where they NEVER run it below 3,000 rpm and consider it a perfect street engine. But they are in the minority.

F-BIRD'88

More compression = more power
Only only true to a point if you have the octane to avoid detonation. A motor that is detonating will not make more power It will make less power, so you'll never get to use the extra power you purpose.
Because you can't open the throttle to get power without
retarded timing back from optimum to avoid detonation.
You won't be any further ahead.

A high compression motor that has to run with retarded timing
and over rich mixtures to survive on low octane gas
does not make more power or get better gas mileage or have any other benefit.

You can have all the timing controls you want , you won't get the effictency back.

A lower compression motor designed to run on the available fuel that will run at full timing under power will make more power than the crippled high compression motor. And last longer. And have better mileage.

This just is not practical at all for a "daily driver". Done right,
the "daily driver" design operating on the available pump gas fuel will out perform your motor in all aspects that are
important to a daily driven car.

There are lots of ways to built a better "street mouse", but exessive high compression isn't it.

BadSS

Chris E,,, you're talking about the -2cc Flat-top Ultra lights,, right?

J&E rates their compression ratios at 0 deck height and a .040 gasket compression.

This piston whether you use the 5.7, 6, or 6.125" rods has a total compression height of 8.99". In a stock block with 9.025 deck height you're looking at 10.12:1 compression with the .040 gasket.

Also, I take it the L98 heads you're refering to are the stock heads off your 91 350 Z,, which are cast iron. If you "unshroud" the valve on this 64 cc head, you'll probably end up with around 66 cc on a 4.03 bore. Without decking the block you're now looking at 9.89:1. If it were me,, I'd take about .006 off the deck and this will bring your compression to right at 10.0:1 (with 66cc chambers),,, which should be no problem running flat tops on the better premium gas even with cast iron heads.

I would beveal/blend or smooth the valve relief edges and I'm sure the chamber work would remove any sharp casting flashes that could have been present in the combustion chamber. Just to be on the safe side.

MikeH

iTrader: (10)

F-BIRD'88 has it right...Ive been down that road....Dont run the compression to the limit for pump gas. Ive pulled my heads once already..Listen to what these guys are saying.

Guest

It really depends on the entire setup you have. The head design has alot to do with it, and the cam profile enters the mix as well. If you have the engine apart, there are things you can do to prevent hot spots from forming, basically that amounts to getting rid of any sharp or abrupt edges, and no thin metal areas where possible. Theres some things you just cant fix, like the ground strap on the plug, unless you run a surface discharge type plug and those run super cold, too cold for a street engine. I have a car with a 224/232 cam, 10.3:1 compression, aluminum heads, no EGR, up to 44 degrees timing (not at WOT but at part throttle sometimes) and it will run on most days with the AC on in hot weather and not see any detonation with 87 octane fuel. With 91, I've never seen any knock that I dont usually have anyway. I figure I could run close to 11:1 without problems on 91 octane. Is it worth it? Probably not, this isnt a race car. I think its doable though, and I know there are better head designs out there than what I have. Blanket statements are dangerous though for everyone, its just that in most cases to be safe you dont wanna run that high of compression. About 9.5-10:1 is the max you want to run with iron heads and a normal cam. Beyond that, you'll just have to see what other people are doing and getting away with. I'd say with L98 iron heads and 10.8:1 compression pistons you're screwed. Putting a thicker gasket on there might work, and then again it just might ruin the quench and make matters worse. I'd look for some different heads or different pistons if I were you.

FJK

I agree with madmax's assessment. On my non-GM engine, I'm running 9.7 meaured CR on 93 octane pump gas with 40 degrees total timing & have no detonation problems. This is with iron heads that have polished combustion chambers. The objective here is to reduce your octane requirement and use that benifit to maximize your static compression ratio, without sacrificing igntion timing advance. As stated in other posts, your "effective" compression ratio plays into this, especially at low speed and is dictated by your cam timing, specifically intake valve closing. So, what are the key players in this balancing act:
Static compression ratio: trying to maximize
Cam timng: how radical do you want to be? (be reasonable)
Total ignition timing: don't sacrifice, you're defeating the whole purpose
Polished combustion chambers
Calibrated vaccum advance; IOW gets out of the way when it should
Reduce inlet air temperature (cold air inlet, blocked heat risers)
Reduce coolant temperature, 180 F min, cooler than that sacrificies ring & bore wear
Using these considerations, I'd say real low 10:1 CR is do-able with reasonable cam timing and cast iron heads. I estimate aluminum heads allow another 0.8-1.0 increase in CR with everything else constant.
If you're really trying to do a nice job to maximize CR for the street and a number of different thickness head gaskets are available for your application, go with a thinner one & if you miss the setup, you have the option of going to a thicker gasket to kill off some CR. When you're off a little, meaning a bit too much CR, you'll start noticing trace detonation as air temps get hotter, say above 85F.
FJK