anyone have any good ideas on how ik can lower my compression
ratio without tearing my motor apart??? i am running trw forged
dometop aluminum pistons. however they are 11.1 compression
which is way too much for me as they ping really bad and especially
with the crappy gas nowadays?? i have been adding octane booster
but it doesnt hep much?? any other suggestions? i guess gthe ideal
compression for the street would be around 10 to 1??? what does
every else think is the best compression for the street with todays
crappy gasoline?? i guess i could run racing fuel but that is kind of
expensive for just running on the street?? please help. thanks

 

http://forums.corvetteforum.com/c1-a...ratio-etc.html


http://forums.corvetteforum.com/c1-a...vs-octane.html


Gerry

 

What engine do you have, what cam is in it, and what's your ignition timing map (initial timing, centrifugal curve, vacuum advance unit spec and is it connected to a full or ported vacuum source)?

A properly tuned 11:1 small-block with a factory cam will run all day long on pump premium with no problem (and no flaky additives).

 

Cut your timing back. Try removing 4 deg and see if that helps. You may have to pull the heads and go with thicker gaskets. Cometic gaskets will make you thicker ones if needed.

 

What John said above. Seems like there's lot's of complaints from California owners that can only get 91 octane instead of 93 like most of us.

My 11-1 (advertised) engine runs fine on 89 octane. 10* lead, stock '63 FI distrubutor curve (can't recall what it is but it's not real quick) and a VC 10 vacuum cannister which is not stock for that engine but comes in/stays in at low vacuum and works well with the 097 cam and the mechanical curve in the distributor.

In other words, unless you have some mechanical alteration from stock, work on your tune. Forget the additives and the airplane gas.

 

I have one and have zero problems with it on pump gas and it runs 180 deg on the temp gauge.

 

i have a 350 small block bored 30 over 4 bolt main. i am running trw
forged aluminum pistons with domes. 11.1 compression i am also
running aluminum trick flow heads 195 cc runner 64cc chambers.
i have changed my msd distributor over to mechanical advance and
have lowered my initial timing down to 8 degrees from 11. i still am
having problems with pinging and dieseling when i shut it down???

maybe i need to go to thick copper head gaskets??? will that lower
myh compression down to 10.1???

i think my compression is 11.1 it might even be 11 1/2 to 1???
dont know for sure???? but it sure pings when i accellerate???

thanks for your help. have a great day.

 

one cause of dieseling is too high an idle.
Bill

 

problem isn't compression..

problem could be a vaccuum leak... that would cause the idle to high, and subsequent diesling problem... and the vaccuum leak will lean out the mixture and cause sparkknock...

check for a cracked baseplate...

on top of that... your aluminum heads will allow you to run about a point higher on pump gas because the aluminum dissipates heat faster.

don't start retarding your timing until you have ruled out the fuel issue.

what cam is in that motor? if you have true 11 to 1,, or 11.5 to 1, and a tiny cam.. the compression could be an issue... but i doub't it.

Aaron

 

Swap the heads with 72-74 cc, that will drop the compression ratio about one point to approx 10 to 1. That's what I did with my 11 to 1 engine. Runs great on 93 octane pump gas.

 

hi, WMF 62 is right, deiseling is a result of a high idle and has nothing (almost) to due with compression, at idle the throttle plates are closed and there is high vacuum in the intake so the actual "compression in the cylinders" is at it's lowest.
GDC 1962, going to (72) 74CC "open chambers" is not the answer, with those open chamber heads you will kill your quinch.

i hate to give you the bad news, but the engine has to come apart---NOW, detonation will kill your engine in an instant. damage may have already been done (imagine beating your engine parts with a sledge hammer, that's what detonation/pinging does).

1- lose the domed pistons, go with flat tops, the domes actually impede flame travel across the combustion chamber, they are old skool.
2- you don't know what your compression ratio is without doing all the measurements due to factory tolerances (how far down in the hole is the piston at TDC? (average is about .015, i measured a small block today that was .017, mine is .027 down, the advertized compression is a blueprint number and the actual compression is almost always lower. measure combustion chamber CCs (a 64 CC head may not actually be 64 CCs)
3- after you do all the measurements find an online compression ratio calculator and punch in all your ACTUAL measurements. then you can play with gasket thickness, combustion chamber volumn (yes, you can grind a little out to reduce compression) to arrive at your target compression.
4- your target compression will vary depending on the cam you are running, the milder the cam, the lower the compression you want to run, with a bigger cam you will want more compression.

to recap or summarize:
use flat top pistons.
use closed chamber heads.
keep the quinch clearance tight to suppress detonation/pinging.
match the compression to the cam.
check ACTUAL TDC to make sure you are timing the engine correctly.

and one last tip:
ported and polished is a long overused buzzword, you can port the intake passages, but don't ever polish them. it is a good thing to smooth/polish the combustion chambers, microscpically you go from this-vvvvvvvv to this-------, there will much less surface area exposed in the combustion chamber, more of the combustion heat will produce power and less will be lost into the cooling system.

i know this is much more than you asked for, but i thought i would just throw it out here for all the forum members who read this to think about...

EDIT: i noticed you live out here in cal. and you mentioned octane booster. that 104 octane booster is crap (NEVER used it), it does nothing (the points are 1/10 of one percent) of course you can't buy it out here anymore (thaks cal) but for increased octane i used to run 10 percent toluene on my 12/1 SBC gasser (which you could buy at any hardware store) with gas. of course you can't buy toluene anymore out here, but it was a cheap solution to the octane problem. that 104 octane booster (if you read the label) it's main ingredeint is toluene...later.

 

I would suggest, before the train jumps the tracks that the questions posed in post #3 be answered before you do anything foolish.

 

Yes, before any fixes we all need to know what's there and how they connect to have a better understanding of possible root causes.

 

I run 12:1 on pump gas all day long..... well 11.86:1 anyways...

 

Lots of good advice so far.
Myself, i'd 1st do a compression test.
If the gauge explodes, i'd change pistons or cam or both.
Otherwise, i'd tune it.