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Topic Review (Newest First)
01-16-2013 06:18 AM
Greg T You can do all that if you like and won't change much. I would just install a water injector and call it a day. If the motor is not built yet, change pistons.
01-15-2013 10:09 PM
prostcelica I want to get the compression down. What about getting a little from everywhere. Run a couple thou thinker gasket, polish the chambers and pick up a cc or 2 and enlarge the valve reliefs slightly (I've heard of people doing this. Not sure if its a good idea)
01-15-2013 06:15 PM
prostcelica So will 10.95:1 work well on 94 ?
01-15-2013 12:36 PM
crownver Yes, but will it cost as much power as lowering the compression to use 91?
01-15-2013 12:34 PM
vinniekq2 Try to use chevron 94 as it contains no ethanol. 10% ethanol will cost power
01-15-2013 12:16 PM
crownver Why don't you just plan on using 94? There's at least 1 gas station near you that sells 94 (10% ethanol). Husky/Mohawk in town and possibly the one south of town near your brothers place. There may be others too. Pretty sure 10% ethanol is ok as long as the carb and fuel lines can handle it.
01-15-2013 11:47 AM
Greg T You may want to consider H2O/meth injection. For a few hundred you can run that compression and fuel without worry. I've used these in the past they work VERY well.
01-15-2013 11:09 AM
F-BIRD'88 I defianlty want to be able to run on 91 with no issue.

91 is not very good fuel. I would not tread over 10.0:1 with no issue.

Start by ccing what you got.

smonty does nice work there, doesn't he.
01-15-2013 10:22 AM
ssmonty prostcelica,
Even though your post was directed to someone else, I thought I might chime in. I have spent alot of time with a set of RPM heads(70cc advertised) cc'ing, and trying to equalize the chambers. Unfortunately I didn't measure them before I started blending sharp edges around the seat inserts, and polishing to give you an idea how much of a difference it made. I know the polishing is just going to get covered with carbon, but from what I've read it will still help a little to reflect heat back into the chamber and avoid hot spots.
The point I was trying to share is that it takes a lot of grinding to make much of a difference. To gain 5cc equally on all chambers without compromising their integrity would take an extremly long time if even possible IMHO. You would have to remove alot of metal that might weaken them or hurt their combustion efficency, but JMO.
I believe you would be much better off buying another set of pistons with a 12cc "D" dish, but JMO.
I'm in process of building a 383 much like the one you described. 0.040 overbore, 5cc vr pistons, 4.166" x .039" gasket, and zero piston to deck height with 71.4-72.0 cc chambers. I get around 10.2-10.3:1 CR. I trying the groove thing for all the insurance I can get, even though I have no proof they work. Forgive the spelling.
FWIW,
ssmonty
01-15-2013 08:49 AM
prostcelica 10.94:1 is defiantly not idealfor what we want to do with the car .I'd rathher see it between 10-10.5:1 (ie 10.2:1). Fbird, what do you think about opening up the chambers 5cc's or so? If I used the texture from the casting as a guide and sanded evenlly across the chamber how many cc's wold I pick up?

I defianlty want to be able to run on 91 with no issue.
01-15-2013 07:37 AM
F-BIRD'88 Its going to be marginal at best. I recomend you use the best available pump gas in it.
Is it worth damaging the engine to save 10 minutes drive?
Critical on daily engine-water and engine inlet air temperature. Which will vary a lot under real world operating conditions. Like in traffic.
All you need is a hot day in traffic and the eninge is now breathing 180+F air. Now the engine is much more likely to Knock @WOT.
I recommend you tread softly on the spark advance setup until you have done enough street-real world testing
at WOT. And sneek up on the tune up. You may be fine one day and your luck may run out on the next day.
and it will knock at WOT. 10.94:1 is very very agressive for any pump gas, reguardless of advertized octane.

And the cam and its position in the motor will not change that much.
Cranking compression and theoretical DCR calcs have no bearing on the actual cylinder Pressure and Temperature or quality of the combustion mix in the cylinders when the engine is actually running at RPM @WOT.
As stated, CCing your heads is a good place to start.

The compression ratio-fuel octane you can get away with on a quick 10sec dyno test or a 10-12dec drag strip test on a cooled motor, breathing cool air under controlled conditons is a whole lot different the widely Different and much more severe
(HOTTER) engine operating conditons that you will experience while street driving. (the engine will get hot and or breath in HOT AIR at some point)
The real fuel octane requirement goes way up under these conditions. Which you cannot control or account for.

If that is the real engine compression ratio I bet sooner or latter the 91 octane gas will let you down under real world conditions. There is a big difference between 91 gas and 94 @WOT.
01-14-2013 11:59 PM
prostcelica Thanks Vin. Sounds like a plan. Ill get a head cc'ed in the next day or 2. I aslo called the guy who ground the cam. He said there will be a part # on the back. I'm going to slide it out and get the part number. He said he would be more than happy to provide me with the specs.
01-14-2013 11:47 PM
vinniekq2 after you get a chance to measure the heads,we can get the master tech to do some math.We need the cam specs too.The cam can be put in a couple degrees late if required.Im more concerned the heads were milled. I tune my car for 92 octane and my CR is 10.75
01-14-2013 11:36 PM
prostcelica They are advertised as a 64cc chamber but I've read sometimes they aren't bang on. I think there is a stations that sell 94 octane but I'm not 100% sure. We want to be able to run it on 91. Don't want to have to drive across town to put gas in it. Hey Vin. What do you think abot opening up the chambers a bit?? Or do you think we will be ok on 91. It will be extremely time consuming to grind and cc every chamber I would think.
01-14-2013 11:26 PM
vinniekq2 Man that car is gonna scoot!measure the combustion chambers.what is the best fuel in your area?
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