383 with 10.3:1 CR too high for pump gas?

[Sweet72]

I've been looking at the 383 stroker flat top rotating assy. from Scoggin Dickey Parts Center www.sdpc2000.com , and want to use the GM Performance Parts Vortec heads w/64cc chamber due to the cost/performance, however this combo supposedly will make a 10.3CR! Will I have to mill the pistons down, or what are my options to make this thing run on pump gas? Thanks. Sam

[Tom]

Run a cam with at least 220* duration and 110* LSA and tune the ignition and you'll be fine on 91, I did. 385 w/ vortecs, 10.2:1 compression, and 214* duration comp cam with a stock HEI

[cdowns]

"pumpgas" ??? we have it available in octanes up to 106 don't see why it wouldn't run on pumpgas

[bigjimzlll]

in CA the highest is 91 octane

[Clancy68]

10.3:1 is marginal at best with aluminum heads in my opinion and to high for iron heads. the cost in milling pistons down is about equal to dished pistons when you figure in the value you pay in the kit. i honestly think you would be taking a step backwards and spending more than you had to, which is a major piss off when your on a budget bacause that usually ends up being extra money your could have spent upgrading to roller rockers or something like that. theres load of these kits on the internet. summit, jegs,PAW,powerhouse,strokerkits.com,EBAY and many others. I would try and find the kit that gives you more like 9:1 - 9.5:1 at the highest. even if you have to spend a little more. thats my .2 cents

--Scott

[1BadSS]

My 11:1 327 ran fine on 94 octane.

[Bryce M]

Run a tight quench and a cam with a fair bit of overlap and you should be able to do it
I have flat tops in mine with 64 cc iron heads as well and it has never knocked once on 91 octane

[Clancy68]

Originally Posted by 1BadSS

My 11:1 327 ran fine on 94 octane.

as bigjimzll pointed out 94 isn't always avalible. also you can usually run more compression with a better rod/stroke ratio like your 327 has, and being that its 11:1 you probably have a pretty bumpity cam in there, which drops low end cylinder preasure. This is made even more pronounced in a smaller dispalcement engine due to lower volumetric effeciency.
a.k.a. the reason that a really hot cam in a 327 will be choppy, while the same cam in a 406 with identical heads compression etc. will be more rumbly, less radical sounding, and more streetable. but on that same token requires a more stable fuel(higher octane) to not have pre-ignition and detonation problems due to natrualy higher low end cylinder preasures.

--Scott

[neonlarry]

I'm planning on running 10.9 on 91 octane. W/aluminum heads, 6.0 rods, 220° w/112° LSA roller and EFI. And I don't plan on keeping a barrel of toluene in the garage.

[CJM72CHEVY]

I'll ask the question that hasn't been asked yet. What are you planning to do with the truck? If you're going to be using it hard and/or towing, or will be running in high ambient temps, it's going to be more prone to detonation, regardless of cam specs, cylinder head type, etc. Hot and/or heavily loaded engines are more likely to experience detonation at a given octane number and compression ratio, period. If you're going to idle around town or blast down the highway with no load, you might be okay, and you could always pull out a couple of degrees of timing in order to dial in a safety factor. Were it my valuable time and money going into the engine, I'd rather be safe than sorry, and would lean toward a little less compression instead of just "getting by".

[bigjimzlll]

Originally Posted by CJM72CHEVY

I'll ask the question that hasn't been asked yet. What are you planning to do with the truck? If you're going to be using it hard and/or towing, or will be running in high ambient temps, it's going to be more prone to detonation, regardless of cam specs, cylinder head type, etc. Hot and/or heavily loaded engines are more likely to experience detonation at a given octane number and compression ratio, period. If you're going to idle around town or blast down the highway with no load, you might be okay, and you could always pull out a couple of degrees of timing in order to dial in a safety factor. Were it my valuable time and money going into the engine, I'd rather be safe than sorry, and would lean toward a little less compression instead of just "getting by".

very valid thinking...something to think about...the actual HP gain from 9.2-1 to 10.2-1 is about 10 HP(using a cam suited for 9.2-1) where the bigger gains come is when you use a bigger cam, heads, exhaust to take advantage of the higher compression. In a non max effort sbc..play it safe..go with a smalle CR and optimise the timing.

[Litdave]

I have an engine similar to yours except it is a 355. I am running flat tops with vortec heads, .025 deck height and .039 gasket (quench about .064). Also, I am running a relatively mild cam (ZZ4) with 208 intake duration.

I was scared to death that I would have gas problems based on what a lot of guys told me. But I run this thing all day long on 89 octane and have run 87 octane and didn't notice and detonation.

I did smooth out any sharp corners or edges in the combustion chamber of the heads that might have caused a hot spot. I don't know if it helped but it sure didn't hurt.

[Motor City]

Just my 2 cents, and please correct me if I am wrong.
I was always told that detonation occurs from heat, and a higher octane fuel, being harder to burn, means less chance of detonation. That being the reason gas stations want you to buy premium, less spark knock they call it. A higher compression ratio, (under 11:1) with a high octane fuel (91+) would be the equivalant of 87 octane with 9:1 comp. (assuming correct A/F ratios, timing is right, etc, etc.)
I have run 11.5:1 on 94, and although the engine ran questionable at best, it was streetable. 106 ran great but at 3.79 a gal. run time was minimal.

[srosa707]

i have "10.5:1" keith black pistons and after the formula was completed on my motor just recently, its actuall 9.5:1 compression. just me 2 cents...

[68c10owner]

i had kb 10.5 to 1 pistons in my old 327 with ported iron heads and a pretty big solid cam and it ran just fine on 91 pump gas. the cam duration helped alot.

[Sweet72]

Thanks very much fellas for all the super replies. I knew you woudn't let me down. In answer to CJM72CHEVY's question: What are you planning to do with the truck? It will be an occasional street driven fun truck, taken to shows, and at times I may require that it be used as it was intended. Definitely not a trailer queen. I called Scoggin Dickey about substituting -12cc dish pistons and was informed that they would rather not break up their kits, and that would be a special order $$$. I see that Jegs sells a set of Speed Pro hypereutectics for $223 and they have the -12cc dished that I'd need to get my compression into the low to mid 9 range and additional tweeking could be done by varying the head gasket thickness. Now I wonder if I could unload the -7cc Speed Pro flat tops that come with the kit on ebay. Oh, cdowns, I guess the airport has a pump that I might be able to get some of that high-test, and maybe I'll consider running a 12.5CR with water injection .

[neonlarry]

Before playing around with head gasket thickness to adjust compression read this:
http://www.speedomotive.com/Building%20Tips.htm

[Sweet72]

"If you have a 10:1 engine with a proper .040" assembled quench and then add an extra .040" gasket to give 9.5:1 and .080" quench you will usually experience more ping at the new 9.5:1 ratio than you had at 10:1."

This must be what you're referring to neonlarry. Thanks for the heads up. (pun intended)