lumley69

Just got the call and my engine is done. hopefully going to get over there in the next few days to get it picked up and I'll post some pics. some details about the engine, 455 .30 over SRP pistons XE268H-10 cam Ga heads with larger valves installed and some minor bowl work. cross overs blocked. My question is the compression ratio all said and done the machinist said it is right hat 10:1 I was hoping for 9.5-9.75:1 but it ended up being 10:1 from what I read that is right about the limit for a iron head engine. was planning to run 91 octane but what should I do in regards to timing I know ill have to be careful but are we too high on compression? thank you for your thoughts and input

TripDeuces

My stock 330 was 10.25:1 and runs fine on 91-93 octane. Your cam is bigger so it should bleed off some cylinder pressure. I think you'll be fine.

lumley69

thanks for the info! was hoping It would be fine.

cutlassefi

I get around 10.25:1, even with a stock deck. That piston only has a 5cc dish and is taller than stock. Plus that's a small cam for that compression. Running only 91 I think you'll need to be real careful with your tune, you may have problems.

And for the record, I have yet to pull apart any Olds that actually had the compression they claimed from the factory. They've always come up short.

lumley69

are you saying it may be less than 10:1 or closer to what you figured at 10.25:1 ?

MDchanic

Mark is saying that with your parts and a stock deck height (yours may have been milled a bit), he estimates an actual compression of 10.25:1, and that George's 10.25:1 engine likely had less actual compression than that, since it was stock, and on stock engines actual compression is always somewhat less than nominal compression.

What's wrong with running 93 octane?

- Eric

lumley69

availability, How effective are octane boosters like royal purple octane booster? said to raise octane 3 points each bottle treats 25 gal of fuel. Im not against using a booster if it actually works.

MDchanic

I have not tried those auto parts store octane boosters, but have used Octane Supreme, which contains tetraethyl lead, and works great, though it's not cheap.

It's tough not having 93 octane available. Two of my "new" cars call for it, so I guess folks who live where it's not available just deal with it.

- Eric

Fun71

So does that mean it will raise 91 octane to 91.3 octane?

lumley69

the way I read It I assumed it meant from 91 to 94

don71

Right, That is the question. When these manufactures market these products are they really saying what they are saying?

At three points, is it 91.0 to 91.3 or is it 91.0 to 94.0? I believe the former not the latter, until proven otherwise.

MDchanic

The convention is to refer to each integer increment of octane value as a point, and each increment one tenth that as a tenth of a point.

I don't know how well those particular products work, but that is the standard usage of the term, and I have never heard it applied otherwise.

I do know that Octane Supreme has delivered the performance that they promised when I have used it.

- Eric

TripDeuces

I agree with Mark that most stock engines aren't at their listed CR and tend to be less. I was just stating that my listed CR is similar except for your rebuild. I get by with 93 octane just fine. I would avoid octane boosters just from a cost stand point if you don't actually need it. I'm assuming this is not a daily driver so if you go the octane booster route more power to you (no pun intended). As a last resort, if needed, you could always go with a thicker head gasket. They are readily available.

lumley69

I just wonder if now is the time to get with the machinist and either A go with a larger cam to agree better with the compression or B go with the thicker head gasket to try and reduce the compression. or just find the happy place on timing with 91 octane fuel and drive the car.

MDchanic

For what it's worth, my nominal 10.25:1 '68 350 could not make it on 93 octane with original heads and a very slightly bigger than stock cam, but after I changed the #5 heads to #6s which had been milled substantially, and on which I had polished the combustion chambers and ported slightly, with measured CC volumes of about 9.75:1, if I recall, I can almost get away with 93 octane, but still have detonation in one cylinder on uphills above 75mph on a hot day, which a few ounces of octane boost will silence.

- Eric

lumley69

spoke with the builder just a little bit ago. plugged in some numbers on summit compression calculator. I came up with 9.72:1 and he was saying he refigured and said I should be @9.8:1 which makes me feel better. so fairly close to what I figured. (im not a engine guru especially on CR) but he said the head gasket was .040 and the heads CC'd at 81cc and took .06 off the block (if I understood correct) and a dish of 5cc on pistons.

bccan

I have used the Octane Supreme - It is said to raise the octane whole "points", not tenths. It is a good product & does work.


I would check w/ machine shop & see just how completely/accurately the CR was calculated. Were heads cc'd? True deck clearance measured? If he just ballparked everything based off 1 or 2 measurements & then filled in a value or two w/ guesstimates you could be off a whole point off, likely lower but can't be sure. My guess is you're cutting it close w/ that cam if CR is accurate, bitch is that you won't know for sure until you install it & drive.


Edit - My handicapped sloth typing speed left me to find your latest post - I would call that good news & should help things to work favorably. With 93 you're definitely good to go but I would guess that a decent tune on 91 should work. Hope install goes smoothly & nuthin but smiles when you drive it!

cutlassefi

It's your money, you need a better explanation from "your builder".
I'm sure he didn't take .060 off the deck, he took .006. SO, knowing that, you're at least 10.25:1 like I said.

4.155x4.155x4.25x12.87=944cc
Gasket = 9cc, piston = 5cc, Chambers 81cc, extra deck (assuming .020 in the hole) 4.5cc= 99.5, let's make it 100.

944+100/100=10.44:1. Maybe you and your builder should go back to school ;-), or maybe you missed something on the calculator. Just sayin.