Realistic Octane Requirements

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Littlekix
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Realistic Octane Requirements

Post by Littlekix » Tue Nov 20, 2018 9:29 pm

I am starting a new SBC 400 build and have some logistics questions.

The goal is to have a reliable and fun street motor that also teaches me a lot about building an engine. Its semi budget but more about learning how to build an engine, something I've never done before.

I just had the donor 400 CI block cleaned and checked for cracks etc and it looks like it will clean up nice at 0.020" over. The Machine shop is checking the line bore (machining if needed), decking and boring cylinders with plate. They are wanting me to mic the pistons to machine the bores to size. I am having some trouble identifying the correct combination such that I don't have to use race gas nor octane boosters.

The rotating kit I'm leaning towards is internally balanced, 4.145" Pistons, 5.7 Rods, Cast Crank from eagle (Kit B13470020 )

This kit, set at a deck height of 9.025", and assuming a head gasket at 0.043" Thick, with 64 cc Vortec heads equates to a CR of ~10.8 which is about what I was targeting.

A deck height of 9.025" and 0.043" Head gasket is a squish of 0.068" which from some of my reading seems to high? If I reduce this further with a smaller head gasket my CR goes to ~11 really quickly. Will this be too much for a street engine with 91-93 octane pump gas? Does anyone have any experience with something similar?

My second question comes in debating hydraulic roller vs flat tappet and cam specs. I am looking at Comp Cams K12-408-8 which is a milder roller cam. Since this will go with a Vortec head I am planning on having to cut the valve seats to allow for over a 0.420" lift.

With the faster closing exhaust will this act to artificially increase the compression ratio further, driving up the octane needs?

These may be myself thinking to much into these things, so please let me know some of your opinions. I've learned a lot reading at speed talk so all is appreciated.

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Re: Realistic Octane Requirements

Post by FC-Pilot » Tue Nov 20, 2018 9:42 pm

Where do you live and how safe do you want to be? I live in the Phoenix AZ area and in the summer with iron heads and in traffic I like to stay on the safe side. The most I would run in my area and environment is 10 to 1. That way if I get a tank of iffy gas I won't rattle it all the way home. Everybody has their own standards, but that is mine. The other thing is the squish distance. I like to be anywhere from .035-.050. If the pistons don't have much rock and have tight clearances then I will lean towards the .035 side. Lose forged pistons and if on a budget (meaning not able to cut the deck to get it closer to the .040 that I want) then I will settle with as much as .050. Any more than that (for me that is) and I will figure out a way to bring that down. Having a tighter squish will help to reduce detonation. Anyway, just my thoughts. It sounds like a good educational build. Have fun and take your time.

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Re: Realistic Octane Requirements

Post by pdq67 » Tue Nov 20, 2018 11:23 pm

I file fit MrG's 4" bore, .020" thick, steel shim head gaskets that I put in my 406 years ago. I had Speed-Pro's way deep dish forged pistons that had the dammed near 1/4", 45 degree chamfer around the top of them to hold my CR down around 9.5 to 1 or so for pump gas use.

I mocked them up on an empty block and reached up inside and scribed a line where they over hung and then very carefully removed the excess metal. Also drilled the needed steam holes in them at this time as well as in the decks of my old double hump heads. Use two drill bits the size of the head/block dowels to locate them. Oh, I used my old 400's head gaskets to locate the steam holes..

I figure with a .025" DITH piston that I was right at .040" deck height.

I used good old Permatex, "Indianhead Shellac Gasket Cement", on both sides real thick, let it tack up and torqued them down in 4 or 5 steps.

Then over a couple of days, I check torqued them three times after heat cycles and never had a problem!! I also put a daub of Indianhead on all my head bolt threads and a skosh under each hex head to stop any coolant leakage issues.

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Re: Realistic Octane Requirements

Post by F-BIRD'88 » Wed Nov 21, 2018 10:36 am

Build it so the finished real compression ratio is 10:1. Not over 10.5:1. Get the net quench clearance right.
Use a dished piston with vortec heads on a 406.

Kb has a nice 18cc d dish piston that is just right.
91 and 93 are not the same. Which is it?
If 91 go on the low cr side. 10:1

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Re: Realistic Octane Requirements

Post by F-BIRD'88 » Wed Nov 21, 2018 10:41 am

I would use one of comps extreme solid street rollers.
Yiu can custom order it on a tighter lsa for some more torque right where you want it.
All you need do is call them.

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Re: Realistic Octane Requirements

Post by Newold1 » Wed Nov 21, 2018 10:56 am

What is the top of your chosen piston look like? The decking of the block (cut) can set your quench height with lets say a -.015" deck height and a .035" gasket compressed gives you .050" quench which is fine for a street 406 like yours.

The IVC will be the better determiner of your dynamic compression which is really what will determine if the engine will function well on 91-93 pump gas with proper timing to prevent detonation.

Use a good dynamic compression calculator to help determine good camshaft event timing and static compression ratio to give you a good street/pump gas safe engine.

With iron heads on the street and since most pump gas today has a little ethanol and that helps a little I would suspect you will find an 8.75 to 1 dynamic compression and 10.0 - 10.5 to 1 static compression will keep you in good shape.

As for what it sounds like you are trying to do and your engine build and tune experience I would HIGHLY recommend a good hydraulic roller camshaft and a good set of hydraulic roller lifters as they can do everything you want and engine to do on a build and use like this and keep valve adjustment and roller maintenance out of your picture.
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Re: Realistic Octane Requirements

Post by My427stang » Wed Nov 21, 2018 1:00 pm

Way too much compression for that cam, and yes I'd deck that block to get to a tighter quench, I'd like to see the piston .005 below or so and squared to the mains with that gasket. You could run a thinner gasket, common on SBCs but a new sealing surface with a square and known deck height would be beneficial

If you want that cam, deck the block to put the pistons .005 below with that gasket and go with a 20-22 cc piston for about 9.25-9.5:1 static

If you want that compression, deck the block and run a lot more cam, 292-ish duration adv

Depends on end use which is the right answer. Option 1 is strong mellow cruiser / healthy truck, Option 2 would be a serious streeter with some gear and a converter
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Re: Realistic Octane Requirements

Post by Littlekix » Wed Nov 21, 2018 2:39 pm

I’m in New England so I don’t have as much heat as others. I have both 91and 93 octane available without driving to far. The deck is going to be squared and cut (still deciding on final values for deck height).

It was my fear that I was looking at cost more then the right setup. Scat has an assembly 1-90950BI that will maintain the internal balance but increase the bore to 4.155, which is not the end of the world I was just trying to save as much wall thickness as I could. Many have bored 400 blocks 30 over before.

This has a 18 cc dish which others have commented works well with vortex style iron heads. This also helps bring down the compression ratio and allows for bwetter gasket thickness choices. With a 0.025 below deck height and a 0.020 to 0.30 head gasket that puts me at a static CR between 9.7 to 10 which is a lot more reasonable. The squish would then be between 45 to 55 thousandths. I try to play with the numbers a bit more to really dial in squish and deck height. (Meaning I need to read a bit more ).

One additional issue is that the rods are now longer at 6.0 instead of 5.7, which from reading means I need to do some clearancing. I’ll see if the machine shop will do it for me if possible.

I’m not really stuck on any particular cam as of yet. Since this car / engine combo is never going to see the track I’ll keep looking at some others.

Thanks for all the comments, looks like I was going down a path I didn’t want to go.....

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Re: Realistic Octane Requirements

Post by My427stang » Wed Nov 21, 2018 2:50 pm

So here is my advice

Post the vehicle it's going into and how you intend to use it. Sounds odd but think carefully on use, parts choice is easy after that. Will you drive it daily, will you idle around more than run hard? What gear ratio? Car weight, transmission? converter? All that goes into the choice long before you buy parts.

After that, pick a cam that matches the use with the heads, intake and exhaust you plan to use, we can help
Pick pistons and the rest of the parts that support that cam and gets you to a nice tight quench (.040-.046) with head gasket, and gets you the compression you need

Depending where you are, you may not need anything special and in the end, it will almost always be cheaper to buy what you need versus making your engine match the kit.
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Re: Realistic Octane Requirements

Post by Truckedup » Wed Nov 21, 2018 3:51 pm

No mention of heads and the role they play in octane requirement?
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Re: Realistic Octane Requirements

Post by rustbucket79 » Wed Nov 21, 2018 11:46 pm

Longer rods don't dictate more clearancing, the style of rod big end does. Go with the Scat kit, internal balance is desirable over the factory 400 external balance. A roller cam will have a higher likelihood of needing clearancing (on the rods) than a flat tappet cam due to the lobe shape.

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Re: Realistic Octane Requirements

Post by Littlekix » Thu Nov 22, 2018 7:27 am

The engine if for a 1976 corvette that has a bone stock engine. This was my dream car as a kid so its been fun to own and work on over the last 10 years. I've done brakes, rear swingers, fan conversion, radiator, water pump, clutch etc etc. The last two pieces for the car are a front suspension overhaul and the engine.

The intent for the car is a Sunday driver that is reliable and a blast to drive. I want to be able to break the tires loose (something I cant do yet) but I don't plan on ever taking the car to the track or racing it. It has the stock 4 speed manual, 3.08 gears in the rear.

The 400 block was chosen over some other options to increase the cubic inches for torque. The goal was for a wide torque band without worrying about trying to reach a crazy peak HP. We found some used Vortec heads that we are reconditioning to match the 400. Vortecs were chosen as even stock Vortecs are a large upgrade to what was stock on the 400. A new rotating assembly for the 400 and new top end cam/lifters/etc was the plan without anything specific chosen yet. Since the plan was Vortec heads Ill eventually get a new intake manifold as well (this is slightly tricky with the low hood clearance of the 76 corvette). The end goal is to have a new exhaust as well, but the keeper of finances says engine first. So this will have the stock exhaust for a year or two till I can get headers and side exhaust.

At first, the cost was driving everything and even tried to hand hone the cylinders etc trying to just reuse as much as I could. I realized that it would be a better option to spend the time (and save up some money) to do it right. It would be cheaper to buy a crate motor but I've never built and engine before so I know Ill spend a bit more but Ill have a lot more fun and continue to learn even more.

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Re: Realistic Octane Requirements

Post by Newold1 » Thu Nov 22, 2018 8:09 am

As your 76 is a carbed Corvette are you looking at making a carb and intake manifold change as part of your 400 build?

If you can afford the added expense the Holley Sniper EFI would be a great upgrade as well as an easy tuning tool.

After reading your car is a 76 Corvette and you do not plan to race it at all, my recommendation of a hydraulic roller camshaft is highly reaffirmed as the best choice! There are plenty of "Camshaft " experts on here who can give you some good recommendations on a hydraulic roller grind that will make the best overall torque in that 1500-4200 rpm range that you will find yourself driving in 95% of the time and still make it easy to spin those Corvette limited tire sizes.

I would also say you should consider using the complete SCAT internal balance rotating assembly kit. Good pricing and quality parts well put together make this type of kit a good choice.

Sounds like you are going to have some fun with your Corvette. Use that extra power safely especially on our winter roads here in the New England area.e
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Re: Realistic Octane Requirements

Post by RevTheory » Thu Nov 22, 2018 8:51 am

Unless things have changed, the manifold selections for the Vette suck. Even a Performer RPM won't fit. I wonder if an EPS will and I bet that would work out pretty well in this application :-k

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Re: Realistic Octane Requirements

Post by GARY C » Thu Nov 22, 2018 10:42 am

One issue I see is with the cam and compression in the OP the cylinder pressure will be sky high and I don't think thats good for pump gas with iron heads.
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