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SBC 383. Blow by

General engine tech -- Drag Racing to Circle Track

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73c34me
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Re: SBC 383. Blow by

Post by 73c34me »

F-Bird. The engine is decked to 9.005”. SRP flat tops, .039” x 4.166” head gasket. 60 cc chamber. I haven’t heard any detonation or feel it. nor do I lug the engine. No run on when shutting off. Cam is a Chris Straub speced hydraulic roller
4.1:1 RP 4speed wide ratio Muncie and a 3100 lb car
The plugs are oily , so looking for tell tale signs is tough.
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Re: SBC 383. Blow by

Post by ptuomov »

So what’s the compression ratio? Oil in the combustion chamber lowers the fuel octane. If you don’t have knock sensors, then I’d ask the question does high octane race fuel reduce blowby?
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Re: SBC 383. Blow by

Post by F-BIRD'88 »

You will not hear nor feel high speed detonation..
The damage happens really fast.
If you rebuild it same same on same fuel it will happen again...
The cr is just too high.
Low speed detonation knock ping that you can hear while driving does not damage the ring lands. Its mostly just annoying.. But high speed wot detonation kills ring lands
. Typical silent damage to high strength forged pistons..
Once the ring lands are damaged the $$$ pistons are toast.

Your cr is well in excess of 11:1 cr.. It will need +/- 100 octane gas..
What pump gas do you run?
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Re: SBC 383. Blow by

Post by F-BIRD'88 »

You can see this high speed detonation while dyno testing or other wot high load stationary engine run.
Sen it on marine engines where you can watch the engine @WOT. running.. You can see the valve cover vent huffing as the rings and lands are being pounded @WOT.
Permanent damage happens fast.
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Re: SBC 383. Blow by

Post by JoePorting »

Before you pull apart the motor, I'd focus on the carb. Might just be a blown power valve and clogged up air bleed holes. Carbs don't like to sit around for years. Maybe try and lean it out too with a lower fuel level and smaller jets. Smoke maybe from a rich carb, or improper mixed air and fuel. Maybe buy a new carb if the old one is old. Might also be a weak electrical system or weak ignition. Maybe try hotter plugs. Try all the cheap/easy options first before you rip the motor apart.
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Re: SBC 383. Blow by

Post by 73c34me »

I did go thru the carb- cleaning, inspection etc. jetted lean and
New power valve. tried a hotter plug. No change. Worked back up to current jetting. It Runs very crisp and excellent throttle response. New wiring MSD wired directly to battery. Additional ground on motor directly to battery.

Accusump. Any chance my accumulator is adding oil volume when it shouldn’t and creating a windage problem?
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Re: SBC 383. Blow by

Post by F-BIRD'88 »

On a SBC intake port when you port them say to a 1206 gasket size and then also mill the heads the intake flange gasket sealing at the bottom of the port opening gets narrow.. This allows the intake gasket to let oil in from the lifter valley. The more you shave down the heads the worse it is. This makes it smoke and use oil too.
The oil gets in the combustion chamber creates detonation which fubars the piston rings and ring lands fast. All accelerated with excessive compression ratio for the fuel octane used .
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Re: SBC 383. Blow by

Post by 73c34me »

Gotcha. I will get an inspection camera and look at the bores and piston tops.
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Re: SBC 383. Blow by

Post by skinny z »

73c34me wrote: Sun Sep 27, 2020 8:00 pm The engine is decked to 9.005”. SRP flat tops, .039” x 4.166” head gasket. 60 cc chamber.
11.45.jpg


Seems high but workable depending on the cam spec.
Still, I'd be concerned about detonation too.
Unless the cylinders were ruined as a result of the leaking carb, I'd be looking at the intake manifold seal as has been suggested. Seems a likely spot to be sucking in oil. Especially considering that you say this issue started to show up after the heads had been cut.
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Re: SBC 383. Blow by

Post by F-BIRD'88 »

This won't tell you much about the condition of the piston ring lands.
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Re: SBC 383. Blow by

Post by PackardV8 »

skinny z wrote: Mon Sep 28, 2020 12:48 pm 11.45 Seems high but workable depending on the cam spec.
Still, I'd be concerned about detonation too.
X2; We'd need to know the cam specs. That high cranking compression on pump gas in a street car is ragged edge.
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skinny z
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Re: SBC 383. Blow by

Post by skinny z »

F-BIRD'88 wrote: Mon Sep 28, 2020 2:33 pm This won't tell you much about the condition of the piston ring lands.
True enough. That said, having a look with an inspection camera may not reveal damage and the broken bits could be out of view.
Case in point. I've beat up a 355 SBC with lots of timing, lean cruise and high compression. In it's lifetime it started to have greater amounts of blow-by. Oil consumption didn't seem much of an issue though. Cranking compression through various iterations, namely head work including milling as well as cam swaps, varied from 200+ (short 274 cam @ 600' elevation) to the current 165 PSI average (288+4 cam @ 3100' elevation). That latter being a less than stellar effort by yours truly.
Plenty of tip in rattle at cruise and it had a few top end WOT lean outs (although generally I'm in the 13-12.5:1 AFR range). But it did see less than ideal drag strip conditions on more than one occasion (some of which is fuel delivery related).
But, despite the dire warnings of many who said the pistons and rings were smoked, having the heads off revealed nothing other than some crud that had to be wiped off the piston tops. No visible damage to the pistons or the cylinders. No gouges and deep scratches to be seen. This doesn't mean that I don't have a pile of broken rings but a visual revealed diddly. The cylinders showing plenty of wear though and my guess is they're tapered, out of round and barrel shaped after years of overly rich, cold start carbureted use! Great engine otherwise.
Any way, what I'm getting at here is that I prefer the forensic approach and like to look for and analyze all of the possible contributors. Unlike many, I'm not so thrilled at the prospect of taking my engine out and spreading it across the workbench unless I'm reasonably certain that'll have to happen.
And for what it's worth, this has to happen in my case as there are other issues.

As this relates to the OP, what I saying is figure out the root cause first obviously. Seems you're on the right track just by being here.
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Re: SBC 383. Blow by

Post by 73c34me »

OP here. Cam card. 1.6 roller rockers intake and exhaust
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Re: SBC 383. Blow by

Post by skinny z »

73c34me wrote: Mon Sep 28, 2020 6:16 pm OP here. Cam card. 1.6 roller rockers intake and exhaust
With that, the dynamic compression is 8.66:1 (or thereabouts). More than I've ever run on a street engine. Iron heads and low 8's (might have been 8.4) gave me grief. But I drove that thing everywhere and under all conditions. Seems the racing engine guys would have no difficulty with that kind of compression. Even welcome it.

Untitled.jpg

I still like the idea of the leakdown test. Which is exactly where I'm going with the build I described above. The first test didn't yield the best results but that may have ben operator error. I've got to give it another go before I take it apart. If I need to that is.
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Re: SBC 383. Blow by

Post by 73c34me »

I ran 93/94 octane with a boost, 180-190 coolant temps and did not lug the engine. Perhaps that compression ratio can’t be done. Track time would have had race gas.

I will start with a camera inspection and see if I can borrow a decent leakdown tester.
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