Author Topic: Quench and Compression Ratio  (Read 574 times)

Offline Ronno6

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Quench and Compression Ratio
« on: January 04, 2019, 07:48:54 AM »
I do tend to get caught up in specs.
A mental bent left over from my years in purchasing mechanical parts and pieces.
My 355 build reached a point where I was ready to install the heads.
But, at TDC my piston decks were .040" below the block deck.
Plugging in my numbers into the equation, I calculated my C/R was gonna be 9.25 :1,
and my squish was gonna be .083"
I had expected a better C/R.
So I am having .012: shaved from the block and plan on using .015" gaskets.
This will yield an optimum .043" squish, but C/R will be 10.1:1.

I have looked at the specs for GM crate engines.
Some have C/R as high as 9.6:1 with 62cc heads.
Near as I can reverse calculate, that puts the squish for that setup at about .070" depending on the dish of the piston.

This gives me pause to turn my palms to the sky, shrug my shoulders and say WTH ???
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline ehjorten

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Re: Quench and Compression Ratio
« Reply #1 on: January 04, 2019, 09:00:43 AM »
You want you quench to be about 0.040" for steel connecting rods.  You can push it a little tighter on engines that don't push over 6,000 RPM to something as little as 0.032" quench.  If you have aluminum connecting rods, they stretch more than steel connecting rods and now you are looking at an ideal quench of around 0.050" - 0.065" quench.  It isn't uncommon to see old engines that had a much larger measurement than this.  Current engine theory shows that having a smaller quench area is much better for overall efficiency and reduces significantly the amount of un-burnt gases.  Old school theory used larger quench to reduce compression and slow the burn rate to reduce detonation, but that proved to be counter-productive.

No problem with 10.1:1 compression, it all just depends on what cam you are running and what octane of fuel you plan to run.  Check you piston to valve clearances!
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline Ronno6

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Re: Quench and Compression Ratio
« Reply #2 on: January 04, 2019, 10:51:43 AM »
You want you quench to be about 0.040" for steel connecting rods.  You can push it a little tighter on engines that don't push over 6,000 RPM to something as little as 0.032" quench.  If you have aluminum connecting rods, they stretch more than steel connecting rods and now you are looking at an ideal quench of around 0.050" - 0.065" quench.  It isn't uncommon to see old engines that had a much larger measurement than this.  Current engine theory shows that having a smaller quench area is much better for overall efficiency and reduces significantly the amount of un-burnt gases.  Old school theory used larger quench to reduce compression and slow the burn rate to reduce detonation, but that proved to be counter-productive.

No problem with 10.1:1 compression, it all just depends on what cam you are running and what octane of fuel you plan to run.  Check you piston to valve clearances!

Thanks for that response!
I am using the LT4 Hot Roller Cam which has a lift of .482" with 1.5 rockers, which I am, 218° and 228° duration
respectively. 112° lobe separation.
I do not anticipate any valve interference as with 1.6 rockers the life would be .525"
Heads are aluminum w/2.02" I and 1.6" E valves.
I am about 270' above sea level and will burn whatever octane gas the setup requires..up to 93.
I am hoping that with good cooling and cold air induction, it will run  ping-free......
This should be a fairly lively setup, or, so I hope...
 
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline VileZambonie

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Re: Quench and Compression Ratio
« Reply #3 on: January 04, 2019, 07:55:24 PM »
Just be careful. Don't bump your compression ratio too high in an attempt to have a tight squish area especially if you are running pump gas. Focus more on the desired compression ratio than the squish area. Ultimately you are going to end up with carbon buildup and variants in fuel quality and you will lose more power to detonation as the quench does not have enough ability to effectively lower combustion temps to a varying threshold. You'll be dealing with adverse effects if you don't stay focused on the bigger picture.
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Offline Ronno6

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Re: Quench and Compression Ratio
« Reply #4 on: January 04, 2019, 10:24:05 PM »
I haven't had any naysayers to 10.1:1 C/R.

I'm keeping the quench on the high end of desirable, .043"
I have read that having a tad more compression with quench in the right range is better than having lower compression and thicker quench layer.
But, having only the minimum machined from the decks leaves me the room to put thicker head gaskets on in the event that things are too hairy at 10.1:1.
I can drop it to 9.4 by using the .043" head gaskets that came with my set,
Quench would then be .071"
Once the combustion chamber size is set, in my case 64cc, the rest is just math: lower C/R = wider quench.
No way around it..........
You can lead a man to water, but yoiu can't keep him from pissing in it.

Offline ehjorten

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Re: Quench and Compression Ratio
« Reply #5 on: January 07, 2019, 09:38:09 AM »
With your LT4 Hot Cam and your engine specs, you might be okay with 87 octane, but could be on the edge of detonation.  Your dynamic compression is right at about 8.0:1 if your intake valve closes at 65° ABDC.  Advertised valve events on GM cams can be hard to find.  Your aluminum heads allow you to bump up the compression ratio a little more due to the increased heat rejection.
Be prepared to either run higher octane fuel than regular 87 or play with the timing, possibly retarding the camshaft a few degrees, or running thicker head gaskets.  Those thin shim gaskets can sometimes be tricky to keep from coolant leaks.  Make sure you re-torque the head bolts after you break it in.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline Ronno6

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Re: Quench and Compression Ratio
« Reply #6 on: January 08, 2019, 10:19:57 PM »
With your LT4 Hot Cam and your engine specs, you might be okay with 87 octane, but could be on the edge of detonation.  Your dynamic compression is right at about 8.0:1 if your intake valve closes at 65° ABDC.  Advertised valve events on GM cams can be hard to find.  Your aluminum heads allow you to bump up the compression ratio a little more due to the increased heat rejection.
Be prepared to either run higher octane fuel than regular 87 or play with the timing, possibly retarding the camshaft a few degrees, or running thicker head gaskets.  Those thin shim gaskets can sometimes be tricky to keep from coolant leaks.  Make sure you re-torque the head bolts after you break it in.

I'll run higher octane if I need to.
As for the shim head gaskets, those are the same as I used on the first engine I ever rebuilt: a 283 in a '57 Chevy wagon in 1969. To us, rebuilding consisted of reaming the ridge, honing cylinders,de-carboning ring grooves, and reassembling with new rings,bearings and gaskets. We had no telescoping gauges, micrometers or calipers, dial indicators, etc. Just a beam torque wrench and a bunch of energy..............
You can lead a man to water, but yoiu can't keep him from pissing in it.

Online bd

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Re: Quench and Compression Ratio
« Reply #7 on: January 08, 2019, 10:34:25 PM »
Red Bull gives you wings!   ;D
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Irish_Alley

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Re: Quench and Compression Ratio
« Reply #8 on: January 09, 2019, 07:13:22 AM »
Red Bull gives you wings!   ;D

cant say that anymore, they got sued for false advertisement
If you can’t tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline ehjorten

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Re: Quench and Compression Ratio
« Reply #9 on: January 09, 2019, 08:41:03 AM »
I'll run higher octane if I need to.
As for the shim head gaskets, those are the same as I used on the first engine I ever rebuilt: a 283 in a '57 Chevy wagon in 1969. To us, rebuilding consisted of reaming the ridge, honing cylinders,de-carboning ring grooves, and reassembling with new rings,bearings and gaskets. We had no telescoping gauges, micrometers or calipers, dial indicators, etc. Just a beam torque wrench and a bunch of energy..............

Of course it can be done, and I have done it!  However, having thrown a rod through the side of a block about a month after rebuilding, I would add measuring and resizing the big-end of your rods as one of the necessities!
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6