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Thread: Dynamic compression ratio
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08-18-2020, 08:10 PM #1LTX Member
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Dynamic compression ratio
Has anyone done any testing on various dynamic compression ratios in the LT1 gen 2 engines, particularly the iron head models?
I ask because I have been finalizing specs for my new engine build for what is really a stock torque curve shape, but higher, type engine for my daily and cruiser RMW, 1996.
Over the years, I have found the "rule of thumb" that says somewhere in the 7.0-8.5 DCR is where engines will make the most power to be quite true unless on racing gas, and along with never going over 8.5 on 91 octane, no ethanol, fuel also pretty good, but a little lower is better on less radical engines that build more cylinder pressure.
My first run on the calculator with the custom built cam I had chosen came back at about 8.9 DCR on a 10.5 static, which was a very big surprise as the cam timing is pretty similar to stock but with fast ramps and higher lift. The stock engine runs fine on midgrade at 89 octane with ethanol even. To see what was going on, I ran the calculator on the stock engine at nominal specs most have measured and got 8.8 DCR. This is on an iron head (bad for detonation) with horrible quench distance at .068" (also bad for detonation). The only explanation has to be the reverse flow cooling keeping the heads cooler, as the rest is all normal style parts.
Has anyone done anything that directly compares similar engines with and without reverse flow cooling to see just how much it will buy you in DCR? I have heard that it will give one point of static, but no testing to back it up.
It appears to me, based on the stock engine being at 8.8 and having no issues with detonation, I should be fine with doing my proposed setup as it will also have quench at .040, which is tight enough to really knock down detonation.
Opinions welcomed and desired.
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10-04-2020, 09:07 AM #2
Static and Dynamic Compression Ratio Concerns
I have a very similar question, that I posed to an advanced engine building group on FB. I will relay the results back to you when I get them; though our builds are slightly different (93 Octane vs 91) and different CR and DCR.
My question:
Static and Dynamic Compression Ratio Concerns
BLUF: I'm afraid my dynamic Compression Ratio may be too high for 93 pump gas. Should I look at a bigger cam to help bleed off cylinder pressure, or bigger cc relief pistons, or will I be fine with my current setup?
I think this Group would have the best input/advice on this specific question. I apologize in advance for the excessive detail but I would rather give more than less information to the experts for review. I appreciate in advance any guidance you could give me that might save me a very large headache in the future.
DETAILS:
I am currently building the powerplant for a C4, 6 speed Vette. I want it to be a good street engine with great reliability and efficiency. I planned originally to run E85 full time but the current gas situation has me worried and want to be able to use 93 if necessary.
Engine: Gen II SBC LT1, (355, 4 bolt main, aluminum heads)
I Plan on running The Holley Terminator X with a flex fuel sensor and running primarily E85 but want to be able to run 93 octane if need be when E85 is unavailable. I chose a medium sized cam to maintain good street manners and drive-ability but perhaps a larger cam will help with my overall numbers.
Through numerous online calculators, I am concerned this setup may have a dynamic compression ratio that is too high for 93 pump gas. I've been primarily using the Pat Kelly DCR calculator but have used others and get about the same results (I included some screenshots below):
Static compression Ratio: 11.0942
Dynamic compression Ratio: 9.26250
quench distance: .037
Over the years, I have researched and found that the "internet rule of thumb" says with pump gas (93) the upper limits are around 8.5-8.9 DCR is where engines will be mostly detonation free. Mine seems like it may be too high even with some better cooling properties (below), however, I may be missing something in my calculations.
Factors on this build that should help with detonation:
LT1 Reverse cooled heads (aluminum)
Large alum radiator
170 deg thermostat with programed fans
AI Ported heads/intake
1 & 3/4 full length headers (ceramic coated)
Colder Plugs
Good Quench: .037
Flat Top pistons: -12.300 cc
LS Ignition (Torque-Head 24X kit with Holley Term X engine management)
Full Details:
RESULTS from PAT KELLY Dynamic CR Calculator:
Static compression Ratio: 11.0942
Dynamic compression Ratio: 9.26250
quench distance: .037
Stroke 3.48
dynamic Stroke length with a 6in rod 2.8485
Rod Length 6.0
Distance of Piston from BDC: .63149
Combustion Chamber: 52cc
Head Gasket Thickness: .027
Head Gasket Bore: 4.04
Piston Deck Clearance: -0.01
Valve relief cc: -12.300cc
CAM: Advanced Induction 234 (2634-10 HR)
LSA: 110
Timing: 4.2 Adv
Overlap @50: 10.6
Intake
Centerline: 105.8
Tappet Lift: .355
Rocker Arm Ratio: 1.6
Valve Lift: .567
Int open @50: 6.3 BTDC
Int Adv open:23.1
Duration @ .050: 226.9
Adv Dur: 261.0
Cam Lift @ TDC: .0690
Int Close @50: 40.6 ABDC
Int Adv close: 57.9 ABDC
Exhaust
Centerline: 114.3
Tappet Lift: .354
Rocker Arm Ratio: 1.6
Valve Lift: .566
Exh close @50: 4.3 ATDC
Exh Adv close: 20.4
Duration @ .050: 234.6
Adv Dur: 266.3
Cam Lift @ TDC: .0619
Exh open @50: 50.3 BBDC
Exh Adv open: 65.9 ABDC
- Squared Deck height now is 9.00'' (.018 cut)
- Align honed main bearings
-Bore and hone with TQ Plates:
4.030,
-Cometic Gaskets Chevy Head Gasket 1992-96 LT1/LT4 5.7L
Part Number: 245-C5645-027
Bore: 4.040"
Compressed Thickness: .027"
-Advanced Induction CAM 226 / 234 - .566" / .566" - 110 LSA
LS6 springs, hardened Pushrods, LS7 lifters, 1.6 Roller rockers
-52MM TB, Air intake,
-Advance Induction Heads :
200cc intake (@ .600 .287int and 196 exh)
Ai Custom Machined Stainless Pro-Flo Valves 2.00" Int & 1.56" Exh
-42lb injectors
-Ported LT1 intake
-CRANKSHAFT:
Stock Crank stroke 3.48 (dynamic Stroke length with a 6in rod 2.8485?)
Balanced
Crank Cut -10 Under?
-PISTONS:
Race Tec: Auto Tec
PN: 1000133
Bore: 4.030
Stroke 3.48
Rod Length: 6.00
Comp Height: 1.250
Type: Flat Top
Valve Relief: -12.300cc
-RODS: Scat 6.0
CR6003 - PBM Performance - 4340 Forged I-Beam Connecting Rod 6.000"-Bushed Pin - 3/8 Cap Screw
Brand: PBM Performance
Category: Forged I-Beam
Make: Chevrolet
Material: 4340 Forged Steel
Notes: Performance Only - Not OEM Replacement
Pin Journal: 0.927
Pin Type: Bushed
Rod Bolt Type: 3/8 Cap Screw
Rod Journal: 2.100
Rod Journal Width: .940
Rod Length: 6.000
-HEADS: Advanced Induction 200cc GM LT1
52cc Chamber
Intake Valves: 2.00 x 5.010 x 3.413
Exhaust Valves: 1.56 x 4.995 x 3.413
Intake @ .550: 282.5
Exhaust @ .550: 192.7Last edited by Cuda99; 10-04-2020 at 09:14 AM.
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10-04-2020, 11:10 AM #3LTX Member
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Thanks for the reply, and I will be eager to see what you get for an an answer. I posed a similar question to the shop that will be doing my machining. They are also a high end race engine, muscle car, street rod engine provider and considered one of the best around here. They were surprised that my calculations came out to 8.9 dynamic on a 10.5 static, but the cam I have speced is quite odd and they thought that was why. I will have -14.5 degrees overlap at .050 and +11.5 degrees at .006 with it. The -15ish looks to be pretty typical of the stock cams for the LT1 gen II engines, based on the specs I have found. It is similar duration, but a bit more, than the stock iron head cam at 200/206, but has much more lift with the faster ramps. I showed them the workup on the stock engine that showed a dynamic compression ratio of 8.8 and he was very surprised as they have nearly zero LT1 experience for designing specs. His bottom line was similar to what I have been thinking in that if the stock engine doesn't detonate at 8.8 with it's .068 quench distance, I should easily be good at 8.9 with .040 quench and that it had to be because the heads have less hot spots with reverse flow cooling.
It appears your pistons will be a two valve cut quench/dish style, which is what I will be using also to get rid of the quench killing four cut stock style. Mine will be Keith Black hypers and fit on the tight side as this will be a low rpm engine. I am bit surprised at the .037 quench you will have with forged pistons. What alloy will they be? How much rpm? I am shooting for .040 quench with the tolerance to tighter, but I don't think I would go under .038 even with tight fit hypers. I haven't been able to find any information on what the reverse flow cooling does to piston temps compared to standard flow as that can be a big deal in kissing the heads with pistons sometimes. It looks like you will be on long rods also, are the pistons going to short skirt?
With the aluminum heads, you would normally get about 1 point of cushion over steel heads and if that is true, you should actually be a bit less gas critical that I will be. You have a lot more cam than I will have and it will look even bigger with 1.6 rockers.
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10-04-2020, 04:37 PM #4
Of course, I didn't want to hijack your thread but I wanted to see if we could both benefit. I too think you will be fine but I would rather measure that with real world data and experience as well. It looks like wen you are finished you will have a very solid and reliable perfomer that should go forever and give you some great power numbers. So far, the engine advice I have received has been positive but these are quirky/oddball motors so I have to take everything with a grain of salt. I have had some good responses and most seem to think it will be alright with a number of folks who didn't fully read (i understand with the very long post).
I've done a lot of research on the quench and honestly with my combination would even feel safe going down to .035. I'm not going to be spinning this past 6,500 rpms and the pistons are made from "premium, high silicon 4032 aluminum alloy" and the rods are 4340 forged steel 6 inch rods. Yes, the pistons are designed for the rods with a short 1.25 compression height. Also, the cam specs on mine are calculated with 1.6 rockers so it's really nothing crazy.
I'll keep you updated if I hear anything of interest and I look forward to seeing you build progress.
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10-04-2020, 07:08 PM #5LTX Member
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Glad to see the 4032 pistons for low expansion not only because of growth up but also because you can fit them tighter so they rock less. When I was asking about short skirts, I was referring to area below the pin that stabilizes the piston against rocking and slap. If they are short skirt like the LS pistons with narrow rings, are and on 6" rods so shorter overall also the rocking can get to be more. If you have big rod and main clearances, floating pins etc, it all adds up. I think the biggest one would be possibility of the pistons running hotter on the warmer cylinder walls with reverse flow. It will interesting to see how well your quench heights match as that is often an issue depending on the dimensional consistency of the parts and the machining. I usually wind up mixing and matching the pistons and rods into the different holes to get the most consistency between cylinders. If I get between .001 and .002 variation I generally figure I did pretty well, especially when compared to a stock engine. There is a reason they leave them with big quench distances, unfortunately. I always find it interesting to measure the rocking also as it is a good final test of a lot of dimensions and is generally good to know.
The plan is to pull the engine in early November once all is buttoned up on the house and yard for the winter and start measuring to see what piston oversize it will take and how much deck will likely be required. It will be going to zero deck so probably about .020 and likely will need to even out besides. I will measure the head CCs first and figure if they need to be angle cut and about how much and get them to the shop first while I get the rest checked completely. When the heads are done, I can do a final CC match with them and calculate the final deck dimension to get the right compression ratio. The iron heads are very small chambers and ports so need to be pretty well matched.
The car (96 Roadmaster wagon) will also get ABS delete, air pump delete, EGR delete as well as a trans rebuild and cooler added and a rear gear change to 3.08 from 2.53 so it will be a busy winter. I will be getting a Jet tuner to do the tune, which shouldn't be a whole lot with the changes being done.
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10-08-2020, 12:10 PM #6LTx Guru
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If you are interested in the cylinder bore temperature profiles, both front to back cylinder-to-cylinder, and top to bottom of the cylinder, there is some limited data (graphics, without actual temperatures) in the paper GM prepared for an SAE presentation, back in 1992. The paper indicates their objective with reverse flow cooling was "...warmer cylinder bores (up to a point) yield lower ring friction."
The reference is:
Paper # - 920673 - New Generation Small Bock V8 Engine, Anil V. Kulkarni, 54-pages.
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10-09-2020, 12:27 PM #7LTX Member
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I may try to dig that one out, although if it is an SAE paper it may now bb pay for type.
What I was hoping to find out was if there was anything the compared the piston and cylinder wall temps for the reverse flow cooling to conventional SBC engines to get a feel for what piston clearance to go for. We know the cylinders run warmer but I have never heard or seen anything on if the pistons follow that or not.
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10-09-2020, 01:19 PM #8LTx Guru
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I'll look thru to see if it mentions the delta T compared to the L98. I got my copy 20+ years ago it cost like $10. I have since lost the digital copy, and as noted, the print version is over 50 pages.
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10-09-2020, 03:14 PM #9LTX Member
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Thanks, I appreciate it
I just looked at my factory service manual to see what the give for stock piston clearances. It shows .001 to .0027" with a wear limit of .0027 (gotta love that new could be at wear limit and pass spec). That seems to be pretty close to other 4"ish bores I have seen IIRC. Will need to dig it out for an early non reverse flow version, though. They did some weird specs on the LTs, on our 96 the main bearing clearances are the same on the center three, tighter on number one, and looser on number 5.
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