9.5:1 Compression

[TR One]

So I'm in the process of building up a motor for my GMC...  The crank on the block I am building will need to be turned, though I think it is only .010 over now, so there should be room.

However, I came across a full Olds Stroker rotating assembly from Eagle for sale locally.   Contains everything from harmonic balancer to flex plate.  Pistons, bearings, rings, etc. and the assembly was pre-balanced.  Looking at the machine work and parts I need for my existing block, buying this assembly ends up being not much more than I would have to spend on re-machining what I have, and I'd pick up some displacement.  (It's a 488 stroker kit)

The main issue I am concerned about, is the compression ratio.  With the Ka heads I have and the pistons that come with the kit, I think I am going to be around 9.5:1.  I haven't cc'd anything, but using stock specs, that's what I am seeing using an online calculator.

I kind of want to stay away from running premium, if I can, but don't think that's possible with the stroker kit.  I am in the process of moving to ESC on my TBI setup, but I don't know if the system is robust enough to support that compression ratio on regular gas.

For those who have more experience with ESC and Olds motors in general, should I just let the kit go?  Or do you think it may be safe to run regular in that setup?  Any other issues you can think of that could influence my decision?  It is forged pistons, rods, etc. but from what I can tell, the alloys used are low expansion, so I don't think I need to worry about the stock tolerances being too tight for the motor....

Thanks in advance for any thoughts or advice on which way I should go!

Mark.

[James Hupy]

 Nope, with that much compression, you "shoud" run premium fuel. Particularly with K/a cylinder heads. 

     Two ways you could go. Dished top pistons or J heads. Either will drop the compression. If you already have a ton of money in the heads, then the pistons are the way to go. Heavy vehicles with tall differential gearing need a ton of bottom end torque. Not high rpm horsepower.

Jim Hupy

Salem, Oregon

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[George Beckman]

Unless you live in absolute flat land and don’t mind buying premium gas, this is a bad deal. 

I used a loaner engine from Manny of Manny’s Trannys—pulled for a diesel install. It was at least 9.5 to 1. Lovely engine. Balanced. Compcam rockers. The fancy-dancy pistons in the engine are pictured. One seller warned to use at least 96 octane for those pistons. It was an oil drinker. About a qt. per 300 miles.

I use EBL (ESC) and had the spark so far retarded the compbustion chamber was getting way too hot. (Spark was running @29*! when pulling and still pinging horribly.) It didn’t ping for the first 30 seconds to a minute of a pull.

On a not-so-steep hill, I forgot to select my super retarded bin. Soon, I was in 2nd, going 20 mph, and every time I touched the gas, it rattled so loud my wife was alarmed. I said, “This engine is tearing itself up.”  I’ve heard pinging all my life and have never heard anything like this. This wasn’t the normal rattle—this was steady like a model airplane engine. I’m surprised I didn’t put a hole in a piston.

Burned an intake valve on that trip. Intakes aren’t made for heat. The more spark is retarded, the hotter the combustion chamber gets because you have to use so much more fuel to do the same work.

I tried water injection. BTW, that shut it up—instantly.  Used a lot of water and ran out. I warped an intake valve on that trip.

 I used premium gas. Puttered up hills. Pulled over and let it rest. (We are not talking water temp. The engine was not hot.) The coach was really no fun to drive because if you needed power for anything more than a freeway overpass, it began to ping. You drove, dreading the need to press the gas.

Did the head myself and managed two short trips. On the third tip, I trashed at least 2 valves. I didn’t even bother with a compression test. I could hear it when the engine cranked, and it was missing on 2 cylinders.

I now have an absolutely stock S&J engine, and it has every bit as much power as the detuned hotrod. The S&J engine runs down the road with spark at 40*.  Yes, it retards on climbs but nothing like 29*. I use regular. Doesn’t ping. Doesn’t use oil.

We need a stump puller, not a dragboat.

On Feb 8, 2024, at 8:33 AM, TR One <mark.s...@gmail.com> wrote:

The main issue I am concerned about, is the compression ratio.  With the Ka heads I have and the pistons that come with the kit, I think I am going to be around 9.5:1.  I haven't cc'd anything, but using stock specs, that's what I am seeing using an online calculator.

I kind of want to stay away from running premium, if I can, but don't think that's possible with the stroker kit.  I am in the process of moving to ESC on my TBI setup, but I don't know if the system is robust enough to support that compression ratio on regular gas.

---------

Best Wishes,

George
gbec...@graestone.org

[bc...@juno.com]

I used water injection on a couple of engines. Yes it was very effective
at stopping knock. But it used a great quantity of water, I thought of
setting up a gauge, and a sump to refill with water condensed by the
air conditioning. It was too easy to run out of water, and sometimes a
water jet would plug up.

Eventually I abandoned all high compression engines. Now much
larger displacement engines here produce just as much torque trouble
free, on the cheapest unleaded. No knock sensors. Bruce Roe
++++++++++++++++++++++++++++++++++++++++++++++++++
From: George Beckman <gbec...@graestone.org>
To:EFI GoogleList <gmcm...@googlegroups.com>
8 Feb 24

[George Beckman]

Yup. The 455 was designed when you could get Chevron 104.

On Feb 8, 2024, at 3:13 PM, bc...@juno.com wrote:

Eventually I abandoned all high compression engines. 

[Keith Vitko]

Has anyone attempted installing an electronically controlled EGR using EBL for knock reduction and better fuel economy when working with these higher compression modifications?

https://www.sae.org/publications/technical-papers/content/860034/

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[TR One]

Ok, thanks guys...  Yes, I'm definitely not looking to build a drag car engine.  My goal is to build a reliable stump puller.  That's part of the reason the stroker kit caught my eye.  Longer stroke =  more low end torque.  

So 9.5:1 is out....  Don't want to mess with premium or water injection, or meth injection or anything like that.  What would be the ideal compression ratio to shoot for on a GMC motor?  I've heard 8.5 to one mentioned a lot.  Would 9:1 still be too high?

If I was able to lower the compression ratio, I still like the idea of stroking the motor.  I don't run my motor more than 3500 or so, and I think the stroked motor would be ideal for that.  So if I go this route, I could lower the compression ratio by:

1. Going with new pistons, as mentioned. 

2. Go with heads with a larger chamber.  The Ka heads actually are large chamber/low compression from what I am able to find.  Same 80cc chambers as the J heads from what I have read.  I believe with small chamber heads like ones used on W30 cars, or aftermarket heads like the edlebrocks, you'd be more like 10:1 with the pistons supplied in the Eagle stroker kit.  Does anyone know of larger chamber heads available to our engines?  

3.  I also looked into running a thicker head gasket to lower compression,  Cometic makes a range of gaskets all the way up to .120"  (I think stock is around .042"), but I heard the change in cylinder quench can cause detonation on it's own...  Negating the drop in compression.

I have no experience doing this, though and am still researching for information on quench.  Has anyone here tried this method to lower compression on an Olds or any other motor?

Any other ideas on how to lower compression that would be viable on a stump puller motor?  

[TR One]

Did a little more research, and it looks like changing pistons is not an option, unless I want to go with custom pistons...  The 22cc pistons in this kit are the largest they make for the Olds stroker application.

Anyone have any experience with running thicker head gaskets or enlarging the chambers on cylinder heads?

[Matt Colie]

Mark, 

Before you go to great lengths trying to improve the low end performance of a 455, there are other things that you might consider.  

In the words of Soichiro Honda (probably a less than accurate translation) when discussing F1 engines,"CC's are heavy and cost money, RPMs are free"  

While the heavy rotating mass of a Olds BB are not conducive to high crankshaft speeds, If these engines are carefully balanced, such is very possible.  Another thing to be aware of is that running an engine at high BMEP and slow crankshaft speeds is a recipe for a short life. 

If I had you desire (and I don't, I have a 23 that crossed the Bighorn with ease), for my money as a multi-decade engine development engineer, I would start saving my pennies to buy a 3.70 final drive and maybe even with LSD option.  Those that are running a 3.70 and a 455 are not reporting and reliability issues and often report better fuel rate than most.  

Best of luck 

Matt Colie

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Virus-free.www.avg.com

[James Hupy]

What Matt says is the real way to go. 4000 RPM equates to about 78+ miles per hour. That is plenty fast enough to be herding one of these coaches down the road. A properly balanced and assembled 455 will run all day at 3750 rpm. Climb hills too.

Jim Hupy

Salem, Oregon

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[Larry Weidner]

I would agree with Matt and Jim’s assessment of the way to get more oomph out of your 455. If the 3.70 is to steep for you there is a 3.50, a 3.42 and a 3.21 all of which will help, but to a lesser degree.  The late, great, Bob Drews after ruining a couple of sets of heads trying to install hardened valve seats, (broke through into the water jackets) finally put down the money to buy a set of Mondello aluminum cylinder heads. He spoke of how impressed he was with how it seemed to wake his 455 up.  Don’t know if I’d get the aluminum heads because of the difference in expansion rates of the two metals possibly leading to head gasket issues. Perhaps Mondello has a set of bench flow tested iron heads with bigger valves that might be worth considering. Those heads with a good set of headers and a well tuned EBL EFI and a taller Final Drive should get you up those hills.  JWIT

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[Johnny Bridges]

My 23 had a stock 455 w/q-jet and stock final drive.  My 26 has a 3.7 final drive and 455 with Atomic MSD fuel injection.  Both ran 65 towing a Jetta or a Ranger pickup over Monteagle without any problems.  I'd skip messing in the engine, put the 3.7 in instead.

--johnny

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[Gerald Work]

I’m not sure why I am getting these emails all the sudden (glad I am), but this conversation is certainly waking up some fond memories.  On our two restored GMC nothing improved the mountain driving like the proper gearing (3.70 was the sweet spot) and the proper cam timing (ask Jim H. How to set that).  Hi to all……

Jerry

Jerry Work

The Dovetail Joint

Fine furniture designed & hand crafted

in the 1907 former Masonic Temple building 

in historic Kerby, OR

http://jerrywork.com

On Feb 9, 2024, at 5:49 PM, Larry Weidner <Weid...@wwt.net> wrote:

I would agree with Matt and Jim’s assessment of the way to get more oomph out of your 455. If the 3.70 is to steep for you there is a 3.50, a 3.42 and a 3.21 all of which will help, but to a lesser degree.  The late, great, Bob Drews after ruining a couple of sets of heads trying to install hardened valve seats, (broke through into the water jackets) finally put down the money to buy a set of Mondello aluminum cylinder heads. He spoke of how impressed he was with how it seemed to wake his 455 up.  Don’t know if I’d get the aluminum heads because of the difference in expansion rates of the two metals possibly leading to head gasket issues. Perhaps Mondello has a set of bench flow tested iron heads with bigger valves that might be worth considering. Those heads with a good set of headers and a well tuned EBL EFI and a taller Final Drive should get you up those hills.  JWIT

To view this discussion on the web visit https://groups.google.com/d/msgid/gmcmh-efi/D020B157-9CF8-4F88-B0AA-1EF8851F570E%40wwt.net.

[TR One]

Appreciate the advice guys...  I'm still on the fence about it, as you say, there are other ways to make the engine pull more.  However, while I don't have 3.70s, my coach does have the 3.42s from Cinnabar I think.  It was installed by the previous owner.  I've not tried the 3.70s, but I'm pretty happy with the way the coach is geared now.  At least happy enough to not want to go though the effort and cost of switching from 3.42 to 3.70 (But the LSD would be nice)

The stroker kit I am looking at buying is brand new, contains the crank, rods, bearings, pistons, rings, balancer, flexplate, ARP rod bolts and is pre-balanced.  Owner is local to me and is asking less than half what the assembly would be new.

To have my existing crank reground, buy bearings, pistons, rings, have the assembly balanced, etc. the kit ends up only being a few hundred bucks more than I would pay to refurb my existing parts.  And I end up with a brand new Eagle crank, forged rods, Mahle pistons, etc and some more displacement.

Assuming I go the stroker route, I could lower the compression to about 8.5:1 by going with a .120 Cometic head gasket.  The main drawback I've been able to find with this, is it's horrible for quench.  However, looking at our engines, we have dished pistons, and pretty much open heads.  From what I can tell, quench is not really an issue with our engines the way the combustion chambers are designed.  

Even if you consider the small area opposite the spark plug that is flat on our combustion chambers, most Olds engines have a deck height somewhere near .020-.030 in the hole ( I already disassembled my engine, so not sure what the deck height is on mine)   Add to that the thickness of a Felpro head gasket is .039 we are looking at a quench number of .06 to .07ish.  From my research, the ideal measurement for quench is around .04, and above .06 the mixing effect is drastically reduced.  So even how most of our engines are set up now, there really is not much of a gain from quench.

Based on that, I am not seeing much drawback to going with the stroker.  Only things I am seeing are the head gaskets are expensive, and I may need to run a thick intake manifold gasket.  

Any other negatives I am missing?  

[George Beckman]

Agreed

On Feb 9, 2024, at 2:05 PM, Matt Colie <matt7...@gmail.com> wrote:

If I had you desire (and I don't, I have a 23 that crossed the Bighorn with ease), for my money as a multi-decade engine development engineer, I would start saving my pennies to buy a 3.70 final drive and maybe even with LSD option.  Those that are running a 3.70 and a 455 are not reporting and reliability issues and often report better fuel rate than most.  

[George Beckman]

I looked into it with the hotrod engine. I found it didn’t make much difference in the compression ratio. I was afraid of blown head gaskets.

On Feb 9, 2024, at 10:16 AM, TR One <mark.s...@gmail.com> wrote:

Anyone have any experience with running thicker head gaskets or enlarging the chambers on cylinder heads?

[TR One]

Thanks George...  Hey, that picture of the piston in your earlier post...  Are you sure that's the correct picture?  I've been researching pistons a little bit, and the part number on that piston ties back to a low compression, cast, Sealed Power part.

466NP - Recessed Head .150 Deep X 3.250 Dia. - 7.9:1 C.R.

https://www.drivparts.com/part-details.html?part_number=466NP&brand_code=BDBR

I was thinking of shooting for 8.5:1...  Is that too high?

[George Beckman]

Mark,

Woops! Those are the wrong engine! Here is the correct picture:

Speed Pro L2323F

I guess I am an old guy who doesn’t know what he’s talking about. *smile*

On Feb 12, 2024, at 7:19 AM, TR One <mark.s...@gmail.com> wrote:

Thanks George...  Hey, that picture of the piston in your earlier post...  Are you sure that's the correct picture?  I've been researching pistons a little bit, and the part number on that piston ties back to a low compression, cast, Sealed Power part.

466NP - Recessed Head .150 Deep X 3.250 Dia. - 7.9:1 C.R.

https://www.drivparts.com/part-details.html?part_number=466NP&brand_code=BDBR

I was thinking of shooting for 8.5:1...  Is that too high?

[TR One]

Thanks George...   That clears it up...  (Hey, one piston looks just like another... ;-)

From what I can tell (Assuming those are .030 over and you are running 80cc  heads) that works out to about a 9.4:1 compression ratio.  18cc dish.

I have a good used set of Speedpro L2390F pistons I could use for this motor.  40CC chambers.  From what I can tell, should be about an 8.5:1 compression ratio.

Would that be a good place to start?

[George Beckman]

Maybe Matt Colli can chime in again. I would think 8.5 would be OK. I read this—but who know if the information is correct:

The compression ratio of engines from about 1964 to 1970 was either 9:1 or 10.25:1. After that, it was 8.5:1.
These are nominal specifications, and the actual compression ratios are about half a point to a point lower, depending on manufacturing tolerances.

On Feb 12, 2024, at 11:20 AM, TR One <mark.s...@gmail.com> wrote:

 From what I can tell, should be about an 8.5:1 compression ratio.

Would that be a good place to start?

[Matthew Colie]

I am off line right now and will supply a clear explanation probably late tomorrow morning.  The situation is relatively simple but the explanation is involved.

Matt_C

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