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LA360 compression

Matt68

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ok engine builders. Here is my dilemma. I bought a Mabbco rebuilt 360 short block from a member. Some upgraded stuff like KB high compression flat top pistons, moly rings, 272 muscle cam (272/272 duration 454-454 lift) Cloyes HD timing chain and sprokets. .30 over bore. Got a nice brand new set of Speedmaster aluminum heads with 68cc combustion chambers.
Here's the problem; my engine is at a builders for finishing. Went by there yesterday and he said this engine will be an absolute dog as it sits now. He doubts it will make 8:1 compression. Yes, the pistons are below deck a little, but he says I am losing a lot of quench with them. He suggests new pistons, rods, roller cam and new crank. That's an expensive prospect, considering what I already have in the motor. Also, this guy is a very well known builder with a great reputation and is well known in the racing circuit. His shop is incredible and he rebuilds engines for a living, and has many many great reviews. He does not even want to assemble the rest because he says he knows I will be disappointed.
What to do? Will this motor be a dog like he says? I don't know that I have the scratch to have him tear down this motor and upgrade it to a stroker.
TIA.
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heminut

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He may have a great reputation for building engines but it sounds to me like he as wanting to turn a bigger profit from you. He is basically wanting to sell you a whole different engine using your block! I would talk to someone who knows how to do the math and supply them with the numbers for the heads and the pistons etc. and find out exactly what your compression ratio will be with what you already have. @Chryco Psycho, member here, might be able to help you out with this.

Just what are you wanting to achieve with this engine? Are you planing on putting it on the track or do you just want a nice strong street cruiser? You definitely don't need to go the stroker kit and roller cam route to achieve the latter! Stroker kits and roller cam setups are definitely more expensive, which equals higher profit margins.
 

pschlosser

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My opinion: Someone, somewhere, will have to take the time to identify the measured compression ratio, and then if it's too low, tearing things down, and measuring components to work through what your options are.

If you're lucky, you may find you only need upgrade pistons or another simpler alteration to get you back to where you want.

But unless you @Matt68 can do this, you may have to pay someone their hourly shop rate to do it for you. With my limited experience, the fastest I could do this is 4-5 hours. I'm guessing a decent shop could not do it faster than 2-3.

Also, those tiny details on quench, compression, piston height, piston configuration, headspace, valve train, etc, its in those details we often find some power gains too good to ignore.

Your machinist will get to invoice you for his time, regardless how strong the engine is when the job is done. But he may want you to be happy, and he may be on track confirming you're going to be happy with the resulting engine he thinks he sees.

And yes, given is experience, he may have upgrades and work to suggest if you want more power from this engine.
 
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Matt68

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He may have a great reputation for building engines but it sounds to me like he as wanting to turn a bigger profit from you. He is basically wanting to sell you a whole different engine using your block! I would talk to someone who knows how to do the math and supply them with the numbers for the heads and the pistons etc. and find out exactly what your compression ratio will be with what you already have. @Chryco Psycho, member here, might be able to help you out with this.

Just what are you wanting to achieve with this engine? Are you planing on putting it on the track or do you just want a nice strong street cruiser? You definitely don't need to go the stroker kit and roller cam route to achieve the latter! Stroker kits and roller cam setups are definitely more expensive, which equals higher profit margins.
@heminut just a nice streetable cruiser. Not looking to break 400 hp.
 

Challenger RTA

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I have to agree with Heminut. what do you want? without a complete rebuild. Might be able to deck the block and heads to raise the compression. check with builder for clearances. he should know.

camshaft option? depending on what you want. I'm not suggesting any brand of cam.
There are cam advance and retard keys. And rocker ratio.

Everyone tends to look at the intake and exhaust lobes as separate movements within the engine’s operation, but what we really need to do is look at the entire function of the camshaft and how all the different events coincide. It is true that many camshafts are designed with advance built into them. The majority of these cams are intended for street use. Here’s how it all comes together.


The cam companies know that when it comes to choosing a camshaft, many enthusiasts tend to suffer from the “More is Better” theory of engine performance. The common approach is that if some additional lift and duration is good, then more is better. So instead of choosing a typical 220 degrees at 0.050-inch tappet lift cam for a mild street engine, many enthusiasts – especially first-time buyers – will upgrade their selection and go with maybe a 236 or 242 degrees at 0.050 cam instead because a buddy told them that’s what they needed.


The unfortunate result is that these longer duration camshafts tend to reduce low-speed torque in favor of top-end power. The camshaft companies know this. One way to improve the low-speed torque on a camshaft with too much duration is to advance the intake lobe in relation to the exhaust. Advancing just the intake lobe opens and closes the intake valve sooner which is one way to help improve the low-speed torque.


We’ve included a Comp Cams illustration of the intake and exhaust lobe lift and duration curves on a simple diagram that makes all this a little easier to understand. If all the cam company did was advance the intake lobe position on the cam, this would also decrease the lobe separation angle and increase the number of degrees of overlap.


ask-11-01



You can learn quite a bit just by studying this Comp Cams illustration of cam timing. Advancing either lobe would move it to the left while retarding would move either lobe to the right on this diagram. So you can see how advancing just the intake lobe (the one on the right) would change the intake centerline but also increase overlap.


Overlap is the amount of time, in degrees, when both the exhaust and intake valves are open. You can see this represented by the small triangle shaped area in the center bottom of the two curves. Overlap was called the “Fifth Cycle” by Ed Iskenderian back in the 1960’s when engine builders were first discovering how much mid-range power can be improved by adding overlap. The downside to increasing overlap is that it tends to hurt idle vacuum. This is also what creates the classic lumpy, uneven idle that everybody wants so their engine sounds like a killer Pro Stock drag race powerplant.


Bu too much overlap can really hurt the off-idle performance so the cam companies typically advance the intake lobe and retard the exhaust lobe to maintain a decent lobe separation angle. For example, almost all Comp street cams use a 110-degree lobe separation angle.


There is a very easy way to tell if the camshaft has been advanced by the cam company when the cam was machined. Look at the cam timing card and compare the lobe separation angle (LSA) with the intake centerline number. If the numbers are the same, then the camshaft has not been advanced.


Let’s look at an example. This first example is a Comp cam we currently have in a 496 big block Chevy we built. It’s a mechanical roller with 261 degrees duration at 0.050-inch tappet lift on the intake and 270 degrees on the exhaust with 0.734-/0.737-inch valve lift (PN 11-851-9). The intake centerline is listed on the cam card at 108 degrees and the LSA is also 108 degrees. In this case, the cam has been ground with no advance as the numbers are the same.


Now let’s evaluate a much milder Comp hydraulic roller camshaft for an LS engine. This is PN 54-456-11 with 219 and 227 degrees of duration at 0.050-inch tappet lift with 0.607-/0.614-inch lift. When we look at the cam card, the intake centerline is listed at 107 degrees while the LSA is listed at 112 degrees. The numbers tell us the intake centerline has been advanced five degrees from 112 to 107 degrees after top dead center (ATDC).


As a simple exercise to put all this in perspective, there is an easy way to mathematically determine the lobe separation angle. This does not necessarily require measuring the camshaft’s intake and exhaust centerlines. But for right now, let’s say we have the numbers with the exhaust centerline at 112 degrees BTDC and the intake at 112 degrees ATDC. Simply add the two numbers together and divide by 2. The equation would be:


112 + 112 = 224 /2 = 112 degrees LSA.


In the case of the LS cam, we know the intake centerline is 107 degrees ATDC and the LSA is 112, so if the exhaust was the same number of degrees from TDC, that would put it at 107, but adding 107 + 107 = 214/2 = 107 and the cam cards indicates that the LSA is 112 degrees. That means that the exhaust centerline has advanced 10 degrees because the difference between the LSA and the intake centerline is 5 degrees. Placing the exhaust lobe centerline at 117 degrees puts the numbers in place:


117 + 107= 224 /2 = 112-degree LSA.


Now because we have so many Dual Over Head Camshaft (DOHC) engines in production today, let’s just take a second and discuss what the OE’s are doing for 21st century engines. Since a DOHC engine uses a separate intake and exhaust camshaft, this allows engineers to move the intake and exhaust lobes separately. So now with hydraulic adjusters, the ECU can move either cam separately to advance or retard the intake or exhaust. So for low=-speed driving, they advance the intake and retard the exhaust to maintain a sewing machine idle.


Then, somewhere in the midrange when the engine would really respond to lots of overlap, they begin to retard the exhaust to create more overlap. In reality, most of the DOHC engines only move the exhaust lobe for emission reasons. As an example, Fords Variable Cam Timing (VCT) engines only change one lobe while the Ti-VCT (Twin Independent Variable Cam Timing) moves both cams.


This is probably more than you wanted to know about camshaft positioning, but this might come in useful at some point. If nothing else, you can use it to dazzle your friends with you incredibly acute knowledge of camshafts and how they work. Or maybe not…


Here’s a breakdown of the advantages of advancing or retarding your cam timing:


Advance Cam Timing



  • Begins Intake Event Sooner
  • Opens Intake Valve Sooner
  • Builds More Low-End Torque
  • Decreases Piston-to-Valve Clearance
  • Increases Piston-Exhaust Valve Clearance

Retard Cam Timing



  • Delays Intake Closing Event
  • Keeps Intake Valve Open Later
  • Builds More High–RPM Power
  • Increases Piston-to-Valve Clearance
  • Decreases Piston-Exhaust Valve Clearance

  • I don't think that is the case in this situation 12 degrees?
 

Slug

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Matt, go into dss racing piston site. They have a compression calculator. Just type in the specs and it spits out your number.
 

moparleo

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If this guy knows that your engine will not perform ask him to explain exactly why. Not just give you a shopping list.
Is he saying the heads are the issue ?
It seems that he has recommended everything but the heads.
Maybe it just needs different heads.
I would contact the short block maker and ask them what heads are intended for that short block build. There had to be a plan .
 

Matt68

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If this guy knows that your engine will not perform ask him to explain exactly why. Not just give you a shopping list.
Is he saying the heads are the issue ?

It seems that he has recommended everything but the heads.
Maybe it just needs different heads.
I would contact the short block maker and ask them what heads are intended for that short block build. There had to be a plan .
@moparleo Well, he says I am losing a lot of quench with the gap. Down .100 doesn't seem like much, but he thinks it will really have no power. I wrote to Mabbco (they built the short block). They don't have a tech line; the woman just gave me the Email to write to the techs.
Alan isn't saying the heads are the issue. He's said that while they would be better for a stroker, he much prefers a better bottom end as opposed to heads and intake, etc. He thinks stroker is the way to go with a small block.
 
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heminut

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.100 down is quite a bit. You could easily get where you want to be (360-400 HP) with just a piston change. You definitely don't need a stroker kit and roller cam for that!
 

Matt68

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.100 down is quite a bit. You could easily get where you want to be (360-400 HP) with just a piston change. You definitely don't need a stroker kit and roller cam for that!
@heminut , for some reason, this guy will not just do a piston change. It's his way or no way. I was about to bite the bullet and just do it, but when I went to leave a deposit today the build price went up 2 grand. SMH.
 

Xcudame

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Those pistons look very odd to me. I've never seen 360 pistons like that. Factory pistons always had some kind of dish on top, and the aftermarket pistons always seem to have valve reliefs and/or a dish on top. Unless I missing something, I've never seen mopar 360 flat top pistons like those. Not wanting to do just a piston swap (you're the customer) sounds flaky to me. Unless I'm mistaken, you just want a cruiser and not a race engine. And the engine builder wants to build you a race engine. Maybe time for a new shop?
 

Xcudame

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I think a set of KB107 pistons will get your compression ratio up for the Speedmaster 68cc heads. I did find Silvolite pistons that do sit about .100" in the bore. So I learned something about 360 pistons. What a lousy piston!
 

heminut

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@heminut , for some reason, this guy will not just do a piston change. It's his way or no way. I was about to bite the bullet and just do it, but when I went to leave a deposit today the build price went up 2 grand. SMH.
You need to distance yourself from that guy! I'm seeing a LOT of red flags! I've worked as a a professional mechanic for 5+ years back in the day, and have been building my own stuff for 50 years. I would run (not walk) from this guy!
 

Matt68

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You need to distance yourself from that guy! I'm seeing a LOT of red flags! I've worked as a a professional mechanic for 5+ years back in the day, and have been building my own stuff for 50 years. I would run (not walk) from this guy!
@heminut funny thing is this guy is a legend around here for building and racing engines. I think he doesnt want his name attached to anything not really well built for speed.
 

heminut

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@heminut funny thing is this guy is a legend around here for building and racing engines. I think he doesnt want his name attached to anything not really well built for speed.
He might be a damn good engine builder, but he wants to build what HE wants, not what YOU want! Find a builder who is willing to work within the specifications you are looking for. I guarantee you will save money in the long run and still have an engine you are happy with.
 

Matt68

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He might be a damn good engine builder, but he wants to build what HE wants, not what YOU want! Find a builder who is willing to work within the specifications you are looking for. I guarantee you will save money in the long run and still have an engine you are happy with.
This builder was chosen by the shop that is pulling my 318 and installing this 360. It was originally sent to him just to finish and balance it, but his computations led him to tell me that I needed to stroke it to make power I’d be happy with. Lost three weeks playing this back and forth. Now I’m back to square one.
 

Steve340

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My 2 cents worth.
Find a rebuilder who will fit a piston that gives you 9:5 compression.
It sounds like that is all you want.
 

pschlosser

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Were I a reputable machinist-engine-builder with a positive reputation, I might cringe given an assignment (by a client) to complete an engine rebuild in a way that could make me look bad.

If this one says he will only do certain work, and not simply a piston change, respect him, his shop, and his request. Go elsewhere. It's not a big deal.
 
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