Double check my engine builder please 340 stroke

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MObarracuda

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I took my 340 to my engine builder for my 1967 Barracuda. I wanted the stock setup for the street driver only. I found a block for $200 I was going to redo the 340 with a 1968 318 forged crank and rods but the builder showed me that the 4” stroke would be equily economical to buy vs reworking all the original parts. I went with the 4” new components.

Here is the build details:
340 block 40 over with one sleeve
Edelbrock Alum heads 63cc pn#60779
Airgap intake
4” Scat crank pn#9-340-4000-6123
Icon forged flat top pistons IC744 (+5cc)
Hydralic flat tappits lifters
Scat 6123 connecting rods (new)
273 adjustable rocker arms (reused)
Custom Cam of New Albany MS
Duration@.050. 230. 238
Lobe lift. .323. .325
Valve lift .491. 1.52x. .494
Lobe separation 113 degree in cl 108
IN oper@.050. 7BTC CL. 42 ABC
EX 57BBT. 1 ATC
340 exht manifolds w/ 2 1/4” duals
904 auto trans
323 8-3/4 rear end

They tell me the cam shaft will bleed down cylinder pressure down to 8.9 to 9.3 to use pump Gas. I looked at the JEGs tool and it says 11:1. I have questions that I’d like a second opinion

1) Is there a way to calculate the compression ratio with my custom cam?
2) They said 87 octain would be fine. Do you agree?
3) Builder recommended 750cfm carb. I like Edelbrock’s AVS carb...is he 650cfm too small or 800cfm to large for a street driver?

Thanks in advance for the advise.

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I had a similar 340 stroker built several years ago but with X heads at around 68cc’s; my builder used Ross dished pistons (-18cc’s) and it calculated out at 10.2 compression. I would be concerned too as your pistons note +5cc’s.
 
I had a similar 340 stroker built several years ago but with X heads at around 68cc’s; my builder used Ross dished pistons (-18cc’s) and it calculated out at 10.2 compression. I would be concerned too as your pistons note +5cc’s.
Would the only
I had a similar 340 stroker built several years ago but with X heads at around 68cc’s; my builder used Ross dished pistons (-18cc’s) and it calculated out at 10.2 compression. I would be concerned too as your pistons note +5cc’s.
i will call my builder in the morning to get confirmation on the deck height of the block. If he left it tall +.10 I’m in the 9:1 range but if he zero decked it
I had a similar 340 stroker built several years ago but with X heads at around 68cc’s; my builder used Ross dished pistons (-18cc’s) and it calculated out at 10.2 compression. I would be concerned too as your pistons note +5cc’s.
I think I should call my engine builder in the morning and confirm deck height. If he left the deck height with pistons below the deck by .10 I’m in the high 8’s range but if he zero decked the block I’m low 10’s

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Re used 273 rockers, they ratio more near 1.4
The cam is too small for 11.1 , in regards to it bleeding down cylinder pressure.
Those heads with those pistons....cam should be in the 246-250@.050 .540-.570 lift.

Maybe they list the pistons wrong. I'd be asking about that.

The calculator you used shows you have the piston .10 in the hole, you know thats 100 thousandths, right?
 
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Way too much static compression.

I have a Scat stroker rotating kit, "63ccc" edelbrock heads (measured at 65+/-1cc) with the 274-S comp cam that at a glance looks close to what you have. I have 9.7:1 measured and CC'd and I run California 91 pump gas and on the edge of pinging on 90 deg days up steep grades. I have the icon 8-IC-745 +070 20.5 cc dish pistons

Way out to lunch with that compression, IMHO. Either he miscalculated the DCR or is bandaiding too much static compression with way too much cam.

But surprisingly the rest seems well thought out. And I believe his recommendation to go stroker is a great call for a hot fun street car you're building.
 
Those ic744-030 5cc dish pistons have the same compression distance as the 20.5cc dish pistons.

Those will not put the pistons 0.10" down in the bore. Unless you are using 6.00" length rods vs Mopar 6.123" rods. I guess he could have done this??

If so, Certainly not the typical way to go. If so, he's not going to get any quench and getting slightly less favorable rod ratio.

Maybe he calculated with the wrong rod length???
 
The DCR on my 340 comes in at 8.195 and I have to pull timing to run 91 octane with iron heads. Static compression is 9.8:1. It’s only a couple of degrees, but it detonates if I don’t pull that timing. Pistons are over the deck on my build, quench isn’t perfect or anything so I’m not saying you can’t do better than I do. But if you go past mid 8’s on the DCR I’m not sure aluminum heads will save you. Not with the gas I run anyway.

I’m with Steve too, those numbers don’t look right.
 
The DCR on my 340 comes in at 8.195 and I have to pull timing to run 91 octane with iron heads. Static compression is 9.8:1. It’s only a couple of degrees, but it detonates if I don’t pull that timing. Pistons are over the deck on my build, quench isn’t perfect or anything so I’m not saying you can’t do better than I do. But if you go past mid 8’s on the DCR I’m not sure aluminum heads will save you. Not with the gas I run anyway.

I’m with Steve too, those numbers don’t look right.

The aluminum head deal doesn't give you the 1 whole point in DCR and SCR's. Imho that's an old wives tale.
 
Yeah I only figure aluminum heads for being good for half a point or so. But my set up isn’t ideal at all. Although your numbers aren’t far off of mine for compression and I know your set up is better so I guess even mid-eights on the DCR may be optimistic.
 
If they are dish i have no opinion. how ever! If it is a flat top with 0 deck......You have WAY more compression than 11.0:1
More like 12.5 to 12.7...................
 
How did you guys figure out the Dcr without the advertised cam duration numbers?
In any case
I ran 11.3/8.6/175psi with a 292/108 in my 367 and it burned 87E10 quite happily.
I have also run 10.9/8.9@184psi with a 270/110,also on 87E
I currently run 10.9/8.7@180,with a 276/110 and still on 87E
The quench was maintained at .039 to .033
Engine currently has over 100,000 on it.

As to your 418, you could be knocking on 11.8/9.5@200psi. There is/was a guy on here a year or two ago who reported that he was running over 200 on pump gas. Actually I seem to remember more than one guy. So while it may be possible to run that high, it's probably not recommended. A much safer number is 180 and with tight quench.You could probably make 200 work with a really,really small carb.


I used an ICA of 64*,and a swept of 856cc,and 79cc as total chamber volume.
Why 64* ? Cuz that's what my 276/286/110&230/237 cam came in at when 4* advanced. To get 57* you'd need a pretty fast roller cam, ...make that incredibly fast.
That calculator is only asking for plus 15* to add to the ICA@.050 which is not enough for a FTH. It should be closer to 22/24
 
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If they are dish i have no opinion. how ever! If it is a flat top with 0 deck......You have WAY more compression than 11.0:1
More like 12.5 to 12.7...................
They are flat tops

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IMHO,

Need 20.5cc dish for 91 octane

Need at least 27cc dish for 87 octane. More?

That will leave the piston around zero to the deck height. Measure. Then pick head gasket to get desired quench.

For reference, the Fel Pro 1008 is .039 compressed.
 
I feel your pain. I am going through the same thing. My head is swimming with heads, pistons and came. Block is at the shop right now and I have to decide which Scat kit and which head. Right now I am leaning towards the Scat kit with the -5 CC the Edelbrock RPM heads with the 63 CC Chambers. I will be watching this thread carefully. Good Luck
 
With an uncut deck, these pistons would be .012" in the hole. With a .039 " thick head gasket, your SCR will be around 11.5 SCR and DCR will be around 9.1 .That is for the brave of heart and the best tuning skills. At last in MO, you have 93 octane premium (I assume).

But..... That assumes that the advertised cam duration on the intake side is around 278 degrees; that is a fairly fast ramp cam so is possible. What you really want to know is the advertised duration and that shows nowhere on the cam card. If this cam has a slower ramp, then advertised duration may be up around 286. Then DCR is around 8.8..... still pretty tough run.

I'm a strong advocate of keeping DCR up for street use but this is more than I would go for 93 pump premium. (Low 8's is where I am comfortable.) Change to the IC745 dish pistons, use a .028" head gasket and put the pistons .007" in the hole (to get a quench gap of .035") and now your SCR is 10.1 and your DCR is 8.0. (Again assuming an actual advertised cam duration of 278*) And you have a good quench gap to help fight any detonation.

IMHO, there are other cam choices that will work nicely with the above combo. I'd probably run a step smaller cam with this combination for a bit more DCR and more fuel economy; but that also gets into what you actually want 'on the street'.

And yes, your last set of DCR computations was for the piston .100" in the hole, as MO noted.
 
How did you guys figure out the Dcr without the advertised cam duration numbers?
In any case
I ran 11.3/8.6/175psi with a 292/108 in my 367 and it burned 87E10 quite happily.
I have also run 10.9/8.9@184psi with a 270/110,also on 87E
I currently run 10.9/8.7@180,with a 276/110 and still on 87E
The quench was maintained at .039 to .033
Engine currently has over 100,000 on it.

As to your 418, you could be knocking on 11.8/9.5@200psi. There is/was a guy on here a year or two ago who reported that he was running over 200 on pump gas. Actually I seem to remember more than one guy. So while it may be possible to run that high, it's probably not recommended. A much safer number is 180 and with tight quench.You could probably make 200 work with a really,really small carb.


I used an ICA of 64*,and a swept of 856cc,and 79cc as total chamber volume.
Why 64* ? Cuz that's what my 276/286/110&230/237 cam came in at when 4* advanced. To get 57* you'd need a pretty fast roller cam, ...make that incredibly fast.
That calculator is only asking for plus 15* to add to the ICA@.050 which is not enough for a FTH. It should be closer to 22/24


To do 200 PSI cranking pressure you need to have you poop wired tight. I'm at 185 now and may advance the cam 2* and see what it does. Or, I may wait until I finish the W-2 heads and just use a thinner head gasket.
 
Ok...more info from the engine builder...I used the calculation tool he uses to verify the numbers. See attached photo

New info...
1) my block was tall and the deck height .018 above the piston
2) To keep the costs down for machining and due to the limited piston selection it drove the builder to select the Icon IC744 pistons are flat top that have 5cc eyebrows cut into the face. This minimized the amount of machining to the block and heads to achieve C.R. ratios and maintain rocker arm geometry.
3) the pistons have (8) .4 cubic inch or 6.44cc gap between the cylinder wall and piston that is .015 wide and .222 deep down to the first compression ring that adds cylinder Volume. (See pictures and the calculation tool allows for this measurement)
4) the cams advertised duration 253/266@20 deg or 276/294@6 deg. The intake closes at 76 degrees (actual zero). I used the 76 degree number in the calculator. Note: asked to install the cam with 4 degrees of advance.
5) the engine builder says the Static compression ratio will be 11.57:1 and the dynamic between 8.1 @ 1160 elevation or 8.4 at sea level.
6) I asked him about crank pressure. He said 225psi. I asked him if that was high. He said he has other engines out there with aluminum heads st 225psi and it is no problem. He said if my heads were iron he would have stopped at 180psi.

It helps when you have all of the answers. I hope these answers help everyone following the tread and who have contributed their thoughts and hands on experiences. I will get the motor assembled and let you know how it goes.

PS my builder said if I was shaky on setting the timing at first to run some 110 octane gas to get it dialed in to avoid detonation while I worked it out the initial timing.

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Did he deck the block with a cleanup cut? I don't think it would be that much more to take off .010 ?? Maybe the others can comment on this?

You are running 273 adjustable rockers. But you can just measure and order new push rods. That would be a labor charge to mock up and calculate. Maybe someone can comment typical on hours billed fo that. New push rods aren't too expensive.

Sounds like he is working with a tight budget. But that's way to much compression.

In your first post you were asking about running 87 octane. You are miles from that. Was that a requirement when you were designing this build with engine builder??

Seems like a lot of duration.
 
Did he deck the block with a cleanup cut? I don't think it would be that much more to take off .010 ?? Maybe the others can comment on this?

You are running 273 adjustable rockers. But you can just measure and order new push rods. That would be a labor charge to mock up and calculate. Maybe someone can comment typical on hours billed fo that. New push rods aren't too expensive.

Sounds like he is working with a tight budget. But that's way to much compression.

In your first post you were asking about running 87 octane. You are miles from that. Was that a requirement when you were designing this build with engine builder??

Seems like a lot of duration.
The requirement was a well mannered, good vacuum, street engine, pump gas and this was a budget conscious build but not to skimp on the requirements.

I don’t understand..if the dynamic ratio at my elevation is 8.1:1 isn’t that pump gas friendly?

Here are pictures of the block

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How did you guys figure out the Dcr without the advertised cam duration numbers?
In any case
I ran 11.3/8.6/175psi with a 292/108 in my 367 and it burned 87E10 quite happily.
I have also run 10.9/8.9@184psi with a 270/110,also on 87E
I currently run 10.9/8.7@180,with a 276/110 and still on 87E
The quench was maintained at .039 to .033
Engine currently has over 100,000 on it.

As to your 418, you could be knocking on 11.8/9.5@200psi. There is/was a guy on here a year or two ago who reported that he was running over 200 on pump gas. Actually I seem to remember more than one guy. So while it may be possible to run that high, it's probably not recommended. A much safer number is 180 and with tight quench.You could probably make 200 work with a really,really small carb.


I used an ICA of 64*,and a swept of 856cc,and 79cc as total chamber volume.
Why 64* ? Cuz that's what my 276/286/110&230/237 cam came in at when 4* advanced. To get 57* you'd need a pretty fast roller cam, ...make that incredibly fast.
That calculator is only asking for plus 15* to add to the ICA@.050 which is not enough for a FTH. It should be closer to 22/24

I remember you stating some different dcr numbers in another thread, I'll have to pull it up.
Seems like everyone's dcr keep magically getting closer to my own, when I'm not supposed to be able to run it that high..lol..but no one on here ran it as high, till now....Okay sure thats possible, I'm not breaking any ground in doing so...just myths and internet babblings put to rest.

You saying you run 8.7dcr 180 psi on 87 with that cam, give the specs so we can see that 276 duration 110 cam make 10.7 into 8.9
I run the 8.8888 @184 dcr and a tight quench, 91 is edgy....
 
The requirement was a well mannered, good vacuum, street engine, pump gas and this was a budget conscious build but not to skimp on the requirements.

I don’t understand..if the dynamic ratio at my elevation is 8.1:1 isn’t that pump gas friendly?

Here are pictures of the block

View attachment 1715103198

View attachment 1715103201

View attachment 1715103202
8.1 is friendly.


What is your elevation?

If it's 3000 plus ft, now the picture is clearer.
But something about this entire post is suspicious...

Why are you hiding the cam specs?
 
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Wrong piston for what you want to do. Price is not an issue as the -20 dish are the same price from multiple vendors.

Doing the dance about elevation helping is folly at best. At 1000-1500 above sea level is not even worth the effort to try and sneak in under the wire.

Putting them down in the hole and running a.039 gasket you are just close enough for quench, but, that is getting iffy if the piston is more than .010 down. IMO, you'd want a .030 gasket to make quench work for you.

Cold days and high atmos pressure are going to be your enemy...
 
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