Suggestions for the Best Comp Ratio, Pistons & Cam for Street 340

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Franko

Franko

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I am rebuilding a 1970 340 for the street, not for racing. My 69 Dart has stock a AVS, intake, 2.02 heads, and stock exhaust manifolds. It is a 4 speed with a 3:55 rear end.
I’ve read that the compression ratio should be less than 10:1 to run on pump gas. I wanted to error on the safe side, so I was thinking about 9:1 to 9.5:1. Of course, in 1972, the 340 CR was reduced to 8.5:1. Maybe that is the way to go.
The block will be bored .030. Any suggestions on the appropriate pistons? Cast or hypereutectic?
The stock cam was Adv Dur- 268/276; Lift- 429/444.
Compcams recommended a step up from the stock old technology: the hydraulic Xtreme Energy cam, No. 20-222-3, which has Adv Dur-262/270; Lift- 462/470; RPM Range- 1,300-5,600. They said this would provide the best street manners.
One more step up for Compcams is No. 20-223-3, which has Adv Dur-268/280; Lift- 477/480; RPM Range- 1,600-5,800.
Does anybody have any suggestions on any of this combination?
Much obliged.
 
The difference in hp from 9:1 to 9.5:1 or even 10:1 ain't that much. A lot of people seem to pick a compression ratio like 10:1 then figure what cam to run for pump gas seems backwards to me. 9:1 CR with your cam choice seems right to me.
 
I am about to yank out my comp 20-223-3, but only because it was the wrong cam for my low CR 318 and automatic. It felt like it really wanted to run once on the highway doing over 60mph. But now I want street torque right from idle, not near 2000rpms in an automatic.

And one thing I have learned is that static CR is one thing, but dynamic CR is another altogether. I think you want to be about 8.5 dynamic CR. Google dynamic CR calculator, and put in your numbers. My comp cam put me at only 6.6 DCR, starting with a static 8.2:1 CR.

do yourself a favor and check that out. DCR and cam specs and hardpoints are linked together.
 
Just build a blueprinted stock 72 340. The stock 340 cam has better manners than that Comp.
 
No more than a 9.5-1 with an iron head and a small cam.
IMO, a 9.0-1 and the MP upgraded 340 cam is a good choice. It's a step larger than the Comp cam with street manors.
Hyperutecic's pistons are fine and available in more ratios than a cast slug which comes as a OE replacement piston. You can do better than a
8.0-1.
 
Franko said:
I’ve read that the compression ratio should be less than 10:1 to run on pump gas. I wanted to error on the safe side, so I was thinking about 9:1 to 9.5:1. Of course, in 1972, the 340 CR was reduced to 8.5:1. Maybe that is the way to go.
Click to expand...

RustyRatRod said:
Just build a blueprinted stock 72 340. The stock 340 cam has better manners than that Comp.
Click to expand...

rumblefish360 said:
IMO, a 9.0-1 and the MP upgraded 340 cam is a good choice. It's a step larger than the Comp cam with street manors.
Hyperutecic's pistons are fine and available in more ratios than a cast slug which comes as a OE replacement piston. You can do better than a
8.0-1.
Click to expand...

I agree with the guys above ^^^^^^^^^^^^as far as compression and a pump gas friendly stock cruiser/driver.

I'm curious as to the cam inquiry? The 340 replacement is fine for a regulation driver, even with 8.5/9.0 compression. I have to wonder if increased performance isn't really in the back of your mind? With the 4spd & 3.55's, there's no reason that the XE262 wouldn't be a suitable option. My personal opinion is, if you were willing to upgrade the carb, the XE268 would be right in my wheelhouse with that setup, regardless of the stock manifold's......Plus it still works with the single valve springs :D.

It really depends on how mild, or how hot, you want it to come out.

Good luck, Rick
 
jerry6 said:
I thought the 1970 340 came with forged pistons . I replaced mine with Mahle pistons , a bit lighter than the stock or replacement pistons . XE 268 and a Holley 670 or 750 street avenger would be good .
Click to expand...

They all came with forged pistons.
 
Franko said:
I am rebuilding a 1970 340 for the street, not for racing. My 69 Dart has stock a AVS, intake, 2.02 heads, and stock exhaust manifolds. It is a 4 speed with a 3:55 rear end.
I’ve read that the compression ratio should be less than 10:1 to run on pump gas. I wanted to error on the safe side, so I was thinking about 9:1 to 9.5:1. Of course, in 1972, the 340 CR was reduced to 8.5:1. Maybe that is the way to go.
The block will be bored .030. Any suggestions on the appropriate pistons? Cast or hypereutectic?
The stock cam was Adv Dur- 268/276; Lift- 429/444.
Compcams recommended a step up from the stock old technology: the hydraulic Xtreme Energy cam, No. 20-222-3, which has Adv Dur-262/270; Lift- 462/470; RPM Range- 1,300-5,600. They said this would provide the best street manners.
One more step up for Compcams is No. 20-223-3, which has Adv Dur-268/280; Lift- 477/480; RPM Range- 1,600-5,800.
Does anybody have any suggestions on any of this combination?
Much obliged.
Click to expand...

We have built a bunch of 340's for the Street.

Either a 'KB Piston' or 'Speed Pro' have worked fantastic, and with nothing
greater than 9.5 to 1 for the Street.

Too many Camshafts to choose from though, but we've found that a '340' 4-Speed
with 3.55 or 3.91 Gears loves a Camshaft with a .470" to .480" lift, and 270* to 280*
Duration for the Street.
 
The cam you chose is 1300-5600 rpm's. The stock cam will run idle to 5600 rpm's no problem. The MP replacement cam claims idle to 5800 rpm's. I used one in a 360 and I shifted at 5500 rpm's. If it would have been a 318/340, might have shifted around 5800.
 
Comp-Cams makes a 'damn nice' kit.

XE-268H ~ {#20-223-8}
Lift.......................... .477"/.480" Lift
Duration.................. 268*/280* Duration
Duration @ .050" .... 224*/230*
LSA......................... 110*
Overlap................... 54*

Valve Springs ~ {#VS-901}
Type....................... Single Coil w/Damper
Valve Closed........... #101 lbs.
Valve Open............. #242 lbs.

RPM Range............. 1600 to 5800

The 54* Overlap works well for a 'Hot Street Performance' 340.

And the Valve Springs are 'not' over Load-Rated, which doesn't beat up the Valves and Valve Seats.

Note; We installed over 300 of these kits in 'Street Performance' Mopar 340's , and not a single complaint.
 
69 Cuda 440 said:
Comp-Cams makes a 'damn nice' kit.

XE-268H ~ {#20-223-8}
Lift.......................... .477"/.480" Lift
Duration.................. 268*/280* Duration
Duration @ .050" .... 224*/230*
LSA......................... 110*
Overlap................... 54*

Valve Springs ~ {#VS-901}
Type....................... Single Coil w/Damper
Valve Closed........... #101 lbs.
Valve Open............. #242 lbs.

RPM Range............. 1600 to 5800

The 54* Overlap works well for a 'Hot Street Performance' 340.

And the Valve Springs are 'not' over Load-Rated, which doesn't beat up the Valves and Valve Seats.

Note; We installed over 300 of these kits in 'Street Performance' Mopar 340's , and not a single complaint.
Click to expand...

I'm using that cam too. I also would like to know the ideal static compression ratio for that cam to run on pump gas. When I say pump gas I mean anything ranging from 87 to 91 octane found in California.
 
Joes Edge,

In the 'Street Performance' 340 Engines we built {New York, New Jersey and Connecticut}
with 93 Octane, we liked to stay around a "9.75 -1" Compression Ratio with the
Comp-Cams XE-268H.

Most engines received the KB-243 Piston {Flat Top; w/2 Valve Reliefs}.

With the Hypereutectic Piston, we could run as high as {10.00 to 1 Compression} without
any 'significant bothersome' issues, but that's the maximum we recommended.

Plus the Piston {Weight @ 583 Grams + Pin @ 132 Grams} was beneficial for quench power.

[URL="https://sdparts.com/sites/default/files/styles/product_display_page/public/product/image/KB243.jpg"][/URL]


But in California, with Octane Levels {87 ~ 89 ~ 91}

Geez......................

Time for 'calculations'.
 
I have a 9.8:1 340 w an XE268, Worked over 2:02 J heads, LD340, Holley 750, TTIs, 3:55 gears w a 4 speed in my 69 swinger. Pulls hard up to 6000 RPM. Runs perfectly fine on our crappy 91. My cylinder pressure is only 150 across the board, just fine for pump gas.
 
Thanks for all the excellent suggestions. I also talked with Mancini and Laysons.

Taking all comments into account, I am leaning toward the original Mopar Performance stock cam P4452782- AD- 268/276 and Lift- 429/444, RPM Range- Idle to 5,800. This may be a better fit with the fact that I will keep the iron intake, iron heads, stock exhaust and the original AVS, and not installing aftermarket aluminum equipment.
Also I lost sight of the fact that I may try to run in Pure Stock Drags just to see what a mostly original 340 will turn (although you can have a slightly larger cam).
It also appears that this cam was stock on lower compression 72 and 73 340s.
The guy at Mancinis also suggested that P4452761- AD- 268/272 and Lift- 450/455, RPM Range- 1,500 – 5,800 could work well with the reduced compression. How would driveablilty and torque be affected by the difference between the difference in the lower RPM Range rating between the 2 cams i.e. Idle vs 1,500?
Also does anybody know who makes full floating pistons (.030 over) that could drop the compression to the 9 – 9.5 : 1 range? I am planning on using Eagle SIR I-Beam Rods 6123CB. The machine shop said the head chamber is currently 71CCs, but may drop to 70CC when the block is decked. They ordered Sealed Power Cast Pistons, 428P, but the Summit Site says they are Press-fit, not bushed for full floating.

Thanks again for your input.
 
id run the stock replacement hi comp pistons and the crane 278hmv or 278 powermax hyd cam.this cam has great manners and a powerband like a table top.specs are as follows : 278/290 duration 467/494 lift 114 lsa. crane lists the "basic rpm range" for the older hmv version at 2200-5800.i have this cam in my current 340 combo.it idles well and pulls good vac.pulls hard to 6000 rpms no problem.so long as i limit my total advance to 34* i dont have any issues running 91 pump swill.
 
Stick with the stock or small comp style cam and try for 9.0 comp or so.
#1 mistake is to pick a cam too big for street driving.
....large cams need a gear,converter and shine at the big end
 
To get 9.0 - 9.5 compression, you will need a clearance volume of 88.1-83 cc.

Your heads and gasket will pretty much take up the 83 cc clearance volume.

You want to find a flat top piston with the top even with the head deck, or with even with 4-5 cc valve releifs to get your desired compression with the heads and fel pro head gasket that comes in the mancini racing gasket kit (12.4cc).
 
Franko said:
Thanks for all the excellent suggestions. I also talked with Mancini and Laysons.

Taking all comments into account, I am leaning toward the original Mopar Performance stock cam P4452782- AD- 268/276 and Lift- 429/444, RPM Range- Idle to 5,800. This may be a better fit with the fact that I will keep the iron intake, iron heads, stock exhaust and the original AVS, and not installing aftermarket aluminum equipment.
Also I lost sight of the fact that I may try to run in Pure Stock Drags just to see what a mostly original 340 will turn (although you can have a slightly larger cam).
It also appears that this cam was stock on lower compression 72 and 73 340s.
The guy at Mancinis also suggested that P4452761- AD- 268/272 and Lift- 450/455, RPM Range- 1,500 – 5,800 could work well with the reduced compression. How would driveablilty and torque be affected by the difference between the difference in the lower RPM Range rating between the 2 cams i.e. Idle vs 1,500?

Thanks again for your input.
Click to expand...


I've run all three of the MP cams in the stock 340 range, I liked the original 340 cam the best, it had better pull. I wanted to step up when I was running the 268/272 .450"/.455" cam.

I also recommend using Rhoades lifters part # 2018. They bleed down at idle and take out about .025" lift and 15° duration and give you the full lift and duration of your cam by 3500 rpm. It's like having an adjustable cam for the low end and without sacrificing top end.

With a 318 with 9.2 compression, a stock 340 cam, rhoades lifters, 4 bbl and dual exhaust with a 2.76 gear, I got 17.75 mpg highway in my 68 Barracuda.


The 340 cam will give you nice power and some fuel economy if run with rhoades lifters.

Read the second article here for more info on rhoades lifters:

http://www.rhoadslifters.com/Pages/Articles.html


You can buy them here:

http://www.summitracing.com/search?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=rhoades 2018
 
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