Compression

[340runner]

so I took a borescope finally and looked in my cylinders, and I found out my pistons in my mopar 340 are something 2385F - with my cylinder head size being 65cc, should put me square at 8.5 to 1 compression. I can't believe someone would put such a piston into a 71 motor, but that all beats me. I have a cam that's very oversized for the motor, previous owner put it in, and I'm not sure if I should slap in a new cam, replace pistons, or do whatever.

For some background info on the car, it's a 71 roadrunner 4 speed, with 4.56 gears in the rear on a mopar 8 3/4 with 28x10.5 tires. It's a 3840lb car plus or minus 15 lbs with me in it ready to go. Runs a 77.x in 1/8th and a 96.x in the 1/4mph wise, but that was with 3.55s and a 295/50r15 tire

Cam card is down below

 

[j-c-c-62]

Yes, that piston/cam match is off.
What are your plans/use/goals for the car?

 

[340runner]

Yes, that piston/cam match is off.
What are your plans/use/goals for the car?

I want it to be a somewhat hot street/strip car, read 4.56 gears, wish to if its possible be at 420ish hp at the flywheel and then shoot a 100-150 shot of gas through it. Engine already has headers, speedmaster alum. Heads, torker 2 intake and a 750cfm carb

 

[Rebelrouser]

I want it to be a somewhat hot street/strip car, read 4.56 gears, wish to if its possible be at 420ish hp at the flywheel and then shoot a 100-150 shot of gas through it. Engine already has headers, speedmaster alum. Heads, torker 2 intake and a 750cfm carb

I am not a fan of thumper cams never seen one run right. I would pick a hydraulic roller around 230 @ .50 duration and about .530 lift on a 110 centerline

 

[Geoff 2]

Agree. Mumper, Whiptrash cams were designed to have a nasty exh note & attract the girls...not win races.

 

[340runner]

I am not a fan of thumper cams never seen one run right. I would pick a hydraulic roller around 230 @ .50 duration and about .530 lift on a 110 centerline

Neither am I right now, it's alright up top but down low there's nothing

Do you have anything for a hydraulic flat? Frankly, I'm broke, I'm in school still. Also, I have stock rockers and pushrods, I don't want to pour over 1500$ into this swap. At that point, I'd find something like a 383 and build it specifically for nitrous.

 

[67MPRFAN]

I don't want to pour over 1500$ into this swap. At that point, I'd find something like a 383 and build it specifically for nitrous.

Wait and shop around, the weather is getting warmer, which means car shows and swap meets, you may be able to find deals that way, and possibly do the swap or the winter

 

[BSB67]

I’m not sure I’m following.

Does the motor already have the Thumper cam in it?
Is 77 and 96 the mph trap speeds?

If so, it runs very well for what it is. I don’t think I would spend $1,500 trying to increase horsepower.

Enjoy the car the way it is. Save your money and make a plan for making some meaningful changes down the road.

 

[340runner]

I’m not sure I’m following.

Does the motor already have the Thumper cam in it?
Is 77 and 96 the mph trap speeds?

If so, it runs very well for what it is. I don’t think I would spend $1,500 trying to increase horsepower.

Enjoy the car the way it is. Save your money and make a plan for making some meaningful changes down the road.

Yes, it's already in and those are the trap speeds. I mean, what would you call meaningful changes? I want to drop in a small block something down the road probably, or a big block whatever takes me and is cost efficient. I want to be able to run high 11s, and I know I need a lot more horse to do that, probably at least 200 more

I also can drop in a hydraulic flat cam in for cheaper, just how much gain am I looking for it?

 

[340runner]

On another note, how would you feel about running some nitrous through it? Like a 125-150 shot, maybe more. At max 200 shot.
Pistons are the 2835 discontinued .030 over pistons. Given I have supposed 8.39 to 1 compression, it should be a bit more forgiving than a 10.5 to one motor to tune as a first time beginner, right? Also talking with a standalone fuel cell running race fuel for the nitrous

 

[PRHeads]

Here’s how I look at it…….

What is your TOTAL budget for the cam swap?

How much of an improvement are you expecting out of just that one change?

Does the expectation match the investment?

Personally, I wouldn’t bother swapping in another HFT cam.
Save your $$ until you can afford a solid.
And in the meantime you can be out enjoying some track time.

 

[68-500]

That cam is definitely oversized for that compression ratio. If you’re wanting to go faster need to get the comp ratio to 10:1.

 

[RemCharger]

On another note, how would you feel about running some nitrous through it? Like a 125-150 shot, maybe more. At max 200 shot.
Pistons are the 2835 discontinued .030 over pistons. Given I have supposed 8.39 to 1 compression, it should be a bit more forgiving than a 10.5 to one motor to tune as a first time beginner, right? Also talking with a standalone fuel cell running race fuel for the nitrous

Do yourself a really big favor

 

[Cranky]

That cam is definitely oversized for that compression ratio. If you’re looking for some street manners maybe advance it a couple degrees and some Rhodes lifters to bleed of some duration. If you’re wanting to go faster need to get the comp ratio to 10:1.

Or maybe even advance the cam 4 degrees. Done that more than once. Just gotta make sure things are right....

 

[68-500]

I edited my post seems like your wanting to go faster.

 

[Cop car wagon]

I just went through this exercise. Well, not the wrong piston part, but getting the compression to match the cam.
First, roller is great, but converting an LA block to get full lifter covererage requires sleeving the lifter bores, but pushrod angle is still off.
So get a magnum block. But in my mind don't stroke it. But thats a whole new engine build.
So back to the 340. I was building a fresh, modern style 340 with Molnar rods, chevy diameter wrist pins and narrow rings with a lightweight forged pistons.
All I could find with a 340 style dome were cast or hyperutectic, which I won't run.
Keeping the rings in the block, with the piston above deck height gives me 10.25cr.
I am getting a mechanical flat tappet around the same advertised spec. as yours, but my cam grinder like everyone else, laughed at the thumper style cam grind.( Even though my crower cam that I was modeling the new one from, was 40 years old, and before a thumper was heard of.) We will see if the new cam makes more power than the old one,
But a point of comparison for you. My old 340, 10.5cr and 3.91 and nearly that cam had a trap speed of 106, but a horrible et as it had no traction. It pulled hard through the traps
My vote is use your budget to pull the motor for a teardown and new pistons.
And if you are going big block...440.
Or 400 block 440 crank build.

 

[340runner]

I just went through this exercise. Well, not the wrong piston part, but getting the compression to match the cam.
First, roller is great, but converting an LA block to get full lifter covererage requires sleeving the lifter bores, but pushrod angle is still off.
So get a magnum block. But in my mind don't stroke it. But thats a whole new engine build.
So back to the 340. I was building a fresh, modern style 340 with Molnar rods, chevy diameter wrist pins and narrow rings with a lightweight forged pistons.
All I could find with a 340 style dome were cast or hyperutectic, which I won't run.
Keeping the rings in the block, with the piston above deck height gives me 10.25cr.
I am getting a mechanical flat tappet around the same advertised spec. as yours, but my cam grinder like everyone else, laughed at the thumper style cam grind.( Even though my crower cam that I was modeling the new one from, was 40 years old, and before a thumper was heard of.) We will see if the new cam makes more power than the old one,
But a point of comparison for you. My old 340, 10.5cr and 3.91 and nearly that cam had a trap speed of 106, but a horrible et as it had no traction. It pulled hard through the traps
My vote is use your budget to pull the motor for a teardown and new pistons.
And if you are going big block...440.
Or 400 block 440 crank build.

Well, my only issue with this is balancing i think is at like 300$, pistons are 700$, and holy hell I'm at half what a cast scat crank 416 stroker kit is. And it's required to balance the crank for new pistons, right? I can't just drop in a set of the stock replacement 70 340 pistons? 16 grams is the difference between what i have now and what i want

Also, what was your old 340 car's weight?

 

[Cop car wagon]

3600 plus my fat *** @200 so comparable to you.
Yes, balancing costs but 72 low comp pistons are not necessarily lighter than 70 pistons.
I am not a fan of strokers. If I was building a truck, or a 500 caddy style motor, maybe. A low rpm torque motor.
Strokers with short skirts, no ability to run for distance at even a moderate 3000rpm. Then you need more port size, and everything else. It ain't cheap.
I like my bores bigger than my stroke.
But I get it, strokers are big inch. Not my kind of driving.
I just ran 200 miles today at 90mph average speed.

 

[340runner]

3600 plus my fat *** @200 so comparable to you.
Yes, balancing costs but 72 low comp pistons are not necessarily lighter than 70 pistons.
I am not a fan of strokers. If I was building a truck, or a 500 caddy style motor, maybe. A low rpm torque motor.
Strokers with short skirts, no ability to run for distance at even a moderate 3000rpm. Then you need more port size, and everything else. It ain't cheap.
I like my bores bigger than my stroke.
But I get it, strokers are big inch. Not my kind of driving.
I just ran 200 miles today at 90mph average speed.

Well i know it won't cost Mallory but I'm just saying even taking it to the shop. I mean, I'm reading forums and people aren't caring over a 20gram or so weight change, so I might run it honestly. With the compression change, i should be golden as far as the rest of my setup goes, right? I don't know what happened, if PO didn't realize that previous PO happened to change out the pistons or something for whatever reason, and then proceeded to treat it like a high compression 340 with cam and single plane intake.

Looks really tempting to be honest with the price
Don't know if it would hold to a 150 shot though

As far as pulling engines go, this should be easy also because I have no heater core, just pulling the radiator hose and unbolting the one engine mount bolts, and the ps pump

 

[Cop car wagon]

I have those pistons in several 340's. I wish they would make a factory style cast oversize in L+R and with valve pockets on one side only. At that cost, you might come in at or under budget.
A minor piston weight change probably won't hurt. As my machinist reminded me, "what does the oil, in and on the piston weigh?" Of course some people believe in underbalancing. Weigh both.
I would not add nitrous to a cast piston. But I know nothing about nitrous tuning with extra fuel.