exhaust valve size increase on J head

[Bodyperson]

We all know the la head is lacking flow on the exhaust side.
In a street car, might we be better off to spend more effort on the exhaust than the intake? I want to run stock manifolds on my 340 or maybe a stroker.
Leave the intake at 1.88 and clean-up the port. Increase the exhaust valve size, work the exhaust port to the max and run an good exhaust system with an x pipe. Maybe a dual pattern cam.
I know it's not that simple and their are variables. Just wondering about some real world experiences with this notion.

 

[Lonewolf3165]

Your best bet would be just run a dual pattern camshaft to improve the exhaust flow. The 1.60 size valve isn't the problem, you'd get a lot more out of it by just a quick clean up around the bowls & back-cutting the valves. IMO just working the exhaust without doing the intake would be foolish. Plus any improvements you gain with whatever you do are going to be compromised by using stock manifolds.

 

[Lustle]

Stock manifolds will really limit what you can do with a stock stroke 340. On a stroker they will definitely kill some serious power. I wouldn't even consider it. I have stock manifolds on my stock stroke 340 that makes around 400 horse and I am SURE I am losing power. I won't be surprised if I pick up 20 horse going to headers. 1-5/8 full legnth headers will give you the best power up to the 500+ horse range. I'm curious why you would want to work on only the exhaust side when you have 1.88 intake? 2.02 intake is a known power maker. 2.02 and 1.60 work great. Clean them up. Cut them proper. Do some gasket match. And you'll be more than pleased with the results. The one area that really needs attention on these heads is the turn in on the intake side. Smooth that out and you get much better flow and less turbulence.

 

[Bodyperson]

OK, good advice Lonewolf. Lets throw out the valve size increase notion but stick with the rest. I don't have the "340" manifolds but I do have the bigger port manifolds. I thought about modifying the pass side by adding a divider between the inner cylinder ports because I AM foolish. LOL

 

[Bodyperson]

Stock manifolds will really limit what you can do with a stock stroke 340. On a stroker they will definitely kill some serious power. I wouldn't even consider it. I have stock manifolds on my stock stroke 340 that makes around 400 horse and I am SURE I am losing power. I won't be surprised if I pick up 20 horse going to headers. 1-5/8 full legnth headers will give you the best power up to the 500+ horse range. I'm curious why you would want to work on only the exhaust side when you have 1.88 intake? 2.02 intake is a known power maker. 2.02 and 1.60 work great. Clean them up. Cut them proper. Do some gasket match. And you'll be more than pleased with the results. The one area that really needs attention on these heads is the turn in on the intake side. Smooth that out and you get much better flow and less turbulence.

I dont really "want" to run 1.88, I am just wondering. I know larger valves and ports can kill low end power.

 

[Lustle]

I dont really "want" to run 1.88, I am just wondering. I know larger valves and ports can kill low end power.

That's a myth. Just like the old "well if I port the heads too much I will lose low end power". An engine is an air pump. Air in, power out. More air in, more power out. Period.

 

[Bodyperson]

That's a myth. Just like the old "well if I port the heads too much I will lose low end power". An engine is an air pump. Air in, power out. More air in, more power out. Period.

Yes.. at high RPM, but the port velocity needs to be there.

 

[Lustle]

Yes.. at high RPM, but the port velocity needs to be there.

At any RPM. And engine is an air pump no matter what RPM it's at. 100-10,000 It's all about flow.

But run whatever valves you want. 1.88 intake and whatever exhaust. With stock manifold. On your stroker. Let us know what happens.

Power to yah.

 

[yellow rose]

I dont really "want" to run 1.88, I am just wondering. I know larger valves and ports can kill low end power.

Total nonsense. Valve size has little to do with torque. Some day, all these wives tales will die off. I hope.

So to tip everyone over, I could make the argument that a 1.600 exhaust valve is too big. My next W2's are getting 1.500 exhaust valves. Most people who port know that a smaller valve with a 92-95% bowl makes more power.

If you ar not willing to pay someone to do the work, use a 2.02/1.60 valve package, get the bowls correct and let it alone.

Some things are counter intuitive and exhaust valve size is one of them.

 

[moper]

Total nonsense. Valve size has little to do with torque. Some day, all these wives tales will die off. I hope.
Some things are counter intuitive and exhaust valve size is one of them.

In total agreement ^^. I use 1.65 exhaust valves. Because normally the seat has to be cut and that means I don't have to have the seats replaced.

Camshaft will be the biggest factor given the usage of manifolds. Strokers with some real science behind them will exceed 550hp with extrude-honed manifolds (FAST class). You will give up power over headers but they won't ruin anything with the right cam. If you seriously think a 4" stroke engine will "be down on torque" because of valve size you have to go drive a few cars with them before you order the 3/4 race cam.

 

[Bill Crowell]

Just thought I'd toss in my experiences with several LA engines that I built with various sized valves, porting and camshafts for my Mopars that are subject to smog regulations. (Hope I'm not hijacking the thread!)

In order to take advantage of porting, you need a sufficient static CR; otherwise you just won't get the flow at low RPMs. I think 9.5:1 is about the minimum CR for a ported LA. Unfortunately, a carbureted LA engine (360 CI) with 9.5 CR won't smog in California. Too much NOx. When I reduced the CR on this engine to pass smog, it was pretty much a dog off the line, but it does quite well once the RPMs are up. I'm pretty happy with the engine, and since it's in a B-200 van, I don't do any street racing with it anyway! The engine has a lot of power on steep hills in this heavy van, once the RPMs are up.
The biggest cam I have ever gotten to smog in California is the dual pattern MP P4452759
(260° - 268°; .430-.450" lift).
Hope somebody finds this info useful.

 

[yellow rose]

Just thought I'd toss in my experiences with several LA engines that I built with various sized valves, porting and camshafts for my Mopars that are subject to smog regulations. (Hope I'm not hijacking the thread!)

In order to take advantage of porting, you need a sufficient static CR; otherwise you just won't get the flow at low RPMs. I think 9.5:1 is about the minimum CR for a ported LA. Unfortunately, a carbureted LA engine (360 CI) with 9.5 CR won't smog in California. Too much NOx. When I reduced the CR on this engine to pass smog, it was pretty much a dog off the line, but it does quite well once the RPMs are up. I'm pretty happy with the engine, and since it's in a B-200 van, I don't do any street racing with it anyway! The engine has a lot of power on steep hills in this heavy van, once the RPMs are up.
The biggest cam I have ever gotten to smog in California is the dual pattern MP P4452759
(260° - 268°; .430-.450" lift).
Hope somebody finds this info useful.

Is there some point in California where you no longer have to smog?

 

[67Dart273]

It has always seemed to me that since exhaust is such high pressure, and that intake is obviously such low pressure, that "most gains" should be made by attending to the "intake" side of things.

 

[Bodyperson]

It has always seemed to me that since exhaust is such high pressure, and that intake is obviously such low pressure, that "most gains" should be made by attending to the "intake" side of things.

I am immediately struck by the notion that the high pressure is actually slowing the piston.
The same could be said for the intake stroke though now being vacuum

 

[Darthomas]

An engine IS an air pump as many point out, however, like guitars, ANYTHING and EVERYTHING about its build affects its nature and inherent resonances.
That's why it's called "tuning, and an ART.
Some people need the reasonance peak between 3500-7000 rpm for racing, others want the natural reasonance to favor lower rpms and torque. It's silly to argue when few builders have exactly the same intentions. I like hydraulic lifters that start to float at 5500 rpm, because now matter how hard I beat the engine, it's going to live generally, so I can drive it to work tomorrow.
Let the Torqueflite upshift and the converter do its multiplication job in the next gear.
For good torque, turbulence is your friend.
I would never port and polish, because I don't bracket race.
But I'd never tell somone else not to do that, because it's magic for high rpm horsepower.

 

[Bill Crowell]

Is there some point in California where you no longer have to smog?

Yellow, don't get me started on that!! Ha. Originally our state Legislature gave us a "rolling" smog exemption, wherein the year of smog exemption would increase with each year that passes. But after a couple of years under the rolling exemption, the feds happened to give the State of California the evil eye about not meeting their air pollution goals, so the state somehow tossed out the rolling exemption. I do not understand how they overrode the vote of the Legislature this way.
Yes, the very first thing the state did to convince the feds they were going to roll over, play dead and let the federal government have their way with them was to offer up the rolling smog exemption as a sacrifice to the nanny state, despite the Legislature's intention to give the citizens a rolling smog exemption, and nobody ever took it to court. So we no longer have the rolling smog exemption and the exemption date is forever written in stone as 1975 and older. Unless somebody takes it to court, perhaps.

 

[70aarcuda]

I am surprised they have not changed the law to read...anything more than 30 yrs old is required to confiscated and crushed....LOL....

 

[yellow rose]

An engine IS an air pump as many point out, however, like guitars, ANYTHING and EVERYTHING about its build affects its nature and inherent resonances.
That's why it's called "tuning, and an ART.
Some people need the reasonance peak between 3500-7000 rpm for racing, others want the natural reasonance to favor lower rpms and torque. It's silly to argue when few builders have exactly the same intentions. I like hydraulic lifters that start to float at 5500 rpm, because now matter how hard I beat the engine, it's going to live generally, so I can drive it to work tomorrow.
Let the Torqueflite upshift and the converter do its multiplication job in the next gear.
For good torque, turbulence is your friend.
I would never port and polish, because I don't bracket race.
But I'd never tell somone else not to do that, because it's magic for high rpm horsepower.

I don't know where you learned that turbulence makes torque, but it doesn't. It kills it. It kills HP. Unless you are trying to get cats to light off, turbulence anywhere in the system is bad.

 

[yellow rose]

Yellow, don't get me started on that!! Ha. Originally our state Legislature gave us a "rolling" smog exemption, wherein the year of smog exemption would increase with each year that passes. But after a couple of years under the rolling exemption, the feds happened to give the State of California the evil eye about not meeting their air pollution goals, so the state somehow tossed out the rolling exemption. I do not understand how they overrode the vote of the Legislature this way.
Yes, the very first thing the state did to convince the feds they were going to roll over, play dead and let the federal government have their way with them was to offer up the rolling smog exemption as a sacrifice to the nanny state, despite the Legislature's intention to give the citizens a rolling smog exemption, and nobody ever took it to court. So we no longer have the rolling smog exemption and the exemption date is forever written in stone as 1975 and older. Unless somebody takes it to court, perhaps.

That sucks.

Where I live, we don't smog. This place never has. Fought it from the beginning. However, the retards are moving in and I suspect we will be full retard in a few years.

 

[Darthomas]

I don't know where you learned that turbulence makes torque, but it doesn't. It kills it. It kills HP. Unless you are trying to get cats to light off, turbulence anywhere in the system is bad.

Once I put a spread bore 73 340 intake manifold on a 71 318.
I did not have a TQ yet so I had an adapter plate made and re-used the two barrel I had.
Huge mismatch on head ports vs manifold ports, but I was amazed at how much more torque that combo gave.
Only stayed that way about 4 months until I found a Thermoquad I could afford,
IThat was even better naturally, but still the torque at liw rpms seemed to like the port mismatch. The 340 went in the next summer so that was a short lived experiment. Still looking for that adapter my father made of stainless steel.
(Maybe that part was the magic)