Rebuilding engine... benefits of higher CR pistons?

[killi6]

Hey, everyone...

So I'm planning the rebuild of my '74 318. I have a question...

I'm assuming I'm going to need to replace pistons after machining the cylinders since there's a noticeable ridge in them at the top of the stroke of the compression rings. My main question is, assuming I'm not really changing anything else except putting a 340 cam in to replace the stock 318 cam, what's it going to benefit me to upgrade from stock pistons to '9.5' pistons? I've read articles with a '5% torque increase per 1.0 CR' rule of thumb... that sound about right?

The higher CR pistons are about another $200 in cost... just trying to decide if it's worth it. FWIW, I'm planning to keep the stock heads, port/polish at home and going to look for a stock iron 360 4bbl intake/carb also. Stock manifolds on the exhaust but want to do an x-pipe dual exhaust.

 

[99ss]

The more compression the higher the octane you need to run. Yes more compression makes the engine more efficient but there is a balance between compression octane and timing. I run premium all the time and I really dont know if 9.5 is low octane friendly... my guess is you would be at the mid octane level at 9.5.

 

[RustyRatRod]

There are SEVERAL different compression heights for stock 318 pistons. Find out what you have now and then choose a longer compression height as compared to it. I don't know where the 200 buck figure came from, but that's total BS. You can raise compression in a 318 very easily by paying attention to the compression height.

 

[Meep-Meep]

I believe a small increase in compression is a good thing for a street engine. If you have 8.5:1 now a .5 point won't hurt anything. If you want a little more upper mid range torque you can select a cam with a slight bit more overlap and duration to take advantage of the slight bump in compression. As mentioned there are a lot of factors that will allow you to run a particular octane fuel but 9.5:1 might be the limit for pump 91 in an older engine. In my own experience it seems that an engine on the lean side with higher octane to control pinging will make more power than a rich engine on lower octane fuel - meaning the richer mixture itself is the method used to control pinging - and that is incorrect. You want to be on the verge of detonation and the mixture toward the lean side for optimum efficiency.

 

[killi6]

Thanks, guys.

The '200 dollar figure' comes from browsing pistons on http://chucker54.stores.yahoo.net/. Keith Black hyper pistons (KB167) are $397.95 with a compression height of 1.81inches versus stock pistons (Sealed Power) at $157 with a 1.741 height.

 

[RustyRatRod]

The benefits of higher compression actually never stop. Because engine efficiency continues to climb. In straight terms of efficiency, a race gas only 14.5:1 engine properly built to take advantage of that compression is much more efficient than a 9.5:1 engine. I'm throwing fuel cost out the window in this comparison.....just strictly from a volumetric efficiency standpoint.

 

[killi6]

So considering that I would rather not bust apart the bottom end of my engine possibly ever again, I should just go with pistons that'll give me a higher compression ratio as it would be worth a couple hundred bucks more, overall. The only kind of rule of thumb I'd heard was the '5% torque per 1.0 of compression ratio' deal as far as a quantifiable idea of what kind of performance improvement higher compression ratio will give besides just 'more' or 'better'.

 

[Dennis H]

9.5:1 in my 440. Pump 91 here in the land of lousy fuel. It still has an occasional knock and have to play with the timing. Any higher and I would be looking for race gas.

 

[Cranky]

Read up on what having good quench (or squish) can do for you. Low compression no quench engines can and do actually ping more than a higher compression engine with good quench will. The total combination also helps. Cams can make or break an engine too. Also, there were two cams for the 340...one for the automatics and one for the stick cars. The stick cam was a bit more lumpy...

 

[99ss]

Good point there Cranky the quench area is very important.

I have one question though for the big cam advocates to reduce compression... If the cam is bleeding off compression what is the result?

In order for the cam to bleed of compression doesnt it have to hang the valve open on the compression stroke? And in doing so arent you actually reducing volume? ( volume is measured with a close cylinder in this case. IF your closing the valve after the piston is on the way up the cylinder you are reducing the length of the chamber/volume).
How does that make a car quicker by taking away cubic inches?
Isnt it better to build for the correct compression per the fuel you are going to run and make more power than tryiing to bleed compression with a cam. ?

 

[RustyRatRod]

Not necessarily. You can also delay the opening of the intake valve, or a combination of both.

 

[99ss]

But you're delaying closing valves isnt the piston on the way up already?

trying to picture it in my head...LOL

 

[Cranky]

On big cam engines, they usually don't make much power down low because of the bleed off but will make killer horse power in the higher rpm ranges. The thought is, with a high compression engine with bigger cams, by the time the engine is making a lot of cylinder pressure in the higher rpm range, there's not enough time for the engine to develop a secondary flame front like it has when running slow with lots of cylinder pressure. The ping you hear is that second flame front colliding with the first one that's still in progress. That's an explanation in a nutshell but that should give you an idea what's happening. It's possible to run pump gas (91) with 11.5-1 but the cam needs to be radical enough to lower the cylinder pressures at lower rpm when the engine is more susceptible to pinging under load. You can achieve pinging with a 9.5-1 engine if you have a cam that creates a lot of cylinder pressure in the lower rpm range. An RV style cam comes to mind in that ap. Most of those have really late intake events. They are designed to make a low compression engine 'think' it has more compression but you have to be careful if the quench isn't there. The RV cam deal is kinda old school thinking with engines with real low compression and lousy quench and the reverse dome piston is more of the newer thinking way to go....imo. You can have a 9-1 (I like to build 10-1 when using iron heads) engine with the reverse dome and run a mild cam and have a pretty decent running engine and still use regular gas in most cases. Of course, it depends on the combo used and how hot you run the engine. My old 71 340 would run great on 91 with the stock cam with 10.25 so long as my temps were not above 180...which isn't too easy where I live but with just a single gallon of av gas to a full tank, never heard it ping.

- - - Updated - - -

Btw, if you open the intake late and close it late, you can pack the cylinder plum full of air and fuel. That's not good for a 11-1 engine nor is it good for a 9-1 with lousy quench.

 

[RustyRatRod]

But you're delaying closing valves isnt the piston on the way up already?

trying to picture it in my head...LOL

That's not what I said. I said delay the opening of the intake valve. By doing that and delaying the closing of the exhaust valve, you can taylor cylinder pressure pretty closely. Of course you don't want the exhaust valve hanging open while the piston is going down, or the intake valve opening too late. It's all a numbers game. You simply have to make an educated guess as to what will work best.

 

[Meep-Meep]

Good point there Cranky the quench area is very important.

I have one question though for the big cam advocates to reduce compression... If the cam is bleeding off compression what is the result?

In order for the cam to bleed of compression doesnt it have to hang the valve open on the compression stroke? And in doing so arent you actually reducing volume? ( volume is measured with a close cylinder in this case. IF your closing the valve after the piston is on the way up the cylinder you are reducing the length of the chamber/volume).
How does that make a car quicker by taking away cubic inches?
Isnt it better to build for the correct compression per the fuel you are going to run and make more power than tryiing to bleed compression with a cam. ?

You are hitting on some important topics. Yes, holding the intake valve open longer (or closing it later) is reducing your compression pressure or dynamic compression ratio but it has advantages.

You are not taking away cubic inches but rather reducing the cylinder pressure as described above. Using your thinking one can say the volume of the sealed cylinder can vary depending on where the intake valve closes but that is not the whole story.

It is always better to plan the compression around the fuel.


In order for all this to work you must think in the proper order. For a street engine, planning for the fuel is the first step. The cam is a way to tailor the power band so by adjusting the intake closing, amount of overlap and duration, you can take advantage of the fuel mixture inertia and exhaust scavenging effects. Cams with more overlap will tend to produce more peak torque but at the expense of a poor idle. With properly selected components, holding the intake valve open longer will allow the already moving intake charge to fill the cylinder even as the piston begins to move upward! An increase in compression ratio is required with a cam that has a later intake closing point to try to preserve any low speed drivability. So you can say compression is like a tool to help the radical cam do it's thing. The result of too big a cam and not enough compression is a non responsive engine, poor idle vacuum, carb difficult to tune and general unhappiness. There is no perfect combination for the older engines without variable cam timing so plan on giving up some high end power or low end torque.

 

[RustyRatRod]

That's why I always say there's more than one way to do it. The optimum way is pretty much how the factory did it. HIGH compression and a fairly moderate camshaft grind. This makes cylinder pressure skyrocket but of course, necessitates the use of race gas. But the optimum way certainly won't work for pump gas, so you have three choices. One, run compression no more then 9.2:1 with a no quench engine. Two, run good quench and cam as needed. Three run a big lumpy cam to knock cylinder pressure down. To me, number two is the best choice. However it requires some research and a learning curve. Remember, engines run best on the ragged edge of detonation, so with a quench motor, everything has to be just right. No sharp edges in the chamber or on the piston tops. Pistons with polished heads or thermal coated to reflect combustion. Polished or coated chambers. Perfect quench distance....we can argue about what that is later. I say around .035" or so. Chambers need to be the same. PIston head volumes need to be the same. Deck surfaces need to be square. So it's not just a case of throw one together anymore with this pump swill we havt to choose from. It takes some time, money and patience to get one to run on pump gas with 10 plus static compression and a cam that doesn't bleed off huge amounts of cylinder pressure, but it can be done. It's done all the time. Just ask Jim Leroy. You don't have to be a professional engine builder to figure it out, but it does take some level of skill. I never get too caught up in valve events.....and I probably should, because I could squeeze a little more out....but geez, I ain't racin anymore so what difference does it really make? Long as it runs on pump gas, that's all I need.

 

[99ss]

So application is probably key here.... You dont want a whole lot of coughing wheezing and sneezing in a street car when you're trying to putt across a parking lot buit in a weekend special or drag car its tolerable..

 

[GrabberOrange69]

killi6,

How much do you drive the car? If it's not very often, you can raise the compression ratio and run octane boost. Some people don’t want to hassle with it and I get it - it is a small inconvenience. I built my '69 Shelby GT350 with 11.2:1 compression and have never regretted it. It has a cam with 224 [email protected], and if I back the time timing down to 8 degrees BTDC I can put around without octane boost - but you can't lean on it. The throttle response on a high compression engine will put a grin on your face!

Low compression engines are lazy and inefficient - one of the main reasons the top engines coming out these days are hitting 11:1 (runs on pump 91 with voodoo electronics, including lightning quick knock sensor ignition retard and careful control of the air/fuel ratio, plus you don't have direct control of the throttle - HAL does) is higher compression = more horsepower AND better fuel economy. On a 350ci class engine, each point of compression is worth about 30 horsepower.

I'm currently building a 512 stroker B for my '65 Coronet. My target is 10.5:1 compression, but I intend to run a cam with around 240@ .050, so based on my previous experience, it should run on pump 91. As pointed out by others, you can control cranking/low RPM cylinder pressure with cam duration and overlap, and ignition timing.

With the relatively mild cam you intend to run, I'd say 9.5:1 should be your limit if you intend to drive the car a lot and avoid octane boost. But I think you should make the effort to get it to 9.5:1, because the benefits are good. More efficient engine, smoother running, crisper throttle response, more economy.

My .02 cents. Good luck.