Building a torque monster 308/355

[Cava454]

Better heads make power. No if's or buts...


An 6.0ltr variety of the LSx would be much better then the holden donk.

 

[Immortality]

Large valve/ fully ported 308 heads are good enough to support a 355 turning 6000+rpm so standard 308 heads with a little tickle will be enough on a 355 that isn't turned hard. Not rocket science. Now seeing as we aren't intending to turn the engine to hard we don't need a huge amount of duration with a very late closing intake valve like previously mentioned before. So, the right cam (not much larger than standard) and suitable compression will work. No need for exotic cams here with late closing valves that need lots of comp and rpm to work on small block. So not a huge variation from a stock cam. I'm not saying it's going to make big block type torque but you can do a lot better than what the 308 came with from the factory.

As mentioned else where in this thread, torque is all about the amount of force that is been pushed on top of the piston/conrod onto the crank shaft. So the point at which the exhaust valve opens is quiet critical to making good torque, there is a bit more to it than just trying to achieve a good cylinder fill. Seeing as we aren't going to be using a camshaft with huge duration you can play with the exhaust valve timing without...........

 

[Immortality]

Ohhh, that's right. Cam shaft design is a black art so I guess that myself and all the other average Joe Blogs will never be able to figure it out........

This thread actually reminds me of another thread a long time ago on another Holden forum. The guy wanted a torque monster but wanted to retain the standard banana manifold as he wanted the engine to look stock under the bonnet. Said engine was stroked to 383cu and was fitted with a standard (mild) cam from the crane catalogue but with one small alteration made to it(at the time the cam grinder said it would never work) but the engine made good torque without needing to be revved to hard (banana's start to choke an engine much over 5K rpm anyway).

 

[VRSenator065]

This thread reminds me of about 40,000 similar I have read or discussions I have been involved with over the years....

We can all have a mental masturbation session and sprout on about valve overlap, brake mean effective pressure, mean piston velocity, volumetric efficiency, yadi yadi yada, but when all is said and done an engine is an engine and a car is a car, or a truck is a truck. I like to simplify things to take the "black art" out of it.

The engine takes in a mixture of air and petrol, a spark explodes it, which causes the piston to go down, which is connected to the wheels which makes them turn.

The bigger the explosion, the more force it exerts on the piston, and so the more rotational force it exerts on the wheels. That's Torque, full stop end of story.

If someone wants a "torque monster" the simplest, easiest, cheapest, best, <insert another adjective here> way is to use a large capacity engine, it just gives a bigger bang. Its just physics, you cant argue with it. Otherwise every truck manufacturer has had it wrong for the last 100 years.

 

[Immortality]

This thread reminds me of about 40,000 similar I have read or discussions I have been involved with over the years....

We can all have a mental masturbation session and sprout on about valve overlap, brake mean effective pressure, mean piston velocity, volumetric efficiency, yadi yadi yada, but when all is said and done an engine is an engine and a car is a car, or a truck is a truck. I like to simplify things to take the "black art" out of it.

The engine takes in a mixture of air and petrol, a spark ignites the mixture, creating a controlled burn, creating pressure which causes the piston to go down, which is connected to the wheels which makes them turn.

The bigger the explosion, the more force it exerts on the piston, and so the more rotational force it exerts on the wheels. That's Torque, full stop end of story.

If someone wants a "torque monster" the simplest, easiest, cheapest, best, <insert another adjective here> way is to use a large capacity engine, it just gives a bigger bang. Its just physics, you cant argue with it. Otherwise every truck manufacturer has had it wrong for the last 100 years.

Fixed it for ya

 

[VRSenator065]

yea yea, ignition timing, flame fronts, stroke verses bore, scavenging, combustion chamber shape, valve shrouding, valve inclination shall I go on?

Bigger bang = more torque

 

[Immortality]

yea yea, ignition timing, flame fronts, stroke verses bore, scavenging, combustion chamber shape, valve shrouding, valve inclination shall I go on?

Bigger bang = more torque

Yeah mate, the rest of the stuff I can give or take but the combustion process being described as an explosion is one of those interweb misnomers that bug me.

 

[VRSenator065]

Yea no worries, just trying to simplify it down to explaina. Point. All good

 

[BlackoutSteve]

You guys are totally right with "better heads equals more power", but what is "better"? You are assuming that a pair of heads, like VNs, that are designed to flow and make good peak torque, as they do, and were designed to do at around 3500 rpm are going to work in a totally different application. Not a car similar to a VN that weighs just a little bit more, but a van that weighes twice the weight empty, and I dare say is capable of easily carrying another ton, or maybe two. Potentially 5 tons.. ?

So, if you build a ~5ltr with VN heads and you capitalize on where those heads are designed to, and do make peak torque, which is ~3500rpm -give or take, then you are restricted to a cam choice that matches those heads as a "package" and is also designed to make peak torque at ~3500rpm. Sure, you can make changes to the cam's LSA and so on, but you're still stuck in the 3500rpm range regardless of what you do because the cross sectional area of the VN's ports are dictating the speed of the air moving through them which is controlling VE and where VE is -based on the size of the engine.
They are less efficient at lower speeds, and become restrictive at higher speeds, hense why they make peak torque at ~3500 which is always max VE, and no cam or intake manifold can change that very much at all, unless you create a mis-match of components.

Now, let's pretend that you make VN-headed 308 and it's makes a very respectable 400ft/lbs @ 3500 and you put that engine in a 3 ton (plus) van. That cam, although small by performance car standards, still has way too much exhaust reversion and overlap at off-idle speeds, which is bleeding-off too much cylinder pressure (Read: VE) to make enough torque to actually get itself and that massive weight holding it back to a speed where it can start performing. The engine and vehicle are a total mis-match and that there is the whole point of where torque needs to be made.
Ever driven a truck with a tacho? It's all low-down for a reason.

You need as much torque as possible, right off-idle, as soon as you crack the throttle open so the engine can pull with loads of torque, immediately.
No turbo will fix that. How are you going to get the turbo spinning at off-idle speeds with an overlapping cam, and you'll blow any boost you make though that overlap anyway.
Or are you going to put 3500 hi-stall in the van and boil the transmission in order to get the engine revving where it needs to be?
Or ride the clutch until it catches fire in crazy efforts to get these heads and cam at their working speeds from a standing start?

Like the HT502 peanut-port example, that is done with cylinder heads with a very small cross sectional port area in order to produce the required amount of air speed at where this engine makes peak torque. The cam it has purely capitalizes on that 2800 rpm, and with little-to-no overlap it has heaps of torque right off idle. So, at 1750rpm, this thing still makes a huge 490ft/lbs - only 20ft/lb less than it's maximum.
If just a cam and comp change were to make this 502 the torque monster that it is, that's all GM would have done using regular heads.
308 heads may be considered too small by performance car standards, but for good for a heavy truck, except you still need more cubes to get the MxTq/VE where you need it..
Very roughly with basic math, 308 heads on a 355 short would bring the torque peak down from ~2800 to ~2400.
With VN heads, even if you could stroke it to 400 cubes, the rpm where they make their peak torque would still be as high as ~2700.

TORQUE.

 

[Immortality]

Looks like I need to forget everything I thought I knew about engines (and transmissions).

Might be time to learn quilting......

Based on the above though, one must wonder what miracle of science ever allowed this truck to move with the current 308 because on the above advice it must either have a cooked auto or burn out clutch..........