306 vs 342

[Knighton]

Coast High Performance 306 vs Roush 342R. Below is a dyno chart of the 306 and below that is a link to the Roush dyno chart (brochure) of their 342R. Very similar hp and tq specs for both motors!

306 makes 460hp and 400 lbs-ft.
342R makes 450 hp and 420 lbs-ft.

Both motors make 325-ish lbs-ft at 3000rpm. Torque curves are near identical (342R a touch more torque). 306 uses hydraulic roller cam but is a "dominator" series motor. Does this mean it is streetable?

Would the 306 rev quicker than the 342? Which would you prefer and why?

I have been under the impression a 342 would produce more torque and do so lower in the rev range making for a more streetable motor...

Roush 342R brochure in pdf

 

[Simon]

Please feel free to ingone this as it's just my own opinion and I'm NOT a technical expert.
My 331 stroker engine (in my Cobra) suffered from awful piston knock from day one. Now I can't say they all will, in fact I know of people with 331 and 347 strokers who have no problem at all, but it just seems to me that having such a short piston skirt is not a good thing on a road engine.
As I said, this is just my opinion based solely on my experience, I'm sure there are far more experienced engine builders than me on the forum.
Simon

 

[Jim Rosenthal]

As you know, I am no expert (someone else is building my motor and in terms of engine longevity this is probably a "good thing") but I will offer the following ideas:
GT40s are not heavy cars. With any reasonable torque and axle ratios they will accelerate like crazy, being light, mid-engined cars with phenomenal traction. The extra torque from a stroker motor may not be needed.
We had a thread on this a couple of years ago (!) and I came away from reading that with the impression that stroker SBF engines, with the pin well up in the ring area, did not hold up as well as normal stroke engines with a longer piston skirt. I suppose that all other things being equal, which is hard to determine, a shorter piston will be apt to rock more, causing slap, and wasting power. And more engine wear.
GT40 engines are not drag motors. What may be acceptable in a motor that is run briefly very hard, and then gets a long rest, may not be okay in a sports car engine which is subjected to repeated duty cycles at varying levels of stress.
In building my engine, we elected to do the following; we kept the displacement at 306 (normal plus cleanup .030), we used a normal stroke but long rods (5.4") which some feel improves piston dwell time at the top of the cylinder and shifts the torque peak down a little bit. We will see how the motor turns out; the way things are progressing, it should be on the dyno in April. Given how light a GT40 is, and how much fun a high-revving engine and 5spd manual gearbox will be in such a car, I would probably opt for a non-stroker engine.

 

[John]

having the longer rod/stroke ratio also makes the engine more resistant to detonation when combined with a tight quench area (distance between piston face and the bottom of the combustion chamber). Long rod also makes for a lighter rotating assembly (because of shorter piston skirt) and reduces sidewall loading, putting less stress on the rings.

shorter rod/stroke ratio though will increase volumetric efficiency by means of having a faster instantaneous piston accelleration. This also puts more stress on the rotating assembly though.

They say that a R/S ratio of around 1.7 - 1.8 is about perfect. A good compromise. A higher rod/stroke ratio is good for high winding endurance engines.

John

 

[Ron Earp]

Seriously guys, I think you are lending too much weight to the pin in the rings. First off, most of the good stroker kits don't have this problem - some designs years ago did and some did wear quickly but the motor got a bum rap. Nowadays there are lots and lots of these in use and they aren't sucking up oil at 1 quart per 1000 miles or anything like it (which, incidently, I read that Porsche thought was okay for their 911 Turbo and normal). Many are in use that deliver lots of hard miles with good reliability.

That being said, I'd build a high RPM short stroke motor and I will do for my next car. I have a feeling my 342 is going to beat the daylights out of my Audi box and it isn't nearly as hairly as some of these strokers like the 392s, 408s, etc. Plus, it is a well known fact I like to drag and launch stuff, so that spells trouble.

 

[Ian K]

As Ron said, stroker motors have improved. Hersh has a
great long rod 306, I'm sure he can safely say he is
100% OK with that choice.

The 306s, 331s and 347s sold these days no longer have
the wristpin intersecting the oil ring, and most pistons
are very sturdy. As usual, caveat emptor. Do you research.
I'm planning on a 331. 306 sounds good too, but I think I
can build a 331 to rev almost as high.

Oh, and just for completeness:

306 is a 302 bored .030 over
331 is a 327 stroker bored .030 over
347 is a 342 stroker bored .030 over

Didn't want to cause any confusion.

Ian

 

[Paul Thompson 'Hooligan']

In case anyone's interested - Roy Smart's 347 comprises a Fontana Alloy 302 that was then bored 4.125" and stroked to 3.25" with a Crower billet crank. Cannot remember rod length but can check if anyone wants to know.

 

[Neal]

Paul, that would be a 3.4" rod length.

A stroker will typically more torque than it's smaller brother. I've built a number of 331 and 347 motors both with and without the oil control ring intersecting the pin and never had problems with consumption or wear. You don't want to rev a 3.4" rod motor beyond 6500 or so though.

Here is the curve on my current 10:1 347 motor w/ webers...

 

[Brian Kissel]

Hello Neal !!!
I am not a Ford man by any means but I think the correct rod length is 5.400. Here is a common build on a 302 which has been decked.

4.030 bore 3.400 stroke 5.400 rod length 1.100 compression height

This combination was using a 8.190 deck height, and is for a 347 engine. Where :
4.030 X 4.030 X 3.400 X .7854 X 8 ==346.95239 cu. in. They are using a 5.400 rod and a 1.100 compression height.
I think my theory is correct but please don't flame me to bad if I'm wrong. I haven't built a small block Ford in years. Please correct me if I'm wrong.

Regards Brian

 

[James]

331 seems close to the ideal! Wonder why I mentioned that? /ubbthreads/images/graemlins/grin.gif

 

[Neal]

You are correct. A minor math lapse on my part!

 

[Ron Earp]

Paul, isn't that motor a Clevland configuration?

I like the idea of using a Dart block and going heavy on the overbore and lighter on the stroke. For example, using a Dart block and going 4.125" bore and 3" stroke would make around 320 inches I think. This, I think, could be setup to make around 325 ft/lb torque but rev to the moon if needed. Right now I'm the other way around, basically Neil's motor with a little less duration on the cam.

 

[Paul Thompson 'Hooligan']

Hi all

The Rods are actually 5.155" (same as 289?) and so the wrist-pin does not intersect the oil ring. Pistons are Custom Ross to Roy's Spec with further machining done in his workshop to better suit the modified TFS heads.

Motor pulled like a train even from low rev's, although was a little reluctant to rev much past 6200 rpm - something that Roy has hopefully sorted now with some new valvetrain - including increased valve lift from an increase in rocker ratio and heavier springing control. Can't wait for the shakedown - then it's off to Santa Pod.

Ron, the block is a Clevor - which as I understand has a Cleveland bottom end (crank) and Windsor top end (heads).

Hope this makes sense - I know that we had a whole lot of work to do to get it all to fit in! The lower part of several bores had to have sections cut away to clear the crank etc. /ubbthreads/images/graemlins/tongue.gif

 

[Gary Gibbs]

The dart block can be taken to 4.185 with 0.25 wall thickness remaining. I just saw a chevy dart block build (rated at the same 4.185 max bore) with 4.200 pistons. The aluminium dart blocks can go to 4.125 because of the sleeves.

 

[Ron Earp]

Then we could go down to a 2.75" stroke and still have 302 inches. And a very high redline. Does this look right assuming 4000 ft/min max?

This would assume fantastic components of course. If it is correct is illustrates what happens to RPM potential with stroke, it takes a serious hit. I know my next motor will be a serious revver - that is for sure. Sounds good and goes fast, plus, you just don't see too many revving V8s - 6500 RPM is a common cutout for the domestic V8.

 

[Kevin M]

Ron,

The numbers are right, and you can actually go higher than that as far as the bottom end components go.

The 4,000 FPM piston speed rule of thumb has been around since the 60’s. Modern Forgings using Vacuum Melt Steels are a fair bit stronger than the forgings from back then and modern slipper skirt pistons are about 2/3 the weight of the old ones, so you can actually go higher than 4,000 FPM with a good block like a DART. There are several production car engines that go as high as 4,500 FPM these days. (Saleen S7 = 4,330 FPM, Ford GT = 4,500 FPM, etc.)

You could use a 3.250” stroke and turn it 7,500 RPM easily, even 8,000, the real problem is still the valvetrain. I wouldn’t want to go much over 7,500 RPMs on an endurance type engine with a pushrod valvetrain even with the new beehive springs and titanium retainers. Plus, a 2-valve head just isn’t very efficient at extremely high revs.

I like the sound of a high revving motor as much as anyone, but the simple fact is that a longer stroke giving the same piston speeds at lower RPMs will almost always make more power and be a lot more reliable. If only it sounded as nice! /ubbthreads/images/graemlins/frown.gif

Kevin

 

[Ron Earp]

Is cylinder filling a big issue? That is, so we have a large bore and we try large valves (>2.02) but we can't fill it efficiently? I had these thoughts in the back of my head as well as what you mentioned about FPM. I remember my dad told me that 4000 ft/min rule back when I was around 12 or so (1979 for those that don't know me) and I have seen it validated over and over again. I should have guessed it has gone up!!

The valve train seems solvable with fancy bits availalbe nowadays.


Ron

 

[Kevin M]

You have two things.

First, to run high RPMs and get the motor to breath well, you need lots of lift and fast opening and closing on the valves. Translation: Mechanical Roller cam. That means high spring pressures (200 lbs. or more on the seat). Once you get to that kind of pressures, you can figure on replacing or rebuilding rockers and lifters from time to time, it goes with the territory.

Second, Friction and Pumping Losses go up by the Square of the RPMs, so you always loose power, relatively, as you go up in RPM. For a racing motor where displacement is limited, you have nothing else you can do so you accept the losses and go as high as you can. In classes that use restrictor plates to limit power, you see things go the opposite way. In LeMans Prototypes like the Audi A8s (and Bentleys that used the same engine) they use restrictor plates and have an upper limit on turbo boost. Because of this Audi went for a larger engine that makes peak power (552 hp) at only 5,750 RPMs. Since the engine is only able to make a certain amount of power in the cylinders, they do everything they can to reduce losses so as to get the highest net output.

This is effectively what happens when you look at a 331 turning 7,380 RPMs and a 393 turning 6,230 RPMs. Both have 4,000 FPM average piston speeds with the same bore size, so power potential is theoretically the same. But the friction and pumping losses on the 331 are 1.40 times as high as they are on the 393 because of the higher RPMs. Add to that the much greater reliability of the valvetrain at 6,200 RPM (heck, you could use a Hydraulic Roller cam) and you can see which is the better approach if you are not racing in a class that is displacement limited.

There is one catch here I should mention that is specific to GT40 kits. If you are using a marginal transaxle (Audi, etc.) your transaxle may not be able to handle the greater torque of the large engine.

The actual torque reaching the tires will be the same with either engine, contrary to common opinion. The 393 is 19% larger and will have 19% more torque at the flywheel, however the 331 turns 19% more RPMs and so will be geared 19% lower for the car to reach the same speed at redline. Since gearing multiplies torque, once you get through the gearbox the torque reaching the wheels will be identical.

Kevin

 

[Ron Earp]

[ QUOTE ]
"The actual torque reaching the tires will be the same with either engine, contrary to common opinion. The 393 is 19% larger and will have 19% more torque at the flywheel, however the 331 turns 19% more RPMs and so will be geared 19% lower for the car to reach the same speed at redline. Since gearing multiplies torque, once you get through the gearbox the torque reaching the wheels will be identical. "

[/ QUOTE ]

I should hvae realized this myself since it is simple physics. I still like something that will reach high RPMs, however impractical or inefficient it might be when compared to a larger displacement engine. Right now the point it mote as the 342 is in and staying for the time being.

 

[Gary Gibbs]

First law of HP:

"Two like vehicles traveling down the road at the same speed with engines making the same HP will produce the same torque at the wheels."

Weither it is a big block putting out 400 HP at 3000 RPMs or a 302 putting out 400 HP at 6000 RPM's (assuming the appropriate gearing differences) the torque to the wheels is the same.

In my opinion the way to look at it is how much HP do you want and what ratio of RPM (max usable to min usable) do you want. Then you can select the combination of displacement and components (thus actual RPM) to get there.