Before anyone says anything, yes, I have searched and read just about every thread on this subject in the advanced section. If I missed the one with my answer I apologize.

Now I know that between 5.5" and 6.125" or 1.5:1 and 1.85:1 you can see no measureable difference in engine performance. My problem is no one ever talks about what happens past those limits.

Basically I was thinking about doing a 4.125 bore X 3.00 stroke engine combo. So to stay within accepted parameters I would have to get a big, probably heavy, piston and that would be counter productive on such an over-square motor.

So my question is, should I use the long rods, or the heavy pistons, or does it not matter one way or the other?

If there is some info you need that I forgot just let me know.

Jeremy

 

What is your thinking behind doing a 3" stroke motor? Application?

 

It should be easier/cheaper to rev a short stroke motor, and the heads/cams are designed for a higher revving engine.

But basically it's a novelty engine, so I might as well get as much novelty into it as I can (cheaply).

Now obviously if there are major problems with running long rods I will either change my mind about heavy pistons, get lighter ones custom built, or get a longer stroke.

Jeremy

 

Jeremy

You know that the 3" stroke-5.700 rod makes it 1.9 to 1 R/R
Smoky Yunack was a firm believer in the long rod in fact he thought anything less that 2 to 1 was rediculus.

Here's what I know for sure - the higher the rod ratio the less head you need to make a given power level. When we ran the real 426 Hemis at PS and NASCAR the factory proved that 1.88 made the best overall power. Bill Jenkins proved that in a 331 a 1.88 proved to make the most power with the heads that he had at that time. 685 to 710 horsepower with a 331 is nothing to laugh about considering that all he had for heads were 292 turbo castings.

When you can learn to think outside of the box you can accomplish things no else can.

Denny Schmidt

 

Thanks Denny,

I'm trying to think outside the box. And I think a DOHC SBC is as outside the box as you will find on this board.

My problem is that my rods will be quite a bit longer with a 400 block, 3" crank, and off the shelf "light as possible" pistons. I'm still looking through some lists to find a piston that will let me use a stock sized rod though, so I can't be sure what the rod ratio will really be.

Another thing to consider is that this will not be a max effort engine. It will be 100% street driven, might take it to the track to see what she'll run once or twice.

So can you give me any of the theory on why you need less head with the high ratio? I thought most of it was just bigger bores?

Jeremy

 

Jeremy,
A light piston should be an easy task for what you are trying to do. The ring package you will need is a very common comp deal. You won't get much love from me on the Rod ratio deal, as long as the rods don't break, I am happy.

Dennis

 

I have a couple sets of those - just need to find something to put them on.

You will need a 4.125" x 1.550" comp height piston for a 6.200" rod which is the closest you are going to get for off-the-shelf stuff. Manley and Scat make rods in that size among others...I would highly recommend the Manley rods due to the kind of rev's you will be turning with that combo. A 6.200" rod/1.550" piston combo is going to cost you 60-70grams over a 6.0" x 1.125" 383CID combo...405g(1.125") 470g(1.550") - piston weight

 

Jeremy

3" stroke 5.7 is 1.9 - 3" by 6" is 2 to 1 but for the same money you can buy a 3.250 crank put 6" rod in it and have 1.85 R/R here you wind up with 348 CID and a really reliable combo. If you really want to get tricky with the light weight piston you could put a 6.125 or 6.250 rod in it since both are shelf rods and the flat top pistons weigh 345 grams form JE.

Denny Schmidt

 

Dennis I'm with you, as long as it holds together I don't care. I just need some reassurance that there are no adverse effects on motor performance past that 1.85:1 barrier. Of course there have been plenty of factory motors that came stock with 1.9 and above.

Machinist One,

The closest I found with JE was 4.125 X 1.445, and then some 4.125 X 1.560 with their SRP line. The only problem is the JEs are 512 grams and the SRP is 504 grams. What manufacturer were you looking at for those lighter pistons? And seeing as how I've never built a high revving engine before, can you tell me what is and isn't appropriate as far as total rotating assembly weight?

Denny I know there are easier ways to build a motor, and with more cubes. Heck there is an easier way to build it with the same cubes. And I am not sure yet whether I will go with the small journal 327 block (which will solve all of these problems) or a large journal 400 block (which will hopefully solve some others). It will probably be whichever I find first, but until then I like to know my options.

Thanks for the help guys.

Jeremy

 

I just think when its all said and done after all your efforts that when you let the clutch out with sticky tires you're going to be disappointed.

 

I appreciate the concern Mark, but I'm inclined to disagree with you on a couple of points.

The first being that I'm not planning on putting this thing on slicks, it might have some good sized rubber on there, but they will be street tires. This motor will go in a 100% street driven vehicle.

The second being that I'm not sure it will be so devoid of torque as you might think. This is mainly due to the 4 valve heads.

But we will see, one way or the other this is going into a <2700lb car.

Jeremy

 

Be sure to keep us posted. Are you using the Arao heads then? I must have missed the part where you talked about the heads. I was not trying to throw a wet blanket over your project. I happen to have a 302 sbc and speak from some limited experience there. As far as the OHC small block thing I really appreciate stuff that's different, but I can't think of any particularly impressive domestic OHC V-8s. LT5 was cool for its day, or even now but it didn't light the world on fire. The large bore / short stroke combination isn't going to work the heads very hard at street rpm.

Seriously though, I wish you much success.

I mention OHC due to the valve configuration. Quite possibly the 4valve cam in block arrangement can be stronger since its not cam limited.

 

Oh you'll hear about it if it ever gets together

But no not the arao heads, now that would be a waste on a motor this small. I have a set of Infiniti M45 heads with the intake that I'm going to try. And I don't think it will be terribly impressive with the HP numbers, i'm hoping for some good low end torque though, with a high revving ability.

I know you aren't trying to get me down, even if you were it wouldn't work. I accepted the fact that if I tried this there would be a few people who would think it probably wasn't worth it.

But as I have said, this is mainly a novelty engine, going in a novelty car, that hopefully will work together well enough to give some tangible performance

Edit: It's DOHC, and very cam limited. I'll either work on that or go Turbo.

 

Jeremy

Now I'm confused - I want the DOHC Heads - where do I get um? Ya, Ya, Ya More power.

I'm not talking about a 327 block - I'm talking about a 400 block or a Cup-Car take-off, I have a guy in Kentucky that can get Cup-Car take offs.

Today I looked in my CV catalog and found the 350 GR pistons for a 350 engine. CP & JE as well as Wisco but there are flat top's.

With the 348/3.250 stroke and the 6.250 rod - 1.100 compression ht you have a great combo going regardless of what head you put on it. Years ago we built one very similar to this and put it in a 1450 LBS dragster that ran 5.0's in the eighth and made 675 BHP @ 7800 on Kazzi's dyno.

Denny Schmidt

 

Hey Denny,

I'm talking about a 400 block also. Now unless i'm mistaken, the 400 came with a 3.75 crank, 5.565 rods, and a 4.125 bore piston. So to use stock pistons with a 3.00 crank I need 5.94 rods right? I actually think I have been doing the math wrong until now. So I can either use L99 rods with a stock height piston, or change both the piston and the rod length.

Someone please tell me if I calculated that wrong, it just doesn't feel right.

But I hear ya Denny, a 3.25 stroke 348 would probably rev plenty high and make more power. But can I get a large journal forged 3.25 stroke crank? Or a small journal 4.125 bore block?

Tell me some more about those Cup blocks, how much does your guy charge for them? You set me up with him and I'll set you up with the guy who will sell you a set of those DOHC heads

Jeremy