Bore Vs Stroke - how it shapes up in the real world --

[David Vizard]

Bore Vs Stroke which way to go given the choice ---


If not for an overload in other directions I think I would have got to this subject before.

As we have seen in recent posts the subject of bore Vs stroke for extra cubes is always a worthwhile subject. More so as it pertains to real life situations rather than just theoretical ones.

So here is my contribution of the bore Vs stroke for a given displacement discussions as it relates to SB Chevy’s.

All the testing I am going to cover took place when I was running my shop out of Riverside Ca. The test was a 4.03 bore x 3.75 stroke 383 (actual 382.67) versus a short stroke, big bore, 382.58 cube build. Why no bore and stroke figs? Well let me tell you how this all came about because it was not quite as simple as might be considered.

The first piece of the jig-saw was brought about by my late and sorely missed friend Roger ‘Dr. Air’ Helgesen. Roger backs up to the shop door in his beat-up Ford station wagon, opens the tail gate and jerks out a very rusty looking block (his hobby was working out) on to the driveway. I am just about to say why do we need this junky block when Roger, through a big grin, announces that the block is a gem that he just got from Long Beach swap meet for a mere fifteen bucks. I comment that it looks like junk but he reaches into his station wagon pulls out my sonic tester and announces ‘”This says otherwise” which suggests that real measurements beat out opinions any day.

Turns out the rust is mostly superficial and the bores walls are a good 50 – 60 thousandths thicker than usual. After having the block oven/ball peened to clean it up it really looked like new. During machining everything cleaned up except a couple of lifter bores so they were all bored to Ford 0.875 size.

By the time we had bored it to 4.165, decked it to 9.00 added a set of splay 4 bolt mains caps and done some cosmetic machining on the otherwise rust pitted surfaces the block really did look the part of a Chevy engine builders dream for a big cube low buck build.

I was all for building a 4 inch stroker into this block but Roger was more inclined to think we would learn more by building a short stroke big bore 383 and it was, after all, his block! After a little thought on this I agreed and said I would see if I could round up the parts involved to build both engines.

I had some good AFR heads that I ported that were known performers. Along with this I had the block, crank and rods for the long stroke build. I knew before I asked Harvey Crane if he could supply a suitable range of cams to make sure each build had an optimized deal I would be OK. To this end Harvey supplied me with 5 cams to specs pretty close to each other (With a recent three years of cam testing Crane grinds on SBC’s to draw on I was pretty sure I could bracket the required cams very closely). So as far as cams went we were in good shape.

Next on the list of stuff needed were pistons. Here I gave my friend Moe Mills a call. He was at the time the boss/owner of Ross pistons. Right off the cuff Moe offered to make the pistons.

Next deal was the crank.

Here I turned to my long standing friend and project supporter Tom Lieb at Scat Enterprises. For the short stroke build we needed a crank with 3.51 inches of stroke so that the displacement of both engines was very close. In this instance the long stroke 383 was 382.67 cubes versus the short stroke one of 382.58 cubes. But there was a bonus here. I had just done some crank windage tests for Tom and just to make sure any variations here were minimized he made similar pattern cranks for both test engines and supplied the rods. This was a very generous offer but hardly unusual as Tom has always been a willing supporter of my many projects.

The long stroke 383 block had only a short time on the dyno so prep for this was limited to boring the lifter bores to 0.875 and giving the cylinder bores a quick pass through on the CK 10 hone.

After a couple of months two short blocks were taking shape. The pistons Moe Mills supplied were Ross’s close skirt clearance forged items with 1/16 -1/16 -3/16 rings. With the intended 64 cc combustion chambers these pistons would yield 11/1 on both short blocks.

At this point I had to consider heads for these builds. Just using the same heads in exactly the same form is not a true back to back test of any advantages of a short stroke, big bore versus a long stroke small bore. Yes there are advantages in friction reduction by having a shorter stroke and a bigger bore. Ring friction increases faster with stroke than it does with the bore. Also there are some small geometry advantages to the bigger bore but right now I have to get back to work so I will start on the subject of the geometry side of this test in the next a few days.

David Vizard

[stokerboats]

Roger was a good friend and mentor. As you know, a phone conversation with Roger was typically lengthy. He frequently spoke of the benefits of a large bore/short stroke. He was an extremely gifted man. He had a love for teaching others. He is thought of often when I make parts choices. His last few years took a great toll on him.

[David Vizard]

stockerboats,

it is good to know that the Roger i knew was the same one you and so many others did.

DV

[John Wallace]

What's the rod/stroke ratio of each engine?

[David Vizard]

What's the rod/stroke ratio of each engine?

John,
We used a six inch rod in both cases. Part of the shorter stroke is the ability to use a longer rod. Back then the side load reduction of a longer rod was worth something so became part of the big bore/short stroke package.
DV

[Walter R. Malik]

Years ago, in the late 80's ... we did this same type testing on a Pontiac headed, big block Chevy Pro-Stock engine.
All the same top end hardware was used ; just the short blocks were different with several camshafts run in both.

4.625" bore x 3.720" stroke and 4.562" bore x 3.820" stroke ... The big bore made slightly more power, (pretty much the same torque number), however, the longer stroke always went a couple hundredths faster down the track. We attributed this to the lighter bob weight of the total rotating assembly, being that both deck heights were the same 9.780" and the rods were the same brand & length aluminum. But, to this day, I still don't know.

Now, we only went to 8,500 RPM so, today's engines go much higher, which would probably sway more advantage toward the big bore combination.

[David Vizard]

Have a few moments here so let me go into the cylinder head package used on each engine.

The long stroke 383 was to be the first engine of the two test subjects that was to go on the dyno. The heads I had were a set of AFR castings that I subsequently ported and cut the seats for a 2.02/1.6 valve combo. I had my own valve and seat equipment so I could be very fussy about the seat work for good discharge co-efficient right off the seat. The seats were conventional multi angle deals that worked well but were not super hi-tech by any means – just good conventional seats. On my bench the heads flowed 282 cfm at 0.700 on the intake and 204 cfm on the ex. Flow at 0.250 for the intake (which I consider a very relevant figure if performance is the goal) was 172 cfm (0.758 CD) and the flow at 0.100 was 64 cfm (0.705 CD)

One of the advantages of a big bore/short stroke build is that there is more room to accommodate bigger valves. For the big bore build to keep the intake valve to bore size constant the intake valve would need to be increased from 2.02 to 2.087. Well they don’t make off-the-shelf valves 2.087 so I settled for 2.08 items.

After dyno testing the heads on the long stroke 383 I set about installing the bigger intake valve and de-shrouding chambers to the limits of the bigger bore (on the 4.03 bore the chambers were matched to the bore on each side of the chamber to minimize shrouding and flow disturbances in those areas. For the big bore I did the same again referencing the effect it had on flow. The small amount of extra de-shrouding around the exhaust due to the larger bore produced an increase in low lift flow of about 4% so that looked like a slightly larger valve.

By the time I had finished I had the intake flow at 0.050 through to almost t 0.250 up by about the same percentage as the valve size increase. All this came from bowl, seat and chamber work. The amount of material removed was minimal but effective in the low lift ranges. Flow at 0.250 rose from 172 to 177.The main body of the port was unchanged and within the accuracy of the bench remained virtually unchanged above about 0.400 lift. After the valve, seat and chamber work was done I faced the heads off in the Bridgeport to bring them down to the original chamber cc’s.

The next issue I had to deal with here was cam related. That is difficult enough when it’s one engine being dealt with but two becomes even more complex. The big question is what event aspect should we freeze?

I'll see if I can get to that by Monday or Tuesday.

DV

[pdq67]

Reading right along so please continue.

And I think about the old mag. article about the compression ratio test where they took, (I think?), a MOPAR 400 engine that had a 100 percent, "quenchless", chambered head and made it, I want to say, 12 to 1 for starters and then went backwards by using head gaskets to lower the CR to down around 8 to 1 as they watched the power output drop.

I think they ended up stacking two .060" thick head gaskets to reach the 8 to 1 CR???

Anyway, it doesn't include bore vs stroke BUT still illustrates very well CR changes AND B&S changes do this also imho.

pdq67

[BigBro74]

The big question is what event aspect should we freeze?

David, hopefully I am not a poor student
I am going to go out on a limb and say the overlap!-(frozen)
LCA change 1 degree
Duration change 2 degrees.................

don't judge me too harshly if I'm out out lunch here..........

enjoying reading as always,
Jason

[David Vizard]

The big question is what event aspect should we freeze?

David, hopefully I am not a poor student
I am going to go out on a limb and say the overlap!-(frozen)
LCA change 1 degree
Duration change 2 degrees.................

don't judge me too harshly if I'm out out lunch here..........

enjoying reading as always,
Jason

Jason,
Might I point out that folk who are prepared to stick their neck out are usually less afraid of the consequences of being wrong and I like that. However your guess here, although not totally spot on, is a very realistic response to the motors needs which in real life would have left you in a better situation than using the same cam that was optimal in the long stroke 383. Your move, I would guess from the results seen, would have picked up about 7 lbs-ft and 10 HP.

DV

[pdq67]

Are we talking about a stock SBC 3.8" stroke 383 vs a 3.75" stroke made-up SBC engine?

And David, my old junk301 engine build that I have been mouthing about for years is a go so offer up any comments you want to make.

It will either be a 4.00" b x 3.00" stroke or a 4.155" b x 3.00" stroke engine so have at it.

I have the heads coming and now am looking for some 6.25" SJ rods!

pdq67

[289nate]

Small bore and long stroke is for emissions and/or for unshrouded multi valve engines. Just my propeller hat home garage goofball opinion. Do you take stroke or bore size to make power is my look at it. Give me bore size to allow more intake valve any day of the week. But, alter valve placement and/or angles and you are no longer playing on the same field if the larger bore doesn't get the same advantage. Plus rpm (and gear which is incredibly important to acceleration) goes up.

Wanting to burn the tires pulling out of a car show on cruise in night is one thing. Wanting to hammer new ' vetted with old school crap and much less displacement is another thing entirely. Do you want to feel fast? Or do you want to be fast?

[289nate]

We should also get into the difference in rotating mass if we are talking WOT performance. Some stuff doesn't show on the average dyno.
But I through my propeller hat in and say the large bore will ALWAYS BE THE WINNER IF properly used. At the same time, the only reason to limit stroke, if it's available ( rules), is if you're afraid to make too much power for the application. Just me and I've been wrong before. Often more stroke costs the same.

[289nate]

Advances in valvetrain technology and allowing faster intake valve movement sure doesn't hurt my case. Back to sniffing glue. Lol

[289nate]

Off the dyno and not for a dyno completion, give me a long rod, only for the weight advantage on the round awkward peice of aluminum hanging on the rod trying to gain rpm. I'll be able to pick a top end that will spin a little higher and gear for it due physical limitations. Torque at the tire? Yum!