Ideal Drag Race Bore and Stroke

[Newbomb Turk]

If I remember right.....
EM did a dyno test of two otherwise identical short blocks, a long rod engine vs a short rod engine..... and didn't find any substantial difference. (bbc).

Well that proves Yupik was stupid then I suppose. Or, might I suggest their test was fucked? Maybe that would be the reason.

IIRC they used the same cam with both setups. That’s not right. They didn’t hook a blow by meter up either. That would tell a whole different story.

EM always (almost always) uses a 300 rpm/sec rpm acceleration rate. That’s too slow to do testing like that.

That’s why you see dual plane intakes look killer on the dyno and then they get their *** kicked at the track.

But that slow acceleration rate sure makes those power numbers look good.

 

[33IMP]

Well, I'm using a 7.1 bbc rod in my 522, cause that's what hooked up the pistons I have to the crank I have. I suppose that's considered a long rod......

 

[33IMP]

Well that proves Yupik was stupid then I suppose. Or, might I suggest their test was fucked? Maybe that would be the reason.

IIRC they used the same cam with both setups. That’s not right. They didn’t hook a blow by meter up either. That would tell a whole different story.

EM always (almost always) uses a 300 rpm/sec rpm acceleration rate. That’s too slow to do testing like that.

That’s why you see dual plane intakes look killer on the dyno and then they get their *** kicked at the track.

But that slow acceleration rate sure makes those power numbers look good.

How would using a different cam make the test more valid? Sounds like an invalid test to me. (I understand the events should change to match the r/s, but that's assuming the events of the cam used are perfect for one length, and not the other. And the perfect cam doesn't exist)
EM is not gonna run five or ten cams thru two shortblocks to optimize both rod lengths. That's Nascar stuff.

 

[33IMP]

Dyno numbers<track numbers

 

[Newbomb Turk]

Dyno numbers<track numbers

Exactly. That’s why guys get pissed when the dyno numbers don’t match the time slip.

It’s 50% car, 48% end user not knowing what the dyno numbers mean and 2% are shitty testing and lying operators.

If you have a 24 inch stroke and a 7 inch rod what do you have?

Rod length doesn’t mean ****. It’s the ratio of the stroke and the rod length.

The short of it is, the EM test was run at way too slow an acceleration rate and if you think the cam timing for both of those extremes are the same you’d be wrong.

The lower the ratio, the quicker the piston moves around TDC. And the slower it moves around BDC.

The higher the ratio, the slower the piston moves around TDC. And the quicker it moves around BDC.

Think about that. All the whiz bang in gym shorts types all agree that IVC is the most critical event of the eight. A two degree change in IVC makes a difference.

If the piston is lolly gagging around BDC with a low ratio and moving around TDC quickly you can see how that changes cam timing.

A lot of what I’ve seen on EM is testing that DF does is for his confirmation bias.

He does have a box he has to work in, so I get it.

But just because EM got a certain result doesn’t mean it’s the final answer.

Especially with water brake dynos.

 

[tvt59]

You totally missed my point.

I’ll add this because it matters. This is just one point.

If rod length doesn’t matter, why not use a 6 inch rod? It’s cheaper. Bearings are easier to find. And it doesn’t matter right?

Or why not a 5.7 rod? Again, it’s a Chevy length so they are easy to find and bearings are everywhere.

The reason it’s not done is because rod length does matter.

The shorter the rod, the heavier the piston. The shorter the rod, the lower the pin height and the piston has more rock around TDC and it hurts ring seal.

Rod length changes cam timing as well.

So when you boil it all down you find in the end that what Smokey Yunick said 50 years ago was and is right as rain.

Use the longest rod you can fit. And EVERYONE does it, but they slam the guy and say he didn’t know what he was talking about blah blah blah.

Of course, with Chrysler junk you can get the rod too long because the deck heights are too high for the stoke length they use. But that’s the exception and not the rule.

Not one single engine builder I know uses a rod shorter the longest they can fit. To the point they run the oil ring around the wrist pin.

That should prove to you that rod length matters otherwise you’d never see a piston that has the oil ring around the wrist pin.

Physics didn’t change. Smokey wasn’t wrong. Rules changed. And shoe polish won the day.

Mr Turk, I'm not trying to argue with anyone. I was always taught that the closer that you could get to 1.9 rod to stroke ratio the better. And I wanted to use a 6.250 or 6.300 rod. We used a 6.125. And bought a nice set of pistons. The engine made 841hp at almost 7500 rpm. It turned out well. It made 80 hp more that I thought it would. So end the end he told me not to worry about it and it worked out better than I thought is would. Maybe I mis-stated. Maybe I should have said it doesn't make much difference instead of it makes no difference.

 

[Newbomb Turk]

Mr Turk, I'm not trying to argue with anyone. I was always taught that the closer that you could get to 1.9 rod to stroke ratio the better. And I wanted to use a 6.250 or 6.300 rod. We used a 6.125. And bought a nice set of pistons. The engine made 841hp at almost 7500 rpm. It turned out well. It made 80 hp more that I thought it would. So end the end he told me not to worry about it and it worked out better than I thought is would. Maybe I mis-stated. Maybe I should have said it doesn't make much difference instead of it makes no difference.

Do the math on the ratio difference on a 6.125 verses at 6.250 and 6.300.

If the longer rods would have put the oil ring around the pin I wouldn’t have done it either.

If not, I would have used the longer rod and and saved .125-.175 of piston height.

It makes the piston lighter.

 

[gregsdart]

I look at my last build, and chose a 4.315 stroke ccw crank i had over a 4.560 stroke crank i had because i felt the ccw may provide better ring seal in an aluminum BMP block, so i got 549 cubes. It made 900 hp at 6400 and 980 at 7600.
The bigger crank would be 580 cubes, and would drop max power rpm down maybe a couple hundred rpm, but add needed torque at 6400 where my stall is now with the 549. If i was wrong on the ring seal idea, and it was the same, i would gain an extra bunch of hp in the first half of each gear, making the bigger motor faster, and at a bit less rpm. Since i have stainless intake valves and best performance was shifting at a recorded 8,000 rpm, it would be a huge benefit to my valvetrain life. So that is how stroke affects my motor building; for me and my needs the more you can use easily the better. My big mistake was not having that ccw crank made (Crower) in 2003 with a bigger 4.625 stroke, which i believe would have been as big as easily fit in the megablock i had, making 588 cubes. Being able to keep rpm down around 7500 would fit my valve strain combo

 

[273]

Most of us aren't building for a highly competitive race series so you don't need every advantage to make X amount of HP, It's generally easier to build a less efficient larger displacement than a highly efficient smaller displacement. Eg. 365 x 1.45 torque per cid = 530 lbs-ft vs 408 x 1.30 tq:cid = 530 lbs-ft.

 

[Newbomb Turk]

Most of us aren't building for a highly competitive race series so you don't need every advantage to make X amount of HP, It's generally easier to build a less efficient larger displacement than a highly efficient smaller displacement. Eg. 365 x 1.45 torque per cid = 530 lbs-ft vs 408 x 1.30 tq:cid = 530 lbs-ft.

Which is why it’s done so much. Easy is better for some.

 

[Icetech]

4.500 x 4.500.

 

[ValiantRacing]

Exactly. That’s why guys get pissed when the dyno numbers don’t match the time slip.

It’s 50% car, 48% end user not knowing what the dyno numbers mean and 2% are shitty testing and lying operators.

If you have a 24 inch stroke and a 7 inch rod what do you have?

Rod length doesn’t mean ****. It’s the ratio of the stroke and the rod length.

The short of it is, the EM test was run at way too slow an acceleration rate and if you think the cam timing for both of those extremes are the same you’d be wrong.

The lower the ratio, the quicker the piston moves around TDC. And the slower it moves around BDC.

The higher the ratio, the slower the piston moves around TDC. And the quicker it moves around BDC.

Think about that. All the whiz bang in gym shorts types all agree that IVC is the most critical event of the eight. A two degree change in IVC makes a difference.

If the piston is lolly gagging around BDC with a low ratio and moving around TDC quickly you can see how that changes cam timing.

A lot of what I’ve seen on EM is testing that DF does is for his confirmation bias.

He does have a box he has to work in, so I get it.

But just because EM got a certain result doesn’t mean it’s the final answer.

Especially with water brake dynos.

From what i understand and heard the longer the rod the less duration you want to run with the camshaft.

 

[273]

I heard with longer rods engine needs more overlap and higher velocity ports, short rods from what I heard starts pulling the intake charge sooner so putting less need for overlap and fast reacting ports to get it going. Don't know if that equals better filling though.

 

[Newbomb Turk]

It’s hard to nail down but posts 62 and 63 are pretty good general rules.

I want to emphasize it’s not really a long verses short rod discussion, it is really about the ratio of the stroke to rod length.

A 6 inch rod and a 3 inch stroke is 2:1. A 6 inch rod and a 6 inch stroke is 1:1.

I think talking about rod length muddies up the waters when talking about this.

IMO you can use a later IVC and and an earlier IVO with a high(er) r/s ratio.

And with that, more overlap and a faster closing ramp on the intake side.

And probably a 6-8 degree spread on the exhaust so you can get the EVO back where it should be. Especially if you want to extend power after peak.

 

[fishmens67]

R/S ratio is an important factor to consider.

 

[fishmens67]

It’s the ratio of the stroke and the rod length.

The most important factor to consider when building a performance engine.

 

[fishmens67]

4.375 x 3.75

451 ci low deck is hard to beat.

 

[Icetech]

I think rod girth matters more.... at least so i'm told....

 

[fishmens67]

I'm still waiting for my wife's honest answer, she knows how to make HP.

length is important, so I'm told by my engine builder, a big bore with a longer rod length. lol R/S ratio

 

[MOPARMAGA]

I think rod girth matters more.... at least so i'm told....

Never heard a complaint of piston being to big for the bore either

 

[fishmens67]

Never heard a complaint of piston being to big for the bore either

adjustable side clearance with slick coat piston.

 

[MOPARMAGA]

adjustable side clearance with slick coat piston.

Lol

 

[tvt59]

I read an article last night that every year for the last few years that the P/S deck heights have been getting shorter each year. I think that they may be below 9" now. I know what the article said the reason for this is. But what are the thoughts in here about this?

 

[Dartswinger70]

All this rod and stroke talk = that's what she said

 

[273]

I read an article last night that every year for the last few years that the P/S deck heights have been getting shorter each year. I think that they may be below 9" now. I know what the article said the reason for this is. But what are the thoughts in here about this?

Darin Morgan said the intakes work better with shorter decks and the gains are enough to offset running a shorter rod ratio.