Increasing bore VS raising compression -- Discuss/debate

[DRIVEN]

It's actually kind of a trick question. For the sake of this argument it's a performance/race engine. I have to assume the entire package is matched in that direction. I tried to explain that the CR increase will allow you to take full advantage of the combination. The example I tried to use was:
Chevy 350 + .60 over and 8.5cr with large cam, stiff springs, 850cfm, headers = gutless pig.
But,
Chevy 350 std bore and 11.5cr with large cam, stiff springs, 850cfm, headers = strong runner.
However, in a bone stock engine I don't think either will be that noticeable. Have you ever seen huge power claims for cams or rockers? If you start with a full tilt 383 race engine making 600hp that used "X" cam then swapped in a stock 305 cam then only got 400hp on the dyno you could claim that "X" cam is worth 250hp. If you start with a stock 305 that dynos at 140hp, then swap in the big 250hp "X" cam do you think you'll dyno at 400hp? Nope. It could actually be even less than the original 140hp.
Has anyone noticed what the OEs have done over the last couple decades? The overall engine sized have actually gotten smaller in many cases. They are focusing on increased compression and torque. The trend has actually been to decrease bore sizes and increase stroke. Fuel delivery systems are more about velocity/atomization than sheer volume. The results are better mpg and emissions along with more useable power. Most of us have driven cars with stock replacement reman engines. Likely with a .030" or .040" over-bore. You can't feel any difference. All things being equal, you CAN feel the difference between an 8.5cr and an 11.5cr engine. Obviously, on a scratch build engine I would use every available opportunity to increase performance. If I HAD to choose. I'll take compression every time.

 

[othtim]

I dont think anyone's mentioned it yet, but another side effect of increasing the bore is allowing you to use larger valves without additional shrouding. Personally, if I had a choice, for an N/A engine, I would much rather have a short-stroke / big bore engine. If you compare an engine with a 4.00 inch bore and an engine with a 4.125 inch bore, i'd bet you that you could stuff bigger valves in the 4.125 bore engine. In a compression-limited engine (like if you have to run on 89 pump gas), I'd rather have as much bore as possible. If I had to build an engine that was limited in displacement and compression, (say max 350 cubes), I'd rather get those cubes by having as big of a bore and short of a stroke as possible.

Something I've been wanting to do for a few years now is build a 350 using a dart SHP block - you can get like 4.155 bores on those things. 3.25 stroke and a 4.125 bore gets you a nascar-style 350. anyone ever done something like that? i'm just curious how the shorter stroke would change the "personality" of an engine - would it affect your cam choices by having the piston dwell less at TDC and BDC, and overall lower piston speeds? I dont know, but i'd assume that it would.

 

[RITTER]

That is like saying you would rather have a short fat guy on your marathon team rather than a tall skinny guy :idea:

 

[othtim]

That is like saying you would rather have a short fat guy on your marathon team rather than a tall skinny guy :idea:

no it's not like that at all 😛

 

[RITTER]

Either way you would go with the bore to stroke debate; less stroke, more strain on the crank ... more stroke, more strain on the outter cylinder wall

 

[DRIVEN]

I dont think anyone's mentioned it yet, but another side effect of increasing the bore is allowing you to use larger valves without additional shrouding. Personally, if I had a choice, for an N/A engine, I would much rather have a short-stroke / big bore engine. If you compare an engine with a 4.00 inch bore and an engine with a 4.125 inch bore, i'd bet you that you could stuff bigger valves in the 4.125 bore engine. In a compression-limited engine (like if you have to run on 89 pump gas), I'd rather have as much bore as possible. If I had to build an engine that was limited in displacement and compression, (say max 350 cubes), I'd rather get those cubes by having as big of a bore and short of a stroke as possible.

Something I've been wanting to do for a few years now is build a 350 using a dart SHP block - you can get like 4.155 bores on those things. 3.25 stroke and a 4.125 bore gets you a nascar-style 350. anyone ever done something like that? i'm just curious how the shorter stroke would change the "personality" of an engine - would it affect your cam choices by having the piston dwell less at TDC and BDC, and overall lower piston speeds? I dont know, but i'd assume that it would.

Like this one http://www.purplesagetradingpost.com/su ... ngine.html ? I agree that big bore / short stroke engines are fun. I'd rather drive a 302 than a 305 or a 377 over a 383 in a performance oriented situation. For a street cruiser or daily driver I might go the other way. That's kind of a whole separate debate.
In the case of the 4cyl race engine they were discussing, shrouding was a non-issue. It's a hybrid of an Lseries Nissan head on a Zseries block. The result is an automatic increase to 11.5:1 compression. His question was whether it would be worth it to bore it to 89mm and install milled down KA pistons for extra displacement and up to 13:1cr. I made a statement that I would save the trouble/time/expense and run it as-is saying that most of his performance gain would be a result of the added cr rather than the added bore size. That was the statement that sparked the debate. I don't necessarily need to be right, just wanted to see what you guys thought. I like to learn.

 

[RITTER]

I made a statement that I would save the trouble/time/expense and run it as-is saying that most of his performance gain would be a result of the added cr rather than the added bore size. That was the statement that sparked the debate. I don't necessarily need to be right, just wanted to see what you guys thought. I like to learn.

I believe that statement is completely correct.

If you were to take a mid-70's smog SBC and throw a set of 64cc double hump heads on it, dyno the engine before and after the swap ... take the block to the machine shop and have it bored, we'll say .030" over, and replace with the same -Xcc pistons that the engine had from the factory. Place the 76cc smog heads back on the block and dyno again.

I guarantee you would have had more of a power gain using the double hump heads on the old shortblock than boring it and replacing it with factory pistons (.030" over of course) with the 76cc smog heads

 

[DRIVEN]

You know, the group of guys I was discussing this with on the other site know their stuff so it's been fun. Here's a couple statements they made that seem to make sense and had me second guessing my argument.
1. You gain roughly 3.5% per point in compression increase (no source given). On a 2.0l that's only 7% going from 8.5cr to 10.5cr.
2. Adding the 4mm increased bore will add .2 liters. That's a 10% increase in displacement.

The mathematical logic makes sense but my experience says otherwise.

 

[jrm81bu]

You know, the group of guys I was discussing this with on the other site know their stuff so it's been fun. Here's a couple statements they made that seem to make sense and had me second guessing my argument.
1. You gain roughly 3.5% per point(3.5% what? I assume power. And it's still a guess.) in compression increase (no source given). On a 2.0l that's only 7% going from 8.5cr to 10.5cr.
2. Adding the 4mm increased bore will add .2 liters. That's a 10% increase in displacement. 10% in displacement not necessarily power.

The mathematical logic makes sense but my experience says otherwise.

So really the math here doesn't matter, lol.

 

[DRIVEN]

Sorry if I made it unclear. I paraphrased what they said there. Yes, 3.5% per point increase in power and the 10% was an increase in displacement and (he implied) power. I don't know where he got the 3.5% figure from and I highly doubt that the increase in displacement will be equate to the same power increase. Again, I have a great amount of respect for the knowledge these guys have. That's why it's weird that they seem so far off on this one.