[Is1BadFord]

I've got a question for you guys who port for a living.

About how much of a flow gain, on a standard inline valve head (2.055/1.60 Victor Jr, AFR, Pro 1, etc with 210-215cc ports) will you see from changing bore size if you started with 4.030, and went to a 4.125? This is assuming a full port job on both intake and exhaust, and a head that flows these numbers:

.1 72/72
.2 134/122
.3 190/172
.4 242/205
.5 283/217
.6 302/220
.7 306/218

I'm considering doing a Dart block build, and it clicked that the bigger bore might actually help me make more power.

Anyhow, thanks in advance.

Cris

 

[dfree383]

I would guess in the range of 2-4% +/- of total flow, could be more depending on heads and combustion chambers.

 

[Is1BadFord]

By 2-4% of total flow, do you mean at each lift point, add appx. 2-4%, or as an average of the whole thing? I'm just curious, even 2% is a significant amount, and I want to model it correctly in my software in order to choose a cam, and have some idea how it will run.

Cris

 

[ou812]

You will gain some low lift flow, but high lift (.500 and up) flow will be relatively unchanged. But that's not the only reason for a big bore. The bigger bore also increases cubic inches in turn making more power, more torque and better breathing. You can also sometimes use a bigger intake valve if the heads want it and do more porting for a bigger gain yet.
Brian

 

[dfree383]

Like he said it may not be an across the board, it all depends on your combo. 2-4% is a guess.

 

[tonys10sec306]

big bore is the way to go on a race engine

 

[Is1BadFord]

On 2006-03-29 06:21, ou812 wrote:
You will gain some low lift flow, but high lift (.500 and up) flow will be relatively unchanged. But that's not the only reason for a big bore. The bigger bore also increases cubic inches in turn making more power, more torque and better breathing. You can also sometimes use a bigger intake valve if the heads want it and do more porting for a bigger gain yet.
Brian

I was assuming a bigger bore with the same cubic inches =). For example, a 4.125 bore Dart 351 block, with a 4" stroke to make a 427, rather than the off square setup using a factory windsor block.

Anyhow, thanks for the answers! Much appreciated.

Cris

<font size=-1>[ This Message was edited by: Is1BadFord on 4/3/06 8:30am ]</font>

 

[ou812]

Personally, I like smaller cubes and higher rpm. Everytime I build a small engine with a good head it seems to run with or out run larger engines with similar combos. Maybe it's me, dont know

But the combo you want to run should be great, and have a ton or torque...just remember that the extra torque from the 4" crank will be mostly below 5000rpm.
Brian

 

[Is1BadFord]

I was using a 427 as an example...if I spent the money on a Dart block...it would be a hard choice which displacement to build. I prefer smaller motors with a lot of RPM...but then again, why not build a bigger motor with a lot of rpm? =). My dad's circle track motor is a 427 (out of a...small block chevy...yuck...although it's a Dart block too), turns 9000rpm, and makes damn near 950hp. He's running alcohol, and 14.5:1 compression though lol. Again like I said...hard call...build a sick 347, or for very close to the same money...a REALLY sick 427.

Anyhow, lol thanks again for the replies!

Cris