Exhaust port...Blown vs. N/A

steelcomp · Post by **steelcomp** » Sat Mar 25, 2006 11:12 pm

What are the different requirements for a blown motor (BB Chev, 496, 10-12#) vs. N/A? These are the areas I'm curious about. This is for a standard style Chev. head, but questions are in general, as well.
Ex port velocity...is it still an issue with blower motor?
Can a blower motor ex port and/or valve be too big?
Is there a limit to in. vs ex. port ratio on a blown application?
Are there preferred characteristics for a blower motor ex port, or is "bigger is better" the way to go?
Thanks in advance.

KennyM · Post by **KennyM** » Sun Mar 26, 2006 12:53 pm

This is a good question to add to the thread on exhaust venturi discussion.

I seem to remember David Vizard stateing in one of his books that blower and nitrous are about the same as far as increased cylinder pressures and gas volume goes and a bigger exhaust valve would be benifical in a specialty combination.

Reading the venturi thread brings a lot of questions still to be answered in the future.

Mouse · Post by **Mouse** » Sun Mar 26, 2006 1:01 pm

If I were building some heads for a direct drive blower, I would go bigger on the exhaust. But if I were going to use a turbo charger, I would go with smaller ports, more in line with NA ports.

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 1:30 pm

KennyM wrote:This is a good question to add to the thread on exhaust venturi discussion.

I seem to remember David Vizard stateing in one of his books that blower and nitrous are about the same as far as increased cylinder pressures and gas volume goes and a bigger exhaust valve would be benifical in a specialty combination.

Reading the venturi thread brings a lot of questions still to be answered in the future.

Ironically, as I posted this question, I saw the venturi thread, and read it in it's entirety.. Incredibly interesting, though mostly above my knowledge.
I know there is a formula to translate boost into a cubic inch equivilant. If one was to approach a blown motor from a shear cubic inch position, could the typical factors be taken into consideration when addressing a port design? Maybe on the intake as well?

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 1:33 pm

Mouse wrote:If I were building some heads for a direct drive blower, I would go bigger on the exhaust. But if I were going to use a turbo charger, I would go with smaller ports, more in line with NA ports.

Mouse, I'm wondering what's behind your reasoning here? My guess is that you're thinking about maintaining ex. gas velocity, trying to keep the turbo's spinning?

cboggs · Post by **cboggs** » Sun Mar 26, 2006 1:48 pm

I think Mouse is dead on with his post.

I build heads for a blown alky customer and it seems like I can't
get a big enough exhaust port in them, .. every time I use a bigger
port they like it.

But, .. my turbo heads I always use a N/A style exhaust port with
relatively high exit air speeds. The turbo engines seem to respond
with better power and it does seem to help spool up the turbo.

Curtis

Engineguy · Post by **Engineguy** » Sun Mar 26, 2006 2:00 pm

I was playing around on Desktop Dyno 2000 a few years ago and stumbled onto something, perhaps... went something like this:

Large bore (? dia) that had room for two 1.6" intake valves, and two 1.4" exhaust valves... 3" stroke... 10psi supercharged.

Changed all 4 valves to 1.5" and got about 3% more horsepower.

Changed to 1.4" intakes and 1.6" exhausts and got another 3%.

Sorta makes sense that on a supercharged engine it's easier to make gains on the exhaust than it is to hurt the intake. But I never did follow through and investigate further... maybe I had wacky cam timing in it or something... and I know that's not the best engine sim software. It would be interesting if someone would play with this scenario on better software (kinda hard to do it on an existing real engine).

Engineguy · Post by **Engineguy** » Sun Mar 26, 2006 2:07 pm

cboggs wrote:I build heads for a blown alky customer and it seems like I can't get a big enough exhaust port in them, .. every time I use a bigger port they like it.
Curtis

Well, turn those heads around!

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 2:10 pm

OK...this is what I'm getting. Go 1.900+ ex valve, knock the throat out of it to the bottom of the 45, make the port as big as possible...don't worry about shape, STR, anything like that? (within reason, of course?)

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 2:11 pm

Engineguy wrote:
cboggs wrote:I build heads for a blown alky customer and it seems like I can't get a big enough exhaust port in them, .. every time I use a bigger port they like it.
Curtis
Well, turn those heads around!

LOL...too funny.

cboggs · Post by **cboggs** » Sun Mar 26, 2006 2:25 pm

steelcomp wrote:OK...this is what I'm getting. Go 1.900+ ex valve, knock the throat out of it to the bottom of the 45, make the port as big as possible...don't worry about shape, STR, anything like that? (within reason, of course?)

What's the application ?????????????

I wouldn't just knock the throat out to the bottom of the 45 on ANY head!
always worry about shape ( turbulence is a killer )

Curtis

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 2:52 pm

Thanks Curtis...I was hoping to get some attention here. LOL.
The question came from a general discussion on another forum. The advice that was given was as follows:

Unshroud the valves, install the largest exhaust valve you can put in there, we prefer the Ferrea for a blown alcohol set up(you will never hurt them). Port the exhaust side as much as possible, don't worry about the intake flow/exhaust flow ratio, just get as big a flow # on the exhaust as possible.

I wasn't sure I agreed with this, so I was looking for some more specific info. I'm also going to be building a mild blown alky BB Chev in the next year...496, (may go 509 if I can get a good sonic on a block), 871 Littlefield, Birdcatcher, etc., so I'm researching as much as I can.
Thanks for the input.

cboggs · Post by **cboggs** » Sun Mar 26, 2006 3:16 pm

what's the other forum you where on ?

Yea if your going to build blown alky stuff maybe you should call me, ..
I do a bunch of 540 and 572 blown alky heads mostly for Monster trucks
and some dragster.

Curtis

MadBill · Post by **MadBill** » Sun Mar 26, 2006 3:27 pm

As I understand it, a high compression N/A engine makes most of its torque at the top of the stroke, as the cylinder pressure drops rapidly due to the matching high expansion ratio and thus loses little from using a small valve opened early, thereby leaving room for a bigger intake.

By contrast, a mechanically blown engine has relatively high cylinder pressure all the way to the bottom of the stroke, so opening a small or moderate-sized valve early incurs a large power penalty vs. a huge valve opened late.

A turbo is a special case, more like N/A if properly sized, due to the high exhaust back pressure...

steelcomp · Post by **steelcomp** » Sun Mar 26, 2006 3:48 pm

cboggs wrote:what's the other forum you where on ?

Yea if your going to build blown alky stuff maybe you should call me, ..
I do a bunch of 540 and 572 blown alky heads mostly for Monster trucks
and some dragster.

Curtis

Sent you a PM