Minimum Cross Sectional Area: Question

[randy331]

Skinny, your putting way too much faith in the results of your engine sim.

Randy

[gmrocket]

Are you aiming for some specific DCR?

Not specifically but I am avoiding those I've found too high and those that are too low.
On a dyno engine with the right fuel, some of the higher DCRs would obviously work. It's here that I'll have to admit that I have to consider the impact of that kind of result on the ultimate destination for this engine which up to this point I've deliberately excluded from the build process (and would like to keep it that way). As for the lower values, it seems there's always room for improvement there.

Not specifically, but not too high or too low?

How do you know what's too high or low?

[skinny z]

Not specifically, but not too high or too low?
How do you know what's too high or low?

By what's worked for me in the past and what hasn't.

[gmrocket]

Not specifically, but not too high or too low?
How do you know what's too high or low?

By what's worked for me in the past and what hasn't.

Ok, so you must have a number in mind...what is it?

[skinny z]

Ok, so you must have a number in mind...what is it?

In around 8 would work for what I have in mind. But having said that, the final engine package and application hasn't been entirely determined. Hence the point of this thread.

[RevTheory]

Hey skinny, you're running EQ iron Vortec's, right? Do you happen to know if they're more resistant to detonation than non-Vortec chambers?

[skinny z]

Hey skinny, you're running EQ iron Vortec's, right? Do you happen to know if they're more resistant to detonation than non-Vortec chambers?

RHS Vortecs actually although they have similar casting I.D. to the EQs. Seems it's an efficient head design overall (port and chamber) with good combustion characteristics. Like OEM Vortecs.
Keeping the quench tight, target .040", has some detonation inhibiting qualities as well (as you know).

[68corvette]

There can be significant differences between engine combinations, even with same cam.
One great example is TPI vs LT1.
You need to have really different cam mainly due to intake manifold, even if rest of the engine would be close to the same.

Here is TPI vs mini ram manifold swap as the only change.
https://www.eecis.udel.edu/~davis/z28/w ... bpage.html

With long runners and high low rpm torque it would be a lot more sensitive for detonation.

[GARY C]

Ok, so you must have a number in mind...what is it?

In around 8 would work for what I have in mind. But having said that, the final engine package and application hasn't been entirely determined. Hence the point of this thread.

Application is going to play a key part, if you are looking at DRC in relation to pump gas then you have a somewhat narrow window to hit that number, if you want to increase rpm then the cam needed is going to lower DCR unless you increase compression in relation to cam but then you start getting out of the range of pump gas.

I only started looking at DCR on my last 2 street engines because my concern was idel quality and drivability and with an iron head and 10.1ish compression I didn't want a cam that put my DCR over 200 psi and run the risk of trying to tune something that should not be on pump gas.

[skinny z]

There can be significant differences between engine combinations, even with same cam.
One great example is TPI vs LT1.
You need to have really different cam mainly due to intake manifold, even if rest of the engine would be close to the same.

Here is TPI vs mini ram manifold swap as the only change.
https://www.eecis.udel.edu/~davis/z28/w ... bpage.html

With long runners and high low rpm torque it would be a lot more sensitive for detonation.

That is certainly true although the TPI seems almost unique in that regard. We've all seen impressive torque numbers but the RPMs just aren't there to make any HP.
Torque has a data field for induction length. It's generally been award off at 11.5" (port plus manifold), but adding an inch to that length pulls the RPM peaks down a few hundred RPM. Something that I may use as a tuning tool when it comes down to it.

[skinny z]

quote]
Application is going to play a key part, if you are looking at DRC in relation to pump gas then you have a somewhat narrow window to hit that number, if you want to increase rpm then the cam needed is going to lower DCR unless you increase compression in relation to cam but then you start getting out of the range of pump gas.

I only started looking at DCR on my last 2 street engines because my concern was idel quality and drivability and with an iron head and 10.1ish compression I didn't want a cam that put my DCR over 200 psi and run the risk of trying to tune something that should not be on pump gas.

I'm trying to leave the application out of it for the purposes of this thread however, from a street perspective I can relate to exactly what you're talking about. Iron heads, 10.4 SCR, DCR nearly 8.5 and cranking compression north of 200 PSI kept me from utilizing a full timing curve. The WOT element was simple. The part throttle cruise...not so much. I didn't help myself with an ultra lean AFR at those speeds but the transition was difficult and with 45+ degrees of timing (14-15 initial, part of the 19 mechanical plus 14-16 vacuum advance) , detonation was evident. Plus this is good old carb and conventional distributor. I know with EFI, the tuning accuracy can be more precise but the old school way involves a lot of work to get it right. And some changes can be complex.

[GARY C]

quote]
Application is going to play a key part, if you are looking at DRC in relation to pump gas then you have a somewhat narrow window to hit that number, if you want to increase rpm then the cam needed is going to lower DCR unless you increase compression in relation to cam but then you start getting out of the range of pump gas.

I only started looking at DCR on my last 2 street engines because my concern was idel quality and drivability and with an iron head and 10.1ish compression I didn't want a cam that put my DCR over 200 psi and run the risk of trying to tune something that should not be on pump gas.

I'm trying to leave the application out of it for the purposes of this thread however, from a street perspective I can relate to exactly what you're talking about. Iron heads, 10.4 SCR, DCR nearly 8.5 and cranking compression north of 200 PSI kept me from utilizing a full timing curve. The WOT element was simple. The part throttle cruise...not so much. I didn't help myself with an ultra lean AFR at those speeds but the transition was difficult and with 45+ degrees of timing (14-15 initial, part of the 19 mechanical plus 14-16 vacuum advance) , detonation was evident. Plus this is good old carb and conventional distributor. I know with EFI, the tuning accuracy can be more precise but the old school way involves a lot of work to get it right. And some changes can be complex.

Thats where my 350 was but 8.4 on Patric's calculator, 205ish psi, Comp XE 262 flat tappet with Harland sharp 1.65 rockers, I ran about 20 initial, around 40 part throttle via adjustable vac advance with one light spring and one medium and 36 total it took a little time to dial it it but worked good. It was in an Elcamino with 1000 over stock converter and a 2.73 gear with a 28" tall tire, Air Gap and a 780 vac secondary, no A/C.

[mt-engines]

Ok, so you must have a number in mind...what is it?

In around 8 would work for what I have in mind. But having said that, the final engine package and application hasn't been entirely determined. Hence the point of this thread.

Application is going to play a key part, if you are looking at DRC in relation to pump gas then you have a somewhat narrow window to hit that number, if you want to increase rpm then the cam needed is going to lower DCR unless you increase compression in relation to cam but then you start getting out of the range of pump gas.

I only started looking at DCR on my last 2 street engines because my concern was idel quality and drivability and with an iron head and 10.1ish compression I didn't want a cam that put my DCR over 200 psi and run the risk of trying to tune something that should not be on pump gas.

Funny, ive had some big block chevys for motor homes with that kinda number at 9.5:1 375hp 461" .. No tuning problems, detonation problems. Once you guys realize DCR is a BS number along with cranking compression , You will start to be on par with real engine builders that dont care what some made up formula says. They learned to try what works vs some arbitrary calculation.

Bottom line. Is you arent building anything different from whats been done before. Yet you want to reinvent the wheel.. A kot of good suggestions were presented. Once you waste all your time and money on a underperforming cam, it will seemingly run great. Because you wont know how well it would have run if you would have listened to people that actually build engines day to day.

Keep it simple stupid.

[skinny z]

Skinny, your putting way too much faith in the results of your engine sim.

Randy

Actually Randy, I have little faith in my simulations otherwise I would have ordered the parts already.
All I'm doing really, seeing as I don't have a dyno nor do I care in the least to make any whvolesale changes after this is buttoned up, is seeing what's been done. And yes, maybe see who is doing it differently.
As the saying goes, I would prefer to stand on the shoulders of giants. This is why I'm here at ST.
I had a recipe in place before all of this started but through some simple investigation and question asking I'm seeing a deeper layer and I'll indulge myself and explore it. For me it is a hobby.

[skinny z]

. Yet you want to reinvent the wheel.. .

I'm not trying to reinvent anything mt.
I asked a question about MCA. Seeing as I'm trying to avoid "something that's been done before", I'll continue to ask.
Yes. Good suggestions have risen up. And I check them all out. Funny in that these suggestions often have a different spin. So what's right? What's rubbish? And what fits my unique combination of parts? And before everyone goes off all half-cocked, they're not unique in the sense that no one else has them. But unique in the sense that I have them.
Once all the dots line up, then I'll be good to go.

By the way mt, while you're here I've a question.
How would you spec a cam for the following dyno engine? 383, 9.8:1, 255@.500-.550 iron heads, 6" rods, RPM Air Gap. Pump gas. (I can provide additional details should you want them). I want good on paper results. Forget any application.