383 sbc first dyno experience

[skinny z]

Get exhaust lobe right and rocker ratio change is almost margin of error stuff.

When it's all said and done, this part to me makes the most sense.
But, further to what I had posted about changing the exhaust system itself, i.e. larger headers or conversely, going from open headers (PipeMax spec'd) to a fully mufflered system, I think will now take that optimum exhaust lobe designed with the rocker ratio in mind and kind of throw it out the window. This may be the ideal spot to work with ratios.
Another job for the sims as I too don't have the resources to do it with hard parts. In as much as I'd like to.

Thanks for that HMQ.

[HQM383]

Get exhaust lobe right and rocker ratio change is almost margin of error stuff.

When it's all said and done, this part to me makes the most sense.
But, further to what I had posted about changing the exhaust system itself, i.e. larger headers or conversely, going from open headers (PipeMax spec'd) to a fully mufflered system, I think will now take that optimum exhaust lobe designed with the rocker ratio in mind and kind of throw it out the window. This may be the ideal spot to work with ratios.
Another job for the sims as I too don't have the resources to do it with hard parts. In as much as I'd like to.

Thanks for that HMQ.

Or re-evaluate exh lobe if a restrictive exhaust system has to be used v non-restrictive changes optimum lobe. But yes, I understand what you're saying.

[steve cowan]

Smooth, quiet flowing port with appropriately sized cross sectional areas trump flow numbers I suspect?

Yes,I agree
I am no expert at all but most of the time now when I use my bench it is more about listening to the port at all depressions and lifts.
I start my lifts at 0.250" and run to 0.100" past max lift on cam I am using.
I like to test at highest depression o can which is 45" on my smaller heads.
My bench has the potential to flow 400cfm @ 28" .
I don't look at overlap flow or low lift flow,I like steeper angles,sinking the valve more.
Less margins on valves,no back cuts.
My opinion is with a 23 degree head you are not valve limited as it's hard to get the throat as the MCSA without going to big on the port for most applications.
The choke is further up stream so making the throat/ valve bigger will only slow air/ fuel down which I don't believe is what you want.
Again it is application specific.

What do you think is more important, minimum cross sectional area or average cross sectional area? Local velocities in specific areas to get right or average velocity to targeted application?

Up until my first dyno session I used a Mach Index of 0.55 which is around 618 ft/ sec in my calculations.
The cast iron heads I figured would peak at 6300rpm using 618 ft/ sec in my equation.
When the engine peaked at 6600rpm and hung on till 7000rpm I started using 0.61 mach which is 690/ ft sec .
This is for MCSA ,its an approximate not set in stone,just a start point.
I believe if you can keep the CSA as consistent as you can the air speed is not speeding up/ slowing down so much .
As far as velocity goes I look at it but I don't target an average because in my opinion due to the lower rpm we use the velocity can be higher, small port can have SSR floor at 400ft / sec and run ok at 6500rpm.

[steve cowan]

That raises a question I've had since the first I read about someone using less ratio on the exhaust as opposed to the intake.
What's the logic there? Is it compensating for a cam spec or working with the specifics of a particular port?

I'm the seeing the answer is in all of the above.
A guy like Steve, who's constantly experimenting on the flow bench, is seeing an advantage (or not) when moving the rocker ratio around. That's with a given cam and an unspecified cylinder head, Unspecified in that it's constantly evolving. The ratio that may have worked yesterday isn't necessarily the ratio of today.
Now, to bring that around to a fellow like myself, who hasn't got a flow bench (or ready access to an engine dyno for that matter ) but I've a set of heads, done and dusted as it were. I've an engine spec pre-decided. That is, these are the parts I've got. Rocker arm ratio included (1.6 all around). I send that off to my cam designer and what he delivers to me is what is best suited to that pile of parts.
At least that's how I see it.
I can't say what changing my engine spec might do to what is an otherwise optimum cam. A change to a single plane from the dual plane I've always run or a move to large headers will undoubtedly have some sort of cascading effect. Even on a more basic level, my spec is based on an open header. It's reasonable to assume that the cam isn't best suited to a fully mufflered car. Might the rocker ratio come into play here?
Anyway, just musing on my part.

In my quest for knowledge and understanding and a bank account that can not afford a dyno or infinite amount of parts to swap out, running some sims returns interesting feedback on reduced rocker ratio on exhaust. Two patterns emerge

1. If the exhaust lobe is tailored closer to what the engine likes (not duration, this was held reasonably constant) changing exh R/R has reduced effect. The opposite is true. It seems to equate to engine wanting specific exh valve movment for targeted rpm gains ie peak tq or peak hp.

2. If the combination yields fast exh movement off seat such as TK lobe with 1.6 R/R then low torque is enhanced below peak. Going extreme to 1.3 R/R lowers tq below peak ans conversely raises tq above. this leads to higher peak hp.

On test engine:

Intake lobe Comp 12558 and 1.6 R/R

TK lobe 4616 on exhaust ratio change effect
1.3 R/R
3500rpm = 403ft/lb and 268hp
5000rpm = 525ft/lb and 500hp
6500rpm = 448ft/lb and 555hp

1.6 R/R
3500rpm = 412ft/lb and 275hp
5000rpm = 522ft/lb and 497hp
6500rpm = 422ft/lb and 547hp

High Energy lobe 4221 on exhaust ratio change effect
1.3 R/R
3500rpm = 392ft/lb and 261hp
5000rpm = 524ft/lb and 499hp
6500rpm = 447ft/lb and 554hp

1.6 R/R
3500rpm = 394ft/lb and 263hp
5000rpm = 524ft/lb and 498hp
6500rpm = 445ft/lb and 551hp

Get exhaust lobe right and rocker ratio change is almost margin of error stuff.

Very similar to my testing results over last few years,you won't notice 10 - 15 hp in a street deal.
Most guys don't want to get after converter, rear gear/ tyres to get the best performance due to want to cruise the hwy at 60mph

[steve cowan]

Get exhaust lobe right and rocker ratio change is almost margin of error stuff.

When it's all said and done, this part to me makes the most sense.
But, further to what I had posted about changing the exhaust system itself, i.e. larger headers or conversely, going from open headers (PipeMax spec'd) to a fully mufflered system, I think will now take that optimum exhaust lobe designed with the rocker ratio in mind and kind of throw it out the window. This may be the ideal spot to work with ratios.
Another job for the sims as I too don't have the resources to do it with hard parts. In as much as I'd like to.

Thanks for that HMQ.

Not to get off topic but when you are stuck with full exhaust street deal,this is when other cost effective things come into play.
At the moment I have all my brakes,wheel bearings apart looking to free up any rolling restrictions that might be there.
I like being able to push my 3500 pound streeter with one hand in the staging lanes.
I had twin full mild steel exhaust and heavy mufflers, threw them in the trash can and went to shorter,small bullet stainless mufflers and saved some weight.
When you have a combination in play already you have to work around the engine and improve everything else.

[HQM383]

Get exhaust lobe right and rocker ratio change is almost margin of error stuff.

When it's all said and done, this part to me makes the most sense.
But, further to what I had posted about changing the exhaust system itself, i.e. larger headers or conversely, going from open headers (PipeMax spec'd) to a fully mufflered system, I think will now take that optimum exhaust lobe designed with the rocker ratio in mind and kind of throw it out the window. This may be the ideal spot to work with ratios.
Another job for the sims as I too don't have the resources to do it with hard parts. In as much as I'd like to.

Thanks for that HMQ.

Not to get off topic but when you are stuck with full exhaust street deal,this is when other cost effective things come into play.
At the moment I have all my brakes,wheel bearings apart looking to free up any rolling restrictions that might be there.
I like being able to push my 3500 pound streeter with one hand in the staging lanes.
I had twin full mild steel exhaust and heavy mufflers, threw them in the trash can and went to shorter,small bullet stainless mufflers and saved some weight.
When you have a combination in play already you have to work around the engine and improve everything else.

Working to get the best out of what you’ve got is the birthplace for seeking knowledge and understanding. What have I got? How does it work? How can I make it better?

Doing great with what you’ve got Steve.

[bob460]

I'm the seeing the answer is in all of the above.
A guy like Steve, who's constantly experimenting on the flow bench, is seeing an advantage (or not) when moving the rocker ratio around. That's with a given cam and an unspecified cylinder head, Unspecified in that it's constantly evolving. The ratio that may have worked yesterday isn't necessarily the ratio of today.
Now, to bring that around to a fellow like myself, who hasn't got a flow bench (or ready access to an engine dyno for that matter ) but I've a set of heads, done and dusted as it were. I've an engine spec pre-decided. That is, these are the parts I've got. Rocker arm ratio included (1.6 all around). I send that off to my cam designer and what he delivers to me is what is best suited to that pile of parts.
At least that's how I see it.
I can't say what changing my engine spec might do to what is an otherwise optimum cam. A change to a single plane from the dual plane I've always run or a move to large headers will undoubtedly have some sort of cascading effect. Even on a more basic level, my spec is based on an open header. It's reasonable to assume that the cam isn't best suited to a fully mufflered car. Might the rocker ratio come into play here?
Anyway, just musing on my part.

I ran those cast iron heads as stock on my 010 383 block several years ago.
10.6 comp
Comp SFT 235 - 242 @ 0.050
500" lift
106 lsa in at 106 but did move around.
Car ran 11.70 - 114 mph
As cast Vic Jr.
I tested rockers on the exhaust
From 1.3 break in rockers
Up to 1.6.
I had 1.7 rockers but never run on exhaust.
I tested tight lash ,loose lash etc
I will be honest that there was really no excessive change at the track,as you know there is a ton of variables.
In my opinion and testing -
For what we are doing, street car application, 6000rpm- 7500rpm operating range I don't think these style of engines care that much.
I have had more rewards by moving the cam close to straight up most times and picked up ET and mph across the whole run.
This might make some guys laugh but I take my moroso speed calculator pretty seriously and make my car run to those increments.

Moroso speed calculator best $10 i ever spent, just modified mine to read up to 2000hp.....lol

[skinny z]

Very similar to my testing results over last few years,you won't notice 10 - 15 hp in a street deal.
Most guys don't want to get after converter, rear gear/ tyres to get the best performance due to want to cruise the hwy at 60mph

This time it'll be drag race first. This is why I had the cam specified around open headers.
I'll take what I get when the full exhaust is in play. It's massively restrictive what with the single muffler crossways at the back. There are plans for change but primarily it'll be uncorked and go.
Converter and tires are on the order list.

[steve cowan]

Very similar to my testing results over last few years,you won't notice 10 - 15 hp in a street deal.
Most guys don't want to get after converter, rear gear/ tyres to get the best performance due to want to cruise the hwy at 60mph

This time it'll be drag race first. This is why I had the cam specified around open headers.
I'll take what I get when the full exhaust is in play. It's massively restrictive what with the single muffler crossways at the back. There are plans for change but primarily it'll be uncorked and go.
Converter and tires are on the order list.

Kevin,
The advantage I see that you have is the the 4 speed with a lock up converter.
Now I know nothing about the 4 speed auto or a lock up converter with sufficient stall speed.
If it's possible you can use sufficient gear multiplation and high stall speed at the track
EG - 8" 5000 stall but still have 4th gear and lock up for hwy.
Is this possible?
The way it works is if you can't get it done in the 60ft you can not make it up in the rest of the quarter.

[skinny z]

Very similar to my testing results over last few years,you won't notice 10 - 15 hp in a street deal.
Most guys don't want to get after converter, rear gear/ tyres to get the best performance due to want to cruise the hwy at 60mph

This time it'll be drag race first. This is why I had the cam specified around open headers.
I'll take what I get when the full exhaust is in play. It's massively restrictive what with the single muffler crossways at the back. There are plans for change but primarily it'll be uncorked and go.
Converter and tires are on the order list.

Kevin,
The advantage I see that you have is the the 4 speed with a lock up converter.
Now I know nothing about the 4 speed auto or a lock up converter with sufficient stall speed.
If it's possible you can use sufficient gear multiplation and high stall speed at the track
EG - 8" 5000 stall but still have 4th gear and lock up for hwy.
Is this possible?
The way it works is if you can't get it done in the 60ft you can not make it up in the rest of the quarter.

Hey Steve.
First contact with Yank converters has specified a 9.5" at 4000 stall. That may be most and still retain the lock-up feature. I've got another half dozen or so vendors to contact still.
I never see past 3rd gear on race days. The OD and lockup and strictly for getting to the track.
The problem with the 4 speed auto is that they're power hungry. At least that's the feedback from other racers. This particular transmission, a 4L60, also has a wide 1-2 ratio split. 3.06 1st, 1.63 2nd. We've found that a good converter will hide some of that.
If I can lay down a 1.6 60', I'm hopeful of reaching my performance goals. Hopeful being the operative word. With the 3.06 1st and 3.73 rear gear, I've got plenty of ratio I think.
New Hoosier QTP bias slicks on the way. LT 26.0 x 9.50-16.