HQM383 wrote: ↑Fri Jan 19, 2024 5:55 pm
skinny z wrote: ↑Thu Jan 18, 2024 9:58 pm
skinny z wrote: ↑Tue Jan 09, 2024 12:22 pm
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.