GC 8v TC & SOHC power/flowtest brain teaser!

[TR-Spider]

If the TC does 174 hp @ 7000 rpm this results out of 175 Nm torque

SOHC makes 177 hp @ 7900 rpm which results out of 157 Nm torque

since torque is proportional to fuel (i.e. air) mass flow the TC must flow some 11% more, which fits somewhat to the flow graph.

The astonishing thing is that the SOHC should breathe 20% less due to its reduced capacity, is it a short stroker with better filling?

The question is now at what rpm you want to get the 180 bhp of the target head?

My proposal for the "target flow formula" looks like this

indicees base engine =1 target engine =2, both same capacity

CFM_2 = CFM_1 * (M_peak_2 * Lift_2) / ( M_peak_1 * Lift_1 * fill_coeff_2)

with

CFM_1 = known flow of base head at Lift_1
M_peak = torque at peak power
Lift = valve lift (some integral lift would be more accurate?)
fill_coeff = reduced filling of target engine due to increased rpm, example 0.95 i.e. 95%)

Thomas

[Acki]

You can change the ignition to get more power.
But you need a good CC design because of knocking.

[SirYun]

well here goes:

The area under the curve under steady state depression as measured by a flow bench does not take into acount that in real life this "flow potential" is used by the varying pressure during the engine cycle.

my guess it that the higher port velocity of the smaller but oversquare engine (the total amount of air ingested during the entire filling cycle will be what can be used to burn fuel) is quite favourable to VE.
A short stroke engine will pull more CC's in per crank degree thus adding to port velicity earlier in the cycle aiding VE.
A more 'active' port will be able to fill the smaller cylinder better for more of the time that the less active head with more cylinder to fill and add more inertial ramming at the end of the cycle..
the SOHC engine having a cam with more overlap and more duration will benefit more from this effect than the bigger engine which had less to begin with.

but this is not really it either i think


Joost

edited

[Guy Croft]

A possible pressure ratio issue across the inlet valve?

GC

[Guy Croft]

I have a modest certainty that I am beginning to unravel the secrets of this issue, rather than post the same thing twice at this stage have posted some of what I have deduced so far (rightly or wrongly) in a reply to a question at:

http://guy-croft.com/viewtopic.php?t=904

& any interesting observations regarding what I have said are very welcome.

GC

[DamirGTI]

Hi !

I was wandering how can race engine builder transform flowbench mesurment from flow gains in HP on modified head ? how can he know that this head will add for example 15Hp , when inside the engine is a little different enviroment with piston moving up and down inlet and exhaust mainfold fitted , change in temperature-pressure , swirl efects exhaust backpressure etc.
It just can't be the same like testing on flowbench table ?!

Regards !
Damir

[Testament]

Although the test conditions are not the same, it has been proved experimentally that gains on the flowbench are a reasonably good (as good as you can get) indicator of gains in engine flow and subsequently power. But it is more of an indicator that there is an improvement, not that 'x'CFM increase = 'x' HP .

Only an experienced engine builder/tuner can make an estimate that the flow increase, when combined with ALL the other engine parameters may give an output of approximately 'x'HP