Reduced low lift flow from porting...

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travis
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Reduced low lift flow from porting...

Post by travis »

If you port a set of heads and end up reducing flow under .300” lift, stay about the same from .300-.400, and pick up flow above .400” lift, is this necessarily a bad thing for a street performance engine? Or even a mild race engine for that matter?

It may be completely wrong, but I can see the reduced low lift flow actually reducing some of the bad effects of overlap on a street performance engine. Could you see increased idle vacuum or better off idle response in this situation?

How would you adjust your cam specs to take advantage of reduced llf? Tighter lsa, more advance? Would a slower ramped cam work better in this situation, or at least “less badly”?
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Re: Reduced low lift flow from porting...

Post by PRH »

So....... if the work you had done....... instead of reducing low lift flow actually increased the low lift flow........would you be happier with that........ or would you be trying to figure out a way that lose that low lift flow you gained?

Some SBC heads I’m reconditioning now flow better at all lifts after reworking them.......I’m good with it.

A- as rec’d
B- after rework

Lift————A/B
.100——65.5/67.2
.200—-121.2/133.2
.300—-170.4/182.8
.400—-200.0/215.8
.500—-215.0/223.3
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Re: Reduced low lift flow from porting...

Post by travis »

PRH wrote: Fri Jun 28, 2019 4:49 pm So....... if the work you had done....... instead of reducing low lift flow actually increased the low lift flow........would you be happier with that........ or would you be trying to figure out a way that lose that low lift flow you gained?
I don’t have a clue. This being the first set of heads that I have run across a flow bench, I don’t have the knowledge base to make any meaningful conclusion out of what I am seeing.

And...the information you find on the ‘net is all over the place.

I do know this. Many years ago, the first 351w I built had a set of stock C9OE heads on it that are practically identical to the D0OE’s I have now (minus the bowl hogging of course). It was a 9.0-1 compression engine with a 216@.050 hydraulic single pattern cam with .484” and 110 lsa. Stock ‘69 iron 4bbl intake, 1850 Holley, stock exhaust manifolds, and dual 2” exhaust. This was in a ‘77 F150 with a stock torque converter and 2.75 gears. I drove it for a couple of years, then had to go through the heads because the valve guides was shot. It got guides and a valve job, and the only porting I did was a bowl blend and laying back/cleaning up the short turn. It went from just barely able to squeak the right rear 31x10.50x15 to being able to leave a 40-50 foot single stripe on the road. Not very scientific especially since I have no clue what it did to the flow numbers.
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Re: Reduced low lift flow from porting...

Post by ClassAct »

It depends on WHY the low lift flow went down, and what other flow bench measurements you took. If I have low lift numbers go up, I don't get too concerned unless it changed something else.

I find mostly I lose low lift when I port, and I mostly don't care, even if it's less than .500 lift.
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Re: Reduced low lift flow from porting...

Post by SpeierRacingHeads »

The thing about low flow is you can keep it or take it by how hard you blend the valve job.
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Re: Reduced low lift flow from porting...

Post by PRH »

The way I look at it....... for these mild street type builds that are going to generally use pretty short cams with modest lift....... my preference is to not lose any low lift flow.

But, what’s considered good or bad low lift flow is relative.
What would be considered good low lift flow for one type of head, would be a huge loss of low lift flow on another type of head.
And yet, for just about any American v8 platform used since the early 60’s you’ll find examples of both really strong running ones...... as well as combinations that are optimistically called “under achievers”........ and some of those platforms use heads that have inherently “good” or “bad” low lift flow.
I’ve seen plenty of good running combos based on heads that have good and bad low lift flow....... enough to know you can get good results with either.

My friends Pontiac stocker runs great with its 30* intake seats, and the corresponding low lift flow that goes along with that.
And since it’s only got .420 net lift at the valve...... we’re not too concerned with what’s going on at .600 lift.

Any Pontiac head with a 2.11 valve and a 30* seat is going to have good low lift flow.
We have a combo that runs pretty well with 30* intake seats...... but there are plenty of other ones out there that don’t.

Whatever you end up with...... good or bad low lift flow....... just try and maximize what you have to work with.
And...the information you find on the ‘net is all over the place.
Part of that is because.......”it depends” on what you’re doing as to how much benefit there is or isn’t as a result of increased or decreased low lift flow.
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Re: Reduced low lift flow from porting...

Post by modok »

Changing the low lift flow has about the same result as changing the cam duration.
if you reduce the low lift flow, and put in a slightly longer duration cam to compensate....are you better or worse off overall?
probably slightly better off IMO, everywhere except the very bottom RPMS.

If you don't find that to be the case, I'd suspect the reason could be that WET FLOW is worse.
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Re: Reduced low lift flow from porting...

Post by mag2555 »

Picking up added high lift flow will of Course make for more hp , but the drop off of low lift flow means the now the motors peak power band is narrower and higher in rpm then in stock form.

For a race motor this can be delt with and used as a plus, but keeping the low lift flow plus the added high lift would be better.

Good or better then stock low lift flow demands the use of a Camshaft to deal with the better scavenging during the overlap phase when a good tuned Exh header is in place!

This also means ( assiming the correct Camshaft once again !) that the Exh vacuum gives a big head start to the getting the Intake flow restarted again even while the piston is parked at TDC, and this all makes for more Torque , a wider and higher power band and since HP is a mathematical durivitive of Torque times rpm divided by 5252 , more hp is made.
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Re: Reduced low lift flow from porting...

Post by Warp Speed »

mag2555 wrote: Sat Jun 29, 2019 7:17 am Picking up added high lift flow will of Course make for more hp , but the drop off of low lift flow means the now the motors peak power band is narrower and higher in rpm then in stock form.

For a race motor this can be delt with and used as a plus, but keeping the low lift flow plus the added high lift would be better.
My results have been quite the opposite. As long as it isn't a poor porting job, but rather a good valve job, it always broadens the curve, and typically allows for more camshaft.......win/win! :wink:
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Re: Reduced low lift flow from porting...

Post by Stan Weiss »

I think like many things low lift is a relative term. Maybe it should be referenced as a percentage of max cam lift? The OP talks about picking up from .400" which could be fine for him but probable not for that Pontiac with .410" max lift.

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Re: Reduced low lift flow from porting...

Post by 77cruiser »

At what point duration wise would you be willing to give up low lift flow & concentrate on higher lift flow? For a street engine.
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Re: Reduced low lift flow from porting...

Post by HDBD »

Speaking generally low lift flow does not equate to low end torque IME. If the improved low lift flow comes at the cost of higher low lift reverse flow this usually is a net loss all around.
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Re: Reduced low lift flow from porting...

Post by Warp Speed »

HDBD wrote: Sat Jun 29, 2019 11:58 am Speaking generally low lift flow does not equate to low end torque IME. If the improved low lift flow comes at the cost of higher low lift reverse flow this usually is a net loss all around.
This exactly!
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Re: Reduced low lift flow from porting...

Post by BigBro74 »

You guys - this is great stuff !- please keep it going! (I am all ears) - except for this question.....

If low lift flow creates the more (adverse) reverse flow that warp is talking about, is this having the negative effects he’s is highlighting at or near IVC ? And does it then need a different closure point—-

Thanks guys- Jason
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Re: Reduced low lift flow from porting...

Post by GARY C »

BigBro74 wrote: Sat Jun 29, 2019 2:52 pm You guys - this is great stuff !- please keep it going! (I am all ears) - except for this question.....

If low lift flow creates the more (adverse) reverse flow that warp is talking about, is this having the negative effects he’s is highlighting at or near IVC ? And does it then need a different closure point—-

Thanks guys- Jason
In general you can have reversion at either opening or closing if your cam timing is wrong for your cylinder head, reversion at opening would probably be more pronounced due to the fact that you have no port speed or velocity to counter it... As Stan pointed out "Low Lift Flow" is used incorrectly and is either loved or hated based on Flow Bench references but few if any have ever correlated "Flow" in reference to the actual engine or the piston position.

I think Low Lift Flow should be Flow At TDC due to the fact that most people who have claimed Low Lift Flow is bad are working on engines with valve size and head designs that have more low lift flow (Flow at TDC) then most of us will ever deal with.

The benefit I see with limiting low lift flow is that you can start the duration/valve event sooner without adding more reversion and in return you can reach peak lift sooner in the intake stroke, beyond this I think the slower change in pressure differentials using a steeper seat plays into a more uniformed intake/exhaust track as well as a more uniformed pressure drop in the cylinder at EVO.
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