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 Post subject: Dyno Tuning Question
PostPosted: Fri Dec 20, 2019 7:24 am 
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Joined: Wed Oct 28, 2009 9:24 am
Posts: 930
A friend has a vintage race car, mechanically supercharged with moderate boost, IIRC about 0.8 atmospheres gauge pressure. Low compression, unsure of the number.

The car was on a chassis dyno and when adding advance the power had pretty much stopped increasing before detonation was detected. This was running premium pump gas, not race fuel.

Here's the question - since the power had peaked (ish) prior to detectable detonation, does that imply that higher octane fuel would not gain much if anything? Or is it possible that higher octane fuel (or maybe a richer mixture?) would allow more advance and thus more power?

I THINK the former, that the power / torque plateau means that the fuel octane is sufficient to allow optimum ignition timing already. Am I correct?

Thanks!


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 Post subject: Re: Dyno Tuning Question
PostPosted: Fri Dec 20, 2019 6:25 pm 
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Joined: Fri Dec 23, 2011 6:59 am
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Location: Spring Hill, FL.
I spend a lot of time on the dyno. The first thing that is important is verify the fuel. I would use 100 oct aircraft fuel or 110 . 93 is hit or miss.
Make some soft short pulls and look for torque peak. This is the detonation prone area (max BMEP) . Stop the pull 500 rpm over this point . Add timing until the tq value levels off and then retard 2 degrees.
For the 9- 10 #s of boost with a 80mm bore , timing may like 22- 25 degrees . Above the tq peak , as the rpms increase , you may like to add 2-4 degrees to the 24 degrees/ but maybe not .
Read the plugs . If the timing added shows no power increase , reduce the lead as you are just knocking. The center plugs are the hottest , most of the time .
If you see a spike in the coolant temp as you make a pull, retard the timing . There is a hot spot in the chamber .
For larger bores the required timing will be less.

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Mike Ogren/Protech Racing, http://www.FWDracingguide.com http://www.ogren-engineering.com/ 352.428-8983 mogren@tampabay.rr.com


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 Post subject: Re: Dyno Tuning Question
PostPosted: Sat Dec 21, 2019 9:55 am 
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Joined: Wed Oct 28, 2009 9:24 am
Posts: 930
Thanks Mike -

Good advice but doesn't really address my question. The friend and car are in the UK, I wasn't there and likely won't be next time.

Car was run on dyno with the fuel (premium pump gas) that was in it. As spark was advanced, torque peaked before detonation occurred. Does that establish that the octane is sufficient for this engine or is it possible that more octane would allow more advance and thus more power?

The question of fuel apparently came up AFTER the dyno session, leaving my friend to wonder if he needs to go back with better fuel, thus the question.

I THINK that the fact that the torque already peaked means there is little or nothing additional to be gained but my IC engine theory is getting rusty!

The engine is a small bore undersquare 1930s design 1300cc or so 4 cylinder so it inherently should not need a huge amount of anti knock - but it IS boosted.


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 Post subject: Re: Dyno Tuning Question
PostPosted: Fri Dec 27, 2019 10:02 am 
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Joined: Thu Nov 07, 2013 2:47 pm
Posts: 109
The definitive answer is "maybe."

Provided you are definitely not seeing knock, it depends on when peak cylinder pressure is occurring during the expansion stroke, which, of course, is expensive to measure. If it's optimal (~15deg ATDC), then there are little/no gains to be had. Here's the lowdown:

Higher octane fuel burns more slowly, which means your peak cylinder pressure will occur later ATDC, but you can advance the timing which compensates.

Lower octane burns more quickly, so the opposite occurs.

In my experience, I have made more power with lower octane fuels and less advance, but with aluminum heads and efficient combustion chambers. In this instance, the earlier you fire, the more "pumping loss" you suffer due to higher cylinder pressure before TDC.

Less efficient chambers are typically "knock limited" and need higher octane to get to the proper peak pressure/deg ATDC relationship.

If I had to guess, I would say you will see some gains with more octane if you have cast iron heads with bathtub or wedge combustion chambers and not much quench.

YMMV

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Jonathan Spiegel
Big Island of Hawaii Region and Colorado Region
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 Post subject: Re: Dyno Tuning Question
PostPosted: Wed Jan 01, 2020 10:17 am 
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Joined: Thu Nov 05, 2009 3:18 pm
Posts: 370
Al Seim wrote:
Thanks Mike -


I THINK that the fact that the torque already peaked means there is little or nothing additional to be gained but my IC engine theory is getting rusty!



I think I agree with you Al provided A/F and everything else is solid. I would not expect any sizeable gains. However, pump gas in that application makes me a little nervous.


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 Post subject: Re: Dyno Tuning Question
PostPosted: Thu Jan 02, 2020 9:58 pm 
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Joined: Fri Jul 29, 2016 3:51 pm
Posts: 21
In the UK Super is 5% ethanol and Premium is up to 10%. So it's a little different than US gasoline. UK fuel is graded at RON and not PON. That is also a consideration. However, the dyno still tells the tale on power generated regardless.

James


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 Post subject: Re: Dyno Tuning Question
PostPosted: Tue Jan 07, 2020 8:43 am 
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Joined: Sat Apr 08, 2017 6:18 am
Posts: 41
Yes there is generally a torque plateau. However, you can definitely raise that plateau by increasing octane. Easiest/cheapest route would be to increase the ethanol content and retune.


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