Selecting a compression ratio

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skinny z
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Re: Selecting a compression ratio

Post by skinny z »

Wesman07 wrote: Sat Jan 28, 2023 11:34 am I’m following you David on how DCR can be useful. I guess I'm questioning if all the other guys are set up correctly?

Using this program, I should be able to run as high as 10:1 static which gives an 8.5:1 DCR IF I tailor the spark curve back. Power picks up everywhere.
Are there examples of similar builds that you could draw from?
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Re: Selecting a compression ratio

Post by Wesman07 »

skinny z wrote: Sat Jan 28, 2023 1:07 pm
Wesman07 wrote: Sat Jan 28, 2023 11:34 am I’m following you David on how DCR can be useful. I guess I'm questioning if all the other guys are set up correctly?

Using this program, I should be able to run as high as 10:1 static which gives an 8.5:1 DCR IF I tailor the spark curve back. Power picks up everywhere.
Are there examples of similar builds that you could draw from?
No, all of the builds I can find on the internet stick to a dcr of 7.0 for 87 and 7.5 for 93. One of the motors I purchased was set up with a dcr in the mid 8’s. I think he had some problems with it and bought another one out of frustration. I found two broken oil rings and a deck that was heavily eroded.
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Re: Selecting a compression ratio

Post by skinny z »

Wesman07 wrote: Sat Jan 28, 2023 1:39 pm
skinny z wrote: Sat Jan 28, 2023 1:07 pm
Wesman07 wrote: Sat Jan 28, 2023 11:34 am I’m following you David on how DCR can be useful. I guess I'm questioning if all the other guys are set up correctly?

Using this program, I should be able to run as high as 10:1 static which gives an 8.5:1 DCR IF I tailor the spark curve back. Power picks up everywhere.
Are there examples of similar builds that you could draw from?
No, all of the builds I can find on the internet stick to a dcr of 7.0 for 87 and 7.5 for 93. One of the motors I purchased was set up with a dcr in the mid 8’s. I think he had some problems with it and bought another one out of frustration. I found two broken oil rings and a deck that was heavily eroded.
Well, if nothing else it points out that that particular engine doesn't like a DCR that high.
At least that's a conclusion I might draw from that example.

Apples to oranges here but FWIW (and I totally understand this is where the debate about DCR comes in), 8.4:1 and an iron headed SBC made for the street had a tough time handling what would be a traditional timing curve. There was a lot in play with that and probably workarounds, a cold air intake for example, but eventually that running compression was dialed back.
Conversations about that opened the other debate of when or if timing trumps compression. Or vice versa.
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Re: Selecting a compression ratio

Post by David Redszus »

Well, if nothing else it points out that that particular engine doesn't like a DCR that high.
At least that's a conclusion I might draw from that example.
DCR is not a stand alone metric to predict octane requirements. DCR must be used in conjunction with inlet air pressure and inlet air temperature in order to be useful.
Conversations about that opened the other debate of when or if timing trumps compression. Or vice versa.
Compression affects chamber temperature at ignition. Temperature affects ignition timing.
They work together. A low compression engine can still self destruct if the inlet air temperature
is excessive.
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Re: Selecting a compression ratio

Post by skinny z »

David Redszus wrote: Sat Jan 28, 2023 6:53 pm
Well, if nothing else it points out that that particular engine doesn't like a DCR that high.
At least that's a conclusion I might draw from that example.
DCR is not a stand alone metric to predict octane requirements. DCR must be used in conjunction with inlet air pressure and inlet air temperature in order to be useful.
Understood. Which is why I've emphasized that particular engine in the quote above.
By this the environment it's running in is taken as part of the whole.
That engine with that air inlet temperature and pressure doesn't like the DCR that high with the ignition timing used.
As stated any one change will have another effect.
Last edited by skinny z on Sun Jan 29, 2023 11:18 am, edited 1 time in total.
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Re: Selecting a compression ratio

Post by skinny z »

David Redszus wrote: Sat Jan 28, 2023 6:53 pm
Conversations about that opened the other debate of when or if timing trumps compression. Or vice versa.
Compression affects chamber temperature at ignition. Temperature affects ignition timing.
They work together. A low compression engine can still self destruct if the inlet air temperature
is excessive.
Referencing the SBC above above, that 8.4:1 DCR that proved a handful was under those conditions which couldn't be changed. The carbed intake was configured as it was with little to no cold air available. Timing was a mix of best WOT and highway fuel economy. Naturally being NA, it ran with the atmosphere of the day.
It's another case of a particular engine which didn't like a DCR under the conditions it was run in.

That said, getting back to air inlet temperature, as mentioned this was never implemented. I fully believe that this was a large contributor to the difficulties encountered. Going forward, this same DCR (slightly less) and fundamentally the same engine is going to back into service. This go round will see the cowl air hood adapted for lower IAT's. It should be interesting under many operating conditions.

How does a drop of 100°F in air inlet temperature play into all of this?
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Re: Selecting a compression ratio

Post by Bill Chase »

PackardV8 wrote: Fri Jan 27, 2023 11:59 am
Most of these guys are using a DCR of 7.0:1 for 87 octane builds and typically don’t adjust timing much. That puts me at about 8.5:1 static for the cam I will be using. My machinist thinks that would be a mistake and I should be in the 9’s.
With all due respect to Wesman07, he has his own reasons and we should give the requested advice and not second-guess.

If one has EFI, with all of today's available sensors active, 87-octane can be accomodated; OEMs do it with 11:1 and a warranty. But with carb and distributor, not so much.

Having said that, it boggles the mind when a customer comes in with a build request which will be between $5,000 and $10,000 long block, says he's using a carb and distributor and then closes with "and it's gotta run on 87-octane."

The most recent rationale we got was, "Well, I might take it to Mexico someday." We're 1600 miles from Mexico and no one he's ever known has driven an off-road truck that far and back, but the internet told him to plan for that contingency.
I too was worried about pump gas (93 octane) compatibility. in hindsight for no more than the car gets driven I wish id went almost a point higher and settled with 75% pump/25% race gas. For the 5-6 tanks a year that get ran through the car I could afford it, the added performance would've been worth it. I understand exactly what you mean.
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Re: Selecting a compression ratio

Post by Bill Chase »

skinny z wrote: Sun Jan 29, 2023 11:16 am
David Redszus wrote: Sat Jan 28, 2023 6:53 pm
Conversations about that opened the other debate of when or if timing trumps compression. Or vice versa.
Compression affects chamber temperature at ignition. Temperature affects ignition timing.
They work together. A low compression engine can still self destruct if the inlet air temperature
is excessive.
Referencing the SBC above above, that 8.4:1 DCR that proved a handful was under those conditions which couldn't be changed. The carbed intake was configured as it was with little to no cold air available. Timing was a mix of best WOT and highway fuel economy. Naturally being NA, it ran with the atmosphere of the day.
It's another case of a particular engine which didn't like a DCR under the conditions it was run in.

That said, getting back to air inlet temperature, as mentioned this was never implemented. I fully believe that this was a large contributor to the difficulties encountered. Going forward, this same DCR (slightly less) and fundamentally the same engine is going to back into service. This go round will see the cowl air hood adapted for lower IAT's. It should be interesting under many operating conditions.

How does a drop of 100°F in air inlet temperature play into all of this?
I would say it did not like the attempts at lean cruising jetting, and over advanced timing with the lean air fuel ratios. reasonable timing, and richer mixtures it would have lived. The specs you have listed for that failed build were not so extreme that it shouldn't have easily lived on 93 octane pump gas. just my opinion based on reading your posts here, thirdgen etc.
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Re: Selecting a compression ratio

Post by skinny z »

Bill Chase wrote: Sun Jan 29, 2023 5:02 pm
skinny z wrote: Sun Jan 29, 2023 11:16 am
David Redszus wrote: Sat Jan 28, 2023 6:53 pm Compression affects chamber temperature at ignition. Temperature affects ignition timing.
They work together. A low compression engine can still self destruct if the inlet air temperature
is excessive.
Referencing the SBC above above, that 8.4:1 DCR that proved a handful was under those conditions which couldn't be changed. The carbed intake was configured as it was with little to no cold air available. Timing was a mix of best WOT and highway fuel economy. Naturally being NA, it ran with the atmosphere of the day.
It's another case of a particular engine which didn't like a DCR under the conditions it was run in.

That said, getting back to air inlet temperature, as mentioned this was never implemented. I fully believe that this was a large contributor to the difficulties encountered. Going forward, this same DCR (slightly less) and fundamentally the same engine is going to back into service. This go round will see the cowl air hood adapted for lower IAT's. It should be interesting under many operating conditions.

How does a drop of 100°F in air inlet temperature play into all of this?
I would say it did not like the attempts at lean cruising jetting, and over advanced timing with the lean air fuel ratios. reasonable timing, and richer mixtures it would have lived. The specs you have listed for that failed build were not so extreme that it shouldn't have easily lived on 93 octane pump gas. just my opinion based on reading your posts here, thirdgen etc.
True Bill. There was some tuning or other refinements that could have been made, but the temperamental side of it and it's use at the time, said to back off. I certainly knew where the line was drawn.
I will say the MPG returned with the lean cruise made me smile as much as a personal best time slip. But again, it was temperamental. A CAI might have been the difference.
FTR, none of those higher DCR engines ever showed signs of damage from detonation. It was audible but subtle. There was the line.
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Re: Selecting a compression ratio

Post by Wesman07 »

David Redszus wrote: Sat Jan 28, 2023 6:53 pm
Well, if nothing else it points out that that particular engine doesn't like a DCR that high.
At least that's a conclusion I might draw from that example.
DCR is not a stand alone metric to predict octane requirements. DCR must be used in conjunction with inlet air pressure and inlet air temperature in order to be useful.
Conversations about that opened the other debate of when or if timing trumps compression. Or vice versa.
Compression affects chamber temperature at ignition. Temperature affects ignition timing.
They work together. A low compression engine can still self destruct if the inlet air temperature
is excessive.

As long as cylinder temps and pressure is under the auto ignition threshold, Is there anything wrong with increasing compression and less ignition lead?
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Re: Selecting a compression ratio

Post by Fusion Works »

Combustion chamber efficiency will drive maximum compression ratio on whatever fuel you use. How about some pics of the combustion chamber and pistons.

What is the deck height of said pistons, what is the quench area in the chambers? What kind of mixture motion can you generate? How efficient can you burn the mixture? How well can you prepare the mixture to burn?

How many engines have been built with a .060 or larger deck height and still had a low compression ratio, compared to the engines that build reasonable compression with a very tight piston to deck clearance that creates lots of mixture motion and by result better knock resistance and efficiency.

What is your budget on this build? Can you use a modern coil on plug or coil near plug to boost spark efficiency? Can you use a modern programmable ECU that can integrate wideband O2 sensors and knock sensors? Can you alter injection positioning and spray pattern?

Are there early and late heads that had different combustion chamber designs? It appears Ford ran 8.8:1 compression ratio on the last versions of this engine.
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Re: Selecting a compression ratio

Post by stealth »

Yes,
9-10:1 static CP ratio
Keep quench tight as possible.

Don't overthink it....
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Re: Selecting a compression ratio

Post by Wesman07 »

Thank you!
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