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Compression Ratio For Longevity
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DANP85
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PostPosted: Thu Apr 01, 2021 6:07 pm    Post subject: Compression Ratio For Longevity Reply with quote

Hey all, i’m trying to figure out a final compression ratio for my sp 1776 34 pict 3 carb 034 distributor 2280 cam engine in a beetle. I have somewhere between 8 - 8.5:1 in mind, is this fine? I’m after factory like 100k mile reliability and longevity and want to avoid pinging on pump fuel. Thanks heaps in advance!
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PostPosted: Thu Apr 01, 2021 11:48 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

YES!
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PostPosted: Mon Apr 05, 2021 12:51 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

for that cam, I wouldn't go higher than 8/1.

The dynamic compression is so high already.
7.5/1 would be my recommendation.
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PostPosted: Mon Apr 05, 2021 1:23 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

Intake closing? Altitude? What's your average summer temps?
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PostPosted: Mon Apr 05, 2021 1:47 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

Depends somewhat on the fuel.
There is a significant difference in efficiency, and especially torque going from 8 to even just 8,3. For medium fuel (95 Euro) I set them up for 8,4-8,5.
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PostPosted: Tue Apr 06, 2021 1:03 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

I think any recommendations from peeps in USA for CR intended for peeps NOT in USA need to be taken for what they are - recommendations for USA fuel. It clearly isn't very good fuel.

8.5:1 stock cam type-4 no pinking in UK.

I also ran a stock 1600 at just under 9:1 without a problem for a few months.

Both in buses on std supermarket cheap fuel.

Again, UK is not Australia, so I would take heed of our Aussie members.
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PostPosted: Tue Apr 06, 2021 8:46 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

Both the US and Aus based people have one major factor to be aware of compared to us semi northeners. That is ambient heat, which of course is a factor that needs to be taken into consideration with an aircooled engine. But as we have discussed a million times before, if you do it right and keep your fuel and ignition trim right, it is not as much an issue as people wanna make it.

The fuel "issue" is also not as much an issue as it is hyped for. The statement that E10 fuel is bad and will damage your engine along with giving you less power, is true, and then not really. If the fuel is sitting a long time in a car/fuel tank there can definitely occur some issues. But once again, if you use some sort of quality fuel stabilizer the issue becomes MUCH less.
The postulate about E10 reducing performance over clean gasoline is correct, - IF - we are talking a firebreathing monster with 90+ hp/l. it can be noticeable. On a normal street car/engine you will barely notice it, if at all.
Next, coolingwise the E10 is actually a GOOD thing looking from an aircooled engine point of view as the combustion temperature is LOWER with E10 than with pure gasoline.

The whole static compression thing is sadly still influenced by the Berg doctrine from the 70´s and 80´s. And no matter how you turn it it is still wrong on everything but lean cruise, and even there we can discuss whether it is correct or not. "Normal" garage mechanics in lack of a better word will have some difficulties in finding the correct static CR for a given engine, but it can and should really be given by calculating Indicated Mean Presure (IMEP) You can also come close by calculating Bmep, but that figure is very much dependant on how free- or restrictive the intake side is and thus can give you a false reading so t speak.

If we are still in the "keep it simple" stadium, using a 009 distributor etc etc it is right to build somewhat conservative. If we are ready to take one or two steps up the ladder and use just a fraction of the things we have learned since then, you can build an engine that runs cool, makes power when you need it, and lasts a long time.
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PostPosted: Tue Apr 06, 2021 9:02 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

The fuel in the US is fine...

I dont know why do many people insist on running low compression.... oh well, sucks for them Laughing

I'm running 9.5CR and plan to run premium, but I honestly think it would be fine on regular too with no pinging. I honestly would prefer if I had it closer to 10...
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PostPosted: Tue Apr 06, 2021 10:39 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

I feel no matter how many times this subject is brought up, VW people as a whole have low cr ingrained into their psyche. Berg really F'd-ya'll up permanently.
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PostPosted: Tue Apr 06, 2021 10:53 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

evanfrucht wrote:
The fuel in the US is fine...

I dont know why do many people insist on running low compression.... oh well, sucks for them Laughing

I'm running 9.5CR and plan to run premium, but I honestly think it would be fine on regular too with no pinging. I honestly would prefer if I had it closer to 10...

This statement is worth nothing for anybody else than you, because it does not state which parts are involved. Standing alone it is just as bad as when Berg Senior and whoever else claims that 7,5 is the right static CR.
bugguy1967 wrote:
I feel no matter how many times this subject is brought up, VW people as a whole have low cr ingrained into their psyche. Berg really F'd-ya'll up permanently.

That´s a bold statement Wink But there is something to it.
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PostPosted: Tue Apr 06, 2021 11:12 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

The OP asked about "longevity". 100,000mi....

Not maximum performance.


I'm the very last guy to buy into the old GBE compression standards. I agree that you can run 9/1 with success.

However, it is known fact that running lower CR will lower head temps.
And thus extend the life of an air-cooled engine.
GBE knew it and that hasn't changed.
You want 100,000mi? keep the CR a bit on the lower side.

Anyways. I have said my piece.
I'm done..
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PostPosted: Tue Apr 06, 2021 11:28 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

My main point was that in the US, at least here in CA, the top tier fuel is fine. We have three octane grades to pick from and as long as you dont buy from the really cheap off-brand stations (not "top tier") the gas is quality and fairly pure. There are a few states who have it slightly worse or better.

As far as the compression ratio statement, yes it's stupid. I should have said it depends on the cam and altitude, and other factors as well. The CR for my engine is irrelevant. I was simply pointing out a tendency I notice where people try to "play it safe" and end up with a compression ratio which is lower than ideal for their setup. I was just starting trouble, sorry Twisted Evil

I cant offer much help other than I would agree 8-8.5 sounds right for your combo. If longevity is the number one priority above perfomance and efficiency then stay on the low side of that range.
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PostPosted: Tue Apr 06, 2021 12:11 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

There is some VERY good info in this thread so far and some.....not so good.

You can't select your compression ratio based off wanting longevity or how hot you think the heads or engine will get. You select your CR based off of your cam, heads and in some instances, the fuel available. The reason I bring up heads is that the better they cool, the more CR you can run without detonation.

I used to get in knock down, drag out brawls on here because I built a 2276 with a W120, my 050's and ran 10.5 to 1 on 87 pump gas with no detonation. It took me years to learn WHY it worked and it's because my heads cool better.

Here are some examples of what I set my engines at.

Stock to Stock-ish------8.25

Performance(W110 and below or similar)-----8.5

High Performance(W120, web 218 and similar)-------9.0

Race motor specs for street(W125 with 1.25's and above)----9.5

Notice I set nothing below 8.25 and my engines do not run hot, do not detonate and do not have longevity or reliability problems.

I am not knocking or judging or telling anyone on here that they are doing it wrong. If it works for you, keep it up. If not, try a change. I am just giving you examples of what works for me.

Longevity and reliability will be the RESULT of choosing the correct CR with the engine specs. Hopefully that makes sense.
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PostPosted: Tue Apr 06, 2021 2:38 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

[email protected] wrote:
There is some VERY good info in this thread so far and some.....not so good.

You can't select your compression ratio based off wanting longevity or how hot you think the heads or engine will get. You select your CR based off of your cam, heads and in some instances, the fuel available. The reason I bring up heads is that the better they cool, the more CR you can run without detonation.

I used to get in knock down, drag out brawls on here because I built a 2276 with a W120, my 050's and ran 10.5 to 1 on 87 pump gas with no detonation. It took me years to learn WHY it worked and it's because my heads cool better.

Here are some examples of what I set my engines at.

Stock to Stock-ish------8.25

Performance(W110 and below or similar)-----8.5

High Performance(W120, web 218 and similar)-------9.0

Race motor specs for street(W125 with 1.25's and above)----9.5

Notice I set nothing below 8.25 and my engines do not run hot, do not detonate and do not have longevity or reliability problems.

I am not knocking or judging or telling anyone on here that they are doing it wrong. If it works for you, keep it up. If not, try a change. I am just giving you examples of what works for me.

Longevity and reliability will be the RESULT of choosing the correct CR with the engine specs. Hopefully that makes sense.


Roy makes a good point here as far as the ratio should be determined more by the cam choice and head choice than anything else. The larger the cam, the more compression you can get away with (and quite frankly, must have. Research dynamic compression vs. static). I tend to run just slightly more on my high performance builds (9.5 with W120, 9.75 with FK8, 10:1 with W125 and 1.25 rockers, etc). Also, an engine with higher than normal compression or really small chambers that has too much deck height (piston too far in the hole) to compensate is a recipe for heat issues and lower life expectancy. Keep the combustion in the chamber as much as possible where the head can disipate it faster than they cast iron cylinder. As with ANY aircooled VW engine, proper timing, a/f mixture, and necessary tins in place for the application will keep temps in check along with longer life
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PostPosted: Tue Apr 06, 2021 5:16 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

Type 4 engine, Manx copy, mild Web street cam, 8.4/1 CR (was told by the Bus guys NEVER GO OVER 7.4 ON THESE ENGINES!!!) only 87 octane avalable here, 10* initial, 31* total 41* part throttle vacuum advance, mid 13s AFR full song any RPM, mid to high 15 AFR cruise. Cooler climate, 90*F max.

Cannot get a ping unless lugged down below 500 and stood on HARD!.

Deeply regret not going closer to 9/1 CR!
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PostPosted: Tue Sep 21, 2021 7:00 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

1776 with a vz 25 cam 40/35 valves at 9:1 daily driver in Los Angeles 68 bug on 91 pump minimum, makes 104hp on the dyno without number corrections. But there is a catch lol, it's running a 2-quart sump, type 4 oil cooler with a Thing dog house shroud that has a welded fan with more blades, and an oil filter, and an oil cooler with electric fan. If I was to do it again I would just buy the thick wall 92mm piston kit and maybe a vz35 cam. the engine would still be daily reliable and maybe even run a tad cooler with the larger displacement. The rest I would keep the same. Here is the dyno clip at chico performance. https://www.instagram.com/p/B8M_pjYB1L8/?utm_source=ig_web_copy_link
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PostPosted: Sun Sep 26, 2021 4:05 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

Considering the low number of pipelines for gas and fuel in this country, do we really believe that many hundreds of different chemical compounds,(additives),are being pumped through them? No, of course not. At the ends of these pipelines are there hundreds of storage tanks holding millions of gallons of differing brands? Of course not. How many different brands of gasoline exist? Don't try to answer that! It all travels through the same pipe in the ground gentlemen and ladies. Some additives are metered in at the tank farms as it's being loaded into the tank trucks that deliver to your local stations, or not as the cheapos can't compete with the big guys.

And some worry about the freshness of their gasoline or their gasohol. How long was the base product in a ship on the ocean, or the actual gas in a million gal. tank with a stuck valve in a city two hours away from your home? Did the end product have water in it because the underground tanks at your neighborhood station don't have vented caps on the above ground vent pipes?
This mini-rant is relevant, but maybe doesn't belong here, excuse please

We cannot know the condition of the fuel we put in our tanks.
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PostPosted: Sun Sep 26, 2021 3:54 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

Igpoe wrote:
Considering the low number of pipelines for gas and fuel in this country, do we really believe that many hundreds of different chemical compounds,(additives),are being pumped through them? No, of course not. At the ends of these pipelines are there hundreds of storage tanks holding millions of gallons of differing brands? Of course not. How many different brands of gasoline exist? Don't try to answer that! It all travels through the same pipe in the ground gentlemen and ladies. Some additives are metered in at the tank farms as it's being loaded into the tank trucks that deliver to your local stations, or not as the cheapos can't compete with the big guys.

And some worry about the freshness of their gasoline or their gasohol. How long was the base product in a ship on the ocean, or the actual gas in a million gal. tank with a stuck valve in a city two hours away from your home? Did the end product have water in it because the underground tanks at your neighborhood station don't have vented caps on the above ground vent pipes?
This mini-rant is relevant, but maybe doesn't belong here, excuse please

We cannot know the condition of the fuel we put in our tanks.



Yes...we can....to some extent. And...while we need some MORE pipelines, there is not really a shortage of them. There are about 135 gasoline refineries in the US. All have pipelines and pipelines run through all 50 states. Just short of 90,000 miles of non-crude oil and non-natural gas pipelines (gasoline and petroleum chemical only).

While it is true that gasoline once it leaves the refinery storage tanks (pipeline)....and goes to bulk storage terminals ....and then to trucks or railcars for final distribution.....it is comingled or mixed. But it is mixed by brand....not BY TYPE. Its not all just generic gasoline. The grades are kept seperate.

And....the EPA requirements for hot weather, winter blends and specific regional blends is sometimes vastly different. Especially in cities that have ozone or smog issues and EPA action pending....the blends are different and done locally.

By law.....fuel as it exits the pipeline...because its comingled...is tested for quality and local additives are blended on site at reginal and/or state/city bulk storage distribution terminals.....and different BRANDS...have different additive packages.

Having lived and worked and regularly travel (a lot of it driving) all over the US.....I can assure you the fuels are different from region to region. I can tell the difference from brand to brand as well. There are several brands whose regular run just as well as premium of other brands.....and numerous brands of regular that do not run well at all....IN MY DAILY DRIVER CAR.

And these days.....they are actually pretty consistent to their specification for where you live. The biggest part of this consistency ...especially between small mom and pop distributors and large chains ....was the EPA directive that required all stations in the US to get rid of metal and concrete tanks by the end of 2000.

A great many small stations just closed and disappeared between about 1998 and 2001 because the level of sales was not worth the cost of replacing the tanks with modern plastic tanks that do not readily leak and also....if you will notice....you no longer see the goose neck, open air the gas tank vent poles/tubes that every station had.

Between the type of filling ports, the commonly leaky tanks that tended to allow the important lighter fractions of gasoline to evaporate and vent to air and allowed water and san/dust to leak in.....the fuel QUALITY now compared to pre-2000.....is worlds away better. Not saying the recipe may be better ...but the fuel quality is better.

As for the "freshness" of the fuel...I doubt its an issue.....very few gasoline stations stay in business that cannot sell at least one tanker every two weeks. The ones near my house (close to the interstate I-35).....get an 11,000 gallon truck about every 2 days.

The refineries work hand to mouth. Storage costs money. Product costs money to refine. You will rarely find more than 10 days worth of production being stored at refinery bulk storage.

Yes...I have friends and relatives in the business. Wink


Back to the main subject......


As Roy noted...lots of good info in this thread. Others have pointed out some important points. Some clarification is needed I think:

1. What vehicle is this going in?

2. What gearing?

3. What tire size?

4. How is it going to be driven?

5. As Alstrup noted....what's the ambient temperature range?

An engine that is set up for "best longevity" for one type or ACVW....may not give best longevity in another vehicle type.

I can only speak to stockish and just hotter than stock type 4 engines.

A for instance: Stock 1.7L type 4 for 411/412/914....

Compression (except for the hideous California version and the Early Euro carbed version in 1968)....had 8.2:1 compression and 80-82hp. The deck was kind of sloppy at 0.079"....but with a domed piston it at least had good compression.

Typical longevity was 125k miles average...to about 150k miles....all of this with good tuning. This did well in hot weather.

It still ran a bit hot in hot weather. The cam kind of sucked...heat wise. These were listed for 98 octane fuel in 1973 (which was about equivalent to current 89 octane).

This entire engine combo actually works better...slightly less heat....by closing up the deck .....same domed piston....and bringing compression up to about 8.4:1 or 8.5:1. No other mechanical changes. A little ignition and fuel mixture tuning. Same longevity or better. The HP typically increased by about 5hp.
A slightly better cam based around the stock version (like a web 73)....helps with this also. 150k miles is common expectation with this engine.


The California version....compression in the 7.3:1 range. It dumped about 9-10 hp...and picked up a good 50-75* constant head temps. Lifespan was less than or up to 100k max. It just ran hot and lean.

The application issue:

This same basic engine went into the 1972 bus.....with lower gearing and a lot of weight...same 7.3:1 compression,.....but a different cam and twin carbs instead of injection.
Yes...it put out about 14 hp less.....and a lot lower gas mileage (and not just because of the weight and gearing of the bus)....but it did routinely make 100k miles or better.

I do not 100% agree that because you have greater weight, bad aerodynamics and lower gearing....that the compression must also be lower. Plenty of people running 411/412/914 engines with the 8.2:1 compression in busses....successfully. But everything else had to be tweaked....ignition curve, fueling, cam (to some extent).

So what you are going to put it in will decide a lot of what compression you can use.

And....high compression ...sane high compression...is NOT a guarantee that it will produce more heat...IF....you do the rest of the modifications and adjustments required to use that compression in your application.

Sorry for the length. Ray
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PostPosted: Mon Sep 27, 2021 1:59 am    Post subject: Re: Compression Ratio For Longevity Reply with quote

Isn't all this talk of dynamic compression and pinging issues more to do with the piss poor AFR mixture and timing control of carbs and distributor ignitions at low speeds ?
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PostPosted: Mon Sep 27, 2021 5:13 pm    Post subject: Re: Compression Ratio For Longevity Reply with quote

Yes and no
Just assuming old=crude is not very wise.
Some OLD carburetor and distributor systems are actually pretty darn good, and some things that are new are not so hot......


But I do agree that.......
you can't drop the compresion ratio low enough to allow for total stupidity.
And even if you could it's not worth it.
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