Guideline for compression

[Paul Jr]

Siting here fiddling with the lash caps trying to figure the best way to clearance them with a frien and he posed a good question.

Does anyone have a basic guideline on cam duration based on compression. As most know I am still learning so the best I can give is the average compression based on the known or more popular cams such as what Ballpark range will work with an FK 8 or an 86 b compared to the average engine size we see talked about here.
I know there are going to be variables but let’s say a basic guideline that will get everyone close.

[UK Luke 72]

Guidelines are kinda useless because we always read between the lines to suit our agenda.

86b was good at 10.2:1
FK10 is good at 11:1

Both in my 2276.

That's enough CR to scare most, but it works.
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[Paul Jr]

So what if the cam had only 224 degrees at 50 compared to 240 compared to 250. What might be a practical guideline?

Reading between the lines might be ok for those with some experiance but what about someone like my friend who just saw a buggy and wanted to try and build something. I would assume running that 224 degree cam at 11 to 1 would be a very hot running engine and not very much fun having to stop and let it cool all the time while everyone else was out riding the trails.

[UK Luke 72]

Almost every cam will have been discussed on here. The search function can cost you hours of your life!

If the search throws nothing up, post up a topic. Throw some numbers into a dynamic compression calculator and get a feel for it.
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[Alstrup]

Or, just learn to calculate Imep and Bmep and figure out how much you can run with the required octane. Then compensate for the ambient temps you are driving in, plus altitude. Then you have made a home run.
But as Luke mentioned, most cams have been dicussed in here.
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[evanfrucht]

Aircooled.net lists a lot of cams and gives a ballpark range that they reccomend for each one. No other website has such descriptions of the cams. I find their numbers to be pretty in line with what gets recommended here and it's a very quick reference to sort thru.

I may be starting trouble here but this is the way I see it: John at ACN is going to give a conservative static CR number as he knows many people have bad engine combos or don't know what they are doing. Same reason the carnival ride says the weight limit is 250 but in reality it's actually 350 before a guaranteed failure... Stupid employees. You get my point...

So where ACN will rec. 8.5 - 9 CR, you can often get the best performance running 9.25-9.75+ depending on the other factors (fuel and atmospheric conditions.)

I like the run the most CR I can in general. Better overall performance if done right.
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[modok]

I agree ACN has general guidelines and they are close enough.

if you want to get more advanced things like head temp, intake temperature and intake runner length, exhaust dilution %... DO matter, and yet are often left out.

[Paul Jr]

So it’s suggested that someone as I described should start out with a formula such as this. (2π Ti × nr )/ncVd ?
I was looking for a way to help someone that might not have the desire to learn every aspect of engine building but wants to build something that will get them around without self destructing and wasting thier hard earned money.

Myself I have been reading experimenting and learning because it interests me as more than a hobby, but I am not at a point where I can explain everything in simple terms. Heck many things still are confusing if your not focused but hopefully next year my studies will be helped along when I start school.

[modok]

yeah I'm sure there are many formulas but they are a bit too simple.
What does the cam do?...it controls the valve.
So to see what the cam is actually going to do.... you need to take into account what the valve is actually going to flow.
There are things you can do to the valve job which can make the low lift flow more or less, and make a small cam seem big or a big cam run small.
The intake runner size is also a big deal in how much torque it makes and where that peak is.

When does the engine have it's torque peak?

Peak torque at 3k go.... 8-1
Peak torque at 4k.... 9-1
Peak torque at 5k.... 10-1
now that's a simple formula
or rather, you can make that into a function if you want, somehow, using math

too simple yes, but IMo it would be be head to head with the cranking compression formula, and you can look that up if you want, also called "dynamic compression" but I don't see anything dynamic about it.

[vdubrookie]

Do you have a link or a topic for this chart. I'm not finding it on the Aircooled.net website?

[stevemariott]

[oprn]

Question:

I understand the problem of not having enough C/R for a hotter cam due to valve overlap but what is the sin in having a higher C/R with a mild cam? Say for instance if someone was looking for a stump puller low RPM engine, went stock-ish cam and 10/1 C/R?
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[Brian_e]

[Paul.H]

I always like to point out the details of the 2.1 WBX Vanagon motor.
10.5:1 cr with a lame ass stock hydraulic cam, ports almost as big as comp eliminators
They pull well as you expect pushing a Vanagon and no det

[oprn]

[Brian_e]

No. That is ignition timing, not cam timing. Very different.

A small cam has less overlap. Overlap bleeds off compression to keep cylinder pressure in a reasonable range. That’s why big cams need more compression to make up for the bleed off.
If you use a bunch of compression on a small cam, there is no time for it to bleed off, and the cylinder pressures shoot up, and then you have detonation.

Brian
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[Paul.H]

[oprn]

Ok so what you are getting at is if the cylinder pressure is high enough the engine starts to act like a diesel. I once built an engine for my Beetle with a stock cam and 9.0/1 C/R. I do not recall it having any problem. I built an engine for my Rabbit at 9.4/1 with stock cam and even put a turbo on it. No issue below 8psi in the summer and it would pull well past 15 psi in the winter.

So what would be a ball park number for maximum cylinder pressure for gasoline? I know there are lots of variables, combustion chamber shape, ambient air temps etc. But just a rough idea, 170psi, 190 psi, 200 psi cranking pressure?

The early VW diesels were 23/1 and the later ones 19/1 for self ignition on diesel fuel but the later ones are turbo engines. I guess what I am asking then is at what point does gasoline self ignite? If we stay just under that point we have a more efficient engine... no?
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[Stripped66]

[Paul Jr]

In my test engine I noticed an increase in engine/ Head temp but haven’t experianced any audible detonation yet. This is now at 11 to one with 250 degrees at 50. If I can source an other set of bearings for this block I would like to try it again with more compression.

From watching one of the older guys that never seems to be able to compete with the others. He recently built a new motor with a camshaft that supposedly had the same exact duration as a scat c35 but more lift. He set the compression at just over 12 to 1. Day one was its only day! Some short runs in the afternoon and then into the evening he played on the hills. While watching I could see the glow from the engine as it got hotter and hotter until it finally started to loose power and eventually seized.
Now I have watched many others and myself beat the crap out of a buggy playing on the same hillclimbs for much longer periods and even with the heat generated being more than a normal trail ride never have had a noticeable loss in power and always was able to turn around and go straight into a trail ride to let it start to cool off.
The entire time this guys buggy was running everyone present could tell it was running way too hot.
So for me the concern isn’t even getting into the territory that would result in detonation but it seems common sense that a poor balance will cause an engine to run much hotter than it should and simply put too much heat means less power as friction increases along with it.
One other guy is constantly having to stop after any slightly hard part of a trail ride due to his high compression low duration in combination with a set of eliminator style heads while at the same time the 2275 I built with CB eliminator heads lots of duration and compression 2 points higher just chugs along unchallenged by temps?
With water cooling I can assume some of this could be accounted for so that could explain why the water cooled mentioned seems less effected.
Also detonation isn’t always as easy to notice as some think and to some extent can be tuned out with very quick responses on computer controlled cars with knock sensors. For example my aunts bmw has knock sensors that will alter cam timing, fuel, and ignition in milliseconds if even the slightest detonation is detected.

As mentioned many times I am new and just learning so feel free to pick my observations apart as it helps me learn!