Compression ratio

Julian West · 06-16-05, 07:41 AM

Just thinking aloud here...!

Reading up the specs on my SVO 302 it has a CR of 9:1.

I'm not an expert, but this seems low. I've been told how the correct CR can seriously affect the BHP.

Would a new set of pistons, or a quick head skim be worth a few more BHP on this engine??

Any opinions greatly appreciated!

Sandy · 06-16-05, 08:52 PM

More compression is generally better [img]/ubbthreads/images/graemlins/smile.gif[/img], but you may potentially have problems with detonation/pre ignition etc depending on your available fuel. Also the matching of components (Cam, intake, carb, heads, pistons) is important so if you want more power, best to build an engine that is aimed at some target numbers. By the time you cut the heads, mill the manifold and do all that might as well just wait for the rebuild and put a well designed package together. Depending on how well your car is set up (tuned) you might find it worthwhile to have it dyno'ed and most tuner can do some magic if things are not up to snuff (ie, distributor curve, carb tuning, etc).

Ultimatly I guess it is the amount you are willing to spend [img]/ubbthreads/images/graemlins/smile.gif[/img]
Sandy

Malcolm · 06-17-05, 07:37 AM

I cannot comment on how to get more CR but a target level of 10.25 to 10.5 will give you a better BHP output and keep reliability.

Nick · 11-26-06, 03:24 PM

Firstly apologies for the following long rambling narrative, it’s the way I am combined with the fact a little bit of knowledge is a dangerous thing.

I’m finding it really hard to understand which way to turn in the options available to me with rebuilding my 351W engine, (it was all so simple 20 years ago with my only other engine rebuild an MGB engine. Rebore the block, buy some oversize pistons, plonk them in and away you go).

I have some I believe forged dished L2378 +30 TRW pistons. I was planning on using them with the following:-

Trick flow twisted wedge heads tfs-51400002
Trick flow stage 1 roller cam tfs-51402000
Roller rockers
Link bar lifters

The guy I am getting my parts from in the US has suggested not touching my TRW pistons with a barge poll as being dished they are only good for turbo charging. He says the super low compression ratio will kill the engine. He has recommended using speed pro hypereutectic pistons.

What he says makes sense to me and for the sake of a few hundred dollars which is not a great deal in the grand scheme of things will not be much more, should I go this way.

However, I am only looking for around 350HP and the car will be for road use would not using the low compression pistons actually help reliability, with the engine not being so high octaine sensative.

I have tried to work out the compression ratios as follows but my calculations may be wrong as they seem low on one and high on the other. I don't really know what I am doing so I have put ? marks next to things I am not 100% sure on.

Engine Cylinder bore size 4.03
Piston Stroke length 3.5 ?
Head gasket Bore diameter 4.10
Compressed head gasket .039
Combustion chamber volume 61cc
Piston dome volume -16.9 ?
Piston deck clearance 0.016?

This gave me a ratio of 9.15:1 (a bit low?)

Using the hypereutectic pistons with a dome volume of 12cc gave me a ratio of 13.03 :1 (a bit high?)

I would be very interested in any advise on if it would be okay to use my forged TRW pistons for my set up or should I ditch them and go for some speed pro hypereutectic ones.

Regards

Nick

Jac Mac · 11-26-06, 04:06 PM

Two things Nick,

Try to keep your piston to head clearance at 0.045" or slightly less[ This is more important than actual compression ratio]You have 0.055" on your above dimensions.

If your TRW pistons are the type with a slot drain in the oil ring
groove rather than several drilled holes you would be well advised to not use them. Along with that TRW are fairly heavy compared to others.

Dont know what part of the planet you are from, but any more than 10 to1 comp ratio is risky on most pump fuels these days.

Oops that was 3 things!

Jac Mac

Jac Mac · 11-26-06, 04:16 PM

Quote:

Originally Posted by Julian West

Just thinking aloud here...!

Reading up the specs on my SVO 302 it has a CR of 9:1.

I'm not an expert, but this seems low. I've been told how the correct CR can seriously affect the BHP.

Would a new set of pistons, or a quick head skim be worth a few more BHP on this engine??

Any opinions greatly appreciated!

The simplest way to cure all these crate SVO 302 problems is to buy a set of aftermarket 289/302Boss rods (5.155") skim the piston Crowns/Valvepockets for zero deck, rebalance and reassemble. Costs a bit but gets all the critical factors right in one swift move.

Jac Mac

Colin Artus · 11-26-06, 06:15 PM

In the UK we are blessed with high octane fuel ( compared to the States) with the premium Shell/BP super unleadeds having a RON of 98 or more. A cr of 10.5:1 will be quite safe with all grades available here on a 2 valve engine with its relatively low VE - it's the dynamic cr that is critical. Definitely aim for the chosen cr with a flat top piston if possible (i.e. I second JacMac's suggestion).

jason · 11-27-06, 08:23 AM

Julian , What cylinder head chamber volume do you have ?

With 58cc you will typically get around 9:1 i think with flat tops. To get 10:1 compression on standard rods, you will need to skim the heads and ensure the pistons are level with the deck height. I think i read that for every 5 cc in chamber volume lost, you gain 1 compression point. Just check the valve clearances. The ammounts to skim off are in the "how to rebuild a small block ford book", Alternatively you can buy 54cc heads, i think Canfield do some.

Ron Earp · 11-27-06, 08:41 AM

Quote:

Originally Posted by sfm6s

In the UK we are blessed with high octane fuel ( compared to the States) with the premium Shell/BP super unleadeds having a RON of 98 or more. .

Not sure there is any difference. In Europe the "Octane rating" of fuel is almost always the RON, while in the US it is (RON + MON) / 2.

So, a 95 octane rating in the UK would have around a 90 to 91 in the US. A 98 in the UK would be around a 93 in the US, commonly available everywhere on the East coast.

Julian West · 11-27-06, 10:24 AM

Quote:

Originally Posted by jasonlycett

Julian , What cylinder head chamber volume do you have ?

With 58cc you will typically get around 9:1 i think with flat tops. ......

Hi Jason,

Thx for the info, my heads are Ford M-6049-X302 which I believe are 64cc.

Lynn · 11-27-06, 12:50 PM

Julian,

I side with the flat top guys and I would steer away from hypereutectic. Rather, I would suggest a good set of forged pistons; the few dollars extra will be worth the peace of mind.

The thing that properly dimensioned flat tops will give you is quench surface. This is a subject that is not well understood (by the masses anyway) and seems to be lumped in with things such as voodoo and other arcane sciences. As near as I can explain it, the flatter, smoother and plainer the surfaces surrounding the compressed air/fuel charge, the less likely that detonation will occur. The reasoning ranges from removing hot spots to controlling local pressure variations to removal of hiding spots for demons & ogres. This means that you can run higher compression ratios, obtaining more efficiency from the explosion, without encountering the destructive phenomenon of detonation (the explosion occurring before it is supposed to.) In the old days, one would just use a higher octane fuel (does not detonate as readily as a lower octane fuel: lets put diesel on the low end and alcohol on the high end for descriptive purposes). Of course that is not so easily done anymore. The industry answer was to lower compression. So, these have been the main two variables that have been manipulated. However the topology of the combustion chamber is a third variable that can be manipulated with satisfying results.

For completeness: In naturally aspirated engines, these three are pretty much it; however, with forced induction, inter-cooling (and its many semantic variants) along with water or water/alcohol injection also come into play.

Regards,
Lynn

Mike Dozier · 11-27-06, 02:01 PM

I didn't see any mention of head material....ally heads are good for 1 point
more compression than equivalent iron heads...although I forgot why...
maybe better detonation resistance....

MikeD

Jim B · 11-28-06, 12:35 AM

IIRC its the aluminums ability to move the heat away and keep the combustion chamber slightly cooler Vs Iron heads.

Jim

CliffBeer · 11-28-06, 05:27 AM

Yes, undoubtedly, the aluminum heads move the heat away from the piston crown more efficiently resulting in a lower overall running temp. In the tests that I have viewed it was only a matter of 26 - 33 degrees however - this, on a surface temperature of between 525 and 745 degrees in the critical areas of the combustion chamber depending where you measure. Consequently, the calculations I saw indicate that the alloy heads can allow for a maximum of roughly .5 increase in CR, all other things being equal.

Agree with Ron above - the fuel you have readily available is roughly equiv to 93 here (as measured here in the US). Common wisdom is that this limits CR to a maximum of 10 with alloy heads and proper matching of cams and ignition, at least for regular and reliable street driving.

Hope this helps.

CliffBeer · 11-28-06, 05:37 AM

Quote:

Originally Posted by llarsen

Julian,

I side with the flat top guys and I would steer away from hypereutectic. Rather, I would suggest a good set of forged pistons; the few dollars extra will be worth the peace of mind.

The thing that properly dimensioned flat tops will give you is quench surface. This is a subject that is not well understood (by the masses anyway) and seems to be lumped in with things such as voodoo and other arcane sciences. As near as I can explain it, the flatter, smoother and plainer the surfaces surrounding the compressed air/fuel charge, the less likely that detonation will occur. The reasoning ranges from removing hot spots to controlling local pressure variations to removal of hiding spots for demons & ogres. This means that you can run higher compression ratios, obtaining more efficiency from the explosion, without encountering the destructive phenomenon of detonation (the explosion occurring before it is supposed to.) In the old days, one would just use a higher octane fuel (does not detonate as readily as a lower octane fuel: lets put diesel on the low end and alcohol on the high end for descriptive purposes). Of course that is not so easily done anymore. The industry answer was to lower compression. So, these have been the main two variables that have been manipulated. However the topology of the combustion chamber is a third variable that can be manipulated with satisfying results.

For completeness: In naturally aspirated engines, these three are pretty much it; however, with forced induction, inter-cooling (and its many semantic variants) along with water or water/alcohol injection also come into play.

Regards,
Lynn

Lynn, agree with you here - not a well understood subject, but one with some significance. The best way I conceive of the notion here based upon my own reading and experience with different CR, heads and pistons is that a complex surface (ie. not flat) will create areas of warmer and cooler temps because the relative surface area for a particular amount of heat created by the burn varies accordingly. In other words, localized hot spots develop around corners and complex shapes because there is inequality in the ability to dissipate heat.

A subset discussion to this one is the debate over whether to polish the piston crown or not - the goal being to more completely burn the intage charge and dissipate heat. My experience has indicated that the difference is nil - the pistons certainly look cool (no pun intended) during assembly however!

Jim c · 11-28-06, 05:45 AM

I read this years ago and it made me look at it a little different.
I have used the technique and it worked well.

Stock engine will generally have static comp of say 9.5:1 and dynamic compression of around 7.5:1.
That is when the intake valve closes you measure how far the piston has come up the bore.
You then use this cylinder capacity to work out the compression ratio lets say it is 7.5:1.
You then put a the modified cam in the engine.
You go through the same process with a dummy assembly with the cam, 2 lifters ect and one piston.
Set your valve timing bring it up till intake valve close ,lift the head and measure the piston travel.
Then work out how much is required to get it back to 7.5:1.

In effect you are putting back in what the cam takes out.
It may have 11:1 static but it is still 7.5:1dynamic

Jim

Lynn · 11-28-06, 01:37 PM

Jim C,

Very good and pertinent point. I left out the whole cam/valve timing issue, nor did I touch on ignition timing which can come into play. Runner length and shape also impacts cylinder filling and thus compression. Related to this is another technology that, until recently, was rarely seen: variable runner length. All of these can have an impact on the dynamic compression ration.

I shouldn't have assumed alloy heads, but I did. Sorry.

Lynn

jason · 11-30-06, 09:22 AM

Julian

I would suggest with 64cc heads, 9:1 may be on the optimstic side, but i dont know what pistons you have. I would certainly look at some aftermarket heads with smaller chambers, with either 58 or 54 cc chambers. With the dollar so weak against the pound there are some good bargains if you are prepared to import them yourself. You will also get the benefit of better head flow and not having to play around with the bottom end of your engine. If you go the head skimming route, bear in mind the inlet manifold will need skimming too to maintain alignment and the manufacturer may have a specified limit of how much material can be taken off.

As regards octane level, i'm not sure what is the equivelant octane level to the US, but 10.5:1 compression is not a problem with our fuels in the UK with a properley set up ignition. Most standard saloons are running close to 11:1 althought they do have knock sensors and management.

My understanding of temperature is that below around 10:1 an iron head is actually better due to the heat retention qualities and , and above 10:1 that Ally is the better option, but thats only what i have been told by my local tuner, i have no experience of that myself.

Julian i guess it's how much work you want to make of it, a mod or change to your heads is the least hassle if you don't want a full rebuild. Hope this helps.