Compression ratio thoughts...
Discussion
Do I consider raising the compression ratio of my Griff to 10.84 (according to my calculations - let's assume it'll be about 11) with a pair of tin head gaskets?
The cam will be a H404 and I understand from various people that wild cams tend to reduce the dynamic compression ratio, and with me being a reformed (well almost
) turbo fanatic I can't help but think this will be like turning the boost down a bit
So what wins on a 5 litre Rover - more timing advance or more compression? Running on standard 95 RON fuel.
The cam will be a H404 and I understand from various people that wild cams tend to reduce the dynamic compression ratio, and with me being a reformed (well almost
) turbo fanatic I can't help but think this will be like turning the boost down a bit So what wins on a 5 litre Rover - more timing advance or more compression? Running on standard 95 RON fuel.
dnb said:
raising the compression ratio of my Griff to 10.84 ... Running on standard 95 RON fuel.
I suspect it's the RON rating that will do you in. No doubt you can get it to survive that, if you back the timing off far enough, but I suspect that the amount of power you'll sacrifice in doing that may outweigh the gains you might have had from raising the compression that high. My 4.6 was fine at about 10.5:1 with really good squish, but the timing was knock limited; more compression would have meant less advance. Engines do run up to 11:1 and higher, presumably relying on the leary cams to reduce the effective compression.
That's the thing, isn't it! If you aren't det limited at the top end, your compression ratio is probably too low... 98 RON fuel is an option to me, but I'd prefer not to given the 11p per litre price difference. NF booster is also an option and that works out at about 5p per litre.
Where's the line, I wonder? If it was just a pair of tin gaskets, I'd risk it, but I don't really want to tear the engine apart once it's running again...
98 RON fuel is an option to me, but I'd prefer not to given the 11p per litre price difference. NF booster is also an option and that works out at about 5p per litre. Where's the line, I wonder? If it was just a pair of tin gaskets, I'd risk it, but I don't really want to tear the engine apart once it's running again...
.You could always have another map handy,for when you have to run the lower octanedavid vizard always used to say of anothe breathing-limited engine (the A series) .. if you can't get much mixture in there in the first place, squeeze teh heck out of whatever you can get in there .. or words to that effect. IE go for high CR and control any ignition issues afterwards ...
also another engine builder I know who builds very very good RV8 engines says go high CR!! apparently the engines respond very well to it
also another engine builder I know who builds very very good RV8 engines says go high CR!! apparently the engines respond very well to it
trackcar said:
also another engine builder I know who builds very very good RV8 engines says go high CR!! apparently the engines respond very well to it
How high though? I don't think it's common to go past about 10.5:1. Some engines go up to 11:1 or even 12:1 but these tend to be the more expensive engines which have been designed with that CR in mind - I don't think you can just raise the compression that far on a standard engine and expect that to work.
My Griff is running 11.5 : 1 and that is high enough the torque has got me into trouble three times now once it was very expensive.
I have to mention the the cam has no effect on the compression ratio at all. unless the cam is badly manufactured. the CR is the volume at BDC to the volume at TDC cams have FA to do with it, or leaking valves or burst gaskets.
A cam that is opening too early or too late will effect the Peak pressure, it cannot alter the compression ratio.
I should charge for teaching you lot thermodynamics.
my BHP is 355 and the torque is 341 ft lbs and that is too much torque.
Steve
I have to mention the the cam has no effect on the compression ratio at all. unless the cam is badly manufactured. the CR is the volume at BDC to the volume at TDC cams have FA to do with it, or leaking valves or burst gaskets.
A cam that is opening too early or too late will effect the Peak pressure, it cannot alter the compression ratio.
I should charge for teaching you lot thermodynamics.
my BHP is 355 and the torque is 341 ft lbs and that is too much torque.
Steve
First off, I said "dynamic compression", not compression. Possibly this is one of my own strange invented terms - it happens... The theory I worked on is thata high lift cam (with necessarily long overlap) will give the appearance of a slightly lower compression. As you say, it has sod all to do with the static compression!
I presume you used non-standard pistons for this engine to get to 11.5:1? I'm trying to work out if I get any valve & piston intersections and I really don't want to drive out to the garage tonight...
355/341 sounds like it might be a lively engine.
I presume you used non-standard pistons for this engine to get to 11.5:1? I'm trying to work out if I get any valve & piston intersections and I really don't want to drive out to the garage tonight...
355/341 sounds like it might be a lively engine.
The revs are important. I've seen static compressions of 16:1 on a yank engine built for using high rpms. They could have started that high to get to a half decent DCR at those high rpm's. On the other hand a lower DCR reduces pumping losses and probably free's up some extra ponies but only if you rev
Do some DCR calcs and aim for about 9:1 which is probably optimum.
http://members.uia.net/pkelley2/Dynam
Boosted.
Do some DCR calcs and aim for about 9:1 which is probably optimum.
http://members.uia.net/pkelley2/Dynam
Boosted.
Edited by boosted ls1 on Saturday 14th April 23:29
dnb said:
Great website Boosted. Thanks!
Yep, it helped me a lot until I wanted a cam for a blown chevy spinning 7k under boost. Then I had to think about low DCR's to get high revs. But I also got a bit confused because I wondered if I should have greater DCR's in order to fill the cylinders at high rpm's. In the end I concluded the boost would fill the cylinders.
Boosted.
High RPM adversly affects the dynamic compression. the compression ratio is not cannot be affected by revs.
the faster your engine revs the worse valve bounce becomes and the more pressure you loose hence stronger or double springs .
Boosted is quite right the best way is to charge the air but who needs a turbo or supercharger just not necessary, fit these to your engines
www.roverv8engine.co.uk/index2.htm
two stage two heads and you don't need a charger with all the problems they bring.
A bloody site cheaper as well
Steve
the faster your engine revs the worse valve bounce becomes and the more pressure you loose hence stronger or double springs .
Boosted is quite right the best way is to charge the air but who needs a turbo or supercharger just not necessary, fit these to your engines
www.roverv8engine.co.uk/index2.htm
two stage two heads and you don't need a charger with all the problems they bring.
A bloody site cheaper as well
Steve
tgriff said:
My Griff is running 11.5 : 1 and that is high enough the torque has got me into trouble three times now once it was very expensive.
I have to mention the the cam has no effect on the compression ratio at all. unless the cam is badly manufactured. the CR is the volume at BDC to the volume at TDC cams have FA to do with it, or leaking valves or burst gaskets.
A cam that is opening too early or too late will effect the Peak pressure, it cannot alter the compression ratio.
I should charge for teaching you lot thermodynamics.
my BHP is 355 and the torque is 341 ft lbs and that is too much torque.
Steve
I have to mention the the cam has no effect on the compression ratio at all. unless the cam is badly manufactured. the CR is the volume at BDC to the volume at TDC cams have FA to do with it, or leaking valves or burst gaskets.
A cam that is opening too early or too late will effect the Peak pressure, it cannot alter the compression ratio.
I should charge for teaching you lot thermodynamics.
my BHP is 355 and the torque is 341 ft lbs and that is too much torque.
Steve
Why is 341 ft lbs too much? Mine has around 350 at the wheels being something like 380-390 at the fly and I love it. Think it depends on the cam; the 404 means that the torque comes in a little later which keeps it all manageble. Mind you I need to switch to R888 tyres because I do need more grip on the track.
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