Hi
Personally I think decompression plates are a bad idea, if they are a perfect match to the chamber shape then they are passable as far as squish is concerned, however they are still a potential sorce of detonation and leakage. In the case of your existing engine surley better just to buy 36cc heads
On the existing engine you are running about 8.5:1 CR
I would get the heads sorted, explain to whoever is rebuilding them that you need all the chamber volume they can leave you and get all the chambers matched with their valves in, don't let them just use 1 pair of valves and move them chamber to chamber you need an exact match. they should be able to "find" up to about 6 or 7 CC in each chamber when they blend in the valve seats and deshroud, but don't let them decrease the size of the squish band, explain to them about the use of flat top pistons and your target CR. Personally I think if you do it right 8.5 even 9:1 CR and 15lb of boost will be quite safe with everything built properly. That should give you well over 450bhp with these heads by which time you should be breaking gearboxes to your hearts content.
Best regards
Mike
Best regrads
Mike
Thats the compression I had wrote down so nice to know it was right
Im about to get the larger valves from Dimitri on this site then let the build commence. Do you know anyone up to the job of sorting my heads?? I have never had any machining work done. Im guessing new seats and the larger valves installed and ported for starters.
Other people I have spoken too say dont go too mad on the porting as its forced induction would you agree with that?? The ports look much bigger already than the rover ones.
Hi
I do not know of any rover head porters. . Chevy man I'm proud to say. . and you don't bother getting those hand ported as a CNC job is about $50 a head at the moment when buying new. I would also recommend just a blend of the seats to the casting, a ballance of the chambers and a light deshroud of the valves, if you are going to fit bigger ones, you could do it yourself if you had an air grinder, just dont go mad with the emery.
For machining work Knight racing services are very good http://www.knightracingservices.co.uk/
They did mine for my chevy rebuild, nice people and their machineist does a top job (absolutly top job). They could do a mild stage 1 port job for you if you asked and ballance the chambers after skimming the heads flat, explain about the engine buildand that you want as many ccs in the chambers as you can get. I really wouldn't go mad on porting, you bought those heads because they already flow very well (compared to rover castings). and too much porting would not necesserily be a good thing.
Best regards
Mike
Many years ago I had a long chat with Sir Eales, and he explicated to me for supercharger application engine produces more power with flat top pistons and opened head chambers instead to increase the dish in piston to reduce CR. Don't ask me why I didn't understand all his explication . Just the fuel/air mixture burns better.
36cc combustion chamber can be opened to 45cc and match 3.702" bore.
Hello Mr Wotland,
he is starting with 300 chambers which are, depending on where you read it either 54 or 58cc chambers, with a squish band of 40 thou that should give an extra 7cc of volume, if you recon 4 or 5cc of valve cut outs then that is a total of between 65 and 70 cc which works out at a CR of between 9.2 and 9.8 to 1. He is aiming at about 8.5:1 so needs 77 cc total volume above the piston, so that will be a chamber volume of around 65cc, as above, if the heads havnt already been skimmed and the porter reworks the chamber then an extra 7 cc should be found from the chambers, maybe even another 11 cc but I would not sacrifice a mm of squish band to achieve it.
My understanding was always that you wanted flattop pistons and all the combustion in the chambers if you could on a turbo engine, with coated tops to the pistons and oil cooling for the bottom of the pistons, mainly to avoid detonation and get all the exhaust gasses out in the early part of the exhaust stroke.
Best regards
Mike
thanks for the replys guys mostly way above my head but Im enjoying learning.
To start with the basics what is the squish band then?? How is that altered?
I cc'd my old rover heads I have the Buick ones on the bench at home I will try and cc them tonight and see what they give, I will get a few pics as well of the heads.
Im keen to do what I can DIY I love messing (im the worst kind :hehe:)
Looking forward to breaking gearboxes soon then haha
Hi
The squish band is the bit between the top of the piston and the underside of the head (the flat face) within the cylinder. It controlls how much Turbulance and therefor how much mixing there is within the cylinder once the valves have closed, also as the piston comes up to top deac centre it "pushes" the combusting charge from the edge of the cylinder into the centre of the chamber, it has an enormous effect of the resistance to detonation and on how fast the charge will burn and how lean you can run the mixture. As a rule it should be between 35 and 45 thou "deep" (gap between top of piston and underside of head) at top dead centre of the compression stroke. I have mine set at 25 thou on the chevy because it in theory will gain me some horsepower and resistance to detonation over it being set at 35 thou, but The machining on my engine build was very good and I have low expansion pistons that are coated. You really don't want to go wider than 45 thou, as the squish effect is pretty well gone over this width hence my comments on decompression plates.
The things that effect it are
1 Gasket thickness/ decompression plates (if you fit them)
2 how far the piston is down the bore at TDC/ sticking out above the top of the deck at TDC
3 if some twit has opened up the chamber to the sides of the cylinder
Best regards
Mike
. Just the fuel/air mixture burns better.
