eric Posted March 6, 2020 Share Posted March 6, 2020 When you look to forged piston in the burton power catalog, the compression ratio is 12: 1 which is too much for a road engine . To low the CR you have to machine the piston . A good road engine is max 10.5 CR , that means some work on the piston . Work that many engine builders are not happy to do . With standard piston, the Compression ratio is from 9.4 to 9.9 if the engine block is machined .So for a road engine : the better choice is perhaps a standard piston if you have not a skilled engine builder with you ? For 1700 and above, you have not the choice . I have tested a 1720 in my Caterham with forged piston, the temperature was too high, impossible to use the car in summer . However, this engine was made by a bad engine builder and after 3000 km, it failed ( lubrication problem ) .So I changed it for a 1600 with Nural piston that are standard ( CR ratio 9.4 ) and all is fine since .My experience with forged piston and a road car was not good . Good engine builder are not so numerous . Many are retired ... Link to comment Share on other sites More sharing options...
elie boone Posted March 7, 2020 Share Posted March 7, 2020 I am sure that Vulcan or Wilcox and a few others i forget can build you a singing and dancing X Flow. The compression ratio is only part of the picture, the hight of the head gasket also has an influence on the CR. And it needs to work together with the valve timing, carburetion and the ignition advance curve. Just my 2p Link to comment Share on other sites More sharing options...
7 wonders of the world Posted March 7, 2020 Share Posted March 7, 2020 Why do you consider 12:1 CR too high for a road engine...? Link to comment Share on other sites More sharing options...
eric Posted March 7, 2020 Author Share Posted March 7, 2020 Because too much stress on the engine and the temperature of running will be too high in a Caterham . Good in winter but not in goo weather . Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted March 7, 2020 Share Posted March 7, 2020 Caterham road-going engines specs (from the Caterham 2012 owner's manual):CSR 260 Duratec is at 12.0:1, R500 Duratec is at 11.75:1, so Caterham isn't shy using 12:1 compression for a warrantied road car. Link to comment Share on other sites More sharing options...
7 wonders of the world Posted March 7, 2020 Share Posted March 7, 2020 My Durtatec is 12.6:1, mapped corrected at Emerald, 82 deg stat, no issues at all (mine is mapped for 95RON since there are gaps in availability of 97/98 in the principality.If the rest of your engine is in correct order the CR will be fine, good torque starts with a good CR.Accralite still make Heron Xflow pistons. Link to comment Share on other sites More sharing options...
7 wonders of the world Posted March 7, 2020 Share Posted March 7, 2020 Set of unfinished crowns here for you..... or use with a semi chambered head maybe..?I feel an alloy head upgrade may be in order .....https://www.ebay.co.uk/itm/303498500214 Link to comment Share on other sites More sharing options...
eric Posted March 7, 2020 Author Share Posted March 7, 2020 OK for modern engine, however a Caterham with a Duratec is hot in summer and CSR 260 is known as fragile .We speak of a Kent engine, something designed in 1967 No sure CR 12 : 1 is good if you not have everything in steel .New MX5 engine is something like 13:1Better torque and fuel efficiency .... but with special cams to manage a such CR Because I read that there is a way to low the CR playing with the duration of the cam . With an old engine , it is more complicated Link to comment Share on other sites More sharing options...
7 wonders of the world Posted March 7, 2020 Share Posted March 7, 2020 If your needing forged pistons then your looking to up the RPM therefore in the crossflow the rods were ok for 7K but theres a good choice of decent steel rods now.Crank balance was always interesting on the crossflow - some were off the scale on our balancer !If your looking to run wild cams then the best money is spent on an Emerald, TB's and wasted spark, you can mapped race cams to be very mild mannered, no waiting for the engine to come on cam with crude carbs and even worse advance curve of a dizzy.Sure duration can affect dynamic CR - not different to any other engine though, depends what your hoping to achieve from your engine.As for Duratecs being hot and fragile..... really ??? the Duratec has got to be one of the strongest best designed 4 pots out their.... Link to comment Share on other sites More sharing options...
aerobod - near CYYC Posted March 8, 2020 Share Posted March 8, 2020 ....Caterham with a Duratec is hot in summer....I think you are confusing the heat put out by the engine which causes a hot footwell due to the level of power produced if you don't insulate it, and the engine running hot. The engine doesn't run hot in my R400D when thrashed lap after lap on the track in 35°C temperatures, if anything it runs cooler than most engines with oil and coolant temps below 100°C when run flat out for an hour, less than any modern car I know of under the same conditions. Link to comment Share on other sites More sharing options...
elie boone Posted March 8, 2020 Share Posted March 8, 2020 Engine design hasn't changed for over a 100 years, manufacturing however has changed significantly since computers aid the manufacturing. Link to comment Share on other sites More sharing options...
eric Posted March 8, 2020 Author Share Posted March 8, 2020 I have a book with old race Ferraris, some have 12:1 CR and some with 9.5:1 CR .... Why not all with 12 if it is always better ?375 F1 CR 11 12 Cylinders 4 493 cc 350 HP at 7000625 F1 CR 12 4 Cylinders 2498 CC 240 HP at 7000156 F1 CR 9.8 6 Cylinders 1481 CC 190 HP at 9400250 GTO CR 9.2 12 Cylinders 2953 CC 270 HP at 7500 rev/mn 250 LM CR 9.7 12 Cylinders 2953 cc 300 HP at 7500 rev/mn158 F1 CR 9.8 1489 cc 210 HP at 11 000 revs/mn312P CR 11.5 2991 cc 450 HP at 10 800 revs / mn512S CR 11.8 4994 cc 610 HP at 9000 revs/mn Link to comment Share on other sites More sharing options...
Roger King Posted March 9, 2020 Share Posted March 9, 2020 Compression ratio is dependent on many things - cylinder capacity, combustion chamber design, camshaft profile, 2 or 4 valves per cylinder, induction system, exhaust system, carburettors or fuel injection, design of the cooling system, type of ignition system, etc, etc.Fifty years ago, it was generally reckoned that 9:1 was about the limit for road cars on 98ron fuel, yet we now see plenty of engines running over 11:1. Here are a few reasons - 4 valve combustion chambers generally have a shorter flame travel, better atomisation and therefore a quicker burn time, fuel injection (or more crudely, one carburettor barrel per cylinder) gives much better control over fuel/air ratio in individual cylinders than a single carburettor which reduces the likelihood of detonation, better designed water passages in the engine reduce the instances of hot spots in the combustion chamber, more powerful ignition systems give a better burn, piston ring design is much more effective which reduces the amount of oil getting into the combustion chamber; oil can lower the effective octane rating of the fuel. I could go on.Suffice to say that for a road going Crossflow with twin 40s or 45s, I'd stick to just under 11:1 as a maximum if using 98ron fuel, although for full race use you can approach 12:1 if you know what you're doing. This is just a generalisation and can't necessarily be applied to any individual engine specification.By the way, if an engine builder isn't happy to machine pistons to alter the compression ratio, he/she isn't an engine builder! Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now