techbod Posted February 19, 2011 Share Posted February 19, 2011 I'm mulling over an engine rebuild for my 1700cc supersprint spec xflow. I'm looking for engine longevity rather than a hike in power, so will probably rebuild with forged pistons (which should bring a small increase anyway). Looking at the various forged pistons - I have two options: standard and long con rod length. Am I correct in assuming that my current rods are 4.9" this corresponds to standard length? What are the advantages/disadvantages of going to a longer length rod and suitable piston? Higher CR perhaps? What length are long rods? I'm assuming I probably won't need to go down this route but I'd like to know what it offers anyway. cheers Link to comment Share on other sites More sharing options...
George C Posted February 19, 2011 Share Posted February 19, 2011 The main advantage are that you can use a shorter pisiton which will be lighter with less friction and there are less sideways forces on the piston. As with all these mods the advantage is relatively small but if you add them all up then you get good a measurable improvement. You probably wouldnt notice the difference by itself. My X Flow was upgraded last year and was fitted longer rods etc with narrow main bearing journals and a steel crank etc. Rods and crank from Arrow. The block was sonic tested and then bored to 1760cc with shorter forged pistons. Kept the 244 cam. Upgraded from Webber 40s to 45s, MBE ignition. So its basically bullet proof, has amazing torque and gave 170bhp+ on the dyno with better driveability than in all the specs (many) it has been in before. Only downside is the cost. There was a supply issue with the shorter pistons and Scholar who built my engine machined them from something from Cosworth. Link to comment Share on other sites More sharing options...
Revin Kevin Posted February 19, 2011 Share Posted February 19, 2011 George, Sorry for the high jack, but if you don't mind me asking who sonic tested your block and who was the builder. as for the long rods as per george, allows for the use of lighter (shorter) pistons and reduces the angular movement of the rod, reducing friction and allowing higher revs. Cheers Chris Edited by - Revin Kevin on 19 Feb 2011 18:38:42 Edited by - Revin Kevin on 19 Feb 2011 21:56:13 Link to comment Share on other sites More sharing options...
George C Posted February 19, 2011 Share Posted February 19, 2011 Chris Scholar in Mendlesham, Suffolk sonic tested the block and said this was essential for 1760cc. Link to comment Share on other sites More sharing options...
techbod Posted February 19, 2011 Author Share Posted February 19, 2011 *thumbup*Thanks for the replies. I assumed the long rod had to have shorter piston since it still has to fit into the same cylinder. I couldn't work out what it would do for me though - I had been wondering if it meant a shallow combustion chamber and was basically going down the wrong path. Thanks for clearing that up. As I'm looking to keep the revs/performance the same as I have now, this confirms that the long rod route isn't one I'll be going down. Can anyone confirm the length for the standard rod? cheers Link to comment Share on other sites More sharing options...
Revin Kevin Posted February 19, 2011 Share Posted February 19, 2011 George, Thanks for that, do you have contact details for suffolk sonic or were they via Scholar. cheers Chris Link to comment Share on other sites More sharing options...
George C Posted February 20, 2011 Share Posted February 20, 2011 Chris I didn't write my post very clearly! Scholar's tel no is 01449 767 711. They did the sonic testing and build and they are based in Mendlesham, Suffolk. George Link to comment Share on other sites More sharing options...
eric Posted February 20, 2011 Share Posted February 20, 2011 too high compression ratio (more than 10,5) 244 cam ( high valves lift) are no good for the longevity. You can use forged pistons (you are obliged over 1700 ) but why not going to 1720 before to go directly to 1760 I am making ( not myself but a professional) a 1720 with an A2 cam (less stress on the valve gear ) with std clearances to have a max engine longevity. 1720 with accralite forged piston, std conrods, std crankshaft, alloy stage 2 head with stainless valves (39mm and 34 mm), FP 400 springs, Duplex chain, A2 cam (for road use) CR about 10.5 Steel flywheel : 5.9 kg Aldon distributor with an ignitor 2 weber 40 with a mecanical fuel pump Uprated high pression , high capacity oil pump Competition gaskets Arp Bolts all balanced Of course I will never have 170 HP but it is not important for me as the max poxer is difficult to use on the road. (specs However if you work on the head (not max size of valves ) you can reach a very repectable power without the hassle of a race engine Eric Link to comment Share on other sites More sharing options...
Revin Kevin Posted February 20, 2011 Share Posted February 20, 2011 George, Many Thanks. Chris Link to comment Share on other sites More sharing options...
DohNut Posted February 21, 2011 Share Posted February 21, 2011 Quoting eric: .... 1720 with accralite forged piston, std conrods, std crankshaft, alloy stage 2 head with stainless valves (39mm and 34 mm), FP 400 springs, Duplex chain, A2 cam (for road use) CR about 10.5..... Of course I will never have 170 HP but it is not important for me... But I wonder how much power an alloy head is worth ? Link to comment Share on other sites More sharing options...
OliverSedlacek Posted February 21, 2011 Share Posted February 21, 2011 A longer rod reduces the acceleration the piston experiences at top dead centre. This reduces stresses, but doesn't usually give more power. Tony Rudd fell into this trap with the BRM F1 engine in the 60s. A long rod to crank throw ratio means the piston spends more time near TDC, which means you lose more heat, and hence power, on the power cycle. Swings and roundabouts. Link to comment Share on other sites More sharing options...
eric Posted February 22, 2011 Share Posted February 22, 2011 The power question : I like too much this kind of question The only way to know is the Dyno . But my engine will not go as I don't want a full power on a new engine .... The subjective way is to compare different cars or the same with different engine . Today I have a 1600 Sprint, as former race engine, it was all balanced and the compression ratio was raised to 10. Normally this kind of engine was rated at 120 HP with the std head, but when I fit the stage 2 Burton head, it was more powerful at low down and high revs. If normally a road sprint is given for 110HP it is reasonable to think that a race sprint with a stage 2 head is about 115 - 125 HP A 1720 with an A2 cam and a compression ratio of 10.5 will be normally more powerfull. However a 1700 supersprint ( Kent 234 cam) is given for 135 HP. So may be this engine will be about 125 - 135 HP, with a lot of torque, low down power, a weak consumption, and a long life. If the alloy head and the stainless valves helps, maybe 130 - 140 HP is possible. The weight gain is more realistic . But the result will be soon known on the road. Link to comment Share on other sites More sharing options...
Area Representative dikko Posted February 22, 2011 Area Representative Share Posted February 22, 2011 Regards to longevity. Can I recommend a Pegasus (Now sold by SkyFord at Hemel Hempstead) cast STEEL CRANK. A lot stronger than standard, just as cheap and will last for donkeys without breaking up and destroying your wonderful X-flow block....2,000 Formula Ford boys can't all be wrong.. Link to comment Share on other sites More sharing options...
eric Posted February 23, 2011 Share Posted February 23, 2011 if you go to steel crankshaft, normally it is normal to go to H steel forged conrod as you have already forged piston. But in fact std conrods and crankshaft are good to 10 000 revs. Above 6800 / 7000, you must change the schells at close intervall that I don't want. Under this kind of revs, forged items are useless except for the selfish pleasure and the absolute ( I know however some steel crankshaft failures) peace of mind. In my race 1600 sprint engine, std pistons, conrods, crankshaft I went to 7000 (my vdo revcounter does not go further ) of course not often but without any problem, and when it raced it was limited to 6800 during 6 years. Without any rebuild and a 20 years use, it is now about 100 000 miles on the clock. A Good Motul syntetic 15w50 oil and a service every 3000 miles If the new engine is as good as the former I will be very happy Link to comment Share on other sites More sharing options...
George C Posted February 23, 2011 Share Posted February 23, 2011 The cast steel crank is an excellent way to more or less guarantee reliability and is cost effective and allows more revs. The standard crank is good for 7000 or 7500 revs depending on the work done to it to imporive it - turftriding etc but at these revs you are on its limits whereas with the steel version you should be able to forget about it. You can use standard rods with the cast steel ctrank and the rods can be improved by polishing, ARP bolts etc. The next step is steel rods and then H section etc. The forged steel crank is the next step and is more expensive but more or less bullet proof. Some of the smaller capacity crossflows reved to silly high figures but a 1700 will end up back in the workshop very quickly with anything like those revs. Its good to have peace of mind and the extra revs and power with steel really make the car fly as you will for example still be acceletarting hard in second gear instead of more slowly in 3rd with an intoxicating sound. You wont worry about the rods or crank breaking either during your exploits - I have experienced this first hand in Formula Ford on exit of fast corner, close to crash barriers - not good - dont want that to happen again! Trouble is because you have been thrashing it, you still end up rebuilding every now and then, but thats the fun and you can then dream up how to make it better again. And during the rebuilds you will be able to see all of those things that you wasted your money on in previous winters! If the car is driven sensibly with the standard modified components it will though last for many miles / years. The Roger King 155bhp spec is a good example of this. Link to comment Share on other sites More sharing options...
eric Posted February 23, 2011 Share Posted February 23, 2011 Of course Roger King engine was the reference but it is not working on this subject anymore. Planet Zog is too far away an engine builder workshop Don't forget that my aim is to have a nice road engine, very reliable and even if I will play with friends around a track, twice a year, I cannot afford hassles of a competition engine. The budget is also a question, because you can spend a fortune in a crossflow ... The road test will give me the answer of the questions . The guidelines were the engine that is still in the car, fumes but works It is an 1600 AX block all std but well balanced and well built . My car (not with me) went to a lot of races ( including Brands hatch and Spa ) and it never broke Link to comment Share on other sites More sharing options...
eric Posted April 9, 2011 Share Posted April 9, 2011 the 7 is now on the road with its new stunning engine. Lot of torque, good power, low consumption. All is what I like. Link to comment Share on other sites More sharing options...
OliverSedlacek Posted April 9, 2011 Share Posted April 9, 2011 Good stuff. Any figures from the rolling road yet? 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