My understanding about flat plane cranks is that there CAN be a power advantage, but it is not a big one. It comes about because exhaust headers can be made like 4 cylinder ones (ie. 4-1 headers on each side) and because of the even exhaust pulse interval (due to the flat crank design) exhaust scavenging is better. I think the power advantage is only something like 3 or 4% at best. And then there are the vibration issues to deal with, but this is largely dependant on the stroke and weight of reciprocating components, and the rpm it is spun to, less is better in all cases. But they do sound cool!
There was a guy in NZ (Dr Roger Freeth) who ran a flat crank 4.0L RV8 in a Toyota Starlet thundersaloon back in the late 1980's/early 1990's. It was originally built by Oselli, and sounded lovely! Went rather well too:)
Conrad
Flat plane crank - Cam
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I agree with all the above, big bore, short stroke 90 degree crank and let it rev. The best gains from a flat plane are in the exhaust so if you have to compromise that at all to fit the vehicle you will be better off at 90 degrees. No point re-inventing the wheel. Best power ever produced from these engines was for drag race use and all had 90 degree cranks.
Alan
Alan
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oneaves2005
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OK, so.... I have built a car for the BCV8 championship and currently have a John Eales full race 3.9 engine with downdraft dellortos. The max safe revs on the engine for continuous race use is 6500 which limits the differential ratio to a 3.3. There are now5 cars running flat plane cranks which they are reliably changing gear at 7500+ which gives them the opportunity for a 3.7 or 3.9 diff. They seem to be a lot quicker down the straights, better throttle response and more usable power delivery....
the cars are quite light for the power, 925kgs on 15x7 wheels that have to run road legal rubber. With a t5 close ratio box most are running out of revs in 5th.
My first race in the car will be in a few weeks so will see how I get on but it's looking like a flat plane crank will be needed to keep up so doing a bit of research before hand.
the cars are quite light for the power, 925kgs on 15x7 wheels that have to run road legal rubber. With a t5 close ratio box most are running out of revs in 5th.
My first race in the car will be in a few weeks so will see how I get on but it's looking like a flat plane crank will be needed to keep up so doing a bit of research before hand.
oneaves2005 wrote:OK, so.... I have built a car for the BCV8 championship and currently have a John Eales full race 3.9 engine with downdraft dellortos. The max safe revs on the engine for continuous race use is 6500 which limits the differential ratio to a 3.3.
Sorry to say that..... but it is just Sir Eales who doesn't like engines rev more than 6500RPM in race application...
The real problem rev more than 6500 rpm doesn't offer really gain because heads.
Last edited by Wotland on Thu Aug 01, 2013 8:39 am, edited 1 time in total.
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oneaves2005
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oneaves2005
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I don't think it is the flat crank that allows these engines to shift gear at 7500. Every component in the engine must be able to support higher rpm, rods, pistons, rockers, cam, manifolds etc.
They will be quicker than you if they are using higher rpm and a shorter diff ratio, as the torque multiplication through the shorter ratio will be better than yours, even if maximum torque is the same for both engines. In other words, you might have the same max torque as they do (possibly even more) but if they have cammed up and shifted the torque higher in the rev range they will be making more hp than you, and will be faster. I hope that makes sense?
What would stop you from running your engine to 7500rpm? Are you running stock crank, rods etc?
Conrad
They will be quicker than you if they are using higher rpm and a shorter diff ratio, as the torque multiplication through the shorter ratio will be better than yours, even if maximum torque is the same for both engines. In other words, you might have the same max torque as they do (possibly even more) but if they have cammed up and shifted the torque higher in the rev range they will be making more hp than you, and will be faster. I hope that makes sense?
What would stop you from running your engine to 7500rpm? Are you running stock crank, rods etc?
Conrad
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oneaves2005
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It is an Yankee Myths you need billet crank for race application.
With short stroke SG crank does the job.
You don't need just to swap part for stronger but you need to concieve the engine for high rev.
I have a this time two 4.5 engines : one clubman and one race. Internal combo is completly different.
With short stroke SG crank does the job.
You don't need just to swap part for stronger but you need to concieve the engine for high rev.
I have a this time two 4.5 engines : one clubman and one race. Internal combo is completly different.
Hi
to rev an engine you need to keep max piston speed in a sensible region, you need to ballance it in an appropriate manner to reduce vibration and con rod load, lastly you need to control any other harmonicly related vibrations in the engine and block you also neeed to keep friction levels down or it isn't worth reving it!
The first one is easiest to achieve by keeping the stroke noice and short, ballancing is then a case of keeping the pistons and rods light and if you rev it you reduce the loads (specific impulse) into these componenets from combustion but if you don't make things lighter you up the loads from accelerating the pistons and rods. The last one is where a flat plane crank wins and looses against a 90 degree crank, one thing you can do with a short stroke flat plane crank is rev the nuts off it, you can get primary and first harmonics cancelled out properly but you get loads of second harmonic vibration in the crank, this, if it ocours at a frequency that resonates in the crank then it will cause high wear and fateague in the crank and block. What you can do is rev it to take these vibrations to a point above where they will resonate in the crank, this is doable in a race engine, you set idle at say 1200revs or somewhere where it does not vibrate too badly and then only use the engine to make power above 6500 revs and up to say 10500 revs, this way the second harmonics are in the region of 325hz to 525hz this is doable on a race engine but not on a road lump. the key is selecting parts that are light and ballance carefully there is no reason you cannot rev an engine if you build it carefully.
My chevy has a 3.75" throw crank, the bottom end is all forged but that is because this is a big power lump, it has a soft rev limiter coming in at 7800 and 50% spark cut at 8200revs, I have now buzzed the hard limmit several times. . . trying to work out why the car will not accelerate any more before hitting the shifter! a well ballance motor will rev without silly components.
Best regards
Mike
to rev an engine you need to keep max piston speed in a sensible region, you need to ballance it in an appropriate manner to reduce vibration and con rod load, lastly you need to control any other harmonicly related vibrations in the engine and block you also neeed to keep friction levels down or it isn't worth reving it!
The first one is easiest to achieve by keeping the stroke noice and short, ballancing is then a case of keeping the pistons and rods light and if you rev it you reduce the loads (specific impulse) into these componenets from combustion but if you don't make things lighter you up the loads from accelerating the pistons and rods. The last one is where a flat plane crank wins and looses against a 90 degree crank, one thing you can do with a short stroke flat plane crank is rev the nuts off it, you can get primary and first harmonics cancelled out properly but you get loads of second harmonic vibration in the crank, this, if it ocours at a frequency that resonates in the crank then it will cause high wear and fateague in the crank and block. What you can do is rev it to take these vibrations to a point above where they will resonate in the crank, this is doable in a race engine, you set idle at say 1200revs or somewhere where it does not vibrate too badly and then only use the engine to make power above 6500 revs and up to say 10500 revs, this way the second harmonics are in the region of 325hz to 525hz this is doable on a race engine but not on a road lump. the key is selecting parts that are light and ballance carefully there is no reason you cannot rev an engine if you build it carefully.
My chevy has a 3.75" throw crank, the bottom end is all forged but that is because this is a big power lump, it has a soft rev limiter coming in at 7800 and 50% spark cut at 8200revs, I have now buzzed the hard limmit several times. . . trying to work out why the car will not accelerate any more before hitting the shifter! a well ballance motor will rev without silly components.
Best regards
Mike
poppet valves rule!
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oneaves2005
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