Exhaust manifold primary diameter to port/valve size ratio

[GORDON STREETER]

Sorry Tim, but I was just reading "The Naked Chef" and I haven't got it right yet !

[Copperbottom]

Originally Posted by GORDON STREETER

Behind the salad !

AAARGH..............My eyes!





That plate looks a bit heavy for just one hand

[morninggents]

Originally Posted by Copperbottom

AAARGH..............My eyes!





That plate looks a bit heavy for just one hand

I told him on post no.22 that the belt's missing from his water pump.

[zefarelly]

he's not hooked the oil tank up to the pump either

[zefarelly]

BOT . . . 1 1/2" ID primaries and 30" long might be a god starting point, but theres a lot of variables and a 264/254 hybrid cam with whatever phasing is going to give you plenty to play with

[knighty]

HI Folks - I have been reading this thread with great interest and thought I'd ask you clever folks opinion on my latest project........its a Honda GX390cc kart engine, single cylinder 2 valve air cooled, so similar in a way to the tuned 1600cc Kent engines.........my GX390 is very highly tuned, truth be known its got £1500 of tuning toys inside from the US, so its a good job I bought it second hand for a fraction of that cost!!!.......general specs are 34mm mikuni carb, 1.3 ratio billet rockers, 88mm bore, enlarged 38mm intake valve, 31mm exhaust valve, 32lb springs, skimmed head and block for high compression circa 11:1, gas flowed ports, stage 3 cam (high lift & duration) chrome-moly push rods, billet flywheel and con rod, flat top piston, revs to 7-8,000rpm, power is 25-30bhp, possibly a tad more as good ones make 33hp.......as a point of note the exhaust port measures 28mm at the exit of the head-flange, this can be enlarged if necessary (personally I think it needs to come out to 30mm ID)

anyway, the original exhaust primary was only 6 inches long and 1-1/4" OD / 1-1/8" ID.......which is both too small in diameter and length for any scavenging effect to take place, by my very rough sums of cylinder volume versus primary volume the length needs to be in the region of 20-30 inches long.......the cam is 360" lift and 260 duration.

I have just purchased an S-bend exhaust manifold based on a very strong reccomendation of another user with same engine spec as mine https://www.kart-schuette.de/karts_e...ld-racing.html which is 24" long , 1-3/8" OD (35mm) and 1-1/4" ID (32mm) which I think is right on the money, but I just want to have a stab at calculating the correct length of the primary based on 1-1/4" ID and my cam spec, I would like peak torque at about 4000rpm .......I will dig out my books tonight but would appreciate your opinions too ;-)

[phoenix]

By my calcs, applying Helmholtz tuning, the 6 inch length is pretty close to the tuned length for 8000 RPM with a 1.25" ID pipe - I make it about 8 inches at 800C EGT mean inluding the length of the exhaust port - but exhaust temperature plays a big part in this and I can only guess at that.

For 4000 RPM with the same 1.125" ID and an average EGT of 750C you would need 31" including exhaust port and for a 31mm ID (same size as the exhaust valve) 36.5".

For a guide, 7000 RPM needs 12" total length, 6000 RPM 16" and 5000 RPM 23-24".

Your 24", 1.25" ID looks to be tuned to about 5100 RPM - but in reality this would be somewhat lower due to the length of the exhaust port in the head. It won't be 4000 RPM though. You will need to add a further 15" to get the tuned peak down to 4000 RPM.

[knighty]

Many thanks phoenix, in reality I'm only really interested in the 1.25" ID results for the new 24" long S-bend exhaust that has just arrived. I will measure the exhaust port length tonight, but I estimate its about 4 inches.......also with my karts exhaust layout and silencer positioning it makes sense to add another 6 inches just before the silencer, this can easily be increased to 11" but due to pipe availability I will step this up to 1-3/8 ID temporarily, once a piece of 1-1/4 ID rears its head I will cut it to size and slide it inside........ so overall very close to the extra 15" for the 4000rpm sweet spot.......anyway all seems to be going in the right direction!

[Notso Swift]

Remember the Phoenix will be talking about lengths from the valve, not the head, so you will need to add a couple of inches

[knighty]

no worries!.......I measured the exhaust port length from the back face of the valve to the flange face and it was a puny 2-1/2 inches!.....I have since completed the exhaust build and the lengths are as follows:

Port (1-1/8 ID): 2-1/2" long
S-bend (1-3/8" OD / 1-1/4" ID): 24" long
Adaptor to silencer (1-1/2" OD / 1-3/8" ID) : 7" long

so total exhaust primary length from the back face of valve to the silencer entry is 33-1/2"......compared to 8-1/2" previously.....I think thats a far better place to be than previously because the mid range performance was almost non existent, anyway they say the proof of the pudding is in the eating, I will be running it tomorrow at the tilbury kart circuit so the seat of the pants dyno will reveal all, I will post some piccys soon. here is some you-tube footage of the kart as I purchased it, you can clearly see how short the exhaust is.

http://www.youtube.com/watch?v=3cu-mPv3spA

http://www.youtube.com/watch?v=DVFA-...feature=relmfu

.

[phoenix]

Next stage, tune the inlet. 34mm ID and 527mm long from back of the inlet valve to the tip of the inlet trumpet.

[knighty]

Hi phoenix. well i must say the exhaust has been a complete success. the engine was noticably coming on-cam at about 5000rpm which was not even happening before. but now i have 2 problems. as the gearing is wrong i can only rev to 6000rpm before i lift off. so am now addong another 5 teeth to the rear sprocket. second problem is my camshaft was designed for us style oval kart racing which is almost all full throttle so i need to consider a cam that will give some midrange performance. also fun was stopped early due to a failing coil. will post some pics soon. the kart generates alot of interest from the predominantly rotax mob.

[knighty]

finally I got some pictures sorted