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Carb sizing for worked 250.

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Hi guys,

 

After opinions and experience on carb sizing for solid cammed 4 bbl crossy reving to 6200rpm.

Currently got 650 dbl pump holley, can be a bit flat up to about 3200rpm with WOT but after that, it sings.

 

I understand that smaller carb = higher velocity and more low down torque/throttle response.

 

How small a carb would you go while maintaining high rpm capability???
Would something in the 450-600cfm be more suitable???

 

Cheers.

 

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10.8:1

 

.560 lift
244 dur @ .50

106 lobe sep

 

3.45 gears.

 

currently t5 manual.

will be getting a manualized c4 in near future, hence why I am rethinking carb as I have read that auto can make over-carbed 'issues' worse??

 

 

 

 

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I ran a 465 on my old engine on a redline manifold, slight porrted head and good flowing exhaust 

 

10;5;1 comp 

108 lobe sep

545lift in/exh

cant remember the duration of the top of my head 

ran a 7.5 powervalve on 58 i think jets, worked awesome 

pulled 147rwkw 

was manual running 3.23 gearing..

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Haha yeah well I was amazed gerg when I put it on the dyno to tune it, was my first ever crossflow build and only going off what I new, was about 98kw when I drove it in to and when I come out I was amazed :)

I had a custom ground camtech cam made up was running yt roller rockers on a D head

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With that cam and compression I belive it will make the most with a 600/650 DP on a 2" open spacer.It has well enough camshaft and compression to use it.X flows love camshaft as much as they love carburation IMO.

 

Bigger issue is the gearing...need to go to 3.9 for best results or it will not be a happy camper :(

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From people with a lot more experience than me I have been told that crossflows LOVE carby spacers and this allow for a bigger carby, but the inlet manifold needs to be of a good design for both of these to work. Also changing the location of the carby on the manifold gains a lot. When TF250 had his crossflow(now mine) when he used a spacers to move the carby closer to the engine he gained 1mph through the bottom end, this doesn't sound much but when drag racing the speed indicate the true horsepower of the engine. So gaining mph means a gain in horespower in real world terms.

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All this talk about hot crossies is making me want to build one. Even if I've got nowhere to put it, I'll just shove it on a stand and look at it with a beer in my hand.

 

Do it Gerg! I have a very loving home that you can sit the motor in. It's a 91xf ute and it's in my carport :) would love to improve on my old pig one day!

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There's a 470 as well, that might be the old 465 but they all appear to have 1.5" throttle bores. The difference in CFM might be in the venturi and booster design.

 

It comes down to whether you want mech or vac secondaries too. The smallest mechanical is a 600 HP I believe. The goal here was to fill out the lower end of the torque curve and make the thing more driveable so mashing the pedal at low rpm with mechanical secondaries is just going to lose velocity and give soggy response. I say go to a good vac secondary unit. One that's set up correctly will have torque down low and seamless opening of secondaries.

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I think the 465 will work, id did for me even to 6200, but as matt said your camshaft is bigger so you need a bigger carb as it wont punch enough to keep it going,

A 500 two barrel is a nice unit, good down low and has good air speed up top I have seen 250k + stock bottom end crossflows with ported head and valve train kit plus smaller cam rev 6100 all day on a 500, and one burnout comps from the torque,

 

But you said your running a 4 barrel mani so that rules out my jibber...

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Well to throw more argument into the big carby idea...Pro Systems in the US quoted me a 740 CFM DP complete with Dominator boosters 4 corner idle adjustable air bleeds jet extensions modified bowls and all in a new style Ultra HP body.And that was for my old solid cammed set up.

 

Just saying.

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Some small carbys being recommended in here I think

One of my slappers had a 750 on it for a long time, and back then I did dyno time more cause I was to dumb back then to work out why a monsta cam crossys don't idle well. The bloke tuning it said that my 650 would not work. I said its a 750 and tune it its what I have where using it. He rung me up the next day and said he would have said it would not work, but it did. And made heaps good numbers to considering it was a $1500 engine. He said to fit a 600vac it would be better and a year later I did. I retuned it for a retune we binned that I idea it droped 20hp and he could not get it back so we refitted the 750

Man ive had some crossys its funny I still have most of them. Need more time to build toys to put them a in but that times wasted seeing what all your guys are doing :D

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When mine was a solid cammed 650 lift 270 duration banger it was a big toe to drive it.Like just touch the throttle to make it move and any harder was very very loud and generally smoke on road tyres.Still my favorite xflow combo it was.That solid cam unit was a monster,a tractor a cranky sounding bitch but it was over early thanks to Mr Holley fuel pump :(  With the webers and the 4 link 5400 stall and stuff it's a tad doghy but still easily driveable and bearable except for the extreme noise...Seems to be a pattern with this ute doesn't there ? Once it's up and rollin your barely touching the throttle. 

 

The 650 was snappy and touchy yet totally driveable just obscenely loud.The webers on the old convertor and old car set up would have been easier but the 31 spline full spool and bigger convertor and 4 link make it more point and shoot unit if I'm honest...exactly how it is meant to be really so I'm not complaining 1 bit :) That said I took it for a cruise on Sunday (even past a cop) and it drove just fine...but yes very noisily ;)

 

This is the old combo v's new...tell me which sounds more bad ass.

 

Camtech solid 650 DP...

 

Clive cams solid roller webers...

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Mine was ok, but like most larger engines it was like a stock engine off camshaft,

It worked for that engine, its a large carby but worked well. power was 3200 to just past 6000 

I used it cause it was what I had I would have liked to try a 650 but never did

 

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