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flat tops in a 250 crossflow with a 200 kit 

 

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 Post subject: flat tops in a 250 crossflow with a 200 kit
Posted: Tue Jun 26, 2007 9:07 pm 
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Hi all

I am putting a 200 kit in a 250 with 30 thou up flat top single ring pistons etc, all brand new

2 questions ~

best cam to achieve best performance for speedway, going to run 4:11 diff gears and top gear ( 3 speed gear box )

and best head to use and whats the simpliest head improvement to get the best result

I have heard alot of rumors and stories about compression, woes, so any tips would be good

thanks
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Posted: Tue Jun 26, 2007 9:10 pm 
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First question: Why use a 200? Are you using this for a certain engine size bracket race?

 

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Posted: Wed Jun 27, 2007 4:23 pm 
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i think what rumboy is referring to is a 250 with the longer 200 rods in it, with pistons to suit. i would fill out the questionnaire that cam companies give you and ask their suggestion.

 

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Posted: Wed Jun 27, 2007 6:26 pm 
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wades 1147b is a top choice i am running a custom grind basically a 1147b it just has a different duration needs performance valve springs and retainers, as for headwork i would go for a port job and maybe oversize valves, if its a acl race series piston kit with the 2 ring flattops the comp ratio will be over 11:1 maybe even run the low comp head
tips: good fuel and a good graphed dissy will avoid detonation.
4:11 with a 3 speed and i am guessing you will want to be running in second gear probly only pull 70kph at 6000rpm, a 3.08 with a 4 speed running in second is a good combo. anyways mate give wade cams a ring thay will match a cam to your needs.
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Posted: Wed Jun 27, 2007 10:39 pm 
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Top gear will use less BHP to run than using 2nd or 3rd in a 4sp.
With a hi comp like 11:1, you'll need a cam around the 40/80 mark with .520" to .560" lift with valve springs to suit, better pushrods to cope, cyl head porting, roller rockers, anti pump-up lifters, windaged sump, hi volume oil pump, engine oil cooler, regraphed dizzy, fully balance the engine, MSD ignition ( 6AL ) to keep the fire going, 600 to 650 CFM Holley ( with centre float bowls), comp style extractors, 3" exhaust, electric fuel pump to keep up, an off set airfilter assembly or hi-rise the air cleaner assembly to fit a large volume capacity airfilter. All this to keep it happy at 3500 to 6500 rpm.
Here's a pic for an idea.

 

 

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Nathans XF Ute 250 Xflow. 001.jpg
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Posted: Wed Jun 27, 2007 11:15 pm 
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cjh wrote:
Top gear will use less BHP to run than using 2nd or 3rd in a 4sp.
speedway is all about torque and acceleration, 2nd is the go.

cjh wrote:
600 to 650 CFM Holley ( with centre float bowls), comp style extractors, 3" exhaust, electric fuel pump to keep up, an off set airfilter assembly or hi-rise the air cleaner assembly to fit a large volume capacity airfilter. All this to keep it happy at 3500 to 6500 rpm.
no crossflow warrants a carby that big it causes flat response and makes the engine lazy, a well tuned 350 holley would pull to 6500 alot quicker.
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Posted: Wed Jun 27, 2007 11:27 pm 
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fastleno wrote:
cjh wrote:
Top gear will use less BHP to run than using 2nd or 3rd in a 4sp.
speedway is all about torque and acceleration, 2nd is the go.

cjh wrote:
600 to 650 CFM Holley ( with centre float bowls), comp style extractors, 3" exhaust, electric fuel pump to keep up, an off set airfilter assembly or hi-rise the air cleaner assembly to fit a large volume capacity airfilter. All this to keep it happy at 3500 to 6500 rpm.
no crossflow warrants a carby that big it causes flat response and makes the engine lazy, a well tuned 350 holley would pull to 6500 alot quicker.


That pic of that engine, had a 4sp, 2.92 LSD, 600 vac 2nd Holley, and went like a raped ape, even went up against a BA XR8 auto sedan and won, twice. It doesn't have a flat spot and picks up rpms like you wouldn't believe.
Had had it on a dyno, was running lean at 5000 to 6000 rpm but was putting out 170HP at the wheels without the MSD ignition fitted at the time.
These are the wheels that were on it to get traction. 265/70R15.

 

 

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Nathans XF Ute with big tyres on 265-70R15 001.jpg
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Nathans XF Ute with big tyres on 265-70R15 002.jpg
Nathans XF Ute with big tyres on 265-70R15 002.jpg [ 359.71 KiB | Viewed 37 times ]

 

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Posted: Thu Jun 28, 2007 10:32 am 
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just seems over kill to me mate i have seen a crossy making 180rwkw running a 350 and the fuel ratio's where good right through, at the apsolute max i would be using a 390 CFM Four Barrel it will get any crossflow with a volumetric efficiency of 100% well over the 6000rpm.
the reason for the lean out on the dyno is probably that the large CMF holley wasn't creating enough vacuum to pull enough fuel through the jets
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