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Discussion Starter · #1 ·
A friend of mine is having some issues during a recent dyno test. It runs fine until 5200 then it starts to miss. The engine is a 434/400 stroker with CHI 3V heads and intake. Hydraulic roller cam, 224/224 @ .050, about 0.600 lift. Lifters are 5.0 replacement roller lifters and is using the Comp Cams roller conversion setup.

He's tried several distributors and wire sets, several carbs including an 850CFM Demon. He's changed valve springs with no effect. Oil pressure is a bit high at 70psi. Could the lifters be pumping up by 5200? Is there any way to rule this out as a possibility? Could the cam be too small? DD2K says this combo should pull to 6000+. Any other possibilities we could be overlooking?
 

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Are you sure it's not running out of fuel? How aggressive is the cam? What happens after 5200? does if fall off or just sort of flatline and slowly start to drop?
 

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I was thinking the same thing. Check the float level and the fuel pressue at higher RPMs.

Maybe valve float? Are the spings matched to the cam?

How about ignition timing?

_________________
Patrick Brown

62 Falcon - 331 stroker - C4

<font size=-1>[ This Message was edited by: pbrown on 12/9/06 3:46pm ]</font>
 

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Also, is he running a choke? Make sure nothing weird is going on with the carb and verify WOT by checking the secondaries to verify they are opening fully
 

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It could be spark blowing out. Decrese plug gap and use a spark enhancer like MSD or Boost a Spark.
 

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How much preload are you using? I found out when I was running those lifters with an aggresive roller cam, they liked about 1/8 turn of preload max. If I just had that little bit of preload, they ran fine up to a little over 7k.
 

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A cam with simular specs makes max power at 5800 rpm in my little 302 with aluminum heads. Get a more aggressive cam in there.
 

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How well do the CHI 3V heads flow? I run a smaller cam then that in my 302 and it pulls to 6400. Being that it's a .600 inch lift cam it doesn't sound like something off the shelf. What type of cam is it?
 

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Discussion Starter · #11 ·
Bear with me, I'm relaying everything via Email. I should have mentioned my friend is half way across the country.

I don't think there was any fuel starvation problems. The guy doing the actual tuning at the dyno is a full time engine builder so he should have the air/fuel/spark basics covered pretty well.

Cam is not that aggressive IMO, power peaks at 5200 then dips about 10HP and recovers the peak power at about 5600. Graph image was cut off above 5700 for some reason.

I'm sure the valve springs are matched to the cam. I don't know the actual seat pressures or anything but this isn't the type of guy to skimp on parts. A second set of springs were tried with no help.

I'm not sure about ignition timing but I have a lot of faith in the tuner's experience. I didn't think about spark blow out at that RPM. I'll mention it just in case.

I mentioned lifter preload also, I don't know how much he's using.

CHI 3V heads flow very well, especially in the hands of a good porter. A similar combo in the Engine Master's Challenge made 618HP @ 6000 with an unknown cam. Other CHI head/Windsor block competitors have made up to 714 (I believe) with 10.5:1 compression and a 6500rpm limit.

They pulled the carb yesterday and tried the EFI conversion that was custom built six months ago for this application. One of the reasons for this dyno session was to do a carb vs EFI comparison. The good news was the miss went away, the bad new is that the engine wouldn't make as much power as the carbs they tried and unfortunately also blew a head gasket.

So, my friend is heading home without his motor for now. I'll try to keep everyone updated when repairs a finished and when/if it goes back on the dyno.
Thanks for all the input.
 

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Have the 3V's been ported? Thats a large displacement for that little cam but I have a feeling there is more to it than that.
Can we have some more specs - bore, stroke, cam centerlines, minimum port cross sectional area or width/height size? Is there any VE data on the sheet?


<font size=-1>[ This Message was edited by: cmf60 on 12/10/06 9:13am ]</font>
 

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Curious what intake he's running. Don't think I saw it mentioned.

Why would a head gasket blow if everything was done right?
 

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My point is that if the dyno operator is a "full time builder" and the engine builder is not a guy to "skimp" then why would it have blown a head gasket. I don't think he should be as confident in them as he is. I'm sure they're good. I'm also sure they've make mistakes just like everyone else.

My buddy just went to MLS gaskets on his supercharged 302. No problems yet including dyno session and a trip to the track. The two guys who recommended them have plenty of experience with blown small block fords. We'll see.

It was mentioned on another site by someone who seems to know his stuff that this may be a small base circle cam. In that case it could have problems at higher rpms with a decent spring pressure apparently. I've heard many people say the small base circle stuff is horrible for any kind of performance application. It could also be a combo of this and something else.
 

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Discussion Starter · #16 ·
The 3Vs are 218s unported. 4.03 bore, 4.25 stroke, Cam LSA is 114, IC unknown. Corrected intake duration is 224/244 @ .050, it's a custom HR grind from Comp cams.

I don't know the cross section or width/height, whatever the 218s are without porting. The heads were originally chosen for a stock displacement 400. The 434 stroker has 10.8:1 compression. I didn't see any VE data on the sheet, at least nothing readable.

The intake is a CHI 3V single plane with port injector bungs custom installed. The head gaskets are ROL graphite. We won't have an idea why it blew until it comes apart.

When the carb was run, the injectors were installed but inoperable. Is there any chance they could have disrupted fuel flow in the ports?

Has anyone had or heard of flex problems using aggressive cam profiles with hydraulic roller, small base circle cams?
 

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What was the A/F ratio during both(carb & EFI) runs?



BTW, post the sheets so we can better evaluate the problem.
 

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Is current peak torque around 3900rpm?
 

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Firstly, I recently read an item efi vs carbs...done by some clever people (not a magizine article). Carbs won and I think the reason was far better fuel atomisation from memory....something to do with the on/off pulsing at higher revs/airflows....something like that? I will see if I can dig it up.

It wouldn't surprise me if the heads are basically getting near/into "choke". If the heads are un-ported and with that big displacement I am pretty sure it is getting close... according to my calculations. These heads have a 2.4sq in cross sectional area, which is pretty small and to feed this thing the actual air speed in the port gets very high, too high and they start to choke. There are several other factors that effect this such as valve lift, diameter and actual timing that you can mess with to "starve off" port choke.
However, my other calculations put peak torque around 4000...and thats not quite right is it...so I may be talking thru a hole in my head, but in stock trim....these heads seem to struggle with large engines.
Forget about EM results, they didn't use off the shelf heads....thats for sure.

<font size=-1>[ This Message was edited by: cmf60 on 12/10/06 2:38pm ]</font>
 
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