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Old 09-14-2012, 03:14 PM
BogiesAnnex1 BogiesAnnex1 is online now
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Originally Posted by Pupsvette76 View Post
anyone care to offer any actual help?

I've got to know the real detail; which includes part numbers and manufacturer if you don't have the specs. A turbo or any blown or laughing gas motor is not a simple accomplishment. It becomes a very large set of compromises the direction being set by the major use, an automatic transmission just adds to the difficulty because building the engine toward the high performance side then controlling it with gear selection makes the overall task a lot simpler.

I just finished an injected, turbo 911 for an owner with no cash flow problems, it took a year to build including extensive chassis, suspension and transmission work but the engine alone used up as much time as everything else and probably chewed up 80 thousand of the 100 thousand this cost him.

On the surface you have what I perceive as a mild cam in that it has a lot of duration and not a lot of lift, so what is called its Hydraulic Intensity appears fairly mild. But having the part number and maker or its opening and closing specs would be really helpful. Blown or juiced engines like a lot of exhaust duration and lift and for the ratio of the intake to exhaust valve size to be closer together than found in a normally aspirated engine. This is because there is more exhaust product to get rid of. This cam would appear to me to be very rampy that is long areas of slow and low lift chewing up lots of duration degrees. This just isn't good with a turbo, but again I need to know the events, I could be wrong on my assumptions.

The 624 head is kind of a do it all, but none too well head. It was found on 350's and 400s and had valve sizes that range from 1.72x1.5 in heavy duty trucks and industrial engines, to 1.94x 1.5 in average use 350s and did show up with 2.02 x 1.6 valves in some 350s and 400s. So when you say these have 1.94x1.5 is that by actual measurement? Supercharged and juiced engines like big valves and good porting. The blower does not overcome porting deficiencies, it just increases pressure not flow, remember gases like air and vaporized fuel are compressible so you can get a pressure change without a flow change.

The 1.5 rockers aren't doing you any favors with this moderate lift cam and lack of valve size (1.94/1.50- (especially exhaust) you're just not getting enough lift for the needed flows with these valves. This in particular backs up the exhaust to where the residual pressure in the cylinder is near the pressure coming off the compressor so the net effect of flow into the cylinder is much less than the pressure shown on the boost gauge. Again, flow and pressure are independent of each other where the flow of gasses is concerned. To a huge extent back pressure in the entire exhaust system is an issue. You need two gauges to really determine true boost pressure delivered to the cylinder as a function of CFM. One you probably have that measures intake pressure. The other gauge needs to read exhaust back pressure. To simplify, the difference between these two gauges of exhaust subtracted from intake is the Effective Pressure going into the cylinder. You will discover that with just 5 pounds of boost it's really easy to have no net gain over atmospheric as getting exhaust pressure under 5 psi takes a lot of attention to detail. This is one reason why guys tend toward higher blower pressure, but even this begets more exhaust pressure but there usually is some space here to mess with before the whole exhaust system has to be redesigned.

I can and will go on as I’m barely scratching this issue. Out of everything i've read (and I’m not negating the contributions of others at all) certainly cooling the intake charge for example is a good way to get around problems of cam timing, port and valve flows and exhaust pressure. But that works so much better when everything else is properly assembled and tuned.

I’m aiming at where I think your biggest problems are, but I’m leaving for a week of vacation and will be off line from this evening through the morning of the 24th.

Last edited by BogiesAnnex1; 09-14-2012 at 03:19 PM.
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