blue thunder

He means the theoretic or calculated cranking pressure. You will never see that in practice due to leakage and a plethera of other uncontrolable variables. He using it as a design parameter and yardstick to correlate one engine to another. Seems straight forward to me.

James

The idea then of running A/F in the 12.1 range then is to reduce the "possibility" of lean rather than use raw fuel as a coolant. ideally the A/F would be in the 14.1 range with SC or N/A engine. If I'm running 1350 EGT @ 12.1 A/F how much cooling is the fuel doing ?. The static compression & running compression has more to do with detonation but at some point is also affected by A/F especially in a lean condition. A lean engine is more likely to detonate due to higher cylinder temps, true ?

I'll bump the timing up from 30 to 32 degrees and see if the EGT comes down

MILD THUNDER

iTrader: (3)

I wonder how getting the afr in the mid to high 11's and 2-4* more timing would work for you?

articfriends

iTrader: (1)

First post he states that he feels safe with x number of compression psi which from My experience is really too much in most applications for a blower motor running any more than 2-3 psi of boost, people read this and assume it to be fact and reliable safe information as he is a "pro engine builder". When questioned about it he restates that this is just a theoretical number that he calculated, although theoretical if the math was done correctly and motor has good ring seal it SHOULD have very close to the "calculated" number, in the end he DOES agree and concedes that that WOULD actually be too much cylinder pressure (about 25% too much) to actually start putting boost on top of. I'm glad he restated what he said but to me I would have rethought/re-posted what he originally stated so as not to sound like he is back tracking, Smitty

articfriends

iTrader: (1)

Even at 12-1 afr your extra fuel is still giving you some extra cooling to the exhaust valve, piston crown and chamber and it is keeping motor from having lean detonation, especially under a wot, sustained load , Smitty

Young Performance

In the first post I stated that as long as I am UNDER 190 psi, I'm Ok with a GOOD TUNE. In the second post I said that 190 psi is the MAX number.Max means up to but not over.
As Thunder mentioned, there are to many variables.....battery condition, type and strength of starter, battery cables, cylinder leakage, etc.
I calculate every engine mathematically if I think it will be close. I then come back and manually check it with a gauge. I'm sure a different comp. gauge may give a different reading. If I come up with say 185 psi mathimatically, I will only show about 165 or so on a gauge.
So far it's worked out pretty good for me.
Eddie

blue thunder

My theoretical calculates out to 175 pounds cranking and I run 6 pounds boost with good results. I don't get anywhere near 175 pounds on my compression tester. Taking this one data point and applying the fact I have run hyper pistons with this 175lb theoretical cranking pressure leads me to believe it is safe and could probably go higher with the right tune. So what eddie suggests with 190 being the breaking point rings true in my mind.

Young Performance

Thanks Thunder. I'm glad you understand. However, you're running with Hyper-u-crackit pistons????
You're braver than me

But, there's no arguing (at least, for most) with success. If it works for ya, stay at it.
Eddie

HaxbySpeed

I think that's where I got confused too. I thought you were pumping 175 and running 6lbs boost! The theoretical psi based on swept volume is of very little consequence unless you're factoring in IVC. You could have a theoretical psi of 220 but with a really late closing might only pump 150 on a gauge which is what is important as that's what the motor sees. To me it's kinda like the whole dynamic comp deal. It's cool to calc all that stuff but most guys who are building engines know what works and it seems to work well. Just like Eddie's 190 psi works for him with his cam selections, obviously the IVC is bleeding enough pressure to get down to the 155 pumping range. You'd have to stray pretty far from the norm, like running a straub cam, to really screw up a basic blower deal.

In my opinion, what would be valuable information in this thread is: Static comp, Boost level, Octane, Cranking psi.

Panther

Agreed, thanks for sharing!

Mine is;

"Your engine summary is as follows: Bore 4.53 inches, stroke 4.5 inches, rod c-c length 6.7 inches, with a static compression ratio of 8.3 :1. Your camshaft specifications call for an inlet valve closing of 54 degrees ABDC (after bottom dead center).

Your chamber volume is 162.81 cc's. With this camshaft your dynamic, or effective stroke is 3.82 inches. Your dynamic compression ratio is 7.20 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 138.55 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 7 PSI is 10.63 :1."

To add, I see my cranking pressures usually pretty darn close, within say 5-7% give or take.