My engine builder specced the engine for 9.0:1, as I have a blower. (400 block, bore 4.155 stroke 3.48 with AFR 220 heads).

Now I have a blowby problem, so the first thing I did is check the compression ratios. One pot was at 140 psi, but all the others were in the range 190-210 - so obviously there is a problem with that one pot.

However, if atmospheric pressure is 14 psi, then 200 psi represents over 14:1 compression ratio??? What am I missing here?

Having pulled the heads yesterday, the pistons are flush with the top of the block at TDC.

Todays problem is removing the oil pan - It's a high volume aftermarket and it will not clear the beams - I'm hoping that undoing the mounts and lifting the engine an inch will do it, as otherwise I have to pull the whole block to get at the big ends.

There are days when I wish I'd bought a 'sensible' car :nonod:

Thanks, Steve

 

On your PSI calculation, I don't think PSI is inversely proportional to the volume and I'll bet it varies by type of gasses. I'm no expert in physics though.

You might have the valves in that one cylinder checked out before you pull
the pan (since you've already got the heads off).

Dan

 

Well, I guessed I was wrong somewhere, as at 14:1 static, adding boost would surely explode the engine.....which it didn't!

Now if only I had a valve spring compressor, I could check the valves in the number 2 pot. They *look* OK! (Except very carbonised, from all the oil that the blowby forced thru the PCV and into the intake).

The smell when I cracked the heads was appalling. Can't describe it, other then burnt!

Off outside now to crawl about under the car in the rain :cry

 

As for your compression, the ratio is calculated by dividing the volume of the cylinder at bottom dead center along with the gasket and chamber volume by the volume at top dead center. It has no real relation to compression in psi. Your camshaft has alot to do with cranking compression depending on valve timing.
For your diagnosis, it would have been easier to figure out with the heads on. I use a compression tester, and hook the air compressor to it and pressurize the cylinder. With the valves closed (loosen the rockers to ensure) listen where the air goes. Through the exhaust? bad exhaust valve, through the intake- bad intake valve, to the crankcase- bad ring seal, to the coolant- bad head gasket, ect....

 

Thanks for that SBNova - That's an explanation that makes sense.

Well, I got that pan off and the pistons out, and the lands on number 2 are badly cracked - so I must have detonated her at some point. There's a slight score in the bore that you can just catch with a fingernail, and the big end shell and crank for number 2 has scoring in it too - so it looks like a little piece of piston made it's way thru the oil ways.

The thing is I knew is was either rings or lands, as the blow-by was pressurising the PCV and blowing sump oil through the inlet tract, so a leakdown test wouldn't really have told me anything new - plus I don't have the kit to make up a leak-down tester!

The question now is do I remove the block for a skim bore, and get the crank ground - or just replace the piston?

Here's the bad piston showing the cracked lands:



...and here's the scoring in the bore:



...and here's the scoring in the big-end shell:

 

With that scoring it looks like you need that cylinder bored or at least honed.

 

I was hoping to avoid having to pull the block :cry

 

Can you catch your finger nail in the sratch in the block?? Ususally the rule of thumb ... If you can... your going to have to send it out.

Doesnt look good. Sorry to heard about our troubles

 

Sadly, yes!

 

Time to pull her.... and time to make her a 383 :yesnod: :yesnod:

 

Steve, Sorry to hear about your mishap.
Just thought I'd add some comments, hopefully others with forced induction experience may add some useful info as well.
210 psi cranking compression sounds high to me for a street blower motor. If I had that much on a normally aspirated motor I'd be quite chuffed if it ran on pump fuel! I know your cam is probably optimised for a blower to reduce overlap, so that might be the norm I don't know. I know you state it is a 9:1 motor, it must have a pretty small cam to generate that much compression. I would have expected around 170psi max. for a 9:1 motor with a decent cam, but again I have no experience with blower motors or blower cams. Good Luck.
By the way, who's your engine builder? :cheers:

 

1) who built your motor? who bought your parts? you did or the builder?. There are possibly got the wrong piston and your AFR 220cc. You may not have a 9:1 c/r but your build should be able to verify that.

Bruce
93LT1 Conv w/ZF6
Blown D1-SC/N2O
ARE built 383ci/Stage II heads

 

Hi Steve ! I checked out your website and now i'm curious about your carbon fibre dash. Did you cut out carbon fibre and overlay the stock patrs or did you fab up complete panels? I too have a fake wood dash kit that came with the car. I'm thinking of yellow carbon fibre. Thanks !!

Dan ! :cheers:

 

The compression ratio for an engine is the volumetric ratio - the ratio of the maximum volume to the minimum volume. It is not the ratio of the pressures. As the air is compressed, it heats up and this also increases the pressure. For a 9.5 volumetric compression ratio, the ratio of the pressures for an ideal process (without heat transfer to the cylinder walls will be 23.3 giving a possible pressure of 329 psi. However, there is significant heat transfer to the cylinder walls and the valves do not open and close at bdc and tdc so the actual pressure rise is much less.

 

Dave and Bruce - The Engine Builder was Randy O'Donohue of O'Donohue Racing. He's the engine builder that Carroll Supercharging use. I told him I wanted something built up on a 400 block, to take a blower and possible nitrous, and he oferred me several variations, of which the 377 was the one I could afford. Although I am a competant mechanic, I've never messed with this kind of engine before, so I had to take his word on the build spec being right. At the time, I didn't know this forum existed!

Agent - the dash is very simple. A previous owner has fitted some of that horrible fake wood, so I seperated the panels from the dash, covered them in self-adhesive carbon fibre film, then stuck them back on. Not as nice as a proper carbon kit, but waaaay cheaper, and I think it looks OK.

White90 - the high school physics comes flooding back now - pressure is proportional to volume, butI'd forgotten the heat caused by the pressure increase!

All - my cam is as follows:

Duration st .006 tappet lift 280 degrees intake and exhaust
Duration at .050 tappet lift 224 degrees intake and exhaust
Gross valve lift .525 intake and exhaust
Lobe separation 110 degrees

which people tell me is not really a blower cam. That wasn't a nice thing to hear, but it's the cam the builder fitted and I'm stuck with it. The car has been a total money pit, and I cannot put more cash into it just now, beyond what's needed to get her running.

I thought the CR was high, but since the rebuild there has never been *audible* detonation. Can't tell if she detonates inaudibly, as I had to delete the knock sensor due to false knock counts.

My base program for the FAST came from White Racing, who supplied the FAST, and said it was for a blown 383. It put me in the right ballpark straight away, and I've been fining in the tune for the last year. The last mod was to fit the Crane ignition, and that made an amazing difference - she pulls like a train right to the red line.

So.....will a skim bore and oversized rings be enough to go from 377 to 383? I guess so, as it's just over half a cubic inch extra per pot! Must admit I'm looking for ways to simply hone that cylinder with the block still in the car. I'm running out of enthusiasm and really don't want to pull the block.

One thing I didn't mention is that the engine has been burning oil ever since the end of the breakin period, so I'm guessing the damage was done when the old computer was still in charge .... and it was overfuelling like mad.

Steve

PS The trick now is to avoid rust build-up in the bores while the car is standing on my drive waiting for new parts!

 

I just went to the BRC piston website and used their engine calculator.
Bore = 4.155 in
Stroke = 3.48 in
AFR 220 chamber volume = 76 cc (advertised)
Deck height = 0
Piston valve reliefs = -5cc
Head gasket = 0.040 in (I had to pick something)
Compression ratio = 9.6:1
Certainly, a few Vette owners have supercharged thier stock engines and not had that happen. No one should run less than 0.040 piston to head clearence. What was the compressed thickness of your head gasket? I'd take a very close look at that piston and rod tolerance stack up on that particular cylinder.

 

Oh, what did the top of the piston look like? That rod bearing looks bad, what does the upper bearing look like? Is it pounded out, like the piston had hit the head a little?
Or was the bearing tolerence a little to loose allowing the piston to be thrown to high in the cylinder? Once again allowing the piston to contact the head.

 

Hi Rynda

There is no sign of the piston having impacted the head - both are flawless. There seems to be no play in the little end bearing either.

The big-end shell damage is classic 'metal-in-the-oil' type damage, and the damaged piston land has a tiny piece of metal missing, so I'm guessing this is what has damaged both the bore and the end shells.

Thanks for doing the work on the compression calculator - at least that backs up what the engine builder was saying. The head gaskets were *very* thick - close to a sixteenth of an inch I'd guess.

Steve

 

I am not to sure about this builder. You say you told to build 9.1 c/r for blown motor. and yet he suggested and installed a cam with 110LSA. You need at least 114LSA ot even 16LSA. Even though they are some people out run a blown motor with cam 112 and still making good power but are not the best. But 110LSA? In my .02, you need to found out what cc piston do you have and cc on your AFR220cc. Example, my piston are -28cc J.E along with my LT1 heads 54cc that give me about 9.1 c/r


Bruce

 

Yep - feedback on my cam is that the builder threw in a cam he had lying around spare, on the basis that the poor limey would know no better (true). The AFR heads, which were sold to me as *new* 220's have both been drilled at the back for dry sump fittings. I called Afr who said they never ship heads that way. Also, there is no sign of 220 stamped on them anywhere, but there is part of what was clearly '200', and which has been part ground off. I also believed I was getting a new crank, but having pulled it and discovered the end shells are 10 thou over, clearly that's second hand too. Next time I'll buy the bits and build it myself. It took him over 8 months to ship the engine to me as well.

What am I to do? After a year and a half of asking, I don't even have a receipt for the 8 grand I paid him for the engine.

OK, so I have the heads off and the pistons out - how on earth do you measure their volume?