Quench Distance Question

Alan454 · 09-07-2009, 07:22 PM

I am totally rebuilding a 454 and trying to select some pistons and set the static and dynamic compression ratios (SCR & DCR).

One of the many parameters to set is the quench distance. I have been reading many very informative threads here that talk about the ideal quench distance being 0.040", mainly for detonation resistance which I am very interested in. However, all the head gaskets that I seem to find for my gen IV engine seem to be 0.039" (or higher). To get a quench distance of 0.040" I would therefore need a piston to deck height of only 0.001" - nothing essentially. Many piston manufactures seem to suggest a piston to deck height of 0.020" though which would give a quench of 0.059" - what gives? I am a bit worried about just setting the deck height to zero. (this would also require custom pistons).

Also, I plan on using 91 pump gas, but occasionally can only get 89 and of course if the boat isn't used for a month that 91 octane will slip. I have Edelbrock 61459 110cc Al heads and am planning to use a Comp XR270HR cam unless the overlap (49deg) is too much for my stock Bravo I exhaust system do to reversion problems. This cam requires a healthy 9.9:1 SCR to achieve a DCR of 8:1. Anybody have a DCR recommendation for me?

Thanks for any help or suggestions!

Alan

blue thunder · 09-07-2009, 08:17 PM

A 10:1 scr would be a good choice.

Find a dealer for cometic head gaskets. You can get them in .001" increments. My pistons are .010 in the hole and I get .027 gaskets. Gives a .037" quench. You are right for getting the quench where it should be on a marine engine. There is no substitution for a well built engine in a marine application.

BT

PatriYacht · 09-08-2009, 08:40 AM

I think it's best to leave the pistons .005 down in the hole in case you need to true up the decks again in a future rebuild. That leaves you at about .045 with Felpro blue teflon head gaskets. Cometics are great but I wouldn't spend the extra money on a N/A build.

vette131 · 09-08-2009, 09:26 AM

i have the .027 cometics great gaskets & a valuable tool to alter compression & quench

Alan454 · 09-08-2009, 08:44 PM

Thanks guys!

Good point on leaving a bit for a future cleanup if I go that route. I did a quick search on those Cometics and appear to be what I need if I can't get the block decked to the right height for those felpros. They are about 2X the price, but in the grand scheme of this rebuild its in the mud ... well I guess $100 saved is $100 earned. Did read that some have trouble with water seal though on cometics.

Does anybody think I will have a reversion problem with 49 deg overlap and stock wet exhaust? I found some neat stock merc cam info on an old post in this forum from Dennis Moore and was able to calculate my stock overlap at 23 deg.

Alan

PatriYacht · 09-09-2009, 10:03 AM

What is the exhaust duration at .050 lift. You will start having trouble at about .230 duration @ .050 lift.

Alan454 · 09-09-2009, 11:26 PM

Exhaust duration @050 is 224 deg so I have about 6 deg margin.

I thought that it is the time that both valves are simultaneously open at the transition from exhaust to intake stroke (overlap) that is the major driver though. Another factor seems like it should be the exhaust valve closing point which on this particular cam is 24 deg ATDC. In other words, the piston is sucking in (not blowing out) though the exhaust valve for 24 deg. From what I understand the momentum of exhaust gas keeps things flowing in the right direction at "higher" RPM. Not quite sure what qualifies as "higher" though. With the intake valve also open you would have the manifold vacuum contributing to the problem.

This problem seems particularly insidious because, unlike detonation say which you would know about immediately, reversion might go noticed for years until severe corrosion occurred at which point it would be too late ...

cubicinches · 09-10-2009, 12:29 AM

Quote:

Originally Posted by Alan454

Exhaust duration @050 is 224 deg so I have about 6 deg margin.

I thought that it is the time that both valves are simultaneously open at the transition from exhaust to intake stroke (overlap) that is the major driver though. Another factor seems like it should be the exhaust valve closing point which on this particular cam is 24 deg ATDC. In other words, the piston is sucking in (not blowing out) though the exhaust valve for 24 deg. From what I understand the momentum of exhaust gas keeps things flowing in the right direction at "higher" RPM. Not quite sure what qualifies as "higher" though. With the intake valve also open you would have the manifold vacuum contributing to the problem.

This problem seems particularly insidious because, unlike detonation say which you would know about immediately, reversion might go noticed for years until severe corrosion occurred at which point it would be too late ...

You're actually correct. Alot of people tend to put a limit on .050 exhaust duration as if it's the end all in the reversion equation. Many factors come into play which determine whether an engine will revert water or not. Exhaust duration only becomes a factor, as it pertains to overlap. Displacement, stroke, rod length... just to name a few, all play an important part. I just finished two 557's, 255 at .050 on the exhaust with CMI headers and wet tails... no issues.

As far as the gaskets, I've seen the water issues with the Cometics. I'd use a .041 Fel Pro MLS, leave the piston a couple thousandths in the hole, and don't lose too much sleep over the total quench.

My .02, as they say around here.

PatriYacht · 09-10-2009, 09:39 AM

It's true that there are a number of factors in play. I was trying to give a rule of thumb that was simple for your manifolds. Your risers are very short so you have to be conservative. Lightning headers have a slightly longer riser and more of a downward slope and they recommend 240 as a limit for their wet system. However, they say that is only a guideline. http://www.boatheaders.com/reversion.htm

cubicinches · 09-10-2009, 09:56 AM

Quote:

Originally Posted by PatriYacht

It's true that there are a number of factors in play. I was trying to give a rule of thumb that was simple for your manifolds. Your risers are very short so you have to be conservative. Lightning headers have a slightly longer riser and more of a downward slope and they recommend 240 as a limit for their wet system. However, they say that is only a guideline. http://www.boatheaders.com/reversion.htm

I see now that he's using stock manifolds... definitely have to be more conservative while using them.

Good link to the Lightning site, they explain it much better than most. I think all header manufacturers give very conservative guidelines, and probably rightly so... they don't want to be held responsible when your engine is choking down water through the headers.