Streetable compression question

Streetable compression question

Joined: January 30th, 2009, 5:17 pm

July 7th, 2011, 2:55 am #1

I have a 351c, the block has been 0 decked and flat top SRP pistons with -3cc dome volume. I am thinking about putting on Trick Flow heads that have 62cc chambers. Currently I am running a COMP cams Hydraulic Roller 236/240 at .050. My question is will this combination have too much compression to run on pump gas (91)?
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Joined: September 5th, 2006, 1:33 am

July 7th, 2011, 3:06 am #2



What's the advertised duration, and what's the intake closing point in degrees after bottom dead center @.050 lift? With that much .050 duration I bet you will be alright, but never hurts to check as close as you can.

A Visigoth explains the sack of Rome: We wanted art, literature...culture! We're sorry we broke some stuff.
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Joined: August 16th, 2005, 1:46 pm

July 7th, 2011, 6:27 am #3

I have a 351c, the block has been 0 decked and flat top SRP pistons with -3cc dome volume. I am thinking about putting on Trick Flow heads that have 62cc chambers. Currently I am running a COMP cams Hydraulic Roller 236/240 at .050. My question is will this combination have too much compression to run on pump gas (91)?
http://www.network54.com/Forum/215655/t ... n+UK+Cobra

Follow the link. New engine specs half way down the thread.

I am running SRP's with 3cc reliefs and the Trick Flows. I have no problem running regular unleaded (UK) but for how much it costs and gets used I tend to stick Super unleaded in most of the time. Your cam has slightly more duration. I don't see the full cam specs but would imagine that the cam you are looking at will have a later IVC point so should in theory have less dynamic compression.

I would say it'll be fine
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Joined: November 25th, 2006, 4:54 am

July 9th, 2011, 10:29 am #4

I have a 351c, the block has been 0 decked and flat top SRP pistons with -3cc dome volume. I am thinking about putting on Trick Flow heads that have 62cc chambers. Currently I am running a COMP cams Hydraulic Roller 236/240 at .050. My question is will this combination have too much compression to run on pump gas (91)?
Hi,

Here's a website where you can calculate the compression ratio of your setup.....there's a few out there.

http://www.kb-silvolite.com/calc.php?action=comp

Few extra bits of info you need:
I assumed 30 thou over bore, ie: 4.030
Gaskit thickness: 0.040
Gaskit bore: 4.100
Engine Deck: 0....have you had it zero decked?

Just want to confirm you have the dome pistons and not the standard flat top with a 3cc valve relief? You described it right with -3 as being a dome and just 3 is a flat top with valve reliefs.

With the pop up piston it shows 11.8:1 and with a flat top with piston with 3cc valve reliefs it's 10.9:1.

The alloy heads on these clevos can get rid of the head much better than the cast iron ones and can take more compression. The chambers are also more efficient in design and you'll find you won't need as much timing to get your peak power. eg: good set of CHIs will make power at about 28-30 whereas some cast iron heads will need more at about 35 degrees total.....just generally speaking.

Is 91 the highest octane pump fuel you guys have...or do you have up to 95? can't remember! If you're at 11.8 I think you'll be on the limit with that cam.

You also need to look at how heavy your car is and what your rear diff gear is and stall converter rpm. If you're running 2.9's, 3500lbs and a 2500 stall, it'll kill ya!
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Joined: May 20th, 2005, 4:48 pm

July 9th, 2011, 12:23 pm #5

I have a 351c, the block has been 0 decked and flat top SRP pistons with -3cc dome volume. I am thinking about putting on Trick Flow heads that have 62cc chambers. Currently I am running a COMP cams Hydraulic Roller 236/240 at .050. My question is will this combination have too much compression to run on pump gas (91)?
A negative volume would be a dome piston.

I looked in the lobe catalog and saw that there were a couple of 236° hydraulic roller lobes. One was at 286° advertised and one was at 288° advertised. I used the smaller one for my calculations.

With a 3cc valve relief, the SCR would be at 11:1 and the DCR would be over 8.4:1.

With a domed piston, the SCR would be at 11.8 and the DCR doesn't matter at that point.

An 8.4 DCR would require everything to be right in the world to run on 93 octane. No hot spots on the pistons, perfect quench, etc.

Knowing that the heads usually cc a little more than advertised and the cams have a little more "true" duration to them seat to seat, I think you could get away with it if you play the rest of your cards right.

Brent Lykins
B2 Motorsports, LLC






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Joined: October 1st, 2004, 8:05 pm

July 10th, 2011, 3:24 am #6

I have a 351c, the block has been 0 decked and flat top SRP pistons with -3cc dome volume. I am thinking about putting on Trick Flow heads that have 62cc chambers. Currently I am running a COMP cams Hydraulic Roller 236/240 at .050. My question is will this combination have too much compression to run on pump gas (91)?
how much compression any motor can tolerate depends upon the fuel octane, the combustion chamber design and camshaft timing.

Lets cover these things one at a time:

Fuel Octane: Most of the world rates gasoline octane based upon the research octane number (RON). The US and Canada use a number that is the average of the RON number and another testing procedure called motor octane number (MON). The MON test is a more difficult test, the MON octane rating will be 8 to 10 points lower than a RON rating for the same gasoline. Since the US and Canada use the average of the two (RON+MON)/2 our octane numbers tend to run 4 to 5 points lower than the rest of the world for the same gasoline. Gasoline rated 91, 95 or 98 octane in Europe or Australia are equivalent to gasoline rated 87, 91 or 93 octane in the US or Canada, respectively. Our 91 equals their 95!

Combustion Chamber Design: The iron 4V heads are fairly detonation resistant, much moreso that wedge designs like the SBF (Weezer) or SBC, and they are better with gasoline than hemi heads like the hemi Chrysler (which does well with alcohol). Folks don't realize that the combustion chamber design was just as important in the Cleveland head design as the intake port, it was not an after thought, it was in fact quite advanced for its day. I've always found 8.0:1 dynamic compression to be a safe limit for iron Cleveland heads and premium pump gas available here in southern California (91 or 92 octane).

Modern high swirl combustion chambers (like those found on the Trick Flow heads) combine elements of Harry Weslake's combustion chambers with Cleveland combustion chamber geometry, the result is an even more efficient combustion chamber. A combination of a better combustion chamber combined with alloy construction enables those heads to tolerate at least at 1/2 point more dynamic compression than the iron Cleveland heads, i.e. 8.5:1.

Camshaft Timing : Dynamic compression is supposed to more accurately represent how much the mixture within the cylindervof an operating motor is compressed. Dynamic compression is calculated using the SEATED intake valve closing spec, in degrees after bottom dead center. Hydraulic cam events based on duration at 0.004" tappet lift are "close enough" to use in computing dynamic compression. 0.004" tappet lift is reasonably close to how much a hydraulic lifter collapses in operation. The problem is the advertised duration of some hydraulic cams is based on 0.004" tappet lift, but others are based on 0.006" tappet lift. When we are comparing cams, or computing dynamic compression, we must be careful to notice what spec the advertised duration is based upon, because it makes a big difference. Although it doesn't sound like much difference, specs based on 0.006" tappet lift will have much shorter duration figures, and will not give accurate results when computing dynamic compression.

If somebody has only the camshaft duration figures they cannot accurately compute dynamic compression, they have to assume too much camshaft data regarding to cam timing. On top of that, duration at 0.050" tappet lift is useless for computing dynamic compression ... not enough information ... we need to know when the intake valve hits the valve seat.

After all is said and done, keep in mind if you push the limit of dynamic compression, the compression ratio needs to be consistant among all 8 cylinders, i.e. the decks must be very level, the combustion chambers cc'd, etc. If these things aren't done, then it will be prudent to back off a little in the compression to allow for a margin of error.


-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
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Joined: November 25th, 2006, 3:22 pm

July 10th, 2011, 12:25 pm #7

vol/ 45%effective efficiency(6*) of overlap~det.prevention/ blah blah blah. Sometimes I feel like I was left behind somewhere lol.
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Joined: November 25th, 2006, 3:22 pm

July 10th, 2011, 12:39 pm #8

design is/was so well engineered I thought about trying to build a efficiency/mpg motor to see how well it would work. I just don't know if I would use a stock stroke cleve or stroke it out. I would use auz 2v cc heads and this Offy duel port intake that I have been trying to sell.
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Joined: July 12th, 2008, 1:55 pm

July 10th, 2011, 1:38 pm #9

how much compression any motor can tolerate depends upon the fuel octane, the combustion chamber design and camshaft timing.

Lets cover these things one at a time:

Fuel Octane: Most of the world rates gasoline octane based upon the research octane number (RON). The US and Canada use a number that is the average of the RON number and another testing procedure called motor octane number (MON). The MON test is a more difficult test, the MON octane rating will be 8 to 10 points lower than a RON rating for the same gasoline. Since the US and Canada use the average of the two (RON+MON)/2 our octane numbers tend to run 4 to 5 points lower than the rest of the world for the same gasoline. Gasoline rated 91, 95 or 98 octane in Europe or Australia are equivalent to gasoline rated 87, 91 or 93 octane in the US or Canada, respectively. Our 91 equals their 95!

Combustion Chamber Design: The iron 4V heads are fairly detonation resistant, much moreso that wedge designs like the SBF (Weezer) or SBC, and they are better with gasoline than hemi heads like the hemi Chrysler (which does well with alcohol). Folks don't realize that the combustion chamber design was just as important in the Cleveland head design as the intake port, it was not an after thought, it was in fact quite advanced for its day. I've always found 8.0:1 dynamic compression to be a safe limit for iron Cleveland heads and premium pump gas available here in southern California (91 or 92 octane).

Modern high swirl combustion chambers (like those found on the Trick Flow heads) combine elements of Harry Weslake's combustion chambers with Cleveland combustion chamber geometry, the result is an even more efficient combustion chamber. A combination of a better combustion chamber combined with alloy construction enables those heads to tolerate at least at 1/2 point more dynamic compression than the iron Cleveland heads, i.e. 8.5:1.

Camshaft Timing : Dynamic compression is supposed to more accurately represent how much the mixture within the cylindervof an operating motor is compressed. Dynamic compression is calculated using the SEATED intake valve closing spec, in degrees after bottom dead center. Hydraulic cam events based on duration at 0.004" tappet lift are "close enough" to use in computing dynamic compression. 0.004" tappet lift is reasonably close to how much a hydraulic lifter collapses in operation. The problem is the advertised duration of some hydraulic cams is based on 0.004" tappet lift, but others are based on 0.006" tappet lift. When we are comparing cams, or computing dynamic compression, we must be careful to notice what spec the advertised duration is based upon, because it makes a big difference. Although it doesn't sound like much difference, specs based on 0.006" tappet lift will have much shorter duration figures, and will not give accurate results when computing dynamic compression.

If somebody has only the camshaft duration figures they cannot accurately compute dynamic compression, they have to assume too much camshaft data regarding to cam timing. On top of that, duration at 0.050" tappet lift is useless for computing dynamic compression ... not enough information ... we need to know when the intake valve hits the valve seat.

After all is said and done, keep in mind if you push the limit of dynamic compression, the compression ratio needs to be consistant among all 8 cylinders, i.e. the decks must be very level, the combustion chambers cc'd, etc. If these things aren't done, then it will be prudent to back off a little in the compression to allow for a margin of error.


-G
____________________________________________________________

Pantera Photos | 351C Historic Information | 351C Technical Information

If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
They have always made me wonder how well they work these dynamic compression Calculators.

Regardless of when the inlet valve closes, wouldn't and engine with a brilliant flowing Head and manifold design, make a lot more dynamic compression than an engine with crappy heads and manifold and the same closing point?

Cheers.
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