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1.70 to 1.80 rockers

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I have a BBC with 1.70 rockers? Jesel Sportsman. How much reliability am I going to sacrifice going to 1.80 ratio. I am running a hyd roller with I/E 242/249 @ .635//647

The reason for change is I increased displacement from 496 to 557 and believe it needs more cam to compensate for the increase in displacement. Due to wet boat exhaust I can't really replace cam with more duration as it would likely increase overlap and engine screwing exhaust reversion, currently at only 13 deg.
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Depends entirely on the current spring, the clearances and the weight of the valve train.
Assuming all good parts and the heads have the room, AND the heads can support the flow, I'd do it.
 

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There in lies some of the rub of increasing lift via higher ratio rockers.

While they get the valve off its seat at a faster and larger amount per degree of crank rotation which can be helpful as the rev’s get high enough to utilize that which will be a function of mixture velocity of the intake tract which is RPM dependent, the next and larger issue is whether the port can support sufficient increase in flow volume at the higher lift. Here your going to have to hunt down the spec’s for the head in question to determine whether that offers a large enough flow gain to bother with.

Th the greatest extent going to higher ratio rockers provides the most gain when teamed with a cam that offers a slow rate of lift rate combined with a modest peak lift, that being cams more like those of the 1960’s. Modern short ramp, fast lift rate between .050 points and high peak lift tend not to be as enhanced by bigger ratio rockers that those more classic cams.

So whether the result is worth the pursuit takes some examination.

Bogie
 

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Another thought on Bogies ideas.
I'd discuss this with a cam designer but I would think some magic could happen with this by possibly by using inverted lobes to reduce the overlap and picking up the curve mid point on up.
 

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Discussion Starter · #7 ·
According to the flow bench

the baseline for the Dart iron heads shows: the intake flow increases with lift thru .700 and then drops off -4cfm @ .750 to [email protected]
the exhaust flow increases with lift thru .900

with slight port clean up: the intake flow increases about 12 %/0.1 step over raw thru .700 and then drops off -8cfm and is flat thru .900
the exhaust flow increases about 8-10%/0.1 step over raw thru .900

More port development to go after bronze guides installed - if they ever arrive!!!

My valvetrain is Lunati 74855 BH springs good for 0.700 180# seated, 408#/in. so I should be good. TS retainers, Manley extreme and marine exhaust not so light. I did what I could.
 

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That doesn't really tell me much.
You're currently at .647, and talking about upping that .038.
The vague info you have, speaks of .700-.900 and is irrelevant. I'd rather see hard numbers.
If you have the .038 worth of room, it's probably fine. Check it anyway......
Might wanna check with the spring company about it and ask some questions about expected spring surge.
 

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Discussion Starter · #9 ·
That doesn't really tell me much.
You're currently at .647, and talking about upping that .038.
The vague info you have, speaks of .700-.900 and is irrelevant. I'd rather see hard numbers.
If you have the .038 worth of room, it's probably fine. Check it anyway......
Might wanna check with the spring company about it and ask some questions about expected spring surge.
Yes that is what the rocker change would do. Im only considering it as the rockers are new and Jesel has been good about exchanging parts n\c in the past so why not. I will attempt to attach my spreadsheet with the initial and mild cleanup numbers, I think it shows promise.
 

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will the heads flow more at more lift?
That doesn't really matter, just as long as the flow curve doesn't fall off badly and start backing up on flow above the peak flow lift point.
The bigger rocker ratio increases the entire area under the curve.

Johnsongrass1 indicated why in post #4
Weedy,
What lifters are you using??
 

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That doesn't really matter, just as long as the flow curve doesn't fall off badly and start backing up on flow above the peak flow lift point.
The bigger rocker ratio increases the entire area under the curve.

Johnsongrass1 indicated why in post #4
Weedy,
What lifters are you using??
in the original post there is no head specified. it matters.
a 550 inch motor probably needs heads that flow at least 400 to get close to optimum output. if the rockers are free then go ahead and trade them.
 

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Yes that is what the rocker change would do. Im only considering it as the rockers are new and Jesel has been good about exchanging parts n\c in the past so why not. I will attempt to attach my spreadsheet with the initial and mild cleanup numbers, I think it shows promise.
Only 4 CFM more on the intake between .650 and .700 so another .038 prolly not going to help much there. The 1.8 help even more on the exhaust side though. Im not sure how that translates to a boat. Usually when my exhaust gets wet, it's a way bigger deal if you know what I mean. Gurgle...gurgle...
Personally, Jesel is good stuff and if free or close to it, I'd do it, but if you have to buy them, a cam change might be a better investment depending on what your trying to accomplish.
 

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Discussion Starter · #14 ·
I'm dense...gurgle gurgle like it's sinking?

My heads are raw Dart Iron eagles for a Gen7 8.1L. not real common, there are only 3-4 head options for this marine/truck engine. Most 8.1s are not hot rodded, their pistons suck. Mine is an 8.8 stroker with non-oem everything. On my heads the intakes are at 378ish now with minimal re-work . The head porter I have is willing to help me with iron heads (lots won't) and confident of easily over 400 with a decent profiled valve job so I am down with that. If 1.8's help and low cost I will go with it as well.

My cam is a Raylar max effort unit made for wet exhaust inboards, in their 8.8 stroker kit, used on non-oem piston 496's, tried and true. They have other cams for stock blocks (trucks, motorhomes, etc) They have some for forced induction but are limited to dry exhaust where overlap is no issue.
 

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Discussion Starter · #15 ·
I went ahead with the exchange of rockers to the 1.8 ratio.

I am looking to run mechanical roller lifters as its a toy not a DD

Currently my springs are about 180 seated/ 470 open

As an example

The spring specs for some solid rollers 7500rpm
Seat Pressure (Lbs):150-275
Open Pressure (Lbs):400-700

others (racier type) are 8000rpm
Seat Pressure (Lbs):250-350
Open Pressure (Lbs):600-850


As it could come down to price/avail....
Is it just the rpm rating that ups the spring pressure requirements. am I good with either since I am on the lower pressure end? or do the racier type need the higher pressure to run properly even if I remain well under the 7500rpm level.

Ideally I would like needless roller bearings with pressure oiled pins for endurance reasons.
 

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Really the seat pressure is more important and to an extent the open pressure is a function of that. The big headache is getting the valve to close without bouncing. The next is getting it to track the cam lobe on the backside, the third is not to loose track over the top of the lobe where unwanted lofting might occur from front of the lobe acceleration, that’s not to say that lofting isn’t desirable in some situations.

High lift rocker ratios add lift and acceleration to that lift so to some extent there can be power added to the engine by enhancing breathing sooner in the lift cycle. The other component is adding over the lobe top max lift. Here this is a function of whether the port gains enough added flow for the added lift to be worth while pursuing.

Increasing the lift over the top is harder on all the components so it there isn’t any benefit to the additional lift if the port is unable to substantially increase flow to feed that additional lift. Then your money needs to include improving the ports or replacing the heads for something that shows significant flow increases at the lift you’re chasing. Short of that the better choice is a cam with a more aggressive lobe rate of lift rather than adding more lift over the top of the lobe. So project number one is to know what your heads flow to valve lift profile looks like. The other is put your money down and experiment. But keep in minds that most heads show a great reduction in the rate of flow increase as the lift gets toward or beyond .5 inch, so I go back to your should have a lift to flow map for the heads you’re using before going further.

Bogie

Bogie
 
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