Comp ratio and a query.....

[Dave81]

Afternoon all.

Well at the weekend the gods must have been smiling upon me, as the right place and the right time happened.

In the space of an hour the moons aligned and im now the proud owner of a set of Eddy Ali heads to go with the 318 im having built later this year. I have also been offered a complete stroker rotating assembly to suit my goals.
Scat forged 4" crank with Scat rods and Icon pistons at +30.

As if that wasnt enough a Comp cams XS274S with lifters is in as part of the offer.

Given the parts issue globally im pig in muck happy.......

When looking through some of the info though, I've believe that the Icon pistons are the flat tops @ +4.72.
Heads are 63cc.
Head gasket is based on felpro at 0.039in compressed thickness with a 4.18in bore.

So working through the figures and online calcs I believe I'll be at 11.45:1 Scr if the block is decked to 0.
The heads need a slight skim as they are used but this is just to make sure the surface is flat.

I've been looking at comp ratios for a few months prior to this and my head is telling me that this is almost race gas territory and will have issues even though I only run the Dart on super unleaded.
In short it won't like street use!

Looking for your input chaps.

[Dave81]

[Blue]

As no doubt you know it’s dynamic compression rather than static compression that causes detonation issues. It is possible to run higher static compression than that without detonation on pump fuel but cam profile, quench, combustion chamber shape and more come into play to make that possible. I tried to build a pump gas friendly 11.6 cr stroker with a fair bit of advice and failed, I couldn’t stop that thing rattling under power on anything less that Race fuel. This time around I’m on just over 10 and a bit and it seems fine so far even with timing locked out at 34. I was advised the power difference between high 9 and 11 to 1 compression really wasn’t worth the chance of detonation. If you didn’t zero deck the block, what would your compression be? My advice would be high 9 to low 10 as a static target, that should give you the leeway for an off the shelf cam grind to not cause detonation issues. You should be able to find an article David Vizard wrote about dynamic compression online, it makes interesting reading.

[Dave999]

http://members.uia.net/pkelley2/DynamicCR.html

mines 12:1
provided your pistons are nice flat and you smooth off any hard edges to the valve pockets
your heads are clean
and you have senible and good mixture timeing and distibution

you can make it work fine

i used pat kellys calulator to help me choose a cam that would work with my daft CR

i aimed at the top end 8.5 DCR which was naive but worked out ok

http://members.uia.net/pkelley2/DynamicCR.html

Mr Vizzard says much the same


i was stuck with a block that had been massacered by the previous owner who had taken a big chunk off the deck to get it to work with chevy pistons
it was howver bored and/or worn to just under the 20 thou over size

and i had a choice
cast pistons in 40 or 60 over for reasonable aussie cost but double once taxed and landed here and levaing the block bored 40 or 60 for no good reason
custom pistions in 40 or 60 over from SRP. performance unlimited in yorkshire are UK agents but the pattern is owned by an Aussie company so SRP needed me to go through the whole custom design so as not to get into contract issues , and performance unlimited wanated their cut naturally for help and asistance, lets just say the pistosn were going to cost the same as a bespoke aluminium head


so i used jeep /magnum 318 pistons at 20 thou over, the ones listed below. the bores were round so they honed out to hyperutectic size no bother only needed about 2 -3 thou clearance for coated pistons

they stuck up 30 thou above the deck (bit more than a 340 or a race hemi 6 but hey if the factory did it fine enough for me)

anyway i needed a profile to edge of chamber in 2 places, and 10 thou off piston crown in an 8 mm band around the edge and a 20 thou gasket, landed me at 12 and bit:1 piston ring to deck distance was still ok with the piston cut.

get a burette and some plastic to Cc yer heads

the right cam, the webers 1.72:1 ratio rockers and taking half the advance out of the dizzy 15 initial 28 total
and i have a perfectly drivable cool running pop pop crack crack trombone sounding motor. With the right driver who knew what they were doing it would do a mid13 . I lumbered it to a 13.9 and i guess there is probably more in it as i say. i drive it with mechanical sympathy for my transmission.

the cam is a "power comes on 2500 t0 6500 rpm cam" solid.
Hemi 6 cams are a bit odd standard profile on hemi 6 is the first or second performance profile for a 318 from most grinders dunno why

you also have option to advance or retard the cam as you see fit if you use a ROMAC or similar timing set, and you can use nice long collectors to bias the torque curve to be towards the lower end of the rpm range. mine join under the rear seat footwell which helps cater for the small loss below 2500 induced by the slightly performance cam.

flat tops flat with the deck KB or speed pro jobs a goodun

id be buying these
https://www.ebay.co.uk/itm/151825630273
pistons and rings for 150 + postage Say no more... you ain't seen me RIGHT... etc etc
But you already have the option on a really nice set of Icon pistons so i will say no more...

the gist of it is if i can turn a sows ear into something that hasn't gone bang yet and doesn't PING
you can take a decent block and a nice set of crank rods pistons and do good things with it

DCR is key

find an old PC and run mr kellys caluclator with your numbers and aim for a cam that gives you 8 - 8.5 :1 DCR
start with a standard hydrulic or soild and just keep changing the cam spec until the end calculation gives 8-8.5
once you have done that you have ballpark for lift duration centrline and the lobe speration angle and can choose your cam from wherever


Dave

[Blue]

Hughes list a whole bunch of cam grinds and you might find one that works if you wanted to go down that route.

[Dave999]

sounds perfect ....if they are free with the info you can just pop one in after the other and find a nice one

a tool can't replace experiance i guess, but it will sure help you avoid building something that won't work properly due to a mechanical/dimensional issue that you can't change easily, i.e an issuse that can't be tuned away

Dave

[Dave81]

Well using the online calculator It was telling me that the Dcr was 9.55:1 when basing it on the cam I have available to me.

Icon do other pistons which are dished at 23cc rather than the 4.72 above, a bit too much. This drops the Scr to 9.43:1 and Dcr to 7.9:1.

Two ends of the scale and I want to be in the middle...... need a pistons that is dished to circa 15 to put me mid 10s on Scr.

I think i've decided that the high comp pistons are a bit too lairy for what I want and I want a good safety margin of running it on super without issue.

Lots more research to do I feel.

This is the cam:
https://www.compcams.com/xtreme-energy- ... 3-360.html

[Blue]

That cam seems a bit mild for a stroker, the extra cubic inches will gobble that up. Hughes also list a good selection of stroker pistons, there’s likely to be something suitable. Is there a thicker head gasket available that would get you in the ballpark?

[Dave81]

I'm just looking at my maths. I have been stating 0 Deck at just that = 0.00.
Looking at most of the info they are more than likely 0.018 or 0.02 in the hole.
Most piston compression guides are set at around that I believe.

Running the same calculator with the 0.02 drops the 11.45:1 down to 10.93:1.
I've just started a spreadsheet of what I think I will need.....the numbers are silly already.
The only pistons i can find that will put me mid 10s on comp ration will cost $1k plus the ride and tax (Diamond).

I believe you used to be able to get a head gasket that would squish to 0.05 rather than 0.039.
That would drop the 10.93 down to 10.63. That was my aim @ mid 10s for a street/strip motor (75%/25%).

I believe I can get a cam that would bleed of some comp ration at the top end too.
Think a replacement cam would be cheaper than new pistons, but just a bit lost at the moment on it all!

I'm doing this once and want a 400+ horse 400+ ftlb motor that puts me in the 12s and will roast tyres on a Saturday and cruise the local villages on Sunday. Maybe a bit cake and eat it, but one can dream!

Out of interest what cam would you spec for what info i have given you above?
Other info: 3.55SG on 28" tall tyres on a uprated 904 with Cope manual valve body. Converter stall will come out of the rest of the motor, but thinking something in the 3000 to 3500 range. Converter is for another day and thread though!

Your input has been great thanks guys!!

[Pete]

I have not got as much experience on non-standard Small Blocks so I will leave it to the experts.

It is not a direct comparison, but I have run 12.2:1 compression on Pump gas in a Big Block.

This did not Ping if driven correctly - i.e.not slogged in high gear at low rpm. Ignition timing was pretty important too.

I think part of the secret of why it worked was the very large lift Cam with a very long duration (as mentioned above).

Once consideration is whether the your Pistons have large enough cut outs in them to take a high lift / long duration Camshaft.

This is a bit of a killer if starting from scratch, as you have to part build the engine to check the valve to piston clearance.
Often the EXH valve causes more issue than the inlet.

You need to really see 0.100" Clearance between the Piston and Valve at its nearest point in the cycle, which you establish using check springs.

The BB engine I had only showed 0.040" VTP clearance on initial mock up ,and I wanted to avoid having the cut outs enlarged as the Rotating Assembly had already been balanced.

The only solution was to re-time the cam to provide better clearance and it was still tight.
Fortunately, it did not hurt the performance or running of the engine, but it was a risk.

That was a 512ci Motor and powered a Dodge Dart to 9.7sec quarters and I ran it on the street.
Drank fuel like it was going out of fashion - which apparently it is....

[Blue]

I think you will be able to cam the engine to suit the pistons you have, especially if the static compression is in the tens. I don’t profess to be an engine builder and speccing cams isn’t my forte but here’s my thoughts. I’d go solid lifter rather than hydraulic, I’d use 1.6 ratio rocker arms, and you need to let it breathe to make the most of those extra inches so I’m thinking .550 lift at least. You could email Hughes with your spec for a cam recommendation for pump gas and plug that into your calculator. They are keen on a lot of lift so don’t be surprised by that. Plenty of good tech on their website too. I don’t think making decent torque will be an issue.

[Dave81]

Thanks gents.

Its all food information as I'm trying to learn about combinations and what can be done/will work for a street/strip car.

I'm binge watching Engine Masters and swatting on various written threads all over the Web. Solid lifter is what I want to build, so that's already in.
Need to see what is the max lift I can run with the Eddy heads. Will be getting the rockers from Hughes anyway, so we'll worth an email with what I have and see if they advise 1.5 or 1.6s and cam. Pretty sure the rotating assembly came from them anyway as the Icon kit, so I'd like to get it all put together using their stuff.

Interesting times ahead. I will update here or the workshop thread with where it all goes.......

[Dave999]

for too high DCR you can also retard the cam, inlet closes later therfore less to compress therfore DCR is smaller might help if trying to use whats offred with available/sensible $$ pistons

also you can use pistons from another brand. if the piston pin is bigger you need rods with small ends in the middle of the casting (some are not) and they can be honed out in some cases
if the pin is smaller you can get em bushed search imperial and metric pistons on sumitt for About the right bore size and exactly the compression height you want to fit with your stroke

new pistons will need a bore and that bore doesn't need to be a standard chrysler oversize. ford or chevy may come up with the goods with a piston just a bit bigger than 3.91 but with the right Compression height. valve cut outs will not be in the middle though

Dave

[Dave81]

Thanks Dave,

I've asked Hughes the question over Cam choice for the rotating assembly I currently have.

The cam available gives me 11.45:1 with a DCR of 9.55:1.
Need to look at how little I can shave the head rather than have it 0 decked.

If I can cut it so the pistons are 0.04" in the hole then it goes down to 10.46:1 Scr and 8.74:1 Dcr.
I could then look at changing the cam to get it below 8.5:1 Dcr. My understanding is that should run on Super (97 RON) okay.

I believe comp cams specify the intake closing at 63. From what I've read they spec it @ 0.006 rather than @ .050 like other manufacturers.
Found an online tool and that equates to 44 @ .050.
Obviously if I go for it I will check with them that this is the case.

Think I can make it work, lots more reading to do before I spend any more money.

[Pete]

Personally I think you are being too cautious, but then again I set everything to "11"....