skinny z

What kind of compression ratio have you built into your iron headed engine?

The camshaft specs are every bit as important as the actual static CR heretoo.

If you have an example, include as much info about the cam as you can(unless you've got secrets to keep!). Hydraulic, solid, roller, adv and.050" duration, lift, ICL, LSA would be helpful.

What I'm trying to get at here is whether or not I've overdone it with mycompression ratio.

Last year’s spec was Vortec headed with an XR276HR cam. The SCR was spot on10:1. Although I needed premium fuel, detonation was not issue. I kept an eyeon the timing curve, kept it cool and had good AFRs.

This year’s engine saw the SCR bumped up to 10.37:1 with a RHS Vortec ironhead and a cam with similar timing specs to the 276 albeit a custom grind withmore lift.

I’m struggling with detonation especially on the initial hit from a standingstart. There’s a bit of carb tuning still to be done however I think I may haveoverdone it compression wise. I was trying to achieve a .040” quench and indoing so I used a .026” head gasket combined with the .014” piston below deckheight.

If I can’t get this sorted out with tuning, (which I’m beginning to doubt),I may switch to a more standard .040” gasket and get the compression to a moremanageable level.

Any insights from builders are appreciated.

If anyone wants detailed specs to run SCR and DCR numbers that’s not aproblem.

JesasaurusRex

iTrader: (3)

9:1 dynamic with good cooling and you'll be fine. Do the math yourself, there are plenty of calculators.

skinny z

I've done the math. What I'd like to know is what others are running and what they're finding the limits are.
My DCR (using Pat Kelly's calculator) is almost 8.4:1. Research says that's approaching the limit for pump gas which arguably falls in the 7.5 to 8.5 range.
If I can make 8.4:1 work, then I'll continue with my timing and fuel tuning. If not (based somewhat on what other peoples experiences are) I'll either lower the SCR or get a cam with later intake valve closing (and generally bigger all around).

9:1 ? What calculator do you use?

Atilla the Fun

Most importantly, what's your cranking compression?

Jonesyfxr

iTrader: (2)

We built my brother in law a 461 BBC with ported 781 cast iron heads. The cam is a hydraulic roller with a victor jr intake, 850 quick fuel stainless valves and roller rockers. The CR is 12.1. We have to mix C12 with 91 octane ethanol free to make 100 octane. The car is an animal with no detonation issues at all. Although we are going to be running a Now Performance water/ meth injection shortly.

skinny z

190 psi (+/- 3%). 600' elevation.
That's were I'm confused. I should be able to run this engine combination on premium pump gas with little trouble. The heads a very efficient and the quench is spot on. Yet, I have intense spark knock at various points in the engine load/rpm range graph. I solved some of that by working on the fuel curve (PV tuning) however it's still going to kill this engine unless I sort it out or drop the compression. The timing curve is another issue as well. I may drop the timing by 5 degrees across the board and then tune again from there.

I'd still like to hear more from others and their combinations.

skinny z

Any cam specs?

What he said.

JesasaurusRex

iTrader: (3)

I've run 9.5:1 on 91 with aluminum heads no prob. I think you can get away with 9:1 on iron heads on 93 with good cooling and lax on the timing. I'd use a .015 gasket if I were you, doubt with that compression/cam that you'll have any piston to valve clearance issues.

skinny z

I'm at 10:37:1 with a .026 gasket already. I may switch to a thicker .040" gasket in an effort to bring the CR down to around 10:1.
My cylinder head guy builds a lot of Vortec headed circle track engines and 9:1 is the target compression ratio. He uses a .026" gasket in his build but that also means a dished piston in a 350 application. These are sealed claimer engines.

JesasaurusRex

iTrader: (3)

I'd go with a tighter quench and get a bigger cam

Orr89RocZ

iTrader: (20)

I'd like to think the more volumetric efficient the engine is, the more detonation prone it will be given a set compression ratio.

Dynamic ratio really isnt dynamic. Its an adjusted static based on when actual valves are closed versus piston position in its compresson stroke. You calculated the volumes available at that point but on a real motor, you could be seeing intake ramming effects improving cylinder fill just before valve closes. This gives you a chance at over 100% ve. Any improvement in ve could lead to higher chamber pressures so it very well could be more sensitive to fuel type.

If it was an efi car you easily adjust fuel and spark at those specific points

You can try lowering cranking comp by moving cam position around. Delay intake closing. 10.37 scr and 8.4 dcr could be borderline if its moving air well. Other than colder plugs and really softening the tune, a bigger cam could fit the combo or drop comp abit more. Just my opinion

skinny z

Interesting point regarding the improved efficiency and higher cylinder pressures. That's probably what I've managed to accomplish with the new heads and higher lift cam (compared to the stock Vortecs).
I've thought about retarding the cam too. I have the Cloyes Hex-Adjust timing chain set so the change is relatively simple. The heads are coming off this winter anyway (pushrod hole clearancing) so everything's clear to do that should I decide to. It would kill some of the low engine speed torque I deliberately built into this combination though.
A bigger cam is the other option I mentioned earlier too I just don't like the idea of putting this nearly new custom grind on the shelf.
That or the thicker head gaskets.
I've got quite a bit of adjustability in my timing though not of the EFI variety. Same with the carb which is a modified Barry Grant VS.
I'm going to make a wholesale drop in spark advance of about 5 degrees across the board and see where that gets me.

skinny z

Thinking about that too.

Orr89RocZ

iTrader: (20)

With heads off, what you think about grinding the chambers? Perhaps alittle valve unshrouding and some work around the plug location? Perhaps cut a few cc's out of it and lower compression that way?

Or even experiment with a Singh groove?

skinny z

I've thought about some chamber work, particularly around the spark plug boss. There looks to be some material that could be removed from there with some benefit too. As far as valve unshrouding goes, that was done prior to reinstalling the heads after I had wiped out the guides because of poor valve train geometry (which by the way appears to remedied).

A Singh Groove?...I had to look that one up. Interesting concept. Patented too. I'm not sure how much that might contribute to increasing the chamber volume. It's difficult to assess the volume of the grooves from his patent application drawings.


Swapping to thicker head gasket (from .026" to .040") gains about 3 cc.
CR goes from 10.37 to 10.02.
Quench goes from .040" to .054".

Orr89RocZ

iTrader: (20)

Search speedtalk on those grooves. I think automotive breath was the user name who did some experimentation. I believe it helps with combustion efficiency and promotes complete fast burns, and somehow seemed to fight detonation. Its been awhile since i read about them

Jonesyfxr

iTrader: (2)

[QUOTE=skinny z;5635096]Any cam specs?

Sorry, no cam specs. My brother in law won't even tell me the specs, because we will be racing in the same class. All I know is the cam is a custom ground dual profile Bullet cam.


As for the OP's question.....

I was thinking about this. You said it only does this at the hit? Before you trace down everything else, why not put in a bigger accelerator pump? Maybe you just need more fuel upon throttle application? Hey, it's worth a shot?

skinny z

Not just at the hit but it's most pronounced there.
Until I worked on the PV circuit, it would rattle quite a bit under part throttle acceleration too. I could see the AFRs dip into the 15s. With the new valve, I can see 11:1 on my AFR gauge and there doesn't seem to be pinging. Much too rich I know but I can live with that for now.
I'll be working on the fuel curve including the accelerator pump once I dial my timing back a few degrees. The detonation is so intense (although brief) that I'm afraid to do any further testing until I tame the tune a little . I don't want to bust a piston in the process.

Orr89RocZ

iTrader: (20)

Back timing out and work the fuel first. Then add timing back in to see what it will like most

Then see if it still pings around.

skinny z

That's the plan at this moment.
I'll be able to answer my own question about compression ratio limits when I'm done.
Still would like to see some hard numbers from another build. Cranking pressures, calculated DCR, type of head, etc, just to get a feel for whether I'm banging my head against the wall here. If I have to run this thing with 25 degrees total timing, then it's really a poor build and I would make the mechanical changes needed to bring it in-line.

FRMULA88

Not a poor build if it makes same power with LESS timing and not knocking.

however if power loss is substantial due to decreased timing I would be more inclined to use a better fuel (more octane) use premium and blend it with 100 octane unleaded. 5 gallons of 92 + 5 gallon of 100 = 96 octane. R+M/2

OR depending on your location, switch to E85 which is 94 to 98 octane
R+M/2 depending the time of year. This is a bit more octane than premium gas and more economical than "race gas".

I would not use octane booster that claim raising octane 5 points... there are 10 octane pts. per octane number so 92 plus a typical 5 point booster = 92.5 = total waste money.

the best octane booster is race gas, and 100 octane unleaded should be easy to get.

FRMULA88

before going too far... try backing off timing a bit, jet up a bit or both.

A bit more jet will not hurt idle quality but you will have to fine tune that.

The engine may just want more fuel when you stab the throttle.

A SBC should not need more than 34 -36 degrees of total mech. timing at WOT.

Vacuum advance is not a factor at WOT so I never include vacuum advance when I talk about timing.

if you can't make it stop knocking after fine tuning fuel and timing I would be more inclined to increase the octane of the fuel not by much..

Atilla the Fun

Have you tried pouring in a bottle of Super 104+ and if so, did it help? Are you running platinum plugs? If so, replace them with de-burred conventional plugs. A woman's emery board for her fingernails works well for that. Still no good? Try retarding the cam 4 degrees before trying retarded spark timing. You'll lose cranking compression, but because the exhaust valves open later you won't lose torque at idle. It'll just pull another couple hundred RPM higher if you have the parts to support it. But you will lose a little torque in the lower midrange, which should help this problem. I forget if you're running stick or auto, but if auto, a looser converter will help. Otherwise more rear gear would help. Either way you reduce the loading on the engine.

FRMULA88

you did not mention jet sizes,

Rule of thumb the secondary jets should be 8-10 sizes larger than the primaries (assuming you have power valve on the primary side).

so 64s on primary means 74 on the secondary

when you jet up go 2 -3 jet sizes min. and do both the primary and secondary.

so if you jet up to 67s the secondaries should be 77s

good luck.

JesasaurusRex

iTrader: (3)

can always go bigger on the gasket and squeeze the f*ck out of it , problem solved

skinny z

I'd like to try a higher octane fuel for sure.
Not sure how the octane boosters work because my last experiment a few ago (trying to improve on some 91 whereas I always use 94) didn't yield anything worth trying a repeat performance.
One of the local used to sell race gas however I have no idea if it's still available.
I have no catalytic convertors so leaded fuel shouldn't hurt although my O2 sensor won't like it for long.

skinny z

Not as of yet. My last experience with an octance booster didn't do a thing for me . That was a few years back. No. Either Champion RS12YC or AC Delco R44LTS conventional resistor plug. Never had a problem with plugs although with this latest round of abuse I'm putting the engine through, I may not have ANY plugs left to examine.The timing is easier to do but I'm keeping the idea of retarding the cam timing in reserve. That'll be an off season project. something I'll do when the heads are off the engine for a little machine work.

My convertor flashes to 4000 on the initial hit although I can only foot brake to 2000-2200 before it starts to creep.
It'll rattle from a standing start or when crusing at something like 30 mph and then mash it. It's absurd the amount of noise it makes.

skinny z

[quote=FRMULA88;5635356]you did not mention jet sizes, [quote]

I'm working my way through the carb and ignition tune at the moment.
My AFR gauge indicates a fairly rich WOT (12:1) and my part throttle cruise is mid to high 15s at 60 mph and a little richer at 75 mph (mid 14s).
Jetting on my Barry Grant/Speed Demon is 68/78 with a .059" PVCR and 10.5" PV.
I like the cruise lean. MPG is as important as trap speed to me but I don't think the lean cruise is affecting the detonation issue.
The accelerator pump shot on the other hand probably is and I'll be working through that as well.

skinny z

Here's the latest and a little more info as this has turned into a tuning thread as much as it is a research one.
Timing before:
Initial: 12
Vacuum advance: 23 (full manifold vacuum adjusted to be all in 1" below idle vacuum which is 11" at 800 rpm).
Total idle advance: 35.
Mechanical: 20 (all in by 2900 rpm).
Max WOT timing: 32.
Maximum available cruise timing: 55 (at 2900 rpm and part throttle 11"+ vacuum)

Timing now: 4 degrees across the board.
Initial: 8
Vacuum advance: 23 (unchanged)
Total idle advance: 31
Mechanical: 20 (unchanged)
Max WOT timing: 28
Maximum available cruise timing: 51

I only had to change the initial setting of the distributor to realize a change in total advance available.
Results: No improvement. Still rattles insanely at the hit. AFRs don't spike lean although the duration is so brief (but loud) that it may skip right past the O2 sensor without registering.
Before anyone says anything, the results, detonation-wise, are the same with the vacuum advance disconnected. Driveability goes in the toilet but the rattle is still there.
Tomorrow brings a compression test to see if I've broken anything. Then a round of accelerator pump testing via the squirter size.

1gary

Hard numbers??.Our customer builds are held with cast iron heads to 9.5 SCR,8.5 DCR,quench to .035 to .040.The logic behind that is pump gas varies abunch.So someones gas in their area isn't the same as yours.It is so bad that the same brand name in you town but different stations locations can and does vary.And those quench numbers being held is a aid helping you out.So those SCR/DCR numbers gives a lead in the variances.And not at a cost of leaving a ton on the table in output or making concessions in ignition timing.

skinny z

Based on that information, it would seem I have a workable combination.
While I understand the vagueness associated with DCRs and relative engine efficiencies, it seems to me my 8.3-8.4:1 is reasonable.
That is to say, if it's been done with a similar engine (traditional SBC architecture and modern cylinder heads) by someone else, then I should be able to duplicate the results. At least from an engine that doesn't blow up standpoint.
I'll be pursuing this and hopefully don't break anything in the process.

FRMULA88

leaded gas + 02 sensors = ~50 miles of useful 02 sensor life before the lead coating the probe makes the sensor lazy eventually rendering it useless.

the best octane booster is race gas, with electronics use unleaded 100. blended 1:1 with pump gas it will increase the octane of the pump gas 4 whole octane numbers

92 + 100 / 2 = 96

104 + etc are all snake oils... they do little to nothing.

of course you could easily convert to E85 and get 94-98 octane for a fraction of the price.

FRMULA88

check the acceleator pump cam... you may not be getting pump shot fast enough..

if your are knocking there is an issue.. AFRS help, but just becuase you are rich at WOT does not mean you are not going to detonate as you have discovered on your own.

basicly two things cause detonation. 1) insufficient octane & 2) excessive heat (cylinder head temp)which does not mean engine temp because you could have a "hot spot" in the cylinder head. happens more with aluminum heads because water jackets are smaller.


If I were you I would pull the spark plugs and read the porcelains with a 10X scope. mild detonation you cannot see with the naked eye

you are looking for silver specs on the porcelains. That silver is aluminum eroding from the pistons, & that is detonation occuring in the corresponding cylinder.

if you see this occuring on the back half of the engine (5 6 7 & 8) you have a cooling & octane issue.. if it is occuring on all 8 run a better fuel and check the cooling sytem. random holes,,, is fuel distribution / octane.

what t-stat are you running? not more than 180 to be safe.
what is the condition of water pump & radiator use a hi flow water pump and make sure it is not underdriven. make sure the radiator is not clogged.
make sure the cooling system is burped of all air.
good luck !

skinny z

[quote=FRMULA88;5635493]

the best octane booster is race gas, with electronics use unleaded 100. blended 1:1 with pump gas it will increase the octane of the pump gas 4 whole octane numbers.
[quote]

If I can find it, I'll try it. It's something I've wanted to experiment with long before this new build came along. I was on the edge with the previous engine and I would have liked to have known how far I could have gone with timing and the like.

skinny z

The pump shot is on tomorrows list of things to do. That, another compression test and a good look at the plugs (again).
I have a 160 stat (temps rarely get above 180), high flow water pump, 3" rad, electric fan and a auto/manual switch for it for track days. Temperature management doesn't seem to be an issue but you never know what one or two cylinders might be doing.

1gary

One full sweep and you could run with what you got. Here's the deal.Sell off over the fall/winter months the cast iron heads you have and apply that money to buy a nice set of aluminum heads that can stand a 10.5 SCR.Clearly it is the SCR that is hurting you with the cast iron heads.From here on out you going to leave alot on the table trying to fix that and still be very close to the boarder line.Again the way gas varies that would not be a good place to be and some of the suggested fixes might in the end cost as much as a set of aluminum heads.

FRMULA88

@ 8-9 $ gallon you can burn a lot of 100 octane unleaded (especially if you are blending with premium) before you pay for the cost new alum heads. not too mention the labor. even your time is worth $.

Racers use aluminum heads for 1 reason, weight reduction.

The fact alum. is a better conductor and transfers heat faster is a mixed blessing.

All things being equal between them (chamber design, intake runners, valve, etc) At the same CR a cast iron head will make more HP (because it retains heat better) provided it does not detonate, than the alum head.

but because alum is a better conductor you can run a bit more compression over cast iron which makes up for the power loss. now some guys get carried away with CR & timing and wind up in the same place they were at with cast iron heads.

At this point you need a better fuel, reduce timing, or reduce CR. Obviously if you don't want to reduce power the simplest thing to do is use better fuel to halt detonation especially if you are stuck with a low octane premium in your area.

you may want to try a different brand of pump gas. I hate to say I knew a lady who worked years ago at a service station and the owner used to fill all the tanks with regular 87. The people paying more for 89 & 93 were simply getting ripped off.

FRMULA88

FYI

I figure you will burn thru at least 200 gallons of blended fuel @ 1:1 ratio before the added cost of the 100 octane pays for the alum heads.

assuming you can get 100 for 9$ gallon, and bare set of alum heads cost 900$ (just to keep the math simple)

200 gallons of blended fuel / 15 gallon tank = 13.33 full tanks of gas.

I am not sure how much you drive your car, but I assume it is not a daily commuter.

at a typical track outing I will burn from 5 to 7 1/2 gallons of fuel including warm up after unloading from the trailer.

1gary

I am not talking buying the cheapest aluminum heads you can find.Many of the imported heads have casting issues.Nor am I talking about AFR's.But something like a middle of the road priced set is where my thinking is.RHS sells good casting ones as a example.Sometimes you can find a show special if you go to a winter swap meet/trade show.

You also want to mate up with your SCR with them and cam profile.Don't want a set that has too big a intake runner and a cam profile that doesn't compliment your runner size or intake manifold.Ideally a 180cc runner size is a good all around size intake runner where you can keep up the volume for a good low end response and still have somewhat a top end hp curve.

skinny z

I've thought about going the aluminum head route and I would have made that decision had I known I'd be using using a different short block than the one I started with. The old block had a stock crank and an uncut deck that left the piston averaging .030" down. I wanted the iron heads to reach the target compression ratio. I felt at the time that aluminum wouldn't make the same HP as iron. I was barely going to see 10:1. With the new short, a steel crank and pistons that are only .014" down, the compression ratio went up. I stubbornly wanted to hit a .040" quench in the hopes that would help reduce detonation.
If I had to do it all again....

skinny z

A couple of issues with the race fuel.
One is that I drive a lot. At times, it IS a daily driver. I'll often drive to meetings and such or even commute with it on a daily basis depending on the job and the weather. My plans this summer where for another cross-country trip. That's 6000 miles right there and sometimes finding even 91 octane is a problem.
The other thing is that I've never seen 100 octane here in southern Ontario. The Chicago suburbs, where I used to frequent, have it for sale at the corner gas station! What a great idea. I've seen race fuel for sale at the entrance to a couple of the local drag strips, some leaded (bad) some unleaded but that's the only place I've seen it.

skinny z

I have in fact considered the RHS heads. I have their iron Vortec version now with 170cc runners. My plan was to build a smallish engine (compared to the 383s and 400s out there) emphasize low speed torque production, max rpm below 6500 and get some decent fuel economy. Keep the compression ratio up, the port velocity high and pick an appropriate cam profile. I've managed all of those things but am struggling with the high compression. I do get over 20 mpg on the highway and can tune for more but racing is out of the question at the moment.
In hindsight, if I had all of the above AND aluminum heads, I'd have exactly what I was after.
Now I either have to tune my way out of it or go back to the drawing board.

FRMULA88

I think you answered you own question. If this car is a "driver" you need to run a CR that will be safe (not marginal) with 89-91 octane pump gas fuel.


if you had EFI you could fine tune the fuel & timing curves at specific points based on RPM, & engine load (MAP sensor) and a knock sensor would also help to keep the engine safe at all times..

Unfortunately as you know a carb is a compromise.

too many people lately are obsessed with quench.

Check your engine math, a thicker head gasket will help lower the compression. but it would not drop it a full point.. how much are you looking to lose? an engine running mid-grade I would not exceed 9.5:1 with iron heads

FRMULA88

if it only knocks when you try to race then just run the best pump fuel you can find 91-93, for the street and just drive it like normal car.

when you go to the track cut it 50% with 100 unleaded and "race" it.

I just saved you a bunch of money !

1gary

I do agree with most of your post.Facts are a good quench number does help prevent detonation.That is the way the mechanics works.

1gary

No truer words where ever posted to this thread..................

Orr89RocZ

iTrader: (20)

Dart shp 180's or jegs/profiler 195's. american made, wont find a better bang for buck. Profiler are good 20 hp over the shp out the box.

skinny z

Yes. I'd even though of a second carb. One small one tuned for maximum economy and another race carb strictly for the track. That and a half a tank of racing fuel, swap the carb, set the timing and blast off.
Fact is I just want to be able to drive to the track, bolt on the slicks, rip off a few respectable ets and go home. You know, like the LS guys do everyday.
As for engine math, a swap in head gaskets from .026 to .040 drops me 10.37 to 10.03. (Don't know where the full point came from?) There's my old CR from the driveable Vortec engine from a few seasons ago.

Performance on the street does have it's advantages. I'd like my build to be driven anywhere. Hence this thread.

Agreed. That's part of what got me into this situation in the first place. You know, blinded by the light. That said, if I ever do sort this out, it might be the quench that allows the high CR as I had planned. I just wasn't anticipating over 10:1.

Yes. And they're mine. Unfortunately the heads were purchased for another short block which didn't work out. Too many miles, too many camshafts sucked through it, too worn out. The new short was a much easier way out but it didn't suit the build as I had envisioned.

I've been looking at all the options although I'm loathe to spend any more money on the heads when I have a nearly new set here. In fact I have a pair of Edlebrock Performer heads sitting on the shelf that I could swap in however they're not Vortecs so I'd need another intake. Still, they're far from I'd choose If I had my choice.

Orr89RocZ

iTrader: (20)

Well if the tune is close for air fuel and still knockin at 28 deg i would try another 4 deg pulled

Max advance of 24-25 deg by 3600. Dont give it any more any lower.

If that dont work, then go big on the cam. 282-286/230-233 intake range and let it eat. Look at the high lift magnum lobes

skinny z

WOT is not a problem. No knock that I can here. That's with the previous advance that had 32 degrees at max. It's getting there that's the issue. I can't get the car car moving with pinging.
May do a cam change. I've built this short block with the intention of being able to go 7000 at the strip or steadily at high rpms for the open road events and flying mile. The heads would hold me back some from a really big cam although something as you suggest is basically the next size up from mine catalog-wise.