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Compression Ratio Limits and Iron Heads

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Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 7:56 am

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 here too.
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.
So would your cranking pressure (and elevation above sea level).
What I'm trying to get at here is whether or not I've overdone it with my compression ratio.
Last year’s spec was Vortec headed with an XR276HR cam. The SCR was spot on 10:1. Although I needed premium fuel, detonation was not issue. I kept an eye on 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 iron head and a cam with similar timing specs to the 276 albeit a custom grind with more lift.
I’m struggling with detonation especially on the initial hit from a standing start. There’s a bit of carb tuning still to be done however I think I may have overdone it compression wise. I was trying to achieve a .040” quench and in doing so I used a .026” head gasket combined with the .014” piston below deck height.
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 more manageable level.
Any insights from builders are appreciated.
If anyone wants detailed specs to run SCR and DCR numbers that’s not a problem.
I've calculated my DCR at 8.4:1.
Cranking compression is 190psi +/- 3% (600' elevation)
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Re: Compression Ratio Limits and Iron Heads

Postby Walter R. Malik » Wed Sep 11, 2013 8:34 am

What application is this vehicle ...?
A racer would just use the correct fuel for whatever the compression ratio is.
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Re: Compression Ratio Limits and Iron Heads

Postby F-BIRD'88 » Wed Sep 11, 2013 9:15 am

Last years fuel was better than this year.

The actual quality of the fuel used is a HUGE factor.
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Re: Compression Ratio Limits and Iron Heads

Postby PackardV8 » Wed Sep 11, 2013 12:23 pm

I’m struggling with detonation especially on the initial hit from a standing start.


1. How hard is this initial hit? Pulling away from a stop or full strip launch? If launch, what RPM?
2. What initial and total timing at what RPM?
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Re: Compression Ratio Limits and Iron Heads

Postby swatson454 » Wed Sep 11, 2013 2:30 pm

Are there any steps you can take to reduce the intake temp and/or increase water management?

Blocked heat risers
Cool air intake ducts
Increased water flow from the block to the heads
Pull water from the back of the intake
OEM water pump?

I doubt if adding squish clearance is going to do you any favors. Just a few thoughts...


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Re: Compression Ratio Limits and Iron Heads

Postby raceczar » Wed Sep 11, 2013 3:19 pm

What spark plug are you using? No way will .4 CR increase go from correct to rattling unless you were extremely borderline to begin with. We regularly run 11:1 on pump gas with iron heads without issues. As you said, camshaft selection plays a HUGE part. Spark plugs are the second part of it. I would suspect you are 2-3 ranges too hot.
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Re: Compression Ratio Limits and Iron Heads

Postby My427stang » Wed Sep 11, 2013 4:46 pm

I'd say you are right on the edge for a streeter with your numbers

You could rock the cam back a bit, lower initial timing and recurve for total, cool it down, fatten up the mixture, all with help

FWIW, I run 10.76:1 SCR in my 489 inch FE Mustang, 10:1 in my 445 FE 4x4 truck, DCR for the Mustang works out to 8.33, and the truck is 8.1, neither rattle on anything. Both have crossovers blocked. the 489 is alum head, the 445 is iron. The Mustang has a computer controlled curve, but the truck is traditional, 12 initial, 36 total, all in by 2800

The truck runs a Comp 270H, but I rocked it back to 110 ICL, seems to be very happy with no loss of torque at all.
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 445 FE, hot rod 4x4
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:16 pm

[quote="Walter R. Malik"]What application is this vehicle ...?
A racer would just use the correct fuel for whatever the compression ratio is.[/quote]

Street/strip/cross country cruiser.
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:17 pm

[quote="F-BIRD'88"]Last years fuel was better than this year.

The actual quality of the fuel used is a HUGE factor.[/quote]

Limited to 94 octane pump gas.
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:23 pm

[quote="PackardV8"][quote]I’m struggling with detonation especially on the initial hit from a standing start. [/quote]

1. How hard is this initial hit? Pulling away from a stop or full strip launch? If launch, what RPM?
2. What initial and total timing at what RPM?[/quote]

It shows in a couple of ways. On the street (a quiet one in the country!) from cruise at 30 mph, pin it, the convertor flashes to 4000, the trans downshifts (auto) and the rattle is massive.
Part throttle acceleration from a standing start, (a full launch would be impossible), the same rattling. It's brief but loud.
Initial timing is (was) 12 degrees with another 20 mechanical all in by 2900.
All of this is above the stall of the convertor so there's no rpm less than 3000 in these tests.
Full manifold vacuum advance connected or disconnected doesn't make a difference in the detonation.
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:26 pm

[quote="swatson454"]Are there any steps you can take to reduce the intake temp and/or increase water management?

Blocked heat risers
Cool air intake ducts
Increased water flow from the block to the heads
Pull water from the back of the intake
OEM water pump?

I doubt if adding squish clearance is going to do you any favors. Just a few thoughts...


Shawn[/quote]

No heat riser.
Cowl hood.
High flow water pump (Weiand)
3" rad and a 160 stat. The temps rarely get past 180. Sensor is in the right side head at the back ( between 6 and 8 ).
Like you, I think the .040" squish is a benefit here.
Last edited by skinny z on Wed Sep 11, 2013 6:39 pm, edited 1 time in total.
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Re: Compression Ratio Limits and Iron Heads

Postby F-BIRD'88 » Wed Sep 11, 2013 6:28 pm

skinny z wrote:
F-BIRD'88 wrote:Last years fuel was better than this year.

The actual quality of the fuel used is a HUGE factor.


Limited to 94 octane pump gas.


Try a different brand of 94 or go to a different gas station to fill up.
Lots of stations play games, becasue now 99% of the cars on the road have active spark knock management and they will
not knock on inferior fuel octane. Most people that just drive their cars will never complain.
The only people that notice or even care are us gear heads.
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:34 pm

[quote="raceczar"]What spark plug are you using? No way will .4 CR increase go from correct to rattling unless you were extremely borderline to begin with. We regularly run 11:1 on pump gas with iron heads without issues. As you said, camshaft selection plays a HUGE part. Spark plugs are the second part of it. I would suspect you are 2-3 ranges too hot.[/quote]

Plugs are typically AC R44LTS or more recently Champion RS12YC or RS9YC. The plugs have always looked good regarding colour and ground strap annealing.
That said, tomorrow will be another compression test to see if I broken anything with all of this testing and I'll be examining the plugs again.
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Re: Compression Ratio Limits and Iron Heads

Postby F-BIRD'88 » Wed Sep 11, 2013 6:36 pm

Something you can try. Does your intake manifold have the extra water ports in back near the distributor?
If so, try connecting the heater/bypass hose from the top of the water pump
to the rear driver's side water port on the intake manifold. near the distributor.

Try it.
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Re: Compression Ratio Limits and Iron Heads

Postby skinny z » Wed Sep 11, 2013 6:37 pm

[quote="My427stang"]I'd say you are right on the edge for a streeter with your numbers

You could rock the cam back a bit, lower initial timing and recurve for total, cool it down, fatten up the mixture, all with help

FWIW, I run 10.76:1 SCR in my 489 inch FE Mustang, 10:1 in my 445 FE 4x4 truck, DCR for the Mustang works out to 8.33, and the truck is 8.1, neither rattle on anything. Both have crossovers blocked. the 489 is alum head, the 445 is iron. The Mustang has a computer controlled curve, but the truck is traditional, 12 initial, 36 total, all in by 2800

The truck runs a Comp 270H, but I rocked it back to 110 ICL, seems to be very happy with no loss of torque at all.[/quote]

Retarding the cam is on the list of potential fixes if I can't tune my way out of this. I have a 106 ICL/110 LSA and dropping back 4 degrees I think will have little impact on low engine speed torque.
Your 8.3:1 DCR is getting up there too. It's encouraging that you've made it workable.
Last edited by skinny z on Wed Sep 11, 2013 6:50 pm, edited 1 time in total.
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