Cranking pressure up and DCR down - How to ???

[sneem26]

Apologies in advance as I'm sure this topic has already been discussed at length but I was running my 388 SBC combo through the various calculators checking for cam suitability.
Without re-igniting a discussion on the merits of DCR or cranking pressure figures I was hoping for some guidance, please.
I have had the Comp XE274H FT (274/288 adv. 230/236 @.50" 110/106 LSA/ICL) recommended to me.
When I use these specs. with my proposed 10.32:1 CR ( zero deck block, 5cc valve relief flat tops, .039" quench and 72cc alloy heads ) and the cams 63 ABDC IVC I get a DCR of 8.4 from both the Pat Kelley and Wallace Racing DCR calculators, which seems too high ???
The Wallace Racing cranking pressure calculator shows 183 PSI, which seems low.
By using the larger XE284's 68 IVC the DCR drops to 8.0 but the cranking pressure also drops to 175 PSI.

My main concern is if the XE274 will be OK with my CR using premium fuel ???

If not, can someone tell me what specs. need to be manipulated to get a DCR of around 8.0 and a CP of 190-200 PSI ???

[travis]

Uh oh...you said “DCR”. Prepare yourself

I have found that most cranking compression calculations are “close” at best, and more often than not off by quite a bit. I think the XE274 is doable if the tune is spot on and your not trying to use something crazy like a 2.73 gear in a 4500 pound car.

My experience...9.4-1 355 with Vortec heads and an XE268, cranked 180, and was fine on 89 octane even with a shitty .080” quench in a 4500 pound truck with 3.08 gears, even when pulling 3-4000 pounds. It seems like the calculated DCR was around 7.7-7.8 iirc.

If a noisy valvetrain doesn’t bother you, then the XE’s do work pretty good

[n2xlr8n]

Ask CamKing.

I've used a couple of Jone's (CamKing) camshafts successfully- both engines turned out to be more than I expected.

[My427stang]

Like someone said, strap in when you talk cranking compression and DCR here

However, with aluminum heads and tight quench, I'd expect yours to do just fine....assuming all your measurements are accurate, and you are not running too much advance, too hot, or too lean. However, getting there should be easy to dial in with your tight quench and relatively small chamber and a flat top.

It's tough to draw parallels, from one engine to another, but my experience has been that you are at the high end of where you want to be, but likely not exceeding it

[Stan Weiss]

Since DCR and cranking compression are linked at the hip. I would love to see a way to have them go it opposite directions.

As far as calculators and cranking compression. I believe most times the problem is GIGO. By that I mean the people have guessed at where IVC is and not the actual IVC as measured.

Stan

[CamKing]

If not, can someone tell me what specs. need to be manipulated to get a DCR of around 8.0 and a CP of 190-200 PSI ???

Can't be done. Since DCR only calculates a form of cranking compression, they will always move in the same direction.

BTW, that cam will be fine with 10:3 compression, aluminum heads, and 91+ octane fuel

[David Redszus]

Apologies in advance as I'm sure this topic has already been discussed at length but I was running my 388 SBC combo through the various calculators checking for cam suitability.
Without re-igniting a discussion on the merits of DCR or cranking pressure figures I was hoping for some guidance, please.
I have had the Comp XE274H FT (274/288 adv. 230/236 @.50" 110/106 LSA/ICL) recommended to me.
When I use these specs. with my proposed 10.32:1 CR ( zero deck block, 5cc valve relief flat tops, .039" quench and 72cc alloy heads ) and the cams 63 ABDC IVC I get a DCR of 8.4 from both the Pat Kelley and Wallace Racing DCR calculators, which seems too high ???
The Wallace Racing cranking pressure calculator shows 183 PSI, which seems low.
By using the larger XE284's 68 IVC the DCR drops to 8.0 but the cranking pressure also drops to 175 PSI.

My main concern is if the XE274 will be OK with my CR using premium fuel ???

If not, can someone tell me what specs. need to be manipulated to get a DCR of around 8.0 and a CP of 190-200 PSI ???

Without knowing your bore, stroke and chamber volume, I can't confirm the DCR numbers for either calculator. The cranking pressures indicated for either DCR, seem low, depending on inlet air pressure.

As Mike indicated, both pressure and temperature will move with DCR, but can be varied with alternate inlet conditions.

The DCR number, by itself has limited value; it is only used to calculate compression pressure and temperature.
If fuel selection is the objective, then compression temperature matters most. Higher compression pressure being useful at higher engine rpms. Build the engine so that compression temperature is compatible with the fuel selected.

On way to lower compression pressure is to partially close the throttle.
The way to lower compression temperature is to lower inlet air temperature.

[F-BIRD'88]

You need to determine the duration and in,ake closing point at the running lash point of that cam.
.016" lash equals .0106" at the cam.

Not the .015" specs (.022" lash )
Garbage in = garbage out.

You can find the lash point durations by analyzing the cam on a cam doctor or ask comp cams for that.
.016" lash. .0106" @ cam.
Without this. The dcr calc is false.

The advertized duration ≠ lash point duration.

[sneem26]

Yeah, I thought they must have both been linked but I've seen guys quoting 8-8.2 DCR and 195-200 PSI cranking pressure figures for their 10:1 SCR small blocks and wondered if these figures could be manipulated or were just unique to certain combo's ???
Thanks to everyone who replied in particular Mr. Redszus for mentioning that different inlet air pressure inputs would vary the results.
I rechecked the figures I put into the Wallace Racing cranking pressure calculator and the uncorrected barometer reading was wrong.
I had used 29inhg and found my local reading is actually 30.5 inhg though i must admit I didn't think it would make a lot of difference if any.
This higher value raised the result from 183 to 193 PSI which seems a bit more realistic.

Mr Cam King ..... I filled in your recommendation form earlier under the name Paul Maloney in case you have a more appropriate cam suggestion for me ..... thanks for the immediate reply.

Thanks again for the responses guys.

[CamKing]

Mr Cam King ..... I filled in your recommendation form earlier under the name Paul Maloney in case you have a more appropriate cam suggestion for me

I email you my recommendation, earlier today.

Cam# SBCH, H70330-72334-110
280/288 @.004"
224/232 @.050"
.330"/.334" Lobe Lift
110 LSA
107 ICL

[GARY C]

Apologies in advance as I'm sure this topic has already been discussed at length but I was running my 388 SBC combo through the various calculators checking for cam suitability.
Without re-igniting a discussion on the merits of DCR or cranking pressure figures I was hoping for some guidance, please.
I have had the Comp XE274H FT (274/288 adv. 230/236 @.50" 110/106 LSA/ICL) recommended to me.
When I use these specs. with my proposed 10.32:1 CR ( zero deck block, 5cc valve relief flat tops, .039" quench and 72cc alloy heads ) and the cams 63 ABDC IVC I get a DCR of 8.4 from both the Pat Kelley and Wallace Racing DCR calculators, which seems too high ???
The Wallace Racing cranking pressure calculator shows 183 PSI, which seems low.
By using the larger XE284's 68 IVC the DCR drops to 8.0 but the cranking pressure also drops to 175 PSI.

My main concern is if the XE274 will be OK with my CR using premium fuel ???

If not, can someone tell me what specs. need to be manipulated to get a DCR of around 8.0 and a CP of 190-200 PSI ???

Kelly's program at 8.4 on my iron head SBC 350 put me just over 200 psi, I ran it with 92 octane but t did require some time getting the timing curve correct. I think I did 7.9 or 8 on my Pontiac 400 and put me at 195.

[GARY C]

Uh oh...you said “DCR”. Prepare yourself

I have found that most cranking compression calculations are “close” at best, and more often than not off by quite a bit. I think the XE274 is doable if the tune is spot on and your not trying to use something crazy like a 2.73 gear in a 4500 pound car.

My experience...9.4-1 355 with Vortec heads and an XE268, cranked 180, and was fine on 89 octane even with a shitty .080” quench in a 4500 pound truck with 3.08 gears, even when pulling 3-4000 pounds. It seems like the calculated DCR was around 7.7-7.8 iirc.

If a noisy valvetrain doesn’t bother you, then the XE’s do work pretty good

The other part of the equation is do all gauges read the same?

[travis]

The other part of the equation is do all gauges read the same?

I wouldn’t bet on it!

[MadBill]

The other part of the equation is do all gauges read the same?

I wouldn’t bet on it!

A problem I've run across more than once is the tester reading low because someone has replaced the special Schrader valve with a standard tire valve.

[GARY C]

The other part of the equation is do all gauges read the same?

I wouldn’t bet on it!

A problem I've run across more than once is the tester reading low because someone has replaced the special Schrader valve with a standard tire valve.

Glad to know they are different so I don't ever make that mistake.