Cranking Compression and Octane Requirements

[Stan Weiss]

Cranking Compression and Octane Requirements

I was having a conversation with someone about this this morning. Can you post up your combination? Next to each item I have shown mine.

Engine type -SBC
Bore - 4.0
Stroke - 3.25
Rod Length - 5.7
CR - 10.0:1
Intake seat-to-seat duration - 286
ICL - 110
Heads (Iron - Alum) - Iron
Cranking Compression - 180
Octane - 91
Location - Sea level

Thanks
Stan

[groberts101]

Engine type -SBC
Bore - 4.040
Stroke - 3.75
Rod Length - 6
CR - 11.5
Intake seat-to-seat duration - 282
ICL - 108(2° retarded)
Heads (Iron - Alum) - Alum
Cranking Compression - 215-220
Octane - 93
Location - 900

[statsystems]

Engine type-340 dodge
Bore-4.04
Stroke-3.313
Rod length-6.123
CR-11.08:1 actual measured
Intake seat to seat duration-281
LSA-105 (installed at 105)
Cranking compression-TBD I have not rated it yet...maybe later today
Octane-92
Location-1100

[GARY C]

Engine type -SBC
Bore - 4.0
Stroke - 3.48
Rod Length - 5.7
CR - 10.0:1
Intake seat-to-seat duration - 262
ICL - 105
Heads (Iron - Alum) - Iron D shape chamber
Cranking Compression - 205
Octane - 92 or 87 & TK7
Location - 500, I think..Dallas Tx.

[MadBill]

Wikipedia says 430 ft.

[turbo2256b]

Engine type -SBF
Bore - 4.0
Stroke - 3.00
Rod Length - 5.09
CR - 9.0 : 1
Intake seat-to-seat duration - 282
ICL - 110
Heads (Iron - Alum) - Iron D shape chamber
Cranking Compression - 180
Octane - 93 , likes 4 Aces added
Location - 800 Central KY.

[Stan Weiss]

Thanks for the posts.

Stan

[statsystems]

Thanks for the posts.

Stan

What are you figuring Stan? I'm kind of curious.

[claysmoker]

SBC
4.155
3.75
6.0
11.1
294
106
AFR Alum 65cc
200
91
5000 ft

[Stan Weiss]

Thanks for the posts.

Stan

What are you figuring Stan? I'm kind of curious.

I just wanted to see if there was a come up with something to help people building pump gas engines. I was looking for data to see if cranking compression might be a good gauge.

Stan

[MadBill]

DV targets 200 psi for a hot street engine. I don't need less compression, just more cam...

[SchmidtMotorWorks]

Thanks for the posts.

Stan

What are you figuring Stan? I'm kind of curious.

I just wanted to see if there was a come up with something to help people building pump gas engines. I was looking for data to see if cranking compression might be a good gauge.

Stan

IIRC there was a thread on this in the past, as I recall it evolved into two tracks, one for aluminum heads and one for iron.

You might also need to include something to compensate for tuned manifolds that can raise trapped mass.

[Xnke]

Engine-Nissan L-28 inline 6 (in a generally detonation prone configuration)
Bore-3.46"
Stroke 3.11"
9cc dished piston-round dish
9.7:1 compression
280 degrees intake seat duration
72 degree intake valve closing point
Cranking compression 120PSI
87 octane unleaded pump fuel
Aluminum head, full-circle open combustion chamber

180RWHP, 210ft-lbs.

[Leftcoaster]

1986 VW Jetta, "RD" 1.8l SOHC 2v iron block, ali head, stock untouched 303,000k engine with 5sp

Bore/stroke 81.0 x 86.4mm, rods 144.0mm, claimed comp 10.0 to 1, mildly dished pistons

Bosch CIS-E ecu with distributor, fine with 87 RON until gasahol was added, now use 91 RON

Tested yearly since 2006 and 205,000k, Sun and Equus guages indicate compression drop from 240 to 230psi

Mild blowby, no discernible loss of performance or economy, rings at 350,00k?

Note, an overspec battery, decent cables, and oe direct drive starter must crank this at 600 - 800rpm

[GARY C]

Thanks for the posts.

Stan

What are you figuring Stan? I'm kind of curious.

I just wanted to see if there was a come up with something to help people building pump gas engines. I was looking for data to see if cranking compression might be a good gauge.

Stan

For me the simple method is 10 to 10.5 compression with a cylinder head sized for the ci and rpm and then I want a cam that puts me at 195 to 200 psi in conjunction with a rocker ratio that will take advantage of as much of the flow potential of the head as possible. This would be for an engine with a very moderate convertor and gear 2000 to 6000 type engine.