Max CR with 87 octane

[johnj92131]

Just to keep this from becoming a Jim-hijacked thread, here is a link to an interesting treatise on SCR, DCR, cam timing, etc., and is focused on Ford inline sixes. They are a push rod, non-cross-flow headed engine like ours, so their data probably has some relevance since they are just as out-dated as our inlines.

http://classicinlines.com/CompressionRatio.asp

The punch line is that the recommended DCR for use with Regular/Mid-grade (87-89 octane) is 7.0-7.7:1 according to their analyses.

Good find. Reading the 67 HotRod Magazine article on the Mustang 6 nodified by Ak Miller was expecially interesting. Miller went step by step from 65 rear wheel horsepower to 100 horsepower with simple changes to the stock engine. He went from 90 hp to 100 hp by milling the head .060 for a compression bump of just over a full point. That was 11% more power at the rear wheels with the switch to premium gas. Almost enough to abandon 87 RUG for Premium. Lets see... from 190 hp to 210 hp just by bumping compression from 8.7 to 9.9??? Sounds a bit high, but tempting. Then I wonder what the DCR was in that stock 200 Ford six?

[Root Moose]

So, I was playing around the compression ratio tool.

+0.030 over
0 deck
0.043 gasket
IC945s
RENIX cam retarded -12 degrees (-4 more than default)

SCR = 10.77
DCR = 7.58

What's wrong with this picture? Could you run this on 87 successfully?

[Jim K in PA]

So, I was playing around the compression ratio tool.

+0.030 over
0 deck
0.043 gasket
IC945s
RENIX cam retarded -12 degrees (-4 more than default)

SCR = 10.77
DCR = 7.58

What's wrong with this picture? Could you run this on 87 successfully?

Good question. That is a LOT of cam offset. I would talk to your builder about that. I suspect it would not ping, but you might wind up with a dog of an engine at low RPM, and all your power at the top end. Here is another link to some generalized info on cam timing and it's effects: http://www.compcams.com/Technical/FAQ/LSAproperties.asp

Why would you want to run that combo? The '91-'95 cam can be installed straight up with the larger dished IC pistons with essentially the same DCR.

[SilverXJ]

I would be concerned about running that much retard. There have a been a few instances of someone being off a tooth on the timing set (~13*) and the engine wouldn't run right.

[Root Moose]

Thanks for the link. I'll take a look when I get a moment to focus.

I guess what I don't understand at this point is the relationship between SCR and DCR... if there is one.

Do we just design for DCR and let the chips fall where they may for SCR?

Also, the RENIX cam is retarded -8°, or so the spec says, so that means that installed per factory procedures that the lobes would be retarded -8° relative to say a 96+ cam? Yeah, something else I'm not sure of... Pointers? Maybe it'll be in the link provided above?

-8° is a mechanical property of the cam versus installation degreeing? Sorry, if I'm not making/asking my point correctly... Not at a keyboard currently.

[Jim K in PA]

Thanks for the link. I'll take a look when I get a moment to focus.

I guess what I don't understand at this point is the relationship between SCR and DCR... if there is one.

Do we just design for DCR and let the chips fall where they may for SCR?

There is a relationship. DCR is what is left of SCR after the effects of the cam are accounted for (IVC, LSA, duration, and lift). The engine will not "see" SCR in typical applications, only DCR. So, within reason, you do in fact design the engine to a desired DCR. If your desired displacement and cam combination results in a DCR lower than you desire, you increase SCR to raise the DCR, and vice versa.

Also, the RENIX cam is retarded -8°, or so the spec says, so that means that installed per factory procedures that the lobes would be retarded -8° relative to say a 96+ cam? Yeah, something else I'm not sure of... Pointers? Maybe it'll be in the link provided above?

-8° is a mechanical property of the cam versus installation degreeing? Sorry, if I'm not making/asking my point correctly... Not at a keyboard currently.

The degree of advance or retard of the cam is relative to a specific event in the crank rotation, usually referenced to TDC. Changing only the cam timing (no other change in the cam) moves all the events earlier or later in the cycle relative to TDC. Advancing or retarding the cam relative to its starting point can be done on the engine with multiple key ways in the cam or crank gear (or with a fancy adjustable gear), or "in the cam" when it is ground by the manufacturer. In fact, all they really need to do is move the key way to change the timing events.

As I mentioned above, and Chris also agrees, that much built in retardation will likely result in a poor running engine, certainly at low-mid RPM. The range of cam "tuning" that I typically see is no more than +/- 4 degrees. I do not know why the factory chose to retard it 8 degrees from the factory in the Renix days, other than to react to an emissions requirement at the least cost.

Hope this helps.

[Cheromaniac]

Also, the RENIX cam is retarded -8°

I do not know why the factory chose to retard it 8 degrees from the factory in the Renix days, other than to react to an emissions requirement at the least cost.

Partially correct. The same cam grind was used in the Renix 4.0 and in the '91-'95 HO 4.0, however it was installed with 1.5 degrees of advance (112* LSA, 110.5* ICA) in the Renix and 8 degrees of retardation (112* LSA, 120* ICA) in the HO. When the HO engine was introduced DC did away with the knock sensors and EGR (probably to reduce costs) so they retarded the cam instead so that the engine would still run on 87 octane. That, together with the higher-flowing HO head and bigger TB, meant that the HO engine produced 13 more HP and the torque peak was shifted to higher rpm.

[Jim K in PA]

Also, the RENIX cam is retarded -8°

I do not know why the factory chose to retard it 8 degrees from the factory in the Renix days, other than to react to an emissions requirement at the least cost.

Partially correct. The same cam grind was used in the Renix 4.0 and in the '91-'95 HO 4.0, however it was installed with 1.5 degrees of advance (112* LSA, 110.5* ICA) in the Renix and 8 degrees of retardation (112* LSA, 120* ICA) in the HO. When the HO engine was introduced DC did away with the knock sensors and EGR (probably to reduce costs) so they retarded the cam instead so that the engine would still run on 87 octane. That, together with the higher-flowing HO head and bigger TB, meant that the HO engine produced 13 more HP and the torque peak was shifted to higher rpm.

Woof! That is a huge difference. I am sure there was a change in the ECM too. 9.5 degrees retardation just to avoid the possibility of detonation? All due to better cylinder filling? That implies that DCR should be even lower than what I am considering . . . hmmm.

[johnj92131]

as for pistons there are a lot of choices for custom out there in a reasonable price range for what the KB's are going for.

Bulltear is another name if have seen and spoke with months ago. Last time I spoke with someone from there they said they had there slugs on backorder. There foreign supplier was backlogged and they said there was not going to be any till something mid year (which is approaching). They get there slugs and machine them down some to there specs <$500 last i saw.

Brian

FYI - I just took a look at the Bulltear site, and they no longer list any pistons for the Jeep inline six.

Called Bulltear last Friday, they are still selling pistons for the stroker. Seemed surprised when I told them it was not listed on the web site. Said maybe it was taken down when they were having delivery problems with their blanks. Price was $475 + delivery.

[Jim K in PA]

Called Bulltear last Friday, they are still selling pistons for the stroker. Seemed surprised when I told them it was not listed on the web site. Said maybe it was taken down when they were having delivery problems with their blanks. Price was $475 + delivery.

Excellent! Thanks for following up on that John. I may just go with them for a custom pin height and dish, especially at that price. I'll give them a call.

[Cheromaniac]

I am sure there was a change in the ECM too.

I expect so and yes, retarding the cam timing would indeed have lowered the DCR.