I put this in FI as well but would like some other opinions from the tech guys here on H-T

A friend/co-worker of mine and i were having a conversation at work about compression on boosted motors. He is a domestic guy and i obviously am an import guy but have a passion for both.

Here's the argument.....I said that a friend of mine should boost his V-8 that is currently at 10.2-10.4cp, the car has all forged internals and aluminum heads, it is stroked and bored, reguardless of this this his argument is that do to higher compression a V-8 motor recieves more stress with higher compression than a 4 cylinder with the same cp. I then told him about the k20 and its 11.1cp and how people have made rediclous numbers with them, but his argument was still that since there are less cylinders there is less stress at the same set compression as opposed to a V8 at the same cp.

I agree to a point with him due to rotating assembly weights but still cant agree with his side of the arguement. We then got into retarding timing, EGT differences and IAT's etc but it led no where aside from our motors running hotter than most V8's.

I know this is kind of a ramble but i would like to get opinions with knowledgable answers aside from i am horribly wrong to he is a complete idiot.

TIA

 

Stress = reciprocating weight + RPM.

By the way, what V4 Honda motor do you have?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">

By the way, what V4 Honda motor do you have? </TD></TR></TABLE>

the only one ever made

lets stay on topic here

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by urbansi &raquo;</TD></TR><TR><TD CLASS="quote">Here's the argument.....I said that a friend of mine should boost his V-8 that is currently at 10.2-10.4cp, the car has all forged internals and aluminum heads, it is stroked and bored, reguardless of this this his argument is that do to higher compression a V-8 motor recieves more stress with higher compression than a 4 cylinder with the same cp. I then told him about the k20 and its 11.1cp and how people have made rediclous numbers with them, but his argument was still that since there are less cylinders there is less stress at the same set compression as opposed to a V8 at the same cp.</TD></TR></TABLE>

He does have somewhat of a point here, although he might not know it. If his car is a pushrod V8, then he has a wedge-style chamber with only two valves and an asymmetrically located sparkplug. Such a setup is FAR more detonation-prone than a four valve pentroof chamber with a centrally located sparkplug (the norm for ANY modern four cylinder engine). The fact that the detonation threshhold is SO much higher with this more modern combustion chamber design means that you can safely run higher compression ratios with this design, whether boosted or allmotor. Boosting just takes you closer to the detonation threshhold, so this matters more. Of course, if he has an import V8 OR a Mustang Cobra V8, then he has the same pentroof chambers found on four cylinder engines.

Another factor is bore diameter. a V8 is probably going to have a larger bore diameter than a four cylinder. This also lowers the detonation threshhold somewhat, albeit not as much as a poor chamber design. Also keep in mind that this factor has a relatively smaller effect, since the actual difference i bore diameter is relatively small between a four cylinder and V8 (86mm in a K20 vs the mid-high 90s in a typical V8).