MTFCA Forum

I’m helping a guy get a car running after 15 years. It starts and runs smooth. It has this after market head. I am assuming it is a higher compression. I think it is a “Giant” brand. When doing a compression test what PSI reading should I get? Thanks

Assuming it is a 5.5:1 head, somewhere under 5.5 x 14.7... so something in the range of 72-75PSI would be quite good

another source puts it at only 4.3:1, which would be something under 4.3 x 14.7, or around 58PSI

hopefully someone who has cc'd one will chime in

According to the Tulsa Club technical page on head design, the Haibe, Giant Power, and Simmons Super Power heads are similar in combustion chamber design and volume. Measured at 255 cc, the resulting compression ratio on an un-milled example would be 4.3:1. http://mtfctulsa.com/Tech/head_design.htm

Keep crankin’
Eric

Not all era replacement head were higher compression, some were just better designs that allowed better firing and combustion.

I have been running a Giant power head on my 13 touring and have been happy on performance. Do not consider it a High compression head.....just a head with better design. The head is cast iron, larger coolant capacity and does not eat out like aluminum heads. Better squish design compared to stock high or low head. The Z and Puris aluminum head have smaller combustion chambers, improved design and provide more compression =better performance. JMHO

Les

THANKS for all the info. Going to try to give the old car a test drive this weekend. Been sitting for a few decades.

I have a Giant Power Head. Before fitting it I measured it and calculated 5.1:1 compression ratio.
It does have the 'squish' areas above the pistons.
It makes a significant improvement to performance.
On most of those hills where in High with the stock '26 head you gradually slow down from 30 to about 22, it holds 30 or close.

Before the Giant, I tried an aluminium head with a higher CR (above 6:1 I think). Not a 'Z' or a Prus, but well made with steel washers counterbored into the top for the bolts. No identifying marks. This also performed well, but I felt it was a bit too much and that something would break sooner or later.
Giant = Goldilocks for me.